HomeMy WebLinkAboutAkutan Geophysical and Geological Summary Report 2-2013
Geothermal Resource Group, Inc.
75-145 St. Charles Place, Suite B
Palm Desert, CA 92211
Phone: 760-341-0186
Preliminary Summary of Findings
Akutan Geophysical and Geological Investigation, August 2012
For the Purpose of Delineating and Targeting the Geothermal Resource
of Hot Springs Bay Valley
Commissioned by
City of Akutan, Alaska
Report prepared by:
Geothermal Resource Group Inc.
With assistance and additional materials from:
William Cummings, PhD, Cummings Geoscience
Nicholas Hinz, PhD, University of Nevada
Amanda Kolker, PhD, AK Geothermal Inc.
Gary Oppliger, PhD, Zonge International, Inc.
Pete Stelling, PhD, Stelco Magma Inc.
Confidential-Not to be reproduced without permission of the City of Akutan
Photo by Nicholas Hinz
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Table of Contents
Table of Figures ............................................................................................................................................. 3
Acknowledgements .............................................................................................................................. 4
Introduction ............................................................................................................................................ 4
Background ............................................................................................................................................. 5
Method selection ................................................................................................................................... 7
Field Work Summary ........................................................................................................................... 8
Magnetotelluric (MT) Survey ........................................................................................................................ 8
Gravity Survey ............................................................................................................................................... 9
Geologic Mapping ....................................................................................................................................... 10
Data processing and interpretation ............................................................................................. 11
MT Survey ................................................................................................................................................... 11
Gravity Survey ............................................................................................................................................. 18
Geologic Mapping ....................................................................................................................................... 19
Results ..................................................................................................................................................... 23
Conceptual Model ....................................................................................................................................... 25
Resource Capacity ....................................................................................................................................... 26
Resource Risk .............................................................................................................................................. 26
Recommendations .............................................................................................................................. 27
Drilling Options ........................................................................................................................................... 27
Preliminary Well Design .............................................................................................................................. 31
Cost Reduction Methods and Considerations ......................................................................................... 31
Works Cited ........................................................................................................................................... 34
List of Appendices ............................................................................................................................... 34
Appendix 1 ............................................................................................................................................. 35
Appendix 2 ............................................................................................................................................. 42
Appendix 3 ............................................................................................................................................. 53
Appendix 4 ............................................................................................................................................. 93
Appendix 5 .......................................................................................................................................... 174
Appendix 6 .......................................................................................................................................... 203
Appendix 7 .......................................................................................................................................... 295
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Table of Figures
Figure 1: Akutan Island and location of 2012 field work. (Courtesy Nicholas Hinz) .................... 5
Figure 2: MT field work photos. (Photos by Mary Ohren and Curtis Caton) ................................ 9
Figure 3: Gravity survey acquisition photos. (Photos courtesy Christopher Kratt) ...................... 10
Figure 4: Geological fieldwork images. (Photos courtesy Nicholas Hinz and Gregory Dering) . 11
Figure 5: Image of 3D MT model, showing two potential well trajectories for reference.
(Courtesy Zonge International, Inc.) ............................................................................................. 12
Figure 6: Map showing top elevation of low-resistivity layer and where it reaches the surface. . 13
Figure 7: Plan map of resistivity model showing 25 m (~82') below mean sea level, the top most
level that the geothermal reservoir will be intersected according to the model. ........................... 15
Figure 8: Map showing MT resistivity model at the middle of the elevation where the geothermal
reservoir is expected based on the model. .................................................................................... 16
Figure 9: Map showing MT resistivity model at the lowest depth that the geothermal reservoir is
expected based on the model. ....................................................................................................... 17
Figure 10: Gravity model showing the interpreted elevation of a dense basement rock. ............. 18
Figure 11: Geologic map of fumarole area and immediate surroundings produced by Hinz, et al,
2012............................................................................................................................................... 20
Figure 12: Cross sections from 2012 geologic report. .................................................................. 21
Figure 13: Images of the alteration and mineral deposition of the Akutan geothermal system,
taken from Hinz et al., 2012, which can be found as Appendix 5. ............................................... 22
Figure 14: MT model slice through center of fumarole area with geologic cross section overlay.
....................................................................................................................................................... 24
Figure 15: MT model of the resistivity at the ground surface. ..................................................... 25
Figure 16: Recommended well pad and well sites, south and southwest of the fumarole area. ... 28
Figure 17: The trajectories of PW 1 and PW-4 inside the resistivity model, with the geo-
referenced geologic cross section A-A' shown. ............................................................................ 29
Figure 18: Preliminary well design for Akutan geothermal production wells. ............................. 31
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Acknowledgements
The research team would like to thank Mayor Joe Bereskin and the residents of the City of
Akutan for their hospitality and assistance during the 2012 fieldwork season and constant
support for the Akutan Geothermal Project. We would also like to acknowledge the assistance of
Rich Koehler of the Alaska Division of Geological and Geophysical Surveys, who spent three
days with the team in the field applying his expertise in recent tectonic activity to aid the
geologic investigation and participated in the well targeting meeting in Seattle, WA.
Introduction
The City of Akutan undertook a program of exploration during the summer of 2012 to augment
previous work to establish and target a geothermal resource in the vicinity of Hot Springs Valley
(HSBV). Exploration methods included geologic mapping, and magnetotelluric (MT) resistivity
and gravity data acquisition. The area targeted was the highland region along the eastern flanks
of the volcano above HSBV, encompassing the fumarole field. The fieldwork was carried out
between August 11th and 24th, 2012. Subsequent to the fieldwork, industry experts in geology
and geophysics processed the data and integrated it with the data from previous exploration
efforts. A meeting was held in Seattle on November 13, 2012 during which the primary scientists
concluded that the preponderance of evidence supported the likelihood of a commercially viable
geothermal resource. During the same meeting, two subsurface targets were selected for
production drilling. The following report and attachments detail these conclusions and the
methods used to reach them.
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Figure 1: Akutan Island and location of 2012 field work. (Courtesy Nicholas Hinz)
Background
An exploration program was established in 2008 by the City of Akutan to define the geothermal
resources of Hot Springs Bay Valley (HSBV) (Figure 1). The program was directed by Amanda
Kolker, PhD, of AK Geothermal Inc. (AKG) and administered through RMA Consultants, Inc.
(RMA). In 2009, the first year of data acquisition, an initial conceptual model of the resource
was developed from a Magnetotelluric (MT) survey, and a review and reevaluation of fluid
chemistry data from the hot springs. A soil gas and temperature survey completed during the
2009 field season did not significantly contribute to the conceptual model. The initial model led
to the selection of four drilling targets for core and temperature gradient wells in the HSBV, to
be drilled during the summer of 2010. Geothermal Resource Group, Inc. (GRG) was selected to
direct the drilling campaign. More in-depth information about the development of the 2009
conceptual model can be found in Appendix 1: Geothermal Exploration at Akutan, Alaska:
Favorable Indications for a High-Enthalpy Hydrothermal Resource near a Remote Market;
Kolker, et al; 2012.
Due to budgetary constraints, only two of the four selected targets were drilled. Wells TG2 (Hot
Springs Well) and TG4 (South Elbow Well) were planned to 457 m (1500’) with the intention of
intersecting the shallow outflow of the resource, sampling reservoir fluids, and establishing
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temperatures, all to improve resource data and establish the extent of the resource. TG2 was
located on the northwest side of the study area, adjacent to the hot springs near the mouth of
HSBV. TG2 intersected a highly permeable, over-pressured interval at 178 m (585’) with a
downhole temperature of 182°C (359°F). Drilling was halted at 254 m (833’) due to problems
related to high borehole temperatures and equipment breakdowns. TG4 was located toward the
southern head of HSBV and was drilled to 457 m (1500’). TG4 showed a high temperature
gradient but no significant permeability. Both wells are now believed to be located outside of the
main reservoir as indicated by a lack of extensive alteration. Further information about the
drilling campaign and results can be found in Appendix 2: Exploration of the Akutan Geothermal
Resource Area, Kolker et al 2012.
Subsequent to the 2010 core drilling, exploration work included a core sample analysis, the
acquisition of geo-rectified aerial photographs of HSBV, and the creation of a digital elevation
map of the valley. During this time, the existing project data was integrated into the conceptual
geological model and by the late fall of 2011 it had been concluded, for a variety of reasons, that
the most promising target for exploitation was the upflow region encompassing the fumaroles.
Unfortunately, there was a shortage of geophysical and structural data of the area around the
fumarole field to more conclusively support this opinion.
Geothermal resource viability relies on three factors: heat, fluid, and permeability. At Akutan
there is no shortage of the first two elements given the nearby ocean, the high rainfall in the area
and the active volcano that created the island. But without having the third element of
permeability it is difficult to extract the heat needed for energy production. Unfortunately, rocks
that have formed from lava flows and ash layers typically have low permeability. These rocks,
when weathered and altered, tend to form clay, which further reduces the permeability. To access
a geothermal resource, the drilled wells must tap into a permeable zone at depth. It is generally
considered that permeable faults and fractures exist as a result of active tectonic forces, and on
Akutan these forces are very active. The presence of the fumarole field is instructive for
indicating the upflow portion of the geothermal resource; there must be some permeability below
this zone or the steam and fluid would not be able to come to the surface. Drilling under the
fumaroles from a surface location to the south has therefore been an attractive proposition.
In the past, terrain and weather conditions have prevented extensive exploration in the highland
area surrounding the fumaroles. Prior to finalizing the surface locations and drilling targets,
further assessment was required, particularly evidence from the western part of the conceived
geothermal field. In order to more firmly establish the limits of the productive reservoir and
minimize resource risk, the City of Akutan instituted a program of work for 2012. The first part
of this work was to develop a strategy that would supplement the existing exploration data,
acquire additional data through fieldwork, and then integrate all data in order to more firmly
establish the limits of the productive reservoir and minimize resource risk. The second part was
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to analyze the results of this work, verify the suitability of the area as a potential geothermal
reservoir, and to identify at least one primary and two secondary drilling targets to be confirmed
in a drilling phase of the project.
Method selection
A variety of methods may be used to identify and characterize geothermal resources leading to
the selection of drilling targets. The factors leading to the selection of the appropriate method
include the location, budget, geological setting, resource type, past exploration techniques,
surface conditions, weather, and potential benefits to the project. All of these factors came into
account in determining exploration methods to use in the 2012 field season for the Akutan
geothermal project.
The MT survey of 2009 found what was interpreted to be a clay cap, potentially created by
geothermal alteration. This is a common indicator used in the interpretation of MT data for
geothermal exploration to indicate the size of the potential resource. Because the 2009 survey
was limited to lower elevations within HSBV, the data necessary to determine the extent of the
clay cap over the fumarole field was absent. The 2012 MT survey was intended as an extension
of the previous survey to the west and to cover the area of the primary surface expression of the
geothermal resource.
The use of a single geophysical method when performing exploration work is often inadequate.
It can be compared to having only one sense without the benefit of the other four. As methods
were being considered, it was clear that a second geophysical technique was needed to augment
the resistivity data gathered during the MT survey. Several techniques were considered. A
gravity survey was chosen for its incremental cost adjunct to deploying the MT survey, low cost
per station, ability to cover the entire field area in the 2-week time frame for exploration, and the
high potential for direct relationship to the MT data for interpretation. A gravity survey measures
density differences in the subsurface, revealing faults, fractures, and altered rock zones that can
be conduits or hosts for the geothermal reservoir. Other techniques were considered, including
induced seismic, ground magnetics, controlled-source AMT, and an aeromagnetic survey. These
were each eliminated based on budget, feasibility or undefined potential benefit to the overall
data set.
The need for geological mapping has been a priority of the project since 2009; there has only
been limited past direct geologic study and none focusing on the geothermal resource
specifically. The small cost of having a skilled structural geologist on the ground is invaluable
for confirmation and interpretation of the geophysical data. In addition to the on-the-ground
mapping, the geologists were able to do a wider analysis of the lineament trends seen in the air
photos and satellite imagery that helped in their interpretation of the stresses present in the area.
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Field Work Summary
The fieldwork phase of the project was carried out from August 11 through August 24, 2012.
The geophysical contractor, Zonge International, provided a magnetotelluric (MT) survey crew,
with one additional crewmember for the gravity survey acquisition. Nicholas Hinz, PhD, and
Greg Dering, structural geologists from the University of Nevada made up the geological
mapping team. Pete Stelling, PhD, an experienced Aleutian volcanologist, and Rich Koehler,
PhD, of the Alaska Division of Geological and Geophysical Surveys, joined them for a limited
number of days. Mary Ohren of GRG directed the fieldwork and Matthew Bereskin of the City
of Akutan provided field assistance where needed. Fieldwork was carried out daily throughout
the 13-day period.
Magnetotelluric (MT) Survey
The purpose of performing MT surveys is to reveal the resistivity properties of the bedrock
within a field area. This assists in determining the extent of the geothermal resource and
boundaries between rock types. The 2012 MT survey sites were selected to cover the fumarole
field and extend coverage outside of the bounds of the 2009 survey. A total of 22 stations were
measured, plus a reference station to aid the processing and interpretation of the data. Some of
the equipment and acquisition activities are shown in Figure 2. A field acquisition report is
included as Appendix 3. In addition to the survey sites for 2012 data acquisition, one site each in
HSBV and on the eastern side of the field area were acquired to remove an ambiguity in that was
present in the 2009 MT survey interpretation.
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Figure 2: MT field work photos. (Photos by Mary Ohren and Curtis Caton)
Gravity Survey
A gravity survey measures density differences in the subsurface, revealing faults, fractures, and
altered rock zones that can be conduits or hosts for the geothermal reservoir. A total of 215
gravity stations were acquired during the August 2012 fieldwork. The gravity stations covered
the entire field area in, and surrounding, the HSBV. The equipment and SHUVRQQHO for acquiring
the survey are shown in Figure 3. The station spacing was approximately 150 m (492’) at the
center of the field and 300 m (984’) at the edges. A field acquisition report is included as
Appendix 4.
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Figure 3: Gravity survey acquisition photos. (Photos courtesy Christopher Kratt)
Geologic Mapping
Structural geologist Dr. Hinz and assistant geologist Dering spent their time in the field area
primarily mapping surface alteration, which indicates the prevalence and surface manifestation
of the geothermal resource. The two geologists were able to determine the orientation of faulting
and fracturing within the field area and to see broader trends of faulting, thus helping to define
the structural makeup of the field and narrowing well target possibilities. Geologic field methods
are the cornerstone that allows the geophysical data to be interpreted. Figure 4 shows images of
the field work in progress. The interpretive geologic report is attached at Appendix 5.
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Figure 4: Geological fieldwork images. (Photos courtesy Nicholas Hinz and Gregory Dering)
Data processing and interpretation
The data processing and interpretation steps are detailed in the respective reports of the
contractors. This phase of the project is necessarily the longest part of the project. The team was
fortunate enough to have the opportunity in the interpretation phase to have contact with other
disciplines and it greatly enhanced the interpretation of the data and enabled the selection of well
targets. Individual reports detailing the data processing and interpretation are attached as
appendices to this report.
MT Survey
The 2009 and 2012 data were combined into a 3-dimensional resistivity model based on
2-dimensional inversions. For more information on how this data was processed refer to
Appendix 6. The resulting model has been a key tool in defining the areas where geothermal
fluids may be circulating. In a typical MT interpretation of a geothermal resource, a layered
model is expected based on the intensity of alteration relative to its position in the system. In an
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actual reservoir, propylitic alteration (defined as a mineral assemblage consisting of chlorite,
epidote, adularia and albite) typically has a resistivity range between 10-60 ohm-meters. The
reservoir is normally covered by a ’clay cap’ containing smectite clays transitioning to illite clay
with increasing depth and temperature. The resistivity range of the clay cap should be 10 ohm-m
or less (Pellerin, L; Johnston, J; Hohmann, G, 1996). In the case of the HSBV resource, the clay
cap is underdeveloped, meaning that the resistivity values found in the layer are higher than
would be expected. The cause of the lack of development can be explained by one or more of the
following: (1) the system rapidly expanded at higher temperatures and has cooled, with a
retrograde alteration that has higher resistivity; (2) the system is immature and alteration a just
begun; or, (3) the system is old and eroded. None of these explanations preclude the presence of
a geothermal system, but require consideration in the data interpretation.
Figure 5: Image of 3D MT model, showing two potential well trajectories for reference. (Courtesy Zonge International, Inc.)
The revealed MT model follows a typical layered model of a geothermal system, with an upper
conductive layer underlain by a zone of intermediate resistivity (50-100 ohm-m), and a resistive
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core defined by resistivity values above 160 ohm-m; an image of the 3D result is shown in
Figure 5.
In looking at the interpreted data set, the conductive layer (defined here as resistivity values
≤40 ohm-m) varies in thickness, averaging 250 m (820’) thick, with a maximum thickness of
about 500 m (1640’). The layer extends to encompass the majority of the highlands portion of
the field area. The top of this conductive layer reaches the surface in the area surrounding the
fumaroles, which would be expected given the intensity of alteration in the exposed rock. The
low resistivity layer is also exposed in the northeastern part of the field, and in the hot springs
area toward the mouth of HSBV. It is thin or absent in the field area directly to the north and east
of the fumarole area as seen in Figure 6.
Figure 6: Map showing top elevation of low-resistivity layer and where it reaches the surface.
The conductive layer is likely indicative of a shallow aquifer circulating close to the surface. The
intermediate resistivity layer is expected to be the host of the geothermal reservoir. The well
targets chosen will intersect this mid-range resistivity layer between the low resistivity layer at
its thickest part and the high resistivity core at the point where it comes closest to the surface.
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The intent is to intersect the reservoir where it should be most active, where the heat source or
intrusive body is coming toward the surface, and the fluids at the surface are circulating most
readily. According to the MT model, the depths below surface that meet these criteria are
between 30 m and 792 m (100’ and 2600’) below mean sea level. Figure 7, Figure 8, and Figure
9 show slices of the resistivity model at the approximate depths of the top, bottom and the middle
of the zone.
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Figure 7: Plan map of resistivity model showing 25 m (~82') below mean sea level, the top most level that the geothermal
reservoir will be intersected according to the model.
16
Figure 8: Map showing MT resistivity model at the middle of the elevation where the geothermal reservoir is expected based
on the model.
17
Figure 9: Map showing MT resistivity model at the lowest depth that the geothermal reservoir is expected based on the
model.
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Gravity Survey
The primary result of the gravity survey was the identification of a dense body extending from
southwest to northeast, aligning with the edge of the conductive layer identified in the MT
model. The high density of this body suggests that it is less altered than the surrounding
formation and has thus far been resistant to fluid intrusion and breakdown, and represents a
boundary to the geothermal system. Analogous resistivity data helps to support this conclusion.
It is suspected that the body is a magmatic intrusion related to the Akutan volcano. The steep
density gradient along the margin of this body is a promising area to encounter fracturing due to
mechanical forces imposed during the intrusion event. A plan map of the interpreted depth to
dense basement is shown in Figure 10. The surface manifestations of the geothermal reservoir at
the western side of the field area do not extend to the south or southeast of the gravity high. This
provides further evidence that this dense body may be a controlling factor on the extent of the
reservoir. For more information on how this data was processed, refer to Appendix 6.
Figure 10: Gravity model showing the interpreted elevation of a dense basement rock.
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Geologic Mapping
The 2012 geologic map and cross sections shown in Figure 11 and Figure 12, identifies three
distinct areas of rock alteration and mineral deposition associated with the movement of
geothermal fluids. The exposed active mineral deposition and alteration type is argillic (clay)
with native sulfur, pyrite, and ferrous and calcite precipitates, as shown in Figure 13. There is
also evidence of past silica deposition in a valley adjacent to HSBV (Long Valley), thought to be
from an extinct geothermal system. This silica deposition should exist deeper in the modern day
system of HSBV. The alteration and mineral deposition are of limited extent and are directly
related to the structural fabric that controls the geothermal system. This suggests that the active
surface manifestations are linked by way of steeply dipping faults and fractures to the geothermal
system at depth. Three fracture orientations intersect in the vicinity of the fumarole field, a
northwest set, an east-west set and a north-northwest set, all with steep dips. It is not possible to
determine the depth that these fractures or faults extend below surface. Even so, the prolific
fracturing in different orientations would create numerous steeply plunging intersections that
should be sources of permeability.
Though the geologic report proposes three possible conceptual models to explain the fracturing
and mineral deposition at the surface, each of the models point to the fumarole area as the most
densely fractured area and the one most connected to the present day center of the system. In the
first model, the permeability created at the intersections of the faults and fractures of the three
distributed fault zones allows for the circulation of hot fluids. This model relies on the island-
wide and regional tectonic forces to create the fracturing. The second model attributes the
fracturing to the mechanical forces of a magmatic intrusion below the geothermal field to create
a network of fractures. Fracturing of this sort was observed in Long Valley, California. The third
proposed model is a combination of the previous two, with both mechanisms at work. Each of
these models are detailed Appendix 5.
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Figure 11: Geologic map of fumarole area and immediate surroundings produced by Hinz, et al, 2012.
21
Figure 12: Cross sections from 2012 geologic report.
22
Figure 13: Images of the alteration and mineral deposition of the Akutan geothermal system, taken from Hinz et al., 2012,
which can be found as Appendix 5.
Figure 13. Examples o f alteration and smficial manifestations of the HSBV geotherma l s ystem in upper
H SBV. A) Blue clay representativ e o f typical sn·ong intensity argillic altered volcaniclas tic rocks (QTv).
B) Hand sample of marbled blue and whi te co lored clay with disseminated p yrite from an outcrop of strong
intensity argillic a ltered volcaniclastic rocks (QTv). C) Active s ulfate (?)precip itate at a hot sp 1ing. D)
Native sulfur depositing in the throat o f an active fumarole . F) Native sulfm exposed in strong intensity
argillic altered volcaniclastic rocks (QTv). F) Activ e fe n icrete precipitate at a spring.
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Results
The combined evidence of past and present assessments strongly supports the presence of a
commercially-productive, high-temperature geothermal resource in the western highlands above
HSBV, in the vicinity of the fumarole field. The 2012 research has provided sufficient evidence
to establish several targets for drilling. The first evidence comes from the surface identification
of three orientations of fracturing that intersect within the proposed geothermal field area. This
fracture complex is the presumed host of hydrothermal fluids, as evidenced by the surface
emanations, and should provide sufficient permeability for effectively exploiting the geothermal
system. Secondly, the completed MT survey revealed a layer of low resistivity within the
proposed field, suggesting the presence of a subsurface fluid body. Figure 14 shows a combined
cross sectional view from the geologic reports and the MT model. Finally, the gravity survey
revealed differences in subsurface density that likely delineates a denser southern field boundary
and a less dense field area. Differences in density may be related to variable permeability.
From the combined evidence, the resource model presented following the 2009 exploration work
and 2010 drilling has been substantiated. This refined model describes fluids heated by an
intrusive body at depth and circulating close to the surface through a network of fractures. The
fumarole field is formed by fluid emanating at the surface, after being partially condensed by
passing through a shallow surface aquifer composed of meteoric water. Substantiating this
model, there is now documentation of three steeply dipping sets of fractures trending northwest,
east-west, and north-northwest, with a primary dip direction to the south. As indicated by the
2012 geologic report (Appendix 5) and by analogy with numerous other geothermal fields
worldwide, the key to having a productive well is to intersect multiple fractures within the depth
interval and reservoir rock that hosts the geothermal fluids. Penetrating fracture intersections
helps to increase the likelihood of locating commercial levels of permeability. Beneath the
fumarole area where the fracturing is most intense, the elevation of the resource is between 30 m
and 792 m (100’ and 2600’) below mean sea level, based on the MT model. From the proposed
accessible location at the surface, the well will need to be drilled to a depth of 500 m to 1250 m
(1640 and 4100’) vertical depth. It is between these depths that the well is expected to intersect
permeable fractures.
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Figure 14: MT model slice through center of fumarole area with geologic cross section overlay. • 500 1000 1500 ~ 2500 ~ ~ 4000 4500 kN ~~ ..... ----------•... J • ---I I 111 r ~ ) th X(IO ~~1_,.., -J Ilf % u,.· .... ~·.,.,.., !!I ~ .::--;::~~' I ---~ I .... ~ I= ~ ~ ~-.. + I+ I + I ~~~ 500 1000 1500 2000 2500 3000 ~ 4000 4500 Geology Report Profile AA with MT Model J4 Resistivity Contours MT Model Resistivitv Log10 Ohm·m Contours: 0.05. 0.2 1 1 1.2 u 1.4 1.5 1.6 1 7 1 8 1.9 2.0 I I I I I I I I I 5000 5500 6000 .. .. " § -~r~ g 5000 ~ § ~ ~ 5500 6000 250 0 250 500 1M 1006 t2!JO 1!100 ,._, ltll.t&f/tmll-~ City of Akutan Geothermal Project, Alaska Magnetotellurles Sul'\ley, AuguM 2012 MT m1ergobon ond model -Reno glo -plotted 20 Nov 12 Zongo lntematlonallnc .
25
Conceptual Model
The conceptual models presented in 2010 and 2011 (detailed in Appendix 7: Akutan Geothermal
Resource Assessment, 2011), were substantiated and significantly enhanced by the 2012 work.
The model describes an upflow-outflow system in which the upflow of the geothermal fluids is
under the fumarole area and outflow of the system comes to the surface at the hot springs area.
What is not confirmed is the path of the outflow, which was in question in the conceptual models
proposed in 2011. One model referred to as CM1 had the outflow along a proposed fault along
HSBV and another (CM2) had flow running north from the fumarole area and then to the east-
northeast to the hot springs area, bypassing the valley. After the drilling of TG-2 and TG-4, CM2
became the more favored interpretation, based on the shape of the temperature curve in TG-4.
TG-4 did not support the presence of a shallow outflow plume in this area. The 2012 resistivity
model supports CM1, provided the outflow along HSBV is only on the northwest side of the
valley. A low resistivity layer extends close to the surface along the north side of the valley, but
the resistivity increases rapidly on the south side of the valley as seen in Figure 15. TG-4 is on
the south side of the valley.
Figure 15: MT model of the resistivity at the ground surface.
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The presence of surface alteration and mineral deposition north and northeast of the fumarole
area, and the existence of a substantial low resistivity feature north-northwest of the hot spring
area, lend credence to the second flow model (CM2). There may also be two outflow paths; the
new data does not eliminate either flow path model. This debate is unlikely to be resolved
without a campaign of slim hole drilling, which would only be necessary if plans are made to
greatly expand extraction of the resource.
Regardless of the direction of outflow, the conclusion reached after the drilling of the two
temperature gradient holes was that the size of the resource that exists in the outflow portion of
the system will not be economically viable given the cost of drilling and plant development in
this remote location. The upflow portion of the resource is located in an accessible location and
is expected to provide sufficient reserves to provide the desired level of power generation.
Resource Capacity
The resource capacity estimates were made primarily based on the geochemical data gathered,
reviewed and interpreted prior to the 2012 work. The previous report concluded that possible
reserves are 20-100 MW. No adjustments have been made to previous findings. Another round
of drilling is required before these estimates can be updated. The previous conclusion was that
there is high confidence of a high enthalpy resource existing in the vicinity of HSBV around the
fumarole area (Appendix 7). The collective 2012 fieldwork provides further evidence to support
the 2011 reserves estimate.
Resource Risk
The previous resource assessment also defined the risks associated with developing the resource
as is proposed in this report; these risks remain. These include the exact location and size of
upflow, exact location and accessibility of permeable structures at depth, volcanic hazards, and
fluid or gas chemistry that can pose a risk during drilling and in maintenance of wells.
Though confidence in the location of the primary reservoir has improved, the volume of the
resource is still in question. The previous estimate used the area of the fumarole field (~0.5 km2)
and a base case number for power density of 15 MW/km2 to estimate a resource capacity of
8 MW after Grant and Bixley (Bixley, 2011). Using the same methodology, but including the
newly documented area of intense alteration and mineral deposition as documented in the 2012
geologic report (Appendix 5), the resource capacity is now suspected of being greater than
8MW. Depending on the relationship of the alteration and mineral deposition to the active
geothermal system the resource capacity may be as great as 34 MW.
27
Recommendations
The next step to advancing the geothermal project at Akutan will be to drill several wells that
reach into the geothermal reservoir, followed by long-term production testing. These wells
should be of production size and grade, as the great expense of drilling on Akutan make
prolonged exploratory drilling impractical. Drilling a slim well to a depth great enough to test the
resource will require fixed costs that could reach two-thirds to three quarters the cost of a full-
size production well. If the test well succeeds in finding the resource, it will still be necessary to
drill a production well toward the same target. The incremental cost of drilling a production-
capable well, rather than a smaller diameter test well is worth the additional expense. Three well
sites and target locations have been selected, two for production (PW-1 and PW-2) and one for
injection (IW-1). The top portion of a third production well (PW-3) is suggested to be drilled as a
second well, sharing the well pad with PW-1. From each proposed location it is possible to drill a
vertical or directional well to intersect numerous fractures and faults identified in the field and
also penetrate the lower resistivity layer that is thought to be the host of the geothermal reservoir.
Drilling Options
The proposed well sites (surface locations and underground targets) have been selected based on
the information gathered, processed and interpreted in 2012, and building on past exploration
work. The locations are shown in Figure 16. In the past the almost 0.6 km2 (150 acre) area of
relatively flat terrain south of the fumaroles has been identified as a suitable location
from which to drill. This location was assessed by the engineering firm Mead & Hunt for access
road alignment. Once there is a road to the ridge any location on the ridge is equally accessible.
Any location in the HSBV would have a starting elevation too low to access the hottest part of
the reservoir, so a previous suggested drilling target at the head of the valley has been
abandoned.
28
Figure 16: Recommended well pad and well sites, south and southwest of the fumarole area.
The surface location of PW-1 is on a northern extension of the flat ridge (WGS84 UTMm
Easting 440611 Northing 5999850). From this location the well will be drilled vertically from
the surface to 335.3 m (1100’), and then drilled directionally with an azimuth of 0° (north),
building at a rate of 3 degrees per hundred feet, to a maximum angle of 30°. The final
configuration of PW-3 from the same well pad has a similar well design but with an azimuth of
45°. This well design, at a total vertical depth of 1250 m (4100’) and a measured depth of about
1442 m (4730’), the total offset from the surface location will be about 997 m (2370’). By
drilling directionally there is a better likelihood to intersect more of the vertical and sub-vertical
fractures than by drilling vertically. Also, from the accessible surface location there is a better
opportunity to reach into the area with the most fracturing at the surface. Figure 17 shows the
track of the PW-1 and the final track of PW-3 inside the 3D model of the resource.
29
PW-2 would be drilled from a pad on the northwestern-most part of the flat ridge. With a similar
well plan and depth as PW-1, this well could reach the proposed permeability target with an
azimuth of 45° or 60°. This secondary well bore needs a minimum of ~300 m (1000’) of offset
from the bottom-hole location of the first well. This will help detrimental interaction between the
two wells when they are in production. This spacing can be achieved by changing the directional
azimuth and well depth.
The IW-1 site was selected relatively close to the extraction area because at this point in field
development strategy, proximity is proposed as an economical way to dispose of heat-mined
fluid. It also has the potential advantage of providing essential reservoir pressure support, which
would significantly increase the life of the resource. Injection sites located too close to the
production wells could cool down the reservoir, so locating injection well(s) properly is
essential. Injection capacity need not be in excess of what is produced, minus evaporation losses.
The injection well could be drilled vertically from the southern edge of the ridge to a maximum
Figure 17: The trajectories of PW 1 and PW-4 inside the resistivity model, with the geo-referenced geologic cross section A-A' shown.
30
of 1829 m (6000’), depending on what depth suitable permeability is encountered. From this
location it should encounter fractures on south-dipping faults or fractures with sufficient
permeability to accept the injected fluid at reasonable pump pressure, but it would have enough
offset from the production wells that it will not cause thermal breakthrough. Table 1 list the
proposed surface location of the four proposed wells.
Table 1: Wellhead locations for proposed wells.
Name
Easting
(UTMm
WGS84
Zone 3)
Northing
(UTMm
WGS84
Zone 3)
PW-1 440579 5999864
PW-2 440220 5999990
IW-1 440736 5999378
PW-3 440638 5999864
31
Preliminary Well Design
The preliminary well design used for the current planning is presented in Figure 18. This design
uses commonly available API grades of materials.
Figure 18: Preliminary well design for Akutan geothermal production wells.
Cost Reduction Methods and Considerations
Aside from resource viability, the overwhelming concern of the investors in the Akutan
Geothermal Project is that the project is economically feasible. Throughout development,
choices will be made and compromises made in order to meet the economic model created by
RMA. With regard to drilling, many costs such as material costs will be fixed and unavoidable.
Others costs will be essential to reduce or respond to risk. The only certain way to reduce drilling
32
cost is to increase operational efficiency and reduce the number of rig days. Several methods of
increasing efficiency are outlined below.
Pre-Setting
All drilling rigs operate within a range of maximum performance. Large drilling rigs designed to
penetrate to the project’s planned total depths have more capacity and higher operating costs than
are required for drilling at shallow depths. GRG recommends that a smaller rig be used to pre-set
the conductor, and surface and intermediate casings. The pre-setting rig will be contracted on a
footage-and materials basis, thereby reducing the financial risk of operating under the
International Association of Drilling Contractors (IADC) time-and-materials day-work contract.
Pre-setting will also provide a level of protection against the cost of downtime on the larger,
more heavily supported rig. Most importantly, pre-setting will compress the project timeline,
reducing both resource requirements and the project stresses imposed on the Akutan community.
Non-Drilling Time Reduction
Most people consider drilling cost in terms of the cost per each foot of hole drilled. In fact, the
greatest cost to a drilling project is so-called ‘flat-time’ when no new hole is drilled. Flat time
occurs primarily during rig-up, casing and cementing, wellhead and BOP installation, tripping,
and testing. It also occurs during unplanned operations such as lost circulation incidents and
fishing (lost-in-hole) operations. A variety of options exist to reduce non-drilling time, and each
should be weighed against overall project cost and potential risk when moving forward with well
design and planning. The options suitable to geothermal drilling operations include:
• Selecting a ’fit-for-purpose‘ rig optimized for the depth interval to be drilled and
designed for rapid rig moves;
• Selecting a rig equipped with BOPE handling and testing system;
• Pre-assembly and testing of BOPE;
• Selecting a rig equipped with a casing handling system;
• Avoiding prolonged welding operations through the use of screw-on wellheads and
preassembled production equipment;
• Installing float equipment on casing before mobilization;
• Using pre-mix tanks and treating and storing mud, rather than mixing new mud in the
primary rig tanks;
• Maintaining micronized cellulose in the mud system to reduce lost circulation and
differential sticking;
• Using rotary steerable tools to eliminate problems imposed by uncontrolled deviation and
to reduce torque and drag on the bottom hole assembly;
• Shift drilling supervisors and toolpushers, providing constant, around-the-clock
oversight;
33
• Onsite engineering and coordination, assuring that each operation is fully prepared for
and able to be carried out efficiently, and ensure that materials and equipment are
available without delay; and,
• Constantly monitoring operations in real time using data acquisition in order to identify
developing trends and adjust for changes in down-hole conditions.
Cost of Quality
One of the most important considerations to the Akutan geothermal project must be the cost of
quality. Using high-grade materials and assuring robustness of well construction is warranted,
given the extreme difficulty and cost of mobilizing rigs and equipment to Akutan Island. There
will be no possibility of ’routine‘ workovers, and replacement drilling will likely be something
that will occur only after the project is quite mature, if ever.
34
Works Cited
Bixley, M. A. (2011). Geothermal Reservoir Engineering 2nd Edition. Academic Press Elsevier.
Pellerin, L; Johnston, J; Hohmann, G. (1996). A numerical Evaluation of
electromagneticmethods in geothermal exploration. Geophysics , 61 (1), 121-130.
List of Appendices
Appendix 1: Geothermal Exploration at Akutan, Alaska: Favorable Indications for a High-
Enthalpy Hydrothermal Resource near a Remote Market
Appendix 2: Exploration of the Akutan Geothermal Resource Area
Appendix 3: Gravity Survey Data Acquisition and Processing Report
Appendix 4: Magnetotelluric (MT) Survey Data Acquisition and Processing Report
Appendix 5: Stratigraphic and Structural Controls of the Hot Springs Bay Valley Geothermal
System, Akutan Island, Alaska
Appendix 6: MT and Gravity Modeling Report, Akutan, Alaska, 2012
Appendix 7: Akutan Geothermal Resource Assessment, 2011
35
Appendix 1
GRC Transactions, Vol. 34, 2010
561
Keywords
Exploration, hydrothermal, geochemistry, magneto-tellurics
ABSTRACT
In summer 2009, the City of Akutan completed an exploration program to characterize the geothermal resource and assess the feasibility of geothermal development on Akutan Island. Aku-tan Island, Alaska is home to North America’s largest seafood processing plant. The City of Akutan and the fishing industry have a combined peak demand of ~7-8 MWe which is currently supplied by diesel fuel. The exploration program included practi-cal access assessments, a geologic reconnaissance field study, soil and soil gas geochemical surveys, a satellite remote sens-ing study, a review of existing hot springs geochemistry data, a magnetotelluric (MT) survey, and a conceptual model analysis. Exploration data indicate a viable resource that could feasibly support planned development for power production and direct use applications. A shallow outflow resource of 155-180°C is likely to be accessible for development at Akutan. A deeper, hotter, upflow resource of >220°C has a greater access risk but will be targeted because of its potentially lower development cost. Alternatively, there is potential for an outflow zone that has greater resource risk but potentially greater access. Two slim-hole exploratory wells are targeted to investigate these reservoirs and determine their likelihood for development and two potential follow-up wells would characterize the potential for the outflow resource.
1. Introduction
Akutan Island is located ~1300 km southwest of Anchorage and 50 km east of Dutch Harbor. The incorporated City of Aku-tan (COA) covers most of the island and currently includes one permanent village with a population seasonally fluctuating from ~350 to ~800, although harbor and nearby airport extensions are likely to increase this. As a volcanic island with accessible hot springs, it has been the subject of geothermal resource studies
since 1979. The COA and Trident Seafoods, a large fish proces-sor on the island, are entirely dependent on diesel fuel imported into the area for power and heat—with an average total demand of 4.3 MW (~7-8 MW peak). In 2008, the base cost of power in the COA was $0.323/kWh (Kolker and Mann, 2009). The geothermal system on Akutan Island is one of the most promising high-temperature sites in Alaska for geothermal development (Motyka et al,. 1993) but until recently only recon-naissance level exploration had been completed at Akutan. In 2009, the COA applied for and received energy grant and loan funds totaling $3.7 million from the state of Alaska (Kolker and Mann, 2009). Part of these funds was used to conduct surface exploration activities in summer 2009. The remainder of the funds will be used to support exploratory well drilling in summer 2010. No drilling has yet occurred at Akutan and, because the Akutan geothermal area is roadless, initial drilling will use a helicopter-supported slim hole rig.This study draws primarily from four datasets in order to con-ceptualize the Akutan geothermal resource and draw conclusions from that model. Those datasets are, in order of their importance in developing the current conceptual model: 1) previously acquired fluid chemical compositional data reported by Motyka and Nye (1988); 2) data from the 2009 MT survey; and 3) soil gas chemi-cal data (Kolker et al., 2010); and 4) background geology and structural studies that provide context for the other data (Motyka and Nye, 1988, 1993; Richter et al., 1998). To address a plausible range of resource size, the probability of exploration success and likely development designs, the Akutan conceptual models are compared to roughly analogous existing geothermal fields.
2. Geoscience Resource Analysis
2.1. Geoscience Background
A state-funded geothermal exploration program was carried out in Akutan Island in the early 1980s (Motyka and Nye, 1988). The investigation included detailed geologic mapping, shallow (<150m) geophysical surveys, soil and fluid geochemical studies, and hydrologic investigations.
Geothermal Exploration at Akutan, Alaska:
Favorable Indications for a High-Enthalpy Hydrothermal Resource
near a Remote Market
A. Kolker1, W. Cumming2, and P. Stelling3
1AK Geothermal, LLC, Portland, OR
2Cumming Geoscience, Santa Rosa, CA
3Stelco Magma Consulting, Bellingham, WA
562
Kolker, et al.
In summer 2009, the COA executed a follow-on exploration program to better characterize the geothermal resource. The 2009 program included practical access assessments, a geologic reconnaissance field study, soil and soil gas geochemical surveys covering a broader area than the 1988 study, a remote sensing study using Landsat thermal infrared data, a review of existing hot springs geochemistry data, a magnetotelluric (MT) survey, and a conceptual model analysis. Kolker et al. (2010) presents the results of the exploration program and implications for geo-thermal development. The next stage of exploration includes access analysis, fumarole sampling and analysis, and the drilling of slim-hole exploratory wells.
2.2. Geology and Structural Context
Akutan volcano is one of the most active volcanoes in the Aleutian Islands, with 32 historic eruptions (Simkin and Siebert, 1994). The modern volcanic complex forms the western half of the island, and volcanic hazards from future eruptions, other than dust-ings of ash, are unlikely to affect the eastern portions of the island (Ancestral Akutan), including Hot Springs Bay Valley (HSBV; Waythomas et. al., 1998). Destabilization of the fumarole area, at an elevation of 350 m near the head of HSBV (Figure 1), may generate debris flows, and such deposits are seen in the valley floor. The valley walls are composed of ~1.4 Ma lava flows, with the SE wall being slightly older and riddled with dikes. The valley was glacially carved, perhaps during the last major glaciation ending ~8,000 years ago, and potentially reworked during a minor glacial event ending ~3,000 BP (Black, 1975). The HSBV is composed of two linear valleys (Fumarole Valley and Hot Springs Valley; Figure 1) joined at right angles, suggesting structural control of glacial flow. Several hot springs are located in Hot Springs Bay Valley (HSBV), ~6 km WNW of the City of Akutan and ~3 km from the fumaroles (Figure 1). Based on the frequent eruptions of Akutan volcano, it is likely the ultimate heat source for the
reservoir is the magmatic system. The neutral cation geochemistry of the hot springs, however, indicates that the reservoir is isolated from the acid core zone around the magmatic system (e.g. Reyes et al., 1993) by an impermeable zone, possibly related to silica and anhydrite deposition (e.g., Wood, 1994). Conduction through the impermeable zone or above episodic dike intrusions on the flank of Akutan volcano could provide heat for an adjacent neutral chloride geothermal convection system.
2.3. Hot Spring and Fumarole Geochemistry
Five groups of hot springs (A-E; E furthest downstream) with about ten vents have been identified, including tidewater springs on Hot Springs Bay beach that are only exposed at low tide. Tem-peratures range from 54-94°C. Most of the hot springs fluids are a dilute chloride to chloride-bicarbonate type (Figure 2).
Figure 1. Map of Akutan Island, showing the geothermal area
and pertinent geologic features. Faults and rift zones are identi-
fied from surface geology and seismic data (J. Power, pers.
comm.). Inset highlights features of Hot Springs Bay Valley.
Figure 2. Chloride, sulfate and bicarbonate ternary plot. Fluids from
groups D and E show likely seawater contamination. Figure generated
from Powell and Cumming (2010).
Cl
HCO3SO410%
20%
30%
40%
50%
60%
70%
80%
90%
Steam Heated Waters
A3A3A3
B1C4
D2
E
563
Kolker, et al.
Geothermometry of the 1988 chemical analyses of the Akutan hot springs from Motyka and Nye (1988) using the liquid chem-istry spreadsheets from Powell and Cumming (2010) are shown in Figure 3 and Table 1.
Table 1. Geothermometry results for Akutan hot springs waters from
groups A-C. Groups D and E are omitted due to likely seawater
contamination. Compositional data from Motyka and Nye (1988); geo-
thermometers computed using Powell and Cumming (2010), based on
Giggenbach (1991) and Fournier (1989).
Sample Temp. °C Quartz Cond. Na-K-Ca Na-K-Ca Mg corr Na/K (Fournier)Akutan HS A3 84 159 189 169 205Akutan HS A3 84 155 185 162 198Akutan HS A3 nd 157 178 162 185Akutan HS B1 47.4 139 179 138 211Akutan HS C4 73.4 155 171 137 196
The Na-K-Mg ternary plot shown in Figure 3 (Giggenbach, 1991) suggests that Akutan hot springs waters are poorly to partially equilibrated and/or mixed, but the trend toward 200 or 220°C is consistent with a hotter, more distant source fluid for an aquifer. The silica geothermometry and the sinter found at hot springs A-C indicate temperatures closer to the hot springs of~160 to 180°C. These geothermometry results are consistent with an interpretation of a geothermal reservoir >210°C, probably closer to the fumarole, supporting a >180°C outflow up to 3 km from the fumarole.
The fumarole consists of low-to moderately-pressured steam vents at 99 °C, steaming ground, and boiling acid-sulfate springs covering an area of about 5,000 m2 (Motyka and Nye, 1988). Akutan fumarole gasses have been sampled in the past (Motyka and Nye, 1988, Symonds et al., 2003) but, unfortunately, both sets of samples were too contaminated by air to provide reliable gas geothermometry.Extensive hydrothermal alteration exposures were not ob-served on Akutan except in the immediate area of the hot springs and the fumarole. Because of the lack of high resolution imagery available for this study, it is possible that undetected alteration exists in areas with difficult access or swampy regions in the valley floor.
2.4. MT Resistivity
The 2009 MT survey detects a resistivity pattern typical of most economically viable geothermal reservoirs where a low resistivity, low permeability hydrothermal smectite alteration layer caps a higher temperature, permeable geothermal reservoir (Figures 4 and 5). The map of resistivity at -400 m elevation in Figure 4 shows a tongue of high resistivity trending from the
Figure 3. Na, K, and Mg ternary plot, with Na-K and K-Mg geothermom-
eters (Giggenbach, 1991). Compositional data from Motyka and Nye
(1988); plot generated using Powell and Cumming (2010).
Figure 4. Map of 3D MT resistivity at -400 m elevation with cross-section
lines used to illustrate the conceptual model (CM1, Figure 5) and the ge-
ometry of shallow aquifers in HSBV (HSV, Figure 6). The conductive clays
in HSBV are lower resistivity (green) than the more resistive (blue) area
around the fumarole. The hot springs have both the A-E labels of Motyka
and Nye (1988) and the 2009 GPS coordinates (UTM WGS84 Zone 3).
Figure 5. MT resistivity cross-section from a 3D inversion of the MT sta-
tions in CM 1 (see Figure 4). The moderately shallow, low resistivity clay
cap is green while the high resistivity, high temperature zone will be in the
blue to purple shaded zone. The much lower resistivity clay cap (red) over
the shallow aquifer that feeds the hot springs is red. MT survey coverage
does not extend to the fumarole area; the resistivity pattern is consistent
with the 3D inversion but not well constrained in that area. Proposed ex-
ploration well locations are projected onto the profile line; note that well
#1 falsely projects on top of a hill. Well #1 will be on the valley floor.
Na
1000 Mg^0.510 K
A3A3B1
C4
6080100120140160180200220240260280300320
Immature Waters
Partial Equilibration
564
Kolker, et al.
fumarole toward the hot springs, consistent with a <180 °C out-flow extending at least as far as the hot springs from a >220 °C upflow near the fumarole. The pattern of resistivity within HSBV suggests that at least one geothermal aquifer exists in the valley and supplies the hot springs. According to Motyka and Nye (1988), the HSBV is covered by a debris flow which is acting as an impermeable cap over the subsurface hydrothermal system. In the cross-section shown in Figure 6, the 2009 MT stations A016, A017 and A013 span the hot springs that have the highest geothermometry and measured temperature. The 1D MT inversions (Figure 6) have higher resolution at shallow depth than the 3D MT inversion (Cumming and Mackie, 2010) and appear to resolve a relatively resistive zone at about -20 to -120 m, likely corresponding to a 100 and 160°C aquifer. Near the hot springs, the MT also resolves a low resistivity clay zone below -120 m elevation that extends down to about -350 m elevation. This low resistivity is consistent with a clay cap over a <180°C outflow in a tabular aquifer below -350 m elevation that locally upflows to a shallower ~100oC aquifer at about -60 m elevation. The low resistivity zone at -120 to -350 m elevation near the hot springs thickens near the shore at the end of the profile in Figure 6 and to the southeast side of the valley as shown in several cross-sections in WesternGeco (2009). The geothermal manifestations are limited to the NW side of the HSBV, consistent with the clay having low resistivity and being thin at that margin. The high resistivity rocks on the southeast valley wall host numerous unaltered or weakly altered dikes that seem likely to be permeable and may promote cold downflow that lo-cally penetrates the clay alteration into the deeper rocks, making the SE margin of the HSBV a higher risk target.
2.5. Soil Geochemistry
Soil geochemical anomalies are clustered at three locations in HSBV. Arsenic (As), mercury (Hg), and carbon dioxide (CO2) all appear in anomalously high concentrations near the hot springs and
at the junction of the Fumarole Valley and the HSBV (Figure 7). This indicates either that Hg is being lost from a reservoir due to boiling and steam loss, probably northwest of the junction, or erosion has carried these elements in sediment from the higher elevation manifestations. The presence of such volatiles in sedi-ments indicates a potential for close proximity to an underlying near-boiling resource.
3. Resource Assessment and Well Targets
3.1. Conceptual Models
Several conceptual models for the geothermal system were prepared to illustrate targets that could be reached from acces-sible locations. The elements of these models include a magmatic core and/or an acid-core of the volcano, a hydrologic barrier between magmatic/acid core and neutral reser-voir, a neutral chloride upflow at >220°C, and a neutral chloride outflow at >160°C. The heat source is conduction from the volcano or from deeper dikes under the fumarole area causing convection. Although fumarole gas geochemistry has not been successfully acquired, the exten-sive alteration zone and minor sulfur emission suggests that it is likely to be associated with a >270°C upflow, whereas the modest rate of sulfur deposition suggests that such an upflow near the fumarole is more likely to be neutral chloride than part of an acid core or magmatic vent sys-tem. The size of the interpreted outflow is not reliably constrained by available MT coverage but the minimum size of outflow system appears to be about ~1000 x 500 m and may be as large as ~3500 x 1000 m. In the example conceptual model given in Figure 8, the upflow is located near the fumarole and outflows to and along the valley as one or more tabular aquifers. One aquifer appears to
Figure 6. Resistivity cross-section HSV showing a 1D inversion of MT data along HSBV. A very
thin clay cap shallower than 50 m is 5 to 15 ohm-m resistivity, shaded orange to light green and
yellow over an aquifer (green) at about 50 m depth. Another zone of high clay content extends to
350 m depth. Other symbols as in Figure 5.
Figure 7. Map showing anomalous concentrations of CO2, Hg, and As in
Akutan soils. All concentrations in ppm.
565
Kolker, et al.
lie at a depth of <100 m and another at depths of ~500 m, beneath an aquitard of low resistivity impermeable clay.
3.2. Proposed Gradient Well Program
The exploration data suggests that the likely up-flow location is in the general vicinity of the fumarole. However, the fumarole is located 350 m up a very steep hillside, posing access limitations. Hence, drill-ing targets are a compromise between accessibility and resource location. Due to the roadless and rugged terrain on the area, drilling rigs for the exploration phase of the project must be mobilized and supported by helicopter, limiting the economically feasible well design to small-diameter wells. A high priority exploration well will be drilled in the Fumarole Valley ~1 km southeast of the fumarole field (Figure 1), to a TVD of 1500 m (3500 ft.). This appears to be the accessible location closest to the high temperature upflow zone. Additional exploration wells will be drilled to 500 m (1500 ft.) in the outflow zone(s). The geophysics suggests that two tabular aquifers could be penetrated at relatively shallow depths, although only the deeper aquifer would be expected to be capable of >1 MW sustained production.
3.3. Resource Capacity and Targeting Risk
The capacity of the geothermal resource at Akutan in terms of electrical power can be assessed using analogies, both rough comparisons to the prospect estimates provided in the Western Governors’ Association report and to geothermal developments at fields with surface indications roughly analogous to Akutan. In the western USA, there are two developed geothermal fields in volcanic systems with different geologic settings but broadly similar geochemistry, the 160 to 175°C, ~40 MW (gross) Casa
Diablo field at Long Valley (Sorey et al., 1991) and the outflow part of the 160 to 180°C, 84 MW (gross) Steamboat Springs Field near Reno (Ormat). In both of these systems, it is entirely or mainly the lower temperature outflow that has been developed. At Akutan, the combination of an apparently non-magmatic flank fumarole, a trend in cation geothermometry to >210°C, and silica geothermometry over 160°C with sinter deposi-tion support the existence of generally analogous resource at Akutan. Because of the dilute outflow chemistry, handicapping the Akutan 160 to 180°C resource by 50% relative to these reservoirs would be reasonable, giving an analogous low temperature resource capacity estimate of 20 MW. Because a high temperature resource might exist, a more optimistic capacity estimate for the entire system would be as high as 100 MW, using the Western Governors’ Association report as-sessments as analogs. However, there are several risks associated with both development options. These potential risks are outlined in Table 2.
4. Conclusions
There is a high probability that a hydrothermal system exists at Akutan. The chemistry of the hot springs strongly suggests the existence of a neutral chloride reservoir with economically devel-opable temperature. The fluid geothermometry tells a consistent story, with cation geothermometry detecting a >210°C reservoir temperature, probably near the fumarole, and silica geothermom-etry and presence of sinter suggesting that 160 to 180°C exists close to hot spring B. Although Akutan is an active volcano with frequent historical eruptions, the most attractive target areas near the thermal manifestations are over 3 km from the active volcanic
Figure 8. Example conceptual model for the Akutan geothermal system. Red arrows indicate the
flow of heated water; blue arrows indicate cold water flow. Note that well #1 falsely projects on
top of a hill. Well #1 will be on the valley floor.
Table 2. Potential advantages and disadvantages associated with the development of the
two major Akutan geothermal resource models.
Model Capacity Range Advantages Disadvantages
Upflow 20-100 MWe
High-enthalpy so po-tentially fewer wells
Lack of fumarole gas sampling increases risk of low permeability, high gas, or cor-rosive conditions
Single pad produc-tion for 20 MW using directional wells
Poorly imaged target geometry due to lack of MT coverage
Transmission lines ~2 km longer
Initial straight hole unlikely to directly test reservoir unless the reservoir is large
Outflow 2-20 MWe
Better constrained resistivity geometry
180°C typically requires more production wells/MW than 270°C spread over larger area because directional wells probably not feasible (160 °C would require still more wells/MW).
Shorter transmission lines
More reinjection wells required
Greater probability of injection break-through
Greater possibility of surface cold water influx
Swampy nature of valley could hinder exploration and construction activities
566
Kolker, et al.
vent. The flank fumarole field has high heat flow and appears to reflect leakage from a geothermal reservoir rather than from a shallow magmatic source or acid-core system. The MT resistivity pattern indicates that a hydrothermally altered clay cap exists near the fumarole and probably overlies an outflow connection from the fumarole to the highest temperature hot springs. Anomalous measurements of mercury and arsenic in soil are consistent with the presented conceptual model and suggest that an upflow near the fumarole may extend over 1000 m to the southeast. Finally, supporting data sets including geological structure and rock types are consistent with the existence of a permeable reservoir associ-ated with structural extension. Based on conceptual models built primarily from MT and geochemical datasets, it appears that development of the Akutan geothermal resource for power and/or direct use may be feasible. These datasets point to a shallow, tabular aquifer(s) of 155-180°C (i.e., “outflow zone”) and a deeper, hotter resource of >220°C (i.e., “upflow zone”) that will be targeted because of its poten-tially lower development cost. The initial exploratory wells will attempt to verify the existence of these aquifers, and determine their potential for development.The resource capacity and the probability of exploration and development success differ for the outflow and upflow targets. The upflow is poorly imaged due to difficult access and, for the same reason, only the periphery of the likely upflow is accessible using a vertical test well. Hence, the probability of encountering a resource using a vertical well at the closest accessible location is lower than at the outflow target, although fewer full-sized directional wells would be required to develop a higher temperature system. In contrast, wells drilled into the outflow zone have a higher probability of encountering a productive resource; however, more wells drilled over a larger area would be required to develop a similar capacity. Because of the modest demand at Akutan either resource target might be adequate.Exploration drilling scheduled for summer-fall 2010 will provide necessary resource parameters that can now only be estimated from surface data.
5. References and Bibliography
Black, Robert F., 1975. “Late quaternary geomorphic processes: Effects on the ancient Aleuts of Umnak Island in the Aleutians.” Arctic, v. 28, p. 159-169
Cumming, W. and R. Mackie, 2010. “Resistivity Imaging of Geothermal Resources Using 1D, 2D and 3D MT Inversion and TDEM Static Shift Correction Illustrated by a Glass Mountain Case History.” Proceedings, World Geothermal Congress 2010.
Fournier R.O., 1989. “Lectures on geochemical interpretation of hydrothermal waters.” UNU Geothermal Training Programme, Reykjavik, Iceland. Report 10, 1989. www.unugtp.is/solofile/33667
Giggenbach, W., 1991. “Chemical Techniques in Geothermal Exploration.” In: The Application of Geochemistry in Geothermal Reservoir Develop-ment, F. D’Amore Ed. 1991 UNITAR/UNDP Guidebook.
Information Insights, 2010. “Akutan Geothermal Energy Demand and Stake-holder Assessment.” Unpublished report to the City of Akutan and the Alaska Energy Authority, 34p.
Kolker, A., and R. Mann, 2009. “Heating Up The Economy With Geothermal Energy: A Multi-Component Sustainable Development Project at Akutan, Alaska.” Geothermal Resource Council Transactions, 33.
Kolker, A., P. Stelling, B, Cumming, A. Prakash, and C. Kleinholt, 2010. “Akutan Geothermal Project: Report on 2009 Exploration Activities.” Unpublished report to the City of Akutan and the Alaska Energy Author-ity, 37p.
Motyka, R., and C. Nye, eds., 1988. “A geological, geochemical, and geo-physical survey of the geothermal resources at Hot Springs Bay Valley, Akutan Island, Alaska.” Alaska Division of Geological and Geophysical Surveys (ADGGS), Report of Investigations 88-3.
Motyka, R.J., S. Liss, C. Nye, and M. Moorman, 1993. “Geothermal Resources of the Aleutian Arc.” ADGGS Professional Paper 114.
Powell, T., and W. Cumming., 2010. “Spreadsheets for Water and Geothermal Gas Chemistry.” Proceedings of the Thirty-Fifth Workshop on Geother-mal Reservoir Engineering, Stanford University, Stanford, California, SGP-TR-188.
Reyes, A.G., W.F. Giggenbach, J.R. Saleras, N.D. Salonga, and M.C. Vergara, 1993. “Petrology and geochemistry of Alto peak, a vapor-cored hydrother-mal system, Leyte Province, Philippines.” Geothermics, 22, 479-519.
Richter, D.H., C.F. Waythomas, R.G. McGimsey, and P.L. Stelling, 1998. “Geologic Map of Akutan Island, Alaska.” U.S.G.S. Open-File Report OF 98-135, 22 p., 1 plate
Simkin, T., and L. Siebert, 1994. Volcanoes of the World. 2nd edition: Geo-science Press in association with the Smithsonian Institution Global Volcanism Program, Tucson AZ, 368 p.
Sorey, M.L., G.A. Suemnicht, N.C. Sturchio, and G.A. Nordquist, 1991. “New evidence on the hydrothermal system in Long Valley caldera, California, from wells, fluid sampling, electrical geophysics and age determina-tions of hot spring deposits.” Journal of Volcanology and Geothermal Resources, 48, pp. 229-264.
Symonds, R. B., R. Poreda, W. C. Evans, C. J. Janik, and B. E. Ritchie, 2003. “Mantle and crustal sources of carbon, nitrogen, and noble gases in Cascade-Range and Aleutian-Arc volcanic gases.” USGS Open-File Report 03-436.
Waythomas, C.F., J.A. Power, D.H. Richter, and R.G. McGimsey, 1998. “Preliminary volcano-hazard assessment for Akutan Volcano east-central Aleutian Islands, Alaska.” U.S. Geological Survey Open-File Report OF 98-0360, 36 p., 1 plate
Western Governors’ Association Clean and Diversified Energy Initiative Report, 2006. http://www.westgov.org/wga/initiatives/cdeac/Geothermal-full.pdf
WesternGeco, 2009. “Magnetotelluric Survey at HSBV, Akutan, Alaska: Final Report – 3D Resistivity Inversion Modeling. Unpublished report prepared for the City of Akutan, Alaska.” GEOSYSTEM/WesternGeco EM, Milan, Italy, 27p.
Wood, C.P, 1994. “Mineralogy at the magma-hydrothermal system interface in andesite volcanoes, New Zealand.” Geology, 22, 75-78.
42
Appendix 2
PROCEEDINGS, Thirty-Seventh Workshop on Geothermal Reservoir Engineering
Stanford University, Stanford, California, January 30 - February 1, 2012
SGP-TR-194
EXPLORATION OF THE AKUTAN GEOTHERMAL RESOURCE AREA
Amanda Kolker1, Pete Stelling2, William Cumming3 and Dave Rohrs4
1AK Geothermal, 864 NE Simpson St., Portland, OR, 97211, USA; info@ak-geothermal.com 2Western Washington University, 516 High St., Bellingham, WA 98225-9080, USA; pete.stelling@wwu.edu3Cumming Geoscience, 4728 Shade Tree Lane, Santa Rosa, CA 95405-7841, USA; wcumming@wcumming.com
4Rohrs Consulting, 371 Gazania Ct., Santa Rosa, CA, 95403-7711, USA; drohrs@sonic.net
ABSTRACT
Gas geochemistry from geothermal fumaroles on
Akutan Island, Alaska, indicates that geothermal
reservoir temperatures could approach 572 °F (300
°C), and probably consists of a brine liquid overlain
by a small steam cap. Fluids produced by core holes
show evidence of chemical re-equilibration to lower
temperatures, with cation geothermometry providing
a range from 392-464 °F (200-240 °C). Geochemistry
of hot spring fluids shows evidence of equilibrating
at still lower temperatures. These data support a
model with a high-temperature upflow system in the
vicinity of the fumaroles that transitions to a lower
temperature outflow zone that mixes with meteoric
water and connects to hot springs 12,000 ft (3600 m)
from the fumarole. This model is supported by MT
resistivity data.
Exploratory drilling targeted the outflow zone with
two core holes 9,200 and 12,000 ft (2800 and 3600
m) from the fumarole. The farther core hole
encountered expected fluid temperatures of 360 °F
(180 oC) at 613 ft (186 m). Static temperature profiles
suggest that the 360 °F zone is drawn from a nearby
fault zone not located directly below the well.
Alteration mineralogy in the two core holes suggests
that the rocks were at temperatures greater than 469
°F (250 °C) in the geological past and have cooled to
present temperatures. The integrated interpretation of
core mineralogy, temperature logs and MT resistivity
suggests that the part of the outflow encountered by
the wells has insufficient volume and too close a
connection to cooler water to support commercial
development, although the higher risk of cooling
during exploitation as a result of either cold water
influx from near-surface aquifers or injection
breakthrough might be offset by flexibility in lower
cost shallow wells. Targeting the area of the fumarole
field with an 8000 ft (2500) m directional well would
have the highest probability of encountering
commercial production at Akutan. This target is
likely to be >430 oF (>220 °C) and could be as hot as
570 oF (300 °C).
INTRODUCTION
The volcanic Aleutian Islands of Alaska have long
been considered a promising setting for geothermal
energy resources. Akutan Island, in the eastern
portion of the Aleutian chain (Fig. 1) holds one of the
most commercially viable geothermal prospects in
the state, Hot Springs Bay Valley (HSBV). The
HSBV resource is approximately 4 miles (six
kilometers) northeast of the only population centers
on Akutan Island, the City of Akutan (COA) and
Trident Seafoods Processing facility. Combined,
these two entities have a peak energy demand of ~7-8
MWe. This demand is currently being met through
diesel generators and heaters, consuming ~4.2 million
gallons of diesel annually. In 2008, the base cost of
power in the City of Akutan was $0.323/kWh
(Kolker and Mann, 2009).
Initial exploration of the geothermal potential of this
area began in 1979 (Motyka and Nye, 1988; Motyka
et. al., 1993). In the summer of 2009, the City of
Akutan initiated a more detailed study of the full
HSBV area (Kolker and Mann, 2009; Kolker et al.,
2010), and in 2010, based on the results of the 2009
efforts, two thermal gradient (TG) wells were drilled
in the floor of the main valley.
Geologic Setting and Background Data
Akutan Volcano is the second-most active volcano in
the Aleutians subduction zone, with 32 historic
eruptions (Simkin and Siebert, 1994; Newhall and
Dzurisin, 1988; Miller et al., 1998; Richter, 1998).
An initial volcanic hazard review indicated that the
proposed geothermal development area was unlikely
to be directly impacted by eruption activity,
excepting ash fall that might cause temporary closure
(see Waythomas et. al., 1998).
The HSBV lies approximately 2.3 miles (3 km) to the
NE of Akutan Volcano, and is composed of two
linear, glacially carved valleys (the SE-trending
Fumarole Valley and the NE-trending Hot Spring
Valley; Fig. 1; Richter et. al., 1998).
1234
Fig. 1. Topographic map of Akutan Island, showing
the geothermal project area and pertinent
geologic features. Hot Springs Bay Valley
(HSBV) is a L-shaped topographic low that
lies at the center of the geothermal project
area.
Five groups of hot springs with more than 10 vents
have been identified in lower HSBV (Fig. 1 and 2).
Temperatures of the hot springs range from 129 to
205 °F (54 to 96 °C); and some have been reported as
boiling. A fumarole complex exists at the head of
HSBV to the west of the hot springs and covers an
area of approximately 1600 ft2 (500m2). Kolker et.
al. (2010) indicate gas geothermometry of reservoir
fluid of 518 °F (270 °C) and higher at the fumaroles,
and that these fluids are re-equilibrating to lower
temperatures along the outflow path feeding the hot
springs. The silica geothermometry of the hot springs
indicate 275 °F (~135 °C) fluid, while cation
geothermometry provides up to 374 oF (190 oC).
Another fumarole field is located at the summit of the
active volcano ~2.5 miles (~4 km) to the west of
Fumarole Valley.
Fig. 2. Map of the Akutan Geothermal area, showing
the four candidate exploration well locations
that were considered for the 2010 program.
The two holes drilled in 2010 are marked
with black arrows.
The two valley sections are joined at right angles,
suggesting structural control of glacial flow. Soil
geochemical anomalies (Arsenic, Mercury, and
Carbon Dioxide) at the junction of the Fumarole
Valley and the HSBV also suggest that the valley
junction is structurally controlled and was initially
interpreted as a potentially important locus of
geothermal fluid flow (Kolker et al, 2010). These
structural features parallel regional fault patterns
associated with subduction stresses and volcanic
processes on Akutan, providing the large-scale
permeability for deep magma migration and storage
(Lu et. al., 2000; John Power, pers. comm.). These
structures are likely to be important in the control
hydrothermal upflow and outflow. One mapped fault
cuts near-perpendicularly across HSBV (Fig. 2). All
of the hot springs are topographically lower than the
fault’s surface trace, consistent with geochemical
indications that they outflow from an upflow near the
fumarole.
MT Resistivity
Kolker et. al., (2010) show that the MT resistivity
pattern of the Akutan geothermal prospect includes a
low resistivity, low permeability clay zone, caps a
higher resistivity, higher temperature, permeable
geothermal reservoir. The resistivity values of >20
ohm-m within most of the low resistivity zone
between the fumarole and hot springs at Akutan,
however, are higher than in the smectite zone of most
developed geothermal fields, although a local pattern
of alteration near the hot springs is more
conventional, with a <600ft (<200 m) thick, 5-15
ohm-m zone interpreted as a smectite clay cap
overlying a higher resistivity geothermal outflow.
Kolker et. al., (2010) interpret the shallow >20 ohm-
m layer between the fumarole and the hot springs as
either an unusually high fraction of dense lavas
causing weak alteration, or relict alteration that
formed at higher temperatures and has experienced
only minor retrograde alteration to smectite clay.
The most serious limitation in the MT survey is the
lack of station coverage in the area near the fumarole
and a large part of a likely direct outflow path to the
hot springs due to steep topography and inclement
weather.
TEMPERATURE GRADIENT DATA
Core Hole Drilling
In 2010, two small-diameter temperature gradient
(“TG”) core holes were drilled in the floor of HSBV
(black arrows in Fig. 2). Well TG-2 was sited to test
the outflow aquifer(s) and was drilled to a Total
Vertical Depth (TVD) of 833 ft (254 m). Between
1235
585 and 587ft (178 and 179 m), a highly permeable
zone, apparently a highly fractured and vesicular lava
flow top, flowed geothermal fluid at 359 °F (182 °C).
Well “TG-4” was sited at the southern part of the
junction between the two perpendicular valleys, to
test the size and extent of the outflow zone. Reaching
the planned depth of 1500 ft (457 m), this well did
not encounter substantial fluid flow. This suggests
that its location is outside the margins of the outflow
zone, vertically or horizontally (or both). However,
the well did encounter a shallow temperature gradient
high enough to imply close proximity to a geothermal
source. Further details on these wells can be found in
Kolker et. al. (2010).
End-of-Drilling Logs
After TD was reached, three P/T logs were recorded
at 12, 24, and 36 hours after circulation ended for
each well. For every run, stops were made in 20 foot
increments. Both wells show very high shallow
temperature gradients, which is consistent with their
proximity to the shallow outflow zone (Figs. 3a, b).
Following production, TG-2 shows a drop in
temperature occurring just above the casing shoe at
603 ft (181 m), corresponding to the hot fracture zone
between 585 and 587 ft (178 and 179 m) that was
cemented in. The apparent cooling is likely the result
of drilling fluid and cement injected across that entire
area. TG-4 shows a relatively rapidly increasing
temperature gradient until ~900 ft (274 m),
transitioning to a slowly increasing temperature
gradient from 900 ft -1500 ft. An injection test
performed on well TG-4 suggested that the well has
generally poor permeability.
TG-2 Equilibrated Temperature Log
The equilibrated temperature survey for TG-2 was
taken nine months after well completion. The
equilibrated log shows a distinctly different shape
from the end-of-well temperature build-up profiles
Fig. 3a. Among the new features to note are: (1) The
well was bleeding while the log was run, resulting in
a minor steam or two-phase section in the upper 60-
70 ft (20-25 m); (2) Apparent cooling of the well
since shut-in is noticeable in the upper 400 ft (122
m). This probably reflects a trickle of water down-
flowing from around 200 ft (61m) measured depth
(MD) and exiting into the formation at about 415 ft
(126m) MD. It can only be a trickle of water because
the water is heating up as it flows down behind the
casing; (3) The highest temperatures occur in the
permeable zone near 585 ft MD (415 ft / 126 m
elevation), with a temperature reversal of about 9 °F
(5 °C) below the permeable zone to the bottom of the
well.
Figure 3. End-of-drilling temperature curves and
equilibrated temperature curves. The end-of-
drilling surveys were taken 12 hours. 24
hours, and 36 hours after circulation; the
equilibrated profile was obtained 9 months
later. (a) temperature curves for TG-2; (b)
temperature curves for TG-4.
The new data shows that the permeable zone at 585ft
MD (415 ft/ 126 m elevation) has thermally
recovered since drilling. Notably, the static
temperatures measured in this permeable zone are
about 338 °F (170 °C), which is lower than the 359
°F (182 °C) temperature measured in this zone when
the well was flowing. Since the MRT reading is
supported by the silica geothermometry, it is likely
that the well was drawing in higher temperature
fluids from an adjacent permeable zone when it was
producing.
TG4 Equilibrated Temperature Log
Unlike TG-2, the equilibrated temperature profile
from TG-4, run about 8 months after well
completion, differs very slightly from the end-of-
drilling temperature profile (Fig. 3b). The new profile
shows that the top 800 ft (244 m) of well TG-4
1236
heated up slightly, but the bottom temperatures
remained extremely close to those measured during
the end-of-well surveys. This is not surprising in light
of the fact that that well was relatively impermeable
and exhibits a temperature profile that shows heating
primarily from conduction for the upper 800 ft (244
m). By contrast, the bottom of the hole is
approaching an isothermal gradient. This suggests
that the conductive heating is from the side (either
from a shallow outflow zone at some lateral distance,
or from a sub-vertical heat source somewhere
nearby), and not from a hot aquifer below.
P/T Data Analysis
Although TG-2 flowed geothermal fluid at 359 °F
(182 °C) during drilling, the equilibrated temperature
logs show a maximum temperature of 338 °F (165
°C) with a reversal at the bottom of the hole. This
implies that the 359 oF fluid was not circulating in the
immediate vicinity of TG-2 but rather was “pulled
in” from elsewhere due to the pressure drop caused
by flowing the well. A likely scenario is that the
productive subhorizontal fracture at 585 ft (178 m) in
TG-2 is connected to a subvertical fracture dipping
west (see Figs. 1, 2). When the subhorizontal fracture
was produced, the subvertical one became a
temporary conduit for fluids in the outflow zone. It is
unlikely that the source of the 359 °F (182 °C) fluid
is directly below Well TG-2 because of the
temperature reversal recorded in the equilibrated log.
A comparison of the static temperature profiles in
TG-2 and TG-4 shows the difference between the
shape of a convectively heated outflow profile in TG-
2, and a conductively heated temperature profile in
TG-4 (Figs. 3a, b). Also, the temperatures in the
upper 800 ft (254 m) of TG-4 are generally lower
than in TG-2, indicating that TG-4 is further from the
outflow path. No strong conclusions can be drawn
from the temperature profiles as to whether additional
high temperature permeable zones underlie either
well, but it appears unlikely based on the shape of the
bottom of both well profiles.
The 359 °F (182 °C) temperature measurement
during drilling of TG-2 is consistent with silica
geothermometry. If the well was producing fluids
that were higher than its static measured temperature,
this suggests that TG-2 was drilled on the margins of
a permeable and hotter outflow path. The higher
temperature fluids drawn into TG-2 during the flow
test suggest that the production zone is in proximity
to the higher temperature zone but that it has a
relatively low volume. The slight temperature
reversal of about 9 °F (5 °C) below the permeable
zone is consistent with the geologic model that the
thermal features in HSBV are sourced from a
vertically restricted lateral outflow from a geothermal
reservoir located further west, or possibly north.
FLUID CHEMISTRY AND THERMOMETRY
Fluid from well TG-2 was collected from the entry
zone at 585 -587 ft (178-189m) MD during a well
discharge and from production zones between 603 ft
(184m) and 833 ft (245m) MD. Air assist was used
to collect fluid samples from well TG-4 due to poor
permeability conditions. New gas chemical data are
from samples obtained from the fumaroles in 2010.
All other analyses used in geothermometry
calculations and chemical modeling were obtained
from past reports (Motyka and Nye, 1988; Motyka et.
al., 1993; Symonds et. al., 2003; Kolker and Mann,
2009; Kolker et. al., 2010).
Chemistry
Chemical analyses of the hot springs water shows
that they are derived from a dilute, near-neutral Na-
Cl reservoir brine. The Akutan hot springs show
slightly elevated HCO3 and SO4 concentrations,
suggesting mixing along the outflow path with dilute,
steam-heated near-surface waters. Hydrogen and
oxygen isotopic data shows that the hot spring waters
are derived from local meteoric water.
The chemistry of the fumarole gases demonstrates
some magmatic affiliation. Gas plots show that the
gases are well-equilibrated and likely to be derived
from a high temperature neutral chloride reservoir. In
addition, the gas concentrations in the flank
fumaroles imply that some fraction of gas is derived
from equilibrated steam, indicating the presence of a
localized steam cap in the reservoir. The chemistry of
the fumaroles are consistent with an equilibrated
geothermal system associated with an andesitic
stratovolcano (Giggenbach, 1991). In comparison,
the gas from the summit fumarole originates from a
more oxidizing environment and exhibits high H2S
concentrations. These all suggest a magmatic
affiliation for the summit fumarole steam but only a
minor magmatic influence in the Fumarole Valley
features that are consistent with a neutral geothermal
reservoir.
Geothermometry
Using silica and Na, K, Ca, and Mg concentrations of
the hot spring and well fluids, we have estimated the
temperature of last equilibration along the outflow
path to be ~338 °F (~170 °C) and ~392 °F (200 °C)
for the two samples from TG-2, following the
calculation methodologies of Powell and Cumming
2010). This temperature is similar to the estimated
entry temperature of 359 °F (182 °C) at 585-587 ft
(178-179 m) MD in well TG-2 (Kolker et al, 2010).
Cation concentrations in hot spring and well
discharge analyses show that the springs and well
fluids are mixed or partially equilibrated fluids. This
is commonly observed along outflow paths where the
1237
fluids are re-equilibrating to lower temperatures and
mixing with near surface waters with elevated Mg
concentrations.
The data from the entry at 585 ft (178 m) MD in core
hole TG-2 and from the hot springs nearest TG-2
suggest that the fluids originate in a deeper reservoir
with temperatures in the range of 428-464 °F (220-
240 °C; Fig. 4).
Fig. 4. Na-K-Mg geothermometry plot for samples
obtained from the Akutan hot springs and
temperature gradient holes. Green squares
=well fluid; purple diamonds=hot spring
fluid; yellow triangle=fumarole condensate.
This compares to a temperature of 412 °F (211 °C)
estimated from the Na-K-Ca geo-thermometer for the
well discharge. Geothermometers that apply Na, K,
Ca, and Mg concentrations tend to partially re-
equilibrate to lower temperatures in the outflow zone,
and so the deep reservoir temperature is likely to
exceed 464 °F (240 °C).
Geothermometry estimates from Fumarole Valley
fumarole gases exhibits very good consistency,
indicating an origin from a mature, equilibrated
neutral chloride reservoir. The gas geothermometry
consistently suggests reservoir temperatures of 518-
572 °F (270-300 °C), as shown, for example, in the
Car-Har plot (Fig. 5)
Geochemical Model
The new geochemical data set confirms the previous
interpretations of the resource distribution in HSBV.
The hot springs represent a shallow outflow from a
high temperature neutral chloride reservoir that exists
further west. The chemistry of the hot springs
indicates that they have experienced significant
mixing with cooler, dilute near surface meteoric
waters. The lack of evidence for mixing of air-
saturated meteoric water in the Fumarole Valley
fumarole gases support an interpretation of a nearby
upflow zone. Geothermometry of the well discharges
and the fumarole gases indicate a likely deep
reservoir temperature of at least 464 °F (240 °C)
based on Na/K geothermometry, with temperatures
possibly as high as 572 °F (300 °C) in the reservoir
based on gas geothermometry. The geochemical data
do not provide any constraints on the reservoir
boundaries to the west nor on the reservoir volume
within the outflow area.
Fig. 5. CAR-HAR geothermometry plot for the
Akutan fumarole gases. Red diamonds
represent samples collected in 2010, squares
from Symonds et al. (2003; yellow=HSBV
fumarole, black=summit), and blue circles
samples from Motyka and Nye (1988). The
Motyka and Nye samples are the poorest
quality, showing depletion in H2 and higher
levels of air contamination.
The non-condensable gas data from the fumaroles
suggest that a steam cap may overlie the deep brine
reservoir. The chloride hot springs in HSBV
represent shallow outflow from the reservoir. The
outflow becomes diluted by mixing with cool
meteoric waters, especially in the near surface
environment. Thus, the new geochemical data from
the fumarole and well TG-2 are very consistent with
the geochemical outflow models suggested by Kolker
et al. (2010).
CORE DATA
In addition to full lithologic logs recorded at the drill
site, core extracted from wells TG-2 and TG-4 was
analyzed at Western Washington University in
Bellingham, WA. The goal of the laboratory analysis
was to determine the hydrothermal history of the
HSBV. Determination of specific mineral species
was conducted through XRD, SEM, and petrographic
observations, whole rock XRF analysis (conducted at
Washington State University) and qualitative
assessment of permeability.
Rock Types and Primary Mineralogy
There are four main lithologies present in the Akutan
core: basalt, andesite, ash tuff, and lithic-rich basalt.
The most common lithology in the core is basalt lava,
Na
1000
Mg^0.510 K
6080100120140160180200220240260280300320340HS A3
HS A3
HS A3*
HS B1HS C4 HS D2
HS E
Fum Sp
TG-2, 585'TG-2, 833'
TG-4,
1500'
Partial Equilibration
Immature Waters
equilibrated
vapor
equilibrated
liquidArgon
Error
-2
-1
0
1
2
3
4
5
012345678log(H2/Ar)log(CO2/Ar)
GRID RH =
2.8
1238
which appears to be subareal in nature and typically
contains plagioclase, clinopyroxene, rare olivine and
primary apatite. Ash tuffs are fine grained rocks
lacking phenocrysts, with groundmass phases of
plagioclase microlites, glass, and alteration minerals
(see below). In TG-2, these units are <3 ft (1 m)
thick. In TG-4, which is ~2 miles (3.2 km) closer to
Akutan Volcano, similar units are as thick as 60 ft
(18 m), although correlations between individual
units was not possible. Units of lithic basalt are
composed of multiple different rock types in a
crystalline matrix. At this time, the origin of this
lithology is unknown.
Secondary Mineralogy, Mineral Paragenesis, and
Hydrothermal History
The core rocks in general appear to be only weakly
altered. Flow margins are characteristically rich in
vesicles and fractures, and thus tend to be more
altered and more readily brecciated than the main
body of the lava. Heavy Fe-oxidation was observed
between flow layers. Alteration minerals occurred
interstitially, in fractures, vesicles, and contact zones.
Alteration assemblages in both wells are dominated
by chlorite, zeolites, epidote, prehnite and calcite, and
this propylitic alteration appears to have happened
multiple times in both wells. The presence of adularia
in specific locations in both wells indicates higher
temperature and permeability conditions existed at
some point in the past. The presence of kaolinite in
TG-2 indicates argillic alteration with lesser extent
and intensity. Illite was identified in both wells,
although much more sparsely in TG-2.
Within the most recent propylitic alteration event in
TG-2, the sequence of zeolite formation shows a
classic trend toward higher temperatures with depth
(Seki et. al., 1969b; Wood, 1994). It is possible that
this trend will continue below the base of the well
(833 ft / 254 m MD). Figure 6a shows that some
higher-temperature minerals (illite, epidote, prehnite,
wairakite and adularia) occur in regions that are
currently much colder than expected for these
minerals. This suggests that the TG-2 region
underwent higher temperature alteration (>469 oF/
250 oC) in the past. The presence of these higher-
temperature minerals at unexpectedly shallow depths
further suggests that a significant portion of this older
alteration sequence has been removed through
erosion, possibly glacial. Overprinting of these
minerals by lower-temperature alteration
assemblages indicates the sampled region has since
returned to a lower-temperature alteration regime
with reduced permeability.
The pattern of alteration in TG-4 is more complex
than TG-2 (Fig. 6b), but still records multiple
propylitic alteration events.
Fig. 6. First occurrence of indicator minerals with
depth in core from wells TG-2 (a) and TG-4
(b). Horizontal arrows indicated formation
temperature ranges for each mineral. Dashed
lines indicate published values; solid lines
indicate the most commonly reported
minimum temperatures.
While the alteration patterns are somewhat different
from those observed in TG-2, they may still be
related to the same thermal histories. The most
recent alteration event may have been stronger in the
TG-2 region, overprinting more completely the
alteration sequence observed in TG-4.
1239
Both cores show an alteration sequence progressing
from an early propylitic event, a narrow band of
adularia-bearing propylitic alteration, followed by a
later propylitic event. The trend from moderate
propylitic to high-temperature adularia-forming
alteration and back to moderate propylitic indicates
that the shallow portion of the HSB field has reached
its thermal peak and has cooled moderately.
Additionally, many of the higher temperature
minerals occur at depths much shallower than
reported in other geothermal fields. Thus is it likely
that 1) this region was hotter than it is currently, and
2) the uppermost portion of the rock column has been
removed by glacial erosion.
Permeability and Porosity of Well Rocks
The primary lithologies do not lend themselves to
high primary permeability. The abundance of isolated
vugs filled with secondary minerals indicates that
fluid flow through microscopic intergranular
networks has been important, but flow rates are likely
very low. Vug filling is especially common in fine-
grained, detrital deposits (e.g., ash tuff), but clay
alteration and fracture mineralization by carbonates
and zeolites reduces permeability in these rocks.
The primary fluid pathways appear to be associated
with brittle fracturing and lithologic contacts, based
on the abundance and degree of alteration and
secondary mineralization. Highly vesicular lava flow
tops have high porosity and, when collapsed, can
have high permeability, although this was not
observed in the core. The majority of alteration and
secondary mineralization occurs along fractures,
however. This is particularly evident in the ash tuffs,
in which the lack of large crystals allows the units to
fracture at prescribed orientations (0o, 30o,45o, 60o
and 90o). The majority of these fractures have
secondary mineralization associated with them, and
some of the larger fractures contain relatively large
amounts of clays and other secondary minerals.
Because the tuff is more susceptible to clay
alteration, these fractures can seal before major
secondary mineralization becomes intense. However,
these units are thin in the wells, so may not have
significant control over the overall fluid flow.
The occurrence of the mineral adularia helps to
elucidate the nature of the permeability beneath
HSBV. Although adularia occurs in all lithologies in
the HSBV cores, the restriction of adularia to
fractures highlights the importance of secondary
permeability, as it does in many fields worldwide.
Adularia is strongly associated with zones that once
had high permeability but each occurrence of adularia
in the core is in veins that are now thoroughly sealed
by mineralization. Therefore, the waxing of a higher
temperature system and subsequent waning has
apparently reduced the permeability in the HSBV
outflow system.
Evidence for large scale structures was not
encountered in Akutan geothermal wells. A number
of brecciated zones were observed in TG-4, but most
were “sealed” with secondary mineral deposits and
therefore probably do not represent active faults.
Minor slickensides observed in cores could be related
to a possible normal fault on the SW side of the
valley near TG-4.
CONCEPTUAL MODELS
Two conceptual models of the Akutan Geothermal
Resource have been presented in previous works
(Kolker et al, 2010), both of which describe the
Akutan geothermal system as a single resource
comprised of two distinct features: a high-
temperature (>500 °F / >240 °C) upflow zone located
at depth somewhere proximal to the fumaroles, and a
lower-temperature outflow aquifer (~360-390 °F /
180-200 °C). Two alternative outflow pathways are
either along the L-shaped path of HSBV (Fig. 7) or
along a northern trajectory from the fumaroles to the
hot springs (Fig. 8).
Conceptual model ‘CM1’ (Fig. 7), follows the HSBV
path. This model was initially preferred because the
flow paths followed major structural features. There
are several lines of evidence, however, that reduce
the likelihood that this model is accurate. The
downhole temperature profile in TG-4 shows little
evidence for conductive heating from below,
requiring that the hot upflow region be significantly
displaced both vertically and horizontally from TG-4.
Additionally, for this model to fit the observed
downhole temperature profiles in both wells, the
outflow along HSBV can only be very thin (vertically
constrained low-permeability) and restricted to the
shallow subsurface.
Temperature differences between fluids flowed from
the permeable zone (585-587 ft MD in TG-2) during
drilling (359 °F; 182 °C) and after equilibration (338
°F; 165 °C) provide additional arguments against
CM-1. First, the hottest fluids must have been
“pulled in” laterally from a nearby source, and CM-1
does not allow for such hot fluids to be so rapidly
available at TG-2,as these temperatures would be ~3
km distant (Fig. 7b). Second, Conceptual model 1
‘CM1’ does not resolve the location of a hotter
outflow resource of 360-392 °F (180-200 °C), for
which there is a substantial amount of geochemical
evidence. Additionally, the rapidity with which this
hotter fluid was drawn in during such a short test
implies that the 338 °F (165 °C) permeable zone in
TG-2 must be restricted in volume and at a higher
natural pressure than the 359 °F (182 °C) adjacent
reservoir.
1240
Fig. 7 Map view of Conceptual model CM1; shallow
outflow following HSBV. Isotherm contour
placement is based on downhole temperature
data, chemical geothermometry, hot springs
and fumarole locations and MT resistivity
data. (a) Map view, with resistivity values for
984 ft (400 m) depth. Angled black line
“CM1” corresponds to the profile trace in
(b). (b) Profile view of CM1, MT resistivity
data based on 3-D inversion model. Black
line at 400 m refers to depth slice for
resistivity values shown in (a).
In conceptual model 2 ‘CM2’ (Fig. 8), the shallow
outflow path takes a northerly trajectory from the
fumarole to the ENE towards the hot springs,
circumventing HSBV altogether. This model appears
more likely based on several lines of reasoning: 1)
the temperature profile for TG-4 shows no evidence
for being along an outflow path, implying that
outflow feeding the hot springs is laterally distal; 2) a
low-resistivity clay cap appears to form a dome
pattern around the northerly outflow path, which is
consistent with the interpretation that the HSBV is
near, but not in, the main outflow path of geothermal
fluids (Figs.7a and 8a); and 3) the isotherm contours
on the CM2 profile (Fig. 8b) are slightly more typical
of an outflowing geothermal system.
Fig. 8 Conceptual model CM2; shallow outflow north
of HSBV. Isotherm contour placement is
based on downhole temperature data,
chemical geothermometry, hot springs and
fumarole locations and MT resistivity data.
(a) Map view, with resistivity values for 984
ft (400 m) depth. Angled black line “CM2”
corresponds to the profile trace in (b). (b)
Profile view of CM2, MT resistivity data
based on 3-D inversion model. Black line at
400 m refers to depth slice for resistivity
values shown in (a).
Both models suggest that producing the outflow
resource entails more risk because much of the data
suggest low permeability conditions in the HSBV. In
addition to the well behavior and alteration patterns
observed in the core discussed above, there is no
well-developed clay cap to indicate that a large, very
permeable reservoir volume at ~360-390 °F (180-220
°C) exists under HSBV. The lack of widespread
surface alteration, geochemical, and ground
temperature anomalies (Kolker and Mann 2009) in
HSBV are consistent with this interpretation.
Additionally, the chemical composition of the hot
springs fluids suggests that outflow fluids become
extensively mixed with cooler meteoric waters near
1241
the surface, raising concerns about cold water influx
into the outflow system with production.
Both models also suggest that the upflow zone could
be an extremely attractive development target.
Geochemical data from the fumaroles suggest that the
area lies fairly near an upflow zone from the reservoir
that a steam cap may overlie the upflow, and that
reservoir temperatures could approach 570 °F (300
°C) within the upflow. The deep reservoir probably
consists of a brine liquid capped by a small two-
phase region (steam cap). Resistivity data suggest
that the upflow reservoir is situated in brittle rocks,
implying propylitic alteration regime and a good
possibility of high permeability.
CONCLUSIONS
The Akutan geothermal resource can be divided into
an upflow zone and one or more outflow zones.
While the conceptual models of the outflow resource
have downgraded its potential for development,
geochemical data from the fumaroles significantly
upgrades the upflow resource as a drilling target.
Studies of alteration minerals in the core suggest that
the outflow region has reached a thermal maximum
and is in a cooling phase. The presence of a thin clay
cap, high resistivity values, and high temperature
minerals occurring at surprisingly shallow depths in
the outflow region suggest the uppermost portion of
the outflow region may have been eroded, possibly
due to glaciation. Alteration and secondary
mineralization in the outflow region has resulted in
“self-sealing” of permeable structures, and the
outflow resource discovered by TG-2 is likely to
have significant permeability limitations. The peak
outflow resource temperature of 359 °F (182 °C)
discovered during slimhole exploratory drilling in
2010 appears to reflect fluid “pulled in” from a
nearby source. A temperature reversal at the bottom
of the stabilized TG-2 profile reduces the possibility
that a hotter or more voluminous reservoir would be
encountered by drilling deeper at that location. New
geochemical data from well fluid and fumaroles
indicates that the upflow region of the Akutan
system, in the vicinity of the fumaroles at the head of
Fumarole Valley, is >428-572 °F (220-300 °C), near-
neutral chloride system with minor volcanic affinity
and a steam cap. Thus, the greatest probability of
successful development is in this region.
REFERENCES
Giggenbach, W., (1991) “Chemical Techniques in
Geothermal Exploration.” In: The Application of
Geochemistry in Geothermal Reservoir
Development, F. D'Amore Ed. 1991
UNITAR/UNDP Guidebook.
Kolker, A., and R. Mann, (2009) “Heating Up the
Economy with Geothermal Energy: A Multi-
Component Sustainable Development Project at
Akutan, Alaska.” Geothermal Resource Council
Transactions No. 33, 11p.
Kolker, A., Cumming, W. and Stelling, P. (2010)
“Geothermal Exploration at Akutan, Alaska:
Favorable Indications for a High-Enthalpy
Hydrothermal Resource Near a Remote
Market.” Geothermal Resource Council
Transactions No. 34, 14p.
Lu, Z., C. Wicks, C., Dzurisin, D., Thatcher, W., and
Power, J. (2000) “Ground Deformation
Associated with the March 1996 Earthquake
swarm at Akutan Volcano, Revealed by Satellite
Radar Interferometry.” Journal of Geophysical
Research, 105, No. B9, p. 21483-21495.
Miller, T.P., McGimsey, G., Richter, D., Riehle, J., Nye,
C., Yount, M., and Dumoulin, J. (1998) “Catalog
of the historically active volcanoes of Alaska.”
U.S. Geological Survey Open-file Report 98-582.
Motyka, R., and Nye, C., eds. (1988) “A geological,
geochemical, and geophysical survey of the
geothermal resources at Hot Springs Bay Valley,
Akutan Island, Alaska.” Alaska Division of
Geological and Geophysical Surveys, Report of
Investigations 88-3.
Motyka, R.J., Liss, S., Nye, C., and Moorman, M.
(1993) “Geothermal Resources of the Aleutian
Arc.” Alaska Division of Geological and
Geophysical Surveys, Professional Paper 114.
Newhall, C.G., and Dzurisin, D., (1988) “Historical
unrest at large calderas of the world.” U.S.
Geological Survey Bulletin 1855.
Powell, T. and Cumming, W (2010) “Spreadsheets for
Geothermal Water and Gas Geochemistry”
Proceedings, 35th Workshop on Geothermal
Reservoir Engineering, Stanford University,
Stanford, CA, Feburary 1-3, 2010. SGP-TR-188
Richter, D.H., Waythomas, C.F., McGimsey, R.G. and
Stelling, P.L. (1998) “Geology of Akutan Island,
Alaska.” U.S. Geological Survey Open-File Report
98-135, 1 sheet, 1:63,360 scale
Seki, Y., Onuki, H., Okumura, K., and Takashima, I.
(1969b) “Zeolite distribution in the Katayama
geothermal area, Onikobe, Japan.” Japanese
Journal of Geology and Geography, 40, 63-79.
Simkin, T., and Siebert, L. (1994) “Volcanoes of the
World” 2nd edition Geoscience Press in
association with the Smithsonian Institution
Global Volcanism Program, Tucson AZ, 368 p.
Symonds, R. B., Poreda, R., Evans, W. C., Janik, C. J.
and Ritchie, B. E. (2003) “Mantle and crustal
sources of carbon, nitrogen, and noble gases in
1242
Cascade-Range and Aleutian-Arc volcanic
gases.” U.S. Geological Survey Open-File Report
03-436.
Waythomas, C.F, Power, J.A., Richter, D.H., and
McGimsey, R.G. (1998) “Preliminary Volcano-
Hazard Assessment for Akutan Volcano, East-
Central Aleutian Islands, Alaska” U.S. Geological
Survey Open-File Report OF 98-0360, 36 p., 1
plate, scale unknown.
Western Governors’ Association Clean and
Diversified Energy Initiative Report, 2006.
http://www.westgov.org/wga/initiatives/cdeac/
Geothermal-full.pdf
WesternGeco (2009) Magnetotelluric Survey at Hot
Springs Bay Valley, Akutan, Alaska: Final Report
– 3D Resistivity Inversion Modeling.”
Unpublished report prepared for the City of
Akutan, Alaska, GEOSYSTEM/WesternGeco EM,
Milan, Italy, 27p.
Wood, C.P. (1994) “Mineralogy at the magma-
hydrothermal system interface in andesite
volcanoes, New Zealand.” Geology, 22, 75-78.
1243
53
Appendix 3
GRAVITY SURVEY
ON THE
CITY OF AKUTAN GEOTHERMAL PROJECT
AKUTAN, ALASKA
FOR
CITY OF AKUTAN, ALASKA
DATA ACQUISITION AND PROCESSING REPORT
ZONGE JOB # 12140
ISSUE DATE: 26 October 2012
ZONGE INTERNATIONAL, INC.
9595 Prototype Court
Reno, Nevada 89521
Phone: (775) 355-7707, Fax: (775) 355-9144
Zonge International Inc. Akutan Gravity 26 October 2012
TABLE OF CONTENTS
INTRODUCTION .................................................................................................................................. 1
INSTRUMENTATION ......................................................................................................................... 1
DATA ACQUISITION .......................................................................................................................... 2
GPS DATA .......................................................................................................................................... 2
GRAVITY DATA .................................................................................................................................. 3
DATA QUALITY ................................................................................................................................... 3
DATA PROCESSING ........................................................................................................................... 5
GPS PROCESSING ............................................................................................................................. 5
GRAVITY PROCESSING .................................................................................................................... 5
DATA PRESENTATION ...................................................................................................................... 9
SAFETY AND ENVIRONMENTAL ISSUES .................................................................................... 9
APPENDIX A. PLOTS ........................................................................................................................ 11
APPENDIX B. GRAVITY METER SPECIFICATIONS ............................................................... 24
APPENDIX C. GPS/GLONASS RECEIVER SPECIFICATIONS ............................................... 25
APPENDIX D. GPS BASE DESCRIPTION ..................................................................................... 27
APPENDIX E: PRODUCTION LOG ................................................................................................ 29
APPENDIX F. GRAVITY BASE DESCRIPTION ........................................................................... 30
APPENDIX G. LOOP CLOSURES .................................................................................................. 31
APPENDIX H. GRAVITY AND GPS REPEATS ........................................................................... 32
APPENDIX I. DATA DVD CONTENTS ........................................................................................... 34
Zonge International Inc. Akutan Gravity 26 October 2012
LIST OF FIGURES
Figure 1. Gravity station locations. ................................................................................... 2
Figure 2. Histogram of gravity repeats. ............................................................................ 4
Figure 3. Histogram of GPS elevation repeats. ................................................................. 5
Figure 4. Terrain Grid ........................................................................................................ 8
Figure 5: Gravity station locations on topographic base. ............................................... 12
Figure 6. Complete Bouguer Anomaly @ 1.90 gm/cc. .................................................... 13
Figure 7. Complete Bouguer Anomaly @ 2.00 gm/cc. ..................................................... 14
Figure 8. Complete Bouguer Anomaly @ 2.10 gm/cc. ..................................................... 15
Figure 9. Complete Bouguer Anomaly @ 2.20 gm/cc. ..................................................... 16
Figure 10. Complete Bouguer Anomaly @ 2.25 gm/cc. ................................................... 17
Figure 11. Complete Bouguer Anomaly @ 2.30 gm/cc. ................................................... 18
Figure 12. Complete Bouguer Anomaly @ 2.40 gm/cc. ................................................... 19
Figure 13. Complete Bouguer Anomaly @ 2.50 gm/cc. ................................................... 20
Figure 14. Complete Bouguer Anomaly @ 2.60 gm/cc. ................................................... 21
Figure 15. Complete Bouguer Anomaly @ 2.67 gm/cc. ................................................... 22
Figure 16. Complete Bouguer Anomaly @ 2.70 gm/cc. ................................................... 23
1
Zonge International Inc. Akutan Gravity 26 October 2012
GRAVITY SURVEY
City of Akutan Geothermal Project
INTRODUCTION
Zonge International, Inc. performed a gravity survey on the Akutan Project, located in the
Aleutian Islands, Alaska for City of Akutan. This survey was conducted during the period of 12
August 2012 to 24 August 2012. The survey area is located in Township 70 South and Range
112 West, and lies within the Unimak (A-6), 15-minute topographic map. Gravity data were
acquired on 22 stations centered on Hot Springs Bay Valley. A total of 217 gravity stations were
acquired. Station locations are shown in Figure 1.
This survey was conducted by Christopher Kratt, Project Geologist, under Zonge job
number 12140. Field assistance was provided by either Mary Ohren of GRG or Matthew
Bereskin from the City of Akutan. This report covers data acquisition, instrumentation and
processing for this job.
INSTRUMENTATION
Gravity data were acquired using a Scintrex CG-5 gravimeter, serial number 792. The
CG-5 gravity meter has a reading resolution of 0.001 milligals and a typical repeatability of less
than 0.005 milligals. Specifications for this instrument are included in Appendix B.
Positioning was obtained with Leica Geosystems model VIVA GS15 GPS/GLONASS
receivers. These are survey-grade receivers capable of centimeter-level accuracy.
GPS/GLONASS receiver specifications are included in Appendix C. Coordinates are given in
UTM, Zone 3, WGS84 and elevations are given in the NAVD88 datum.
2
Zonge International Inc. Akutan Gravity 26 October 2012
Figure 1. Gravity station locations.
DATA ACQUISITION
GPS DATA
Carrier-phase GPS data were acquired for three to five minute sessions at each station
during simultaneous acquisition at a fixed GPS base station. The majority of stations were post-
processed with the exception of thirteen stations on 24 August 2012 when the base station was
operating in RTK mode and the base data were not saved to the memory card; Nine stations for
this day that did not have RTK solutions were post-processed using two CORS base stations. A
discussion of positioning quality is given under the section titled Data Quality, GPS.
A single GPS base station was used for this survey. The position of the GPS base
3
Zonge International Inc. Akutan Gravity 26 October 2012
station was determined by submitting data to the National Geodetic Survey (NGS) On-line
Positioning User Service (OPUS). OPUS processed these observation files with respect to 3
Continuously Operating Reference Stations (CORS). The GPS base station and CORS stations
point specifications are listed in Appendix D.
GRAVITY DATA
Gravity measurements were made in a series of looped-traverses that were closed within
a maximum of eleven hours. At least two measurements were made at each occupation.
The station spacing was variable in order to optimize data coverage. Station spacing in a
few cases was greater than 600 meters, but more commonly were within 200 meters. Repeat
measurements were made at fourteen stations (6 percent) for evaluation of data quality.
A single gravity base station was used for this survey. Logistical and time
considerations made it impractical to tie the survey to the nearest absolute gravity station, which
is located in Dutch Harbor. The regional gravity value for the base station (9000) was assigned
an estimated value based on interpolation of data contained in Saltus et al., (2006). The gravity
base was located in the City of Akutan and is shown with coordinates in Appendix E.
DATA QUALITY
The average loop closure for the local survey was 0.027 milligals. Individual loop
closures are tabulated in Appendix F.
Gravity measurement precision is evaluated by making repeat readings at selected
gravity stations. Thirteen gravity measurements were repeated. The average difference between
repeat measurements is 0.024 milligals, and the maximum difference is 0.096 milligals, which is
observed between measurements made on 22 August 2012 and on 14 August 2012. During the
course of the survey four other stations were repeated from the 14 August 2012 loop, having an
average difference of 0.024 mGals. One station from 22 August 2012 was repeated on 17
August with a difference of 0.008 mgals. Repeated gravity measurements are tabulated in
4
Zonge International Inc. Akutan Gravity 26 October 2012
Appendix G.
Figure 2. Histogram of gravity repeats.
An important factor that determines the accuracy of the reduced measurement is the
accuracy in determining a station’s location, particularly the elevation. The vertical gradient of
the earth’s field is approximately -.308596 milligals per meter of increase in elevation. The
Bouguer correction is .1119 milligals per meter of elevation increase, for a density of 2.67
gm/cc. This results in a total error in the Bouguer Anomaly of .1967 milligals per meter of
elevation error, for a reduction density of 2.67 gm/cc.
GPS positioning precision is evaluated by making repeated GPS measurements at
randomly selected stations. Fourteen duplicate GPS measurements that were made over a range
of field conditions and baseline lengths, show a maximum elevation difference of 39 cm and an
average difference of 5 cm. A tabulation of repeated GPS measurements is presented in
Appendix G. The maximum elevation difference was at station 2052. The second elevation for
this station was from the processing against the CORS stations and had a poor quality solution.
5
Zonge International Inc. Akutan Gravity 26 October 2012
Figure 3. Histogram of GPS elevation repeats.
DATA PROCESSING
GPS PROCESSING
Locations of the gravity stations were determined as baselines from the GPS base in
WGS-84 coordinates and ellipsoidal heights. Due to uncertain helicopter availability during
inclement weather, the GPS base was located in the City of Akutan to ensure access. In nearly
all cases, RTK solutions were not available and the GPS observations were processed after data
acquisition (post-processing) using Leica Geo-Office™ software. The “CLASS” field in the
master data base and principal facts file uses “RTK”, “PP”, and “CORS” to indicate the type of
gps solution.
GRAVITY PROCESSING
The basic processing of gravimeter readings to the Complete Bouguer Anomaly was
made using the Gravity and Terrain Correction software version 7.1 for Oasis Montaj by Geosoft
LTD. of Toronto, Canada.
6
Zonge International Inc. Akutan Gravity 26 October 2012
The observed gravity is the gravitational acceleration, in milligals, that is determined by
relative measurements made in a loop from a gravity base, after the meter readings have been
corrected for instrument height, instrument scale factor, instrument drift and earth tides.
The long-term instrument drift is the rate at which each particular instrument
accumulates error due to instrument factors such as vibration, battery voltage changes, and
elastic relaxation, among others. It is minimized by proper technique, and warm up of the
instrument.
Earth tides cause variations in observed gravity for land-based surveys of up to
approximately .03 milligals per hour (Siegel, 1995). Corrections are computed by use of pre-
programmed theoretical tide tables that are a part of the Geosoft™ gravity reduction software.
The effect of earth tides can be further minimized by frequently tying loops to local gravity bases
(Butler, 1991).
The observed gravity is a function of position (geographic latitude and elevation) and
variations in the density of the subsurface material. A series of reductions are made to remove
the gravity variation caused by position so that the gravity variations caused by subsurface
density distribution remain.
A latitude correction must be made to the observed gravity measurements because the
earth is not spherical, but has a slightly larger radius at the equator. It includes terms for both the
Newtonian attraction of the earth as a flattened spheroid and the centrifugal force caused by the
earth’s rotation (Siegel, 1994). The latitude correction is calculated for the International
Ellipsoid of 1967 (International Association of Geodesy, 1971).
)](sin000023462.0)(sin005278895.01.[846.978031 42
1 g
where:
1g = theoretical gravity in milligals (latitude correction)
= Latitude of the station
Where:
1g = latitude correction
= latitude of the station
7
Zonge International Inc. Akutan Gravity 26 October 2012
ys = station distance north of the grid origin in meters
The elevation correction has two parts: the Free Air correction and the Bouguer
correction. The free air correction compensates for the variation of the earth’s gravitational field
with distance away from the center of the earth. The approximate and often-used correction is -
0.308596 milligals per meter above the ellipsoid. For this survey all elevations are referenced to
the Geoid by use of the GEOID 09 model in the NAVD88 datum.
The free air correction is calculated in the Geosoft Montaj program using the following formula:
Where, safahggg308596.01
gfa = free air anomaly in milligals
ga = observed gravity
gl = latitude correction
hs = station elevation in meters
The Bouguer correction compensates for the mass of material located between the
station elevation and the Geoid (mean sea level). The Bouguer correction is calculated on the
basis of the gravitational attraction of a horizontal slab of infinite extent whose thickness is equal
to the elevation difference between the stations of interest and mean sea level:
gba = gfa – 0.0419088*[hs]
Where,
gba = Simple Bouguer anomaly in milligals
gfa = free air anomaly
= density of rock
The Bullard B correction is used to correct for the fact that the mass of rock between the
Geoid and the station elevation is a spherical shell as opposed to an infinite horizontal slab. The
correction is based on the formula given by LaFehr (1991):
BBg = 2k(sh - R),
where
BBg = Bullard B Correction
R = Earth radius to the station (R0 + h, where R0 is the earth’s radius)
8
Zonge International Inc. Akutan Gravity 26 October 2012
2k is the simple Bouguer slab formula; and are dimensionless coefficients whose
definitions are given in the appendix of LaFehr’s 1991 paper.
The Complete Bouguer Anomaly includes those corrections found in the Simple
Bouguer Anomaly, as well as, corrections for the effect of the surrounding topography.
Corrections for the gravity effect of variable Terrain tcg are made from a combination of
operator measurements and digital elevation data. A high-resolution digital terrain model
supplied by RMA Consulting was converted to a 5 m digital elevation model (DEM) and used
for terrain corrections for radii of 10 meters to 50 kilometers. A National Elevation Dataset
(NED) 2 Arc-Second (60 m) DEM was used to terrain-correct 45 stations that fell outside the
area covered by the 5 m DEM. The average slope from the station to a radius of 10 m was
determined by an inclinometer. Correction for bathymetry was not applied.
Figure 4. Terrain Grid
9
Zonge International Inc. Akutan Gravity 26 October 2012
If the density of the near-surface rocks differs from the reduction density, then an
elevation-dependent error will result. This error is approximately 1.25 microgals per 0.3048 m
for each 0.1 gm/cm3 difference in density (Hinze, 1990). Simple and Complete Bouguer gravity
anomalies were computed for densities ranging from 1.50 gm/cm3 to 3.00 gm/cm3 and contour
maps provided at selected reduction densities. The file containing principle facts provides
densities ranging from 1.50 gm/cm3 to 3.00 gm/cm3.
DATA PRESENTATION
Plan maps are provided as fit-to-page scale plots in Appendix A and as Geotiff files on
the data DVD. These include a station location map and Complete Bouguer Anomaly maps for
the following density reductions: 1.90, 2.0, 2.1, 2.2, 2.25, 2.3, 2.4, 2.5, 2.6, 2.67, 2.7.
Digital data and image files are included on the data DVD. A description of the data
DVD contents is presented in Appendix H.
SAFETY AND ENVIRONMENTAL ISSUES
No health, safety incidents or accidents occurred during the course of this survey. No
environmental damage was sustained as a result of the survey progress.
Respectfully submitted,
__________
Christopher Kratt
Project Geologist
Zonge International, Inc.
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Zonge International Inc. Akutan Gravity 26 October 2012
REFERENCES
Butler, D.K., 1991, Tutorial—Engineering and environmental applications of
microgravity. In Proceedings of Symposium on the Application of Geophysics to
Engineering and Environmental Problems, ’91. Knoxville, TN. 139.
Hinze, W.J., 1990, The role of gravity and magnetic methods in engineering and
environmental studies. In, Ward, S. H., ed. Geotechnical and Environmental
Geophysics, Society of Exploration Geophysicists, Investigations in Geophysics,
5, 75-126
LaFehr, T.R., 1991a, An exact solution for the gravity curvature (Bullard B) correction:
Geophysics, 56. 1179-1184.
Saltus, R. W., Brown II, P. J., Morin, R. L., and Hill, P. L., 2006, 2006 Compilation of
Alaska Gravity Data and Historical Reports. USGS Data Series 264.
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Zonge International Inc. Akutan Gravity 26 October 2012
APPENDIX A. PLOTS
12 Zonge International Inc. Akutan Gravity 26 October 2012 Figure 5: Gravity station locations on topographic base. -f-250 0 250 500 750 1000 1250 (meters) WGS 84 I UTM zone 3N Gravity Station Locations 444000 446000 + City of Akutan Geothermal Project, Alaska Gravity Survey, August 2012 Zonge lnternlltionllllnc.
13 Zonge International Inc. Akutan Gravity 26 October 2012 Figure 6. Complete Bouguer Anomaly @ 1.90 gm/cc. + -t-\: WGS 84 I UTM zone 3N \ 444000 Complete Bouguer Anomaly@ 1.90 gm/cc (mgals) • Gravity stations Ci of Akutan Geothermal Pro· eel, Alaska 114.5 117.1 119.8 122.5 125.2 127.8 130.5 133.2 GrovltySurvoy,August2012 Contour .nteNal 0.2, 1.0 mgats Zongo lntornotlonollnc.
14 Zonge International Inc. Akutan Gravity 26 October 2012 Figure 7. Complete Bouguer Anomaly @ 2.00 gm/cc. ·Ill J77lr I ~ I Complete Bouguer Anomaly@ 2.00 gm/cc (mgals) • Gr•vtty stations 114.5 117.0 119.5 121.9 124.4 126.8 129.3 131.8 ....000 Ci -+ + WGS 84 I UTM zone 3N Gravity Survey, August 2012 Conlout i'lteNII 0 2 1 0 mgall Zonge lntem•tlonallnc.
15 Zonge International Inc. Akutan Gravity 26 October 2012 Figure 8. Complete Bouguer Anomaly @ 2.10 gm/cc. • Gravity stations
16 Zonge International Inc. Akutan Gravity 26 October 2012 Figure 9. Complete Bouguer Anomaly @ 2.20 gm/cc. 440000 ·J6:1•o.r.xr 444000 446000 + i + I • Gravity stations
17 Zonge International Inc. Akutan Gravity 26 October 2012 Figure 10. Complete Bouguer Anomaly @ 2.25 gm/cc. 440000 440000 • Gravity stations -IM'~ 444000 442000 Complete Bouguer Anomaly@ 2.25 gmlcc (mgals) 114.8 116.7 118.6 120.6 122.5 124.5 126.4 128.4 446000 + ~ City of Akutan Geothermal Project, Alaska Gravity Survey, August 2012 Contour lnt~rvoat 0.2, t.O mgafs zonge lnteEnatlonallnc.
18 Zonge International Inc. Akutan Gravity 26 October 2012 Figure 11. Complete Bouguer Anomaly @ 2.30 gm/cc. 440000 • ~~ -f-~ I t WGS 841 UTM zone 3N 440000 442000 Complete Bouguer Anomaly@ 2.30 gm/cc (mgals) • Gravity stations City of Akutan Geothermal Project, Alaska 114.8 116.6 118.5 120_4 122.3 124.1 126.0 127.9 Gravity Survey, August 2012 Contour .-t!e.rval· 0.2. 1.0 mgals Zonge International Inc.
19 Zonge International Inc. Akutan Gravity 26 October 2012 Figure 12. Complete Bouguer Anomaly @ 2.40 gm/cc. c·ty ' of Akutan Geoth Gravity surv ermal Project, Alaska ey, August2012 Contour ml.erval: 0.2, 1.0 mgals Zongc International Inc.
20 Zonge International Inc. Akutan Gravity 26 October 2012 Figure 13. Complete Bouguer Anomaly @ 2.50 gm/cc. + + ~ I y of Ak~tan Geothermal Project Alaska ravity Survey, August 2012 ' Contour lnt~rvoat 0.2, t.O mgafs Zonge lnteEnatlonallnc.
21 Zonge International Inc. Akutan Gravity 26 October 2012 Figure 14. Complete Bouguer Anomaly @ 2.60 gm/cc. 444000 44>6000 • Gravity stations
22 Zonge International Inc. Akutan Gravity 26 October 2012 Figure 15. Complete Bouguer Anomaly @ 2.67 gm/cc. + -rw~ Complete Bouguer Anomaly@ 2.67 gm/cc (mgals) • Gravity stations 111.7 113.6 115.6 117.5 119.5 121.4 123.4 125.4 + WGS 84 I UTM zone 3N City of Akutan Geothermal Project, Alaska Gravity Survey. August 2012 Contourintervai:0.2, 1.0 mgals Zonge lntcrnaUonallnc.
23 Zonge International Inc. Akutan Gravity 26 October 2012 Figure 16. Complete Bouguer Anomaly @ 2.70 gm/cc.Complete Bouguer Anomaly@ 2.70 gm/cc (mgals) • Gravity stations City of Akutan Geothermal Project, Alaska 111.0 113.0 115.1 117.2 119.2 121.3 123.4 125.4 Gravity Survey, August 2012 Contow interval: 0.2, LO mgats Zong.e International Inc.
24
Zonge International Inc. Akutan Gravity 26 October 2012
APPENDIX B. GRAVITY METER SPECIFICATIONS
CG-5 SPECIFICATIONS
Sensor Type :
Reading Resolution:
Standard Fie ld Repeatability :
Operating Range:
Residual Long-Term Drift:
Automatic T ilt Compensation:
Fused Quartz using electrostati c nulling
1 microGal
<5 m icroGal
8,000 mGal without resetti ng
Less than 0.02 mGaVday (static)
±200 arc sec
Tares: Typically less than 5 microGals for shocks up to 20 G
Automated Corrections: Tide, Instrument Ti lt, Temperature, Drift, Near Terrain, Noisy Sample, Seismic
Noise Filter
Operating Temperature: -4o•c to +45°C (-4Q •F to 113°F)
Ambient Temperature Coefficient: 0 .2 microGaiJOC (typical)
P1e~~Uit! CueffiGit!lll. 0 .15 llliG IUGCI I!kPCI (lyiJiGCI I)
Magnetic Field Coefficient 1 microGai/Gauss (typical)
Memory: Flash Technology (data security)
D imensions: 30 em (H) x 22 em x 21 em (12" (H) x 8.5" x 8")
Weight (including batteries): 8 kg (17.5 lbs)
Battery Capacity: 2 x 6.6 Ah (1 1.1 V) rechargeable Li thium-ion Smart Batteries. Full day
Power Consu mption:
Standard System:
GPS
operation i n normal survey conditions with two fully charged batteries
4.5 w at +25°C (77°F)
CG-5 Console, Tripod base, 2 rechargeable batteries, Battery Charger
1101240 V, External Power Sup ply 110/240 V, RS-232 and USB Cables,
Carrying Bag, Data dump and utilities software, Operating Manual (CD),
Transit Case
Enables GPS station referencing from an external 12 channel smart GPS antenna bei ng connected via the
RS-232 port. Standard GPS accuracy: <15m OOPS (WAAS) <3m. C lient has the option to use other higher
accuracy GPS receivers outputting NM EA data stri ng through seria l port.
RF Transmitter
The CG-5 Autograv g ravity meter is equipped w ith a radio frequency remote start transmitter to allow
measurements to be t aken without disturbing th e meter by touch.
OPTIONS
High Temperature Option -For use in climates that may exceed the operation temperature of +45°C (1 13°F).
Allows operat ing temperatures of up to +55°C (131 °F). Th is option is intended to be used in c li mates above
freezing and needs to be o rdered at the time of purchase.
Battery Belt -Suggested for cold weather operation.
T raining Programs
Software Packages
ISO 9001 :2000 reg istered company.
Specification Sheet Part Number 867711 Rev. 1
(\
SCINTREX
CANADA
Scintrex
222 Snidercroft Roa d
Concord. Ontario L4K 2K1
Telephone: +1 905 669 2280
Fax: +1 905 669 6403
e-mail: sdntrex@sejntrexltd tom
Website : www .scin trex .com
All specifications are subject to change w ithout notice.
MICRotg\
LACOSTE
USA
Mlcro.g LaCoste
1401 Haizon Avenue
Lafayeue . co 80026
Telephone: +1 303 828 3499
Fax: +1 303 828 3288
e-mail: jofo@mjt:rog!aeoste eqo
Website: www.microglacoste.com
25
Zonge International Inc. Akutan Gravity 26 October 2012
APPENDIX C. GPS/GLONASS RECEIVER SPECIFICATIONS
Leica Geosystems Viva GS15 survey receiver
GNSS technology Leica patented SmartTrack+ technology:
• Advanced measurement engine
• Jamming resistant measurements
• High precision pulse aperture multipath correlator for pseudorange
measurements
• Excellent low elevation tracking
• Very low noise GNSS carrier phase measurements with <0.5 mm precision
• Minimum acquisition time
No. of channels 120 channels
Max. simultaneous tracked satellites Up to 60 Satellites simultaneously on two frequencies
Satellite signals tracking • GPS: L1, L2, L2C, L5
• GLONASS: L1, L2
• Galileo (Test): GIOVE‐A, GIOVE‐B
• Galileo: E1, E5a, E5b, Alt‐BOC
• Compass1
• SBAS: WAAS, EGNOS, GAGAN, MSAS
GNSS measurements Fully independent code and phase measurements of all frequencies
• GPS: carrier phase full wave length, Code (C/A, P, C Code)
• GLONASS: carrier phase full wave length, Code (C/A, P narrow Code)
• Galileo: carrier phase full wave length, Code
Reacquisition time < 1 sec
Accuracy (rms) Code differential with DGPS /
RTCM
DGPS / RTCM Typically 25 cm (rms)
Accuracy (rms) with Real‐Time (RTK)
Standard of compliance Compliance with ISO17123‐8
Rapid static (phase) Horizontal: 5 mm + 0.5 ppm (rms)
Static mode after initialization Vertical: 10 mm + 0.5 ppm (rms)
Kinematic (phase) Horizontal: 10 mm + 1 ppm (rms)
Moving mode after initialization Vertical: 20 mm + 1 ppm (rms)
Accuracy (rms) with Post Processing
Static (phase) with long Horizontal: 3 mm + 0.5 ppm (rms)
observations Vertical: 6 mm + 0.5 ppm (rms)
Static and rapid static (phase) Horizontal: 5 mm + 0.5 ppm (rms)
Vertical: 10 mm + 0.5 ppm (rms)
Kinematic (phase) Horizontal: 10 mm + 1 ppm (rms)
Vertical: 20 mm + 1 ppm (rms)
26
Zonge International Inc. Akutan Gravity 26 October 2012
On the Fly (OTF) Initialization
RTK technology Leica SmartCheck+ technology
Reliability of OTF initialization Better than 99,99%
Time for initalization Typically 8 sec
OTF range up to 50 km
Network RTK
NetWork technology Leica SmartRTK technology
Supported RTK network solutions VRS, FKP, iMAX
Supported RTK network standards MAC (Master Auxiliary Concept) approved by RTCM SC 104
27
Zonge International Inc. Akutan Gravity 26 October 2012
APPENDIX D. GPS BASE DESCRIPTION
NGS OPUS SOLUTION REPORT
========================
All computed coordinate accuracies are listed as peak‐to‐peak values.
For additional information: http://www.ngs.noaa.gov/OPUS/about.jsp#accuracy
USER: DATE: August 13, 2012
RINEX FILE: 4580225s.12o TIME: 00:42:01 UTC
SOFTWARE: page5 1108.09 master3.pl 0718123 START: 2012/08/12 18:51:00
EPHEMERIS: igu17010.eph [ultra‐rapid] STOP: 2012/08/12 22:57:00
NAV FILE: brdc2250.12n OBS USED: 8930 / 9268 : 96%
ANT NAME: LEIGS15 NONE # FIXED AMB: 47 / 49 : 96%
ARP HEIGHT: 1.039 OVERALL RMS: 0.012(m)
REF FRAME: NAD_83(2011)(EPOCH:2010.0000) IGS08 (EPOCH:2012.6144)
X: ‐3630344.998(m) 0.007(m) ‐3630346.008(m) 0.007(m)
Y: ‐920270.129(m) 0.010(m) ‐920269.078(m) 0.010(m)
Z: 5145498.398(m) 0.021(m) 5145498.781(m) 0.021(m)
LAT: 54 8 1.59195 0.015(m) 54 8 1.58032 0.015(m)
E LON: 194 13 28.18347 0.008(m) 194 13 28.11369 0.008(m)
W LON: 165 46 31.81653 0.008(m) 165 46 31.88631 0.008(m)
EL HGT: 20.340(m) 0.016(m) 21.073(m) 0.016(m)
ORTHO HGT: 3.803(m) 0.028(m) [NAVD88 (Computed using GEOID12)]
UTM COORDINATES STATE PLANE COORDINATES
UTM (Zone 03) SPC (5010 AK10)
Northing (Y) [meters] 5998683.660 396161.338
Easting (X) [meters] 449329.633 1666147.931
Convergence [degrees] ‐0.62847223 8.14812931
Point Scale 0.99963150 1.00010612
Combined Factor 0.99962832 1.00010293
US NATIONAL GRID DESIGNATOR: 3UVV4932998683(NAD 83)
BASE STATIONS USED
PID DESIGNATION LATITUDE LONGITUDE DISTANCE(m)
DI9676 KOD5 KODIAK 5 CORS ARP N573703.695 W1521136.266 931490.0
DJ3025 BAY5 COLD BAY 5 CORS ARP N551124.983 W1624225.701 230256.1
DJ3031 KEN6 KENAI 6 CORS ARP N604029.296 W1512100.503 1126783.8
NEAREST NGS PUBLISHED CONTROL POINT
UW0097 AKUTAN VILLAGE CHURCH CROSS N540800.258 W1654629.948 53.4
This position and the above vector components were computed without any knowledge by the
National Geodetic Survey regarding the equipment or field operating procedures used.
28
Zonge International Inc. Akutan Gravity 26 October 2012
CORS_ID - AV09
CORS This is a GPS Continuously Operating Reference Station.
DESIGNATION - HAYSTACK__AK2004 CORS ARP
PID - DG7414
STATE/COUNTY- AK/ALEUTIANS EAST BOROUGH
COUNTRY - US
USGS QUAD - UNALASKA C-2
______________________________________________________________________
DG7414* NAD 83(2011) POSITION- 53 52 32.29385(N) 166 32 30.54267(W)
ADJUSTED
DG7414* NAD 83(2011) ELLIP HT- 104.940 (meters) (08/??/11)
ADJUSTED
DG7414* NAD 83(2011) EPOCH - 2010.00
DG7414* NAVD 88 ORTHO HEIGHT - **(meters) **(feet)
CORS_ID - AC10
CORS - This is a GPS Continuously Operating Reference Station.
DESIGNATION - CPSARICHEFAK2008 CORS ARP
PID - DM7475
STATE/COUNTY- AK/ALEUTIANS EAST BOROUGH
COUNTRY - US
USGS QUAD - UNIMAK C-3
______________________________________________________________________
DM7475* NAD 83(2011) POSITION- 54 31 21.30288(N) 164 53 12.15283(W)
ADJUSTED
DM7475* NAD 83(2011) ELLIP HT- 169.780 (meters) (12/??/11)
ADJUSTED
DM7475* NAD 83(2011) EPOCH - 2010.00
DM7475* NAVD 88 ORTHO HEIGHT - **(meters) **(feet)
DM7475
______________________________________________________________________
29
Zonge International Inc. Akutan Gravity 26 October 2012
Appendix E: Production Log
Date Operator Assistant Comments Stations
Read
Stations
Repeated
8/9/2012 Travel from Reno to Anchorage
8/10/2012 Travel from Anchorage to Dutch Harbor
8/11/2012 Travel from Dutch Harbor to Akutan
Unpack and set up Equipment for charging
8/12/2012
No helicopter fuel. Established GPS Base
Acquired GPS over benchmark
Obtained OPUS solution for GSP Base
8/13/2012 Chris
Kratt
Matt
Bereskin
Arrived in upper study area.
Realized gravity base not re ad
Waited ~2 Hours for helicopter assistance.
Production started at 12:17
15
8/14/2012 Chris
Kratt
Matt
Bereskin
Strong Winds in upper study area
Steep slopes, wind, vegetation make
leveling gravimeter difficult
19 1
8/15/2012 Chris
Kratt
Matt
Bereskin
Cloud Ceiling Low. Studied Valley Floor
Used meter platform to improve production 20 1
8/16/2012 Chris
Kratt
Mary
Ohren Cloud Ceiling Low. Studied West Valley Floor 23 1
8/17/2012 Chris
Kratt
Mary
Ohren
Low Ceiling. Started at Saddle.
Ascended against wind and rain 20 2
8/18/2012 Chris
Kratt
Mary
Ohren
Good Weather for working
around fumaroles 25 3
8/19/2012 Chris
Kratt
Mary
Ohren
Ascended 500' to cross ridgeline NW
of fumaroles. Limited time on backside
due to high wind/rain. Began long descent
to acquire stations on northern periphery
11 2
8/20/2012 Chris
Kratt
Matt
Bereskin
Circumnavigated Hot Springs
Bay Valley Floor 24 2
8/21/2012 Chris
Kratt
Mary
Ohren
Very windy. Meter Unstable. Hard rain
and winds too high for helicopter.
Began hiking out at 1530
61
8/22/2012 Chris
Kratt
Matt
Bereskin
Good Weather. Able to land near fumaroles.
Steep terrain. 500'+ of ascending 18 3
8/23/2012 Chris
Kratt
Matt
Bereskin Pleasant Weather 19 3
8/24/2012 Chris
Kratt
Mat t
Bereskin
Pleasant Weather
Packed up gear 22 1
8/25/2012 Travel from Akutan to Anchorage
8/26/2012 Travel from Anchorage to Reno
Total Stations 222
Total Repeats 20
30
Zonge International Inc. Akutan Gravity 26 October 2012
APPENDIX F. GRAVITY BASE DESCRIPTION
Photo shows the Russian Orthodox Church in the City of Akutan. A piece of rebar was driven
next to the rock hammer circled in red. This is the gravity base monument.
Lat. 54:08:00.39 Long. -165:46:30.22 Elevation: 4.29 m
WGS84 UTM Z3 449358.28 E 5998646.24 N
Assigned Gravity: 981540.000 (note this is not tied to IGSN71, It is an estimated value and is
floating).
31
Zonge International Inc. Akutan Gravity 26 October 2012
APPENDIX G. LOOP CLOSURES
Date Base Loop # Duration Closure (mGal)
Abs_Closure
(mGal)
8/13/201
2 9000 1 5:00 0.021 0.021
8/14/201
2 9000 1 8:14 -0.003 0.003
8/15/201
2 9000 1 7:48 0.022 0.022
8/16/201
2 9000 1 10:32 0.024 0.024
8/17/201
2 9000 1 10:07 0.035 0.035
8/18/201
2 9000 1 11:01 0.037 0.037
8/19/201
2 9000 1 9:27 0.060 0.060
8/20/201
2 9000 1 9:44 -0.008 0.008
8/21/201
2 9000 1 7:37 0.031 0.031
8/22/201
2 9000 1 9:21 0.030 0.030
8/23/201
2 9000 1 8:19 0.016 0.016
8/24/201
2 9000 1 9:05 0.035 0.035
Average Closure 0.027
32
Zonge International Inc. Akutan Gravity 26 October 2012
APPENDIX H. GRAVITY AND GPS REPEATS
Easting Northing Date Time
1186 440378.30 5999807.93 451.34 981459.617 8/14/2012 11:26
1186 440378.27 5999807.85 451.36 981459.627 8/18/2012 10:10
0.03 0.08 0.02 0.010
2001 442737.66 5999564.03 32.85 981545.955 8/14/2012 16:27
2001 442737.76 5999563.99 32.85 981545.949 8/15/2012 9:28
0.10 0.04 0.00 0.006
1195 440463.16 5999604.19 455.59 981458.653 8/13/2012 12:17
1195 440463.13 5999604.28 455.61 981458.587 8/14/2012 10:49
0.03 0.09 0.02 0.066
1185 440497.93 5999906.05 447.30 981459.824 8/14/2012 11:43
1185 440498.18 5999905.99 447.29 981459.847 8/18/2012 10:23
0.25 0.06 0.01 0.023
1080 440860.73 5998836.94 373.19 981476.841 8/14/2012 10:00
1080 440860.67 5998836.83 373.20 981476.937 8/22/2012 15:32
0.06 0.11 0.01 0.096
1081 440707.48 5999044.99 359.02 981481.276 8/14/2012 10:10
1081 440707.42 5999045.01 358.99 981481.276 8/22/2012 15:45
0.06 0.02 0.03 0.000
1018 444314.50 5999886.30 109.15 981528.760 8/15/2012 16:36
1018 444314.37 5999886.36 109.08 981528.738 8/17/2012 10:33
0.13 0.06 0.07 0.022
1023 444068.51 6000900.54 12.32 981548.786 8/15/2012 15:46
1023 444068.72 6000900.57 12.32 981548.799 8/20/2012 18:27
0.21 0.03 0.00 0.013
2015 442409.31 5999897.14 155.52 981521.093 8/16/2012 18:07
2015 442409.36 5999897.07 155.53 981521.093 8/20/2012 11:56
0.05 0.07 0.01 0.000
Station
WGS ‐84 UTM Z3N Elevation
NAVD88 Obs Grav
33
Zonge International Inc. Akutan Gravity 26 October 2012
Easting Northing Date Time
2021 441922.89 5999117.57 139.93 981524.941 8/17/2012 17:07
2021 441922.97 5999117.43 139.93 981524.949 8/22/2012 17:23
0.08 0.14 0.00 0.008
1202 440240.98 5999973.77 460.64 981457.416 8/18/2012 10:47
1202 440240.88 5999973.84 460.67 981457.430 8/19/2012 9:52
0.10 0.07 0.03 0.014
1114 439957.79 6000017.87 482.77 981451.836 8/18/2012 10:59
1114 439957.36 6000017.60 482.78 981451.844 8/19/2012 10:07
0.43 0.27 0.01 0.008
2052 443943.35 5999871.66 28.27 981544.141 8/20/2012 16:01
2052 443943.06 5999871.77 28.66 981544.190 41145 12:07
0.29 0.11 0.39 0.049
Station
WGS ‐84 UTM Z3N Elevation
NAVD88 Obs Grav
34
Zonge International Inc. Akutan Gravity 26 October 2012
APPENDIX I. DATA DVD CONTENTS
DVD contains raw, processed and plot images.
Note: The gravity base station absolute value was assumed and the base is not tied to an absolute
gravity station.
Gravity\: Main directory folder containing all pertinent folders.
Gravity\Data
Akutan_Gravity_Principal_Facts_2012.csv: Gravity principal facts in .csv format
Akutan_Gravity_Principal_Facts_2012.gdb¹: Geosoft database containing gravity
principal facts for this survey.
The following are columns included in this file.
Station: Station Number
Reading: Meter reading
Time: Local time (GMT-8)
Date: Date
Elevation: Station elevation, meters NAVD88 vertical datum
Gravimeter_Height: Gravity meter instrument height measured from the GPS reference
point on the ground to the bottom of the gravity meter, meters
Slope: Slope for 10 m radius from station, calculated with 5 m DEM or by
operator where 5 m DEM data were not available
SD: Standard deviation of gravity meter readings
X: UTM Easting, Zone 3N, meters. WGS84
Y: UTM Northing, Zone 3N, meters. WGS84
Latitude: WGS84 latitude DMS
Longitude: WGS84 longitude DMS
Source: Name of the daily gravity dump file where data originated
WGS84_LATDD: WGS84 latitude D.dddddd
WGS84_LONDD: WGS84 longitude D.dddddd
WGS84_E_HT_M: GPS Ellipsoid height
GEOID_09: GEOID09 Elevation
35
Zonge International Inc. Akutan Gravity 26 October 2012
GPS_3D_Qual: GPS 3D quality indicator in meters
GPS_STD_Ht: GPS height quality indicator in meters
GPS_Class: PP – post-processed GPS solution; RTK – Real Time Kinematic
solution; CORS- Continuously operating reference stations AC10
and AV09 used for post-processing
Gravity: Observed Gravity, milligals
TideCorr: Geosoft tide correction. milligals
FreeAir: Free air gravity, milligals
TC_100: Terrain corrections for a radius of 10 m to 50 km calculated for a
density of 1.00 gm/cc
Curv_C: Bullard B curvature correction for a density of 2.67 gm/cc. This
value was applied to the Simple Bouguer Gravity and Complete
Bouguer Gravity fields. milligals
SBG150, etc: Simple Bouguer Anomaly. Density: 1.50 gm/cc to 3.00 gm/cc
CBG150, etc: Complete Bouguer Anomaly. Density: 1.50 gm/cc to 3.00 gm/cc
Gravity\Data\Daily
CG5_CK_DDMMYYY.gdb¹: Daily gravity instrument Geosoft database files. X, Y in
WGS84 UTM Zone 3N. Elevation in NAVD88 vertical datum.
Gravity\Maps
Digital images in geotiff format and as Geosoft maps¹. Coordinates in WGS84, UTM
Zone 3N, meters.
o Station locations
o Complete Bouguer Anomaly, density reduction at the following densities (gm/cc):
1.90, 2.0, 2.1, 2.2, 2.25, 2.3, 2.4, 2.5, 2.6, 2.67, 2.7
Gravity\Terrain_Grids
DTM_DEM_proprietary_merge_5m_.grd: merged NED 2 arc second DEM and client-
provided proprietary 5 m DEM. Geosoft format grid.
Horizontal datum: WGS84UTM Zone 11N
Vertical datum: NAVD88
This proprietary DTM grid is not provided on the Data_DVD due to restrictions on its
distribution.
36
Zonge International Inc. Akutan Gravity 26 October 2012
Gravity\Report
12140_Akutan_Gravity_Data_Acquisition_Report: Gravity data acquisition and
processing report in Adobe PDF and Microsoft Word format.
Gravity\Survey
Gravity_station_coordinates.csv: Station coordinates in .csv format.
Station: Station Number, *”RP” denotes repeated station
X: UTM Easting, Zone 3N, meters. WGS84
Y: UTM Northing, Zone 3N, meters. WGS84
Elevation: Station elevation, meters NAVD88 vertical datum
WGS84_LATDD: WGS84 latitude D.dddddd
WGS84_LONDD: WGS84 longitude D.dddddd
WGS84_E_HT_M: GPS Ellipsoid height
GEOID_09: GEOID09 Elevation
GPS_3D_Qual: GPS 3D quality indicator in meters
GPS_STD_Ht: GPS height quality indicator in meters
GPS_Class: PP – post-processed GPS solution; RTK – Real Time Kinematic
solution; CORS- Continuously operating reference stations AC10
and AV09 used for post-processing. CTRL- GPS base station
93
Appendix 4
MAGNETOTELLURIC SURVEY
ON THE
CITY OF AKUTAN GEOTHERMAL PROJECT
AKUTAN, ALASKA
FOR
CITY OF AKUTAN, ALASKA
DATA ACQUISITION AND PROCESSING REPORT
ZONGE JOB # 12140
ISSUE DATE: 26 October 2012
ZONGE INTERNATIONAL, INC.
9595 Prototype Court
Reno, Nevada 89521
Phone: (775) 355-7707, Fax: (775) 355-9144
Zonge International Inc. Akutan MT 26 October 2012
TABLE OF CONTENTS
INTRODUCTION .....................................................................................................................................1
SURVEY CONTROL ...............................................................................................................................1
SURVEY PRODUCTION ........................................................................................................................2
INSTRUMENTATION .............................................................................................................................4
MEASUREMENT CONFIGURATIONS ...............................................................................................4
SITE INSTALATION PROCEDURE .....................................................................................................6
DATA RECORDING PARAMETERS ...................................................................................................7
DATA PROCESSING ...............................................................................................................................8
MT PARAMETER DEFINITIONS AND COMPUTATIONS .............................................................9
DATA QUALITY .................................................................................................................................... 12
DATA PRESENTATION ....................................................................................................................... 17
SAFETY AND ENVIRONMENTAL ISSUES ...................................................................................... 17
REFERENCES ........................................................................................................................................ 18
APPENDIX A: MT STATION ARRAY TYPES AND ELECTRODE COORDINATES ............... 19
APPENDIX B: MERGED SURVEYS IMPEDANCE TENSOR COORDINATES......................... 23
APPENDIX C: PRODUCTION LOG .................................................................................................. 26
APPENDIX D: GEOMAGNETIC INDICES ...................................................................................... 28
APPENDIX E: DISCUSSION OF MAGNETOTELLURICS ............................................................ 31
APPENDIX F: INSTRUMENT SPECIFICATIONS .......................................................................... 38
APPENDIX G: FILE STRUCTURES .................................................................................................. 42
APPENDIX H: IMPEDANCE DATA PLOTS ..................................................................................... 43
APPENDIX I: IMPEDANCE RESISTIVTY AND PHASE PLAN MAPS ....................................... 67
Zonge International Inc. Akutan MT 26 October 2012
LIST OF FIGURES
Figure 1: MT array locations. ............................................................................................ 3
Figure 2: MT, 400 meter, 8 channel array schematic ....................................................... 5
Figure 3: MT, Double-L, 6 channel array schematic. ....................................................... 6
Figure 4: Telluric-MT, Double-L, 4 channel array schematic ........................................... 6
Figure 5: Examples of lowest quality impedances (Rank 3.5 and 4) ............................... 14
Figure 6: Examples of typical quality impedances (Rank 2 and 3) ................................. 15
Figure 7: Examples of best quality impedances (rank 1 and 1.5) .................................... 16
Figure 8: MT Determinant Resistivity (color grid) with Z Polar Diagrams at 128 Hz .... 68
Figure 9: MT Determinant Phase (color grid) with Phase Tensor Ellipse at 128 Hz ....... 69
Figure 10: MT Determinant Resistivity (color grid) with Z Polar Diagrams at 8 Hz ...... 70
Figure 11: MT Determinant Phase (color grid) with Phase Tensor Ellipse at 8 Hz ......... 71
Figure 12: MT Determinant Resistivity (color grid) with Z Polar Diagrams at 1 Hz ...... 72
Figure 13: MT Determinant Phase (color grid) with Phase Tensor Ellipse at 1 Hz ......... 73
Figure 14: MT Determinant Resistivity (color grid) with Z Polar Diagrams at 8 secs .... 74
Figure 15: MT Determinant Phase (color grid) with Phase Tensor Ellipse at 8 secs ....... 75
Figure 16: MT Determinant Resistivity (color grid) with Z Polar Diagrams at 64 secs .. 76
Figure 17: MT Determinant Phase (color grid) with Phase Tensor Ellipse at 64 secs ..... 77
1
Zonge International Inc. Akutan MT 26 October 2012
City of Akutan Geothermal Project
Magnetotelluric Survey
INTRODUCTION
Zonge International, Inc. performed a magnetotelluric (MT) survey on the Akutan
Project, located in the Aleutian Islands, Alaska for City of Akutan. This survey was conducted
during the period of 12 August 2012 to 24 August 2012. The survey area is located in Township
70 South and Range 112 West, and lies within the Unimak (A-6), 15-minute topographic map.
MT data were acquired in and around Hot Springs Bay Valley at 22 receiver sites employing
standard orthogonal and multiple orthogonal electric dipole lines (arrays) which yielded 46 MT
impedance tensor soundings at spacing's of 100 meters to 800 meters.
This survey was designed to augment and expand coverage to the northwest of a MT
survey conducted in 2009 by Geoystem (GSI).
Curtis Caton, Geophysicist for Zonge International, Inc., supervised this survey in the
field. The survey is referenced as Zonge job number 12140. This report covers data acquisition,
instrumentation, and processing to impedance tensors. It presents merged Zonge 2012 and GSI
2009 impedance tensors as data files and maps showing polar diagrams, phase tensor ellipses,
determinant resistance, and phase at five frequencies.
SURVEY CONTROL
Zonge personnel established survey control using Garmin® hand-held GPS model
GPSMAP 60CSx. These GPS receivers are Wide Area Augmentation System (WAAS) enabled
and accuracy typically ranges from 2-5 meters. Positioning is in the UTM Zone 3N, WGS84
datum. MT array types, E line azimuths, and GPS surveyed electrode coordinates are tabulated
in Appendix A: MT Station Array Types and Electrode Coordinates.
2
Zonge International Inc. Akutan MT 26 October 2012
By convention, the MT impedance tensors (stations) coordinates are defined as centered
between each 100 meter-spaced electrode pair in the array's Ex direction.
The electric and magnetic fields were measured using a right-hand coordinate system
with Ey positive (Northward) and Ex positive (Eastward) and the Z axis positive up. The array's
Ex Azimuth is defined as degrees clockwise from geographic North.
For example: a receiver array with an azimuth of 30 degrees or N30E, indicated the Ex
and Ey electric lines were, respectively, aligned and assigned positive polarity toward the N30E
and N60W geographic directions. Impedances from the 2009 MT survey were converted to this
convention prior to being merged with the current survey.
MT impedance tensor locations for the current survey (46 stations) and the 2009 GSI
survey (52 stations) with tensor ID numbers are shown in Figure 1. Tensor IDs for the 2009
survey are defined here as 10 times their original station number. Impedance tensor IDs and
coordinates for the two merged MT surveys are presented in Appendix B.
SURVEY PRODUCTION
Curtis Caton, Geophysicist for Zonge International, Inc., supervised this survey which
was conducted during the period of 12 August 2012 to 24 August 2012. The base camp was in
the City of Akutan. Primary transport of crew, equipment and batteries to survey area, 5 km
away, was via helicopter. One to four stations were acquired daily. Daily productivity depended
primarily on the weather and visibility which restricted the helicopter's availability. On many
days, the crew continued to work through weather conditions grounding the helicopter by hiking
from the survey area back to town. Overall, 22 receiver sites were measured with either standard
orthogonal or multiple orthogonal electric dipole lines (arrays). These yielded 46 MT impedance
tensor soundings at spacing's of 100 meters to about 800 meters. The daily field logistics and
measurement progress log is presented in Appendix C.
3
Zonge International Inc. Akutan MT 26 October 2012
Figure 1: MT array locations.
~
1750 1~;050 o0
1iti
0 o0
15050 20050 111to50 00
6050
0
0 0 1050 tl 3350
0 3No5o 00 7050
3050 e:J.
4350
414fo5() (!t'
405() 00
13050
0
10050 •
442000
530 [']
+
80
70 [']
[']
21050
0
200 ~
() ..... 270
[')
260 .!21
340 ~
·IM•sr.JO"
444000
+
30 [']
90 t:J 100
[']
110 120 ['] ['] ~ 17V9~~
17350°0
170 [;}.
160 (']
230~ 19350
19#
50()co
210 G ~ 220q!f
310+-
290 {']
11JW() [']
280 ,15~~ 300
~11~~ [']
380 [']
360 ['] 111615?5~ 370
MT Station with Tensor ID
1050 0 Zonge 2012
530 ['] GSI 2009 MT Im pedance Tenso r IDs
250 250 500 750 1000 1250
(meters)
WGS 84 I UTM zone 3N
446000
+
40 ['] +
140 130 ['] [']
180 [']
190 [']
I ,
240 [') 250 [']
320 t:J 330 [']
390 ~ 400 ['] 410 '{j;
City of Akutan Geothermal P roject, Alaska
Magnetotelluric survey, August 2012
~-Inc.
4
Zonge International Inc. Akutan MT 26 October 2012
INSTRUMENTATION
Data were acquired with five Zonge model GDP-3224 multiple purpose receivers. The
front panel numbers for the GDPs used were 114, 175, 255, 297, and 299. The Zonge GDP-3224
instrument is a backpack-portable, 24 bit, microprocessor-controlled receiver that can gather data
on as many as eight channels simultaneously.
The electric-field signal was measured using non-polarizing ceramic Cu-CuSO4 porous-
pot electrodes connected to the receiver with insulated 14-gauge wire. Magnetic field
measurements were made with Zonge ANT/4 antennas all survey MT sites. (ANT/4 serial
numbers: (1464, 1474, 1534, 1544, 1554, and 1564). The ANT/4 antennas have a bandwidth
response of 0.0005Hz to 1 kHz and were used during the acquisition schedule which included
three measurement bands.
Band Frequency Range Sampling rate
High 256Hz-10Khz 32768 Hz
Mid 4-256Hz 1024 Hz
Low 0.0059-8Hz 32 Hz
At the remote reference site, two Zonge ANT/6 antennas (s/n 1796, 1846) were installed,
in addition to two ANT/4 (s/n 1114, 1184) and signals from all four magnetic sensors were
recorded simultaneously over the three acquisition bands synchronized to the survey area MT
stations. Telluric-MT type recovery of impedances at survey stations in the 1k Hz - 10 kHz band,
appears possible based on processing tests conducted for four sites.
More on instrument specifications can be found in Appendix F.
MEASUREMENT CONFIGURATIONS
Three array types were employed: 1) an MT, 400 meter, 8 channel (Figure 2); 2) an MT,
Double-L, 6 channel (Figure 3); and 3) a Telluric-MT, Double-L, 4 channel (Figure 4). MT,
here, refers to the installation of orthogonal horizontal magnetic sensors near the center of
5
Zonge International Inc. Akutan MT 26 October 2012
electric field array. Telluric-MT refers to sites measuring only the electric (telluric) fields. An
approximate impedance tensor is obtained at Telluric-MT sites by substituting a set of horizontal
magnetic fields measured concurrently at another survey area site.
Measurements for the 400-meter array used six electric-field receiver dipoles (4 Ex and 2
Ey) of 100-meter length with a pair of magnetic-field antennas ( Hy and Hx) located in the
center of the spread. Measurements for the Double-L array used four electric-field receiver
dipoles (2 Ex and 2 Ey) of 50-meter length, with a pair of magnetic-field antennas.
Measurements for the Telluric-MT site used only the four electric-field receiver dipoles (2 Ex
and 2 Ey) of 50-meter length without orthogonal horizontal magnetic sensors.
In the current survey, the Telluric-MT array was employed for only Tensor 16050, due to
its elevation and visibility-limited access by helicopter. During the 2009 MT survey, about 80
percent of impedance tensors were derived from the Telluric-MT method. No issues were
apparent in the merged MT and Telluric-MT impedances from either survey.
For each Double-L array, two tensors separated by 50 meters were initially generated.
During processing it was determined that the increase in data quality provided b y merging the
sounding into one central tensor per double-L was the best practice due to modest signal to noise
levels present in the 50 m electric dipoles. Hence the Double-L stations map to single
impedance tensors.
Figure 2: MT, 400 meter, 8 channel array schematic
6
Zonge International Inc. Akutan MT 26 October 2012
Figure 3: MT, Double-L, 6 channel array schematic.
Figure 4: Telluric-MT, Double-L, 4 channel array schematic
SITE INSTALATION PROCEDURE
Planned MT sites were navigated to by handheld GPS. If necessary, based on
topography, streams and soil conditions, the site center and E line azimuth was adjusted to
optimize potential measurement quality. The "as-installed" electrode locations were measured
with handheld GPS.
7
Zonge International Inc. Akutan MT 26 October 2012
To minimize wind vibration and thermal variations, magnetic sensors were placed in 25
cm deep trenches and checked by the crew chief for level and azimuth prior to burial. Similarly,
electrode pots were buried in 30 cm deep holes. E line wires were repositioned to lie directly on
the ground and weighted at about 1 meter intervals with soil or rock to prevent wind induced
movement. Contact resistances were measured and additional pots and moist soil added to
minimize it.
DATA RECORDING PARAMETERS
MT time-series were simultaneously recorded with two to five synchronized GDP-3224
receivers. One to four GDP receivers were deployed daily, dependent on weather and access, at
MT stations while an additional reference receiver was at a stationary, relatively low-noise,
easily accessible site located approximately 5 km southeast of the survey area. The reference
GDP was located at 447775E + 5997561N, UTMWGS84.
Time series data were acquired in three frequency bands for scheduled time periods.
Table 1 shows the bands, sampling rate, duration and start times for these acquisition bands.
Table 1: Time-series acquisition bands and durations.
Acquisition
Band
Frequency
Band
Sampling
Rate
Length of
Collection
Time Series
Blocks
Total Daily
Collection
Recording
Start Time
High 256Hz-
10Khz 32768 Hz 8 seconds 90 12 Minutes 1900
Mid 4-256Hz 1024 Hz 14 seconds 90 21 Minutes 1930
Low 0.0059-8Hz 32 Hz 30 Minutes 4 2 Hours 2000
Low 0.0007-8Hz 32 Hz 4 Hours 2 8 Hours 2200
Measured Daily Total ~10.25 Hours
8
Zonge International Inc. Akutan MT 26 October 2012
DATA PROCESSING
MT processing was completed using an integrated set of Zonge programs.
MTFT24: Times-Series Review and Acceptance and Fourier Transformation. This
module allows interactive quality assessment and selection of each block of time-series
recorded. Assessment is facilitated by displays of stacked time-series channels including
remote reference channels with corresponding amplitude spectra.
Selected time-series are next processed via cascade decimation and Fourier
transformation to produce un-averaged collections of individual Fourier coefficients.
Applies the magnetic-field antenna and instrument calibrations.
MTEDIT: Applies a robust impedance processing method to estimate apparent resistivity,
phase, coherence and error. It allows interactive review and acceptance of individual
impedance solutions derived from small subsets spectral cross-power based impedance
solutions.
Quality assessment is facilitated by a plot of apparent resistivity and impedance phase
versus frequency and error estimates derived from a weighted average of parametrically
qualified spectral-cross-power-based impedance solutions. These averaged impedance
solutions may be queried on a per frequency basis to display the underlying cross-power
impedance solution sets as a pair of point cluster plots of real vs. imaginary impedance
values and magnetic signal amplitude vs. coherency of E predicted vs. H measured.
The selected processing flow does not apply smoothing or earth-model based filtering or
interpolation to the impedance vs. frequency solutions. This maintains the original
indications of the impedance estimate quality and consistency.
The output from this program is an ASCII weighted average impedance file in the Zonge
AVG-format.
ASTATIC: This module spatially orders (by stations, coordinates or lines) the individual
impedance soundings for comparison in stacked curves of resistivity and phase vs
frequency and resistivity, phase pseudo-sections. Spurious impedance points may be
flagged to be skipped or dropped. Static corrections, if required are applied here. Input
and output are in the AVG format.
NSSKEW computes all tensor parameters including rotations, skew and determinant
resistivity and plot impedance polar diagrams. Input and output are in the AVG format.
Additional outputs include impedances with standard parameters which can be output as
an ASCII csv tables and the SEG EDI impedance format.
Fence sections may be defined and output for 2D modeling with Zonge program SCS2D
which produces resistivity-depth sections using smooth-model inversion.
9
Zonge International Inc. Akutan MT 26 October 2012
MT PARAMETER DEFINITIONS AND COMPUTATIONS
Impedance estimates for each pair of electric and magnetic field components are
calculated. The impedance tensor Z relates horizontal electric and magnetic field components
as
Y
x
yyyx
xyxx
y
x
H
H
ZZ
ZZ
E
E
or equivalently
yyyxyxyyxyxxxxHZHZEHZHZE and .
Note that since Ex and Ey are associated with two different equations, estimates of Zxx and Zxy are
separate from estimates of Zyx and Zyy.
Impedance magnitudes are transformed to Cagniard apparent resistivity values
where is radial frequency in radians/second, is magnetic permeability in henries/m, and Zxy
is electrical impedance in ohms (Cagniard, 1953). The magnetic permeability is usually taken as
the magnetic permeability of free space, 0.
Similarly, Zxx, Zyx and Zyy magnitudes can be scaled to xx, yx and yy. The accepted
scientific convention is to use SI units with electric field values in V/m and magnetic field values
in A/m. But since geophysical signal strengths are so low, it is convenient to scale E-field values
to nanoVolts/meter (nV/m) and H-field values to picoTesla (pT). Note that although pT values
should be annotated as B-field magnetic flux density, H-field labels and pT units are used in
most of Zonge’s documentation and plot annotation.
meters)-(ohm 1 2
xyxyZ
10
Zonge International Inc. Akutan MT 26 October 2012
Apparent resistivity and frequency can then be used to calculate an approximate depth of
investigation:
(meters) 356 fz
where is resistivity in ohm-m and f is frequency in hertz, and the constant 356 comes from
dividing the skin depth constant 503 by . Depth of investigation increases in proportion to
frequency1 so measurements over a range of frequencies can be used to estimate
resistivities over a range of depths.
Impedances are complex, therefore they have both magnitude |Z| and phase :
mrad )real(Z
)imag(Zarctan1000 Z phase
xy
xy
xyxy
.
Impedance phase is related to the change in apparent resistivity as a function of frequency:
. mrad log
log141000
f
Impedance phase is a measure of the slope of the apparent resistivity sounding curve, adding
information about sounding curve shape at each data point. Plots of apparent resistivity
sounding curves, produced by Zonge, often include impedance phase information represented as
a vector posted at each data point. If impedance phase values are consistent with apparent
resistivity curve shape, the posted vectors are tangent to the sounding curve.
In addition to apparent resistivity and impedance phase representations of each
impedance tensor element, additional derived parameters can be constructed (Vozoff, 1987,
Simpson and Bahr, 2005), of which a subset is described below.
The impedance tensor coordinate system can be rotated to generate an impedance tensor for an
arbitrary (x, y) electromagnetic component orientation. Impedance tensor rotation is used to
estimate electrical strike by finding an (x, y) orientation for which the magnitude of the tensor’s
diagonal trace is minimized, in effect identifying the EM component orientation for which the
geological structure appears to be most two-dimensional. In this fashion, the strike direction min
11
Zonge International Inc. Akutan MT 26 October 2012
is the Ex azimuth that minimizes |Zxx|2 + |Zyy|2 and maximizes |Zxy|2 + |Zyx|2. (In data files on
the DVD accompanying this report, the quantity min is denoted MaxResEAzm.) It should be
noted that the strike direction calculated in this manner is non-unique to 90 degrees.
In addition to information describing the electrical strike, parameters derived from the
impedance tensor can be used to estimate the degree of multidimensionality, namely with the
rotationally-invariant parameter skew:
yxxy
yyxx
ZZ
ZZskew
In one- and two-dimensional geology, the skew is near 0. Values of 0.2 or less have been used to
imply one- or two-dimensional geologic structure, while areas with skew values greater than 0.3
indicate measurements near the corners of three-dimensional structure. Simpson and Bahr,
(2005) argue that while values of 0.2 and greater are indicators of influence of 3D structure,
values of less than 0.2 cannot be used to infer that the geologic structure is two-dimensional.
Skew values are tabulated in the data DVD for Zonge 2012 data and merged Akutan data (Zonge
2012 and Geosystem 2009), which is included on the DVD.
Rotationally-invariant versions of the apparent resistivity and phase can be used to average the
multidimensional information into single pseudosections and plan view presentations. The
determinant apparent resistivity and phase combine impedance tensor elements into a single
apparent resistivity and impedance phase values:
(mrad) arctan1000
meters)-(ohm 1
det
det
det
2
detdet
Zreal
Zimag
Z
where Zdet = (ZxxZyy-ZxyZyx)1/2.
Ranganayaki (1984) concluded that determinant apparent resistivity and impedance phase were
the best summary parameters to use for one-dimensional, layered-earth modeling in areas with
complex two- and three-dimensional geology (in the absence of more sophisticated inversion
algorithms or sufficient data density).
12
Zonge International Inc. Akutan MT 26 October 2012
Small-scale, near-surface resistivity structure and topography can cause a frequency independent
effect termed “static shift”. Static effects shift the entire apparent resistivity sounding curve up
or down without changing sounding curve shape. Tensor parameters are calculated using
Zonge’s NSSKEW program and are output in .CSV file format which is included on the data
DVD-ROM.
Impedance polar diagrams are presented in Appendix I. The outer curve of this plot represents
the magnitude of |Zxy| as it is rotated through 360 degrees in 4 degree increments. The inner
curve represents the magnitude of the diagonal element |Zxx| as described above. A circular
polarization is usually considered as evidence of 1-D structure. More detail on the MT method
can be found in Appendix E: Discussion of Magnetotellurics.
DATA QUALITY
System calibrations: An Analog-Input to Impedance-Tensor-Curve calibration of the total
instrument and processing chain (without antenna) was conducted in Reno immediately prior to
shipment. This calibration verified accuracy and uniformity of all 38 channels of the five
GPD32-24 receivers over the bandwidth of 0.01 Hz to 10 kHz. The frequency calibration tables
of all magnetic antenna were verified in Tucson prior to shipment. In the field, standard
procedure involved a pre-burial, side-by-side comparison waveform amplitudes of the pair of
magnetic antenna. See the SITE INSTALATION PROCEDURE section for additional field
quality control measures.
Several standard measures of data quality are recorded in the provided Zonge and EDI data files.
Overall, during the survey, natural source signal strengths were sufficient to get acceptable,
usable quality data over nearly all of a frequency range extending from .008 to 256 Hertz.
The island's geography and remoteness necessarily influenced survey design and signal-to-noise
factors. In particular, the ideal placement of the remote reference magnetometer (Gamble, et.al,
1985) at a quiet location, 15-30 km from the survey area, was not logistically practical. As a
compromise, which allowed daily servicing of station from the in-town base camp, the reference
station was placed 1.3 km (across the bay) from the town, and 4 to 8 km distant from the field
area. Although power line (60 Hz and harmonics) and cultural electrical noise were very low in
13
Zonge International Inc. Akutan MT 26 October 2012
the study area, they were moderate at the remote site. Additionally, coastal wave-induced
ground roll (microseism) around 6-10 second period was expected to be ubiquitous at all
distances from the coast and on neighboring islands. This was expected to reduce the benefit of
placing a remote reference on a neighboring island. Larger islands and mainland sites hundreds
of km distant were beyond the range of beneficial MT signal correlation. Microseism
degradation of impedances was apparently significant during the 2009 MT survey, apparently
due to historically low geomagnetic signal levels overlapping with a period of storms. During
the current survey, data gaps in the 6-10 second band were less common and not problematic.
This improvement could be the result of moderately higher geomagnetic signal levels and less
extreme winds.
In the natural low-level-signal band between 0.1 and 5 Hz, noise from wind and coastal wave
microseism (6-10 second) caused some frequency gaps in some of the impedances. These were
most apparent between 5 Hz to 2 Hz and to a lesser degree near 6 to 9 seconds. Weak predicted
E field coherencies, seen in the impedance curves figures are a measure of low signal-to-noise
conditions. Weak values are most apparent in bands at 1 Hz to 5 Hz and 6 to 8 seconds, and
where coherencies become extreme are associated with poor and missing impedance estimates.
The widths of these poor or missing data are typically less than one-half decade frequency. Only
5 of 46 impedance tensors had poor or missing impedances over continuous frequency gaps of a
decade or more.
Signal levels in the absorption band of 300 Hz to 5000 Hz, which depend on thunderstorm
lightning in the local survey region, were consistently below usable strengths during the survey.
For many stations, the highest recovered impedance frequency was 200 Hz..
To summarize data quality and usability a ranking system was applied. The overall usability of
each impedance curve was assessed and assigned a rank of 1 to 4. ( 1 = ideal; 2 = fully usable;
3= mostly usable; 4 = partly usable; 5 = not usable ). These quality rankings are listed in the
impedance tensor coordinate table in Appendix B. Additionally, Figures 6, 7, and 8 respectively,
show impedance curve examples from three quality percentiles: the bottom 10%, Middle 70%,
and the top 20% .
14
Zonge International Inc. Akutan MT 26 October 2012
Figure 5: Examples of lowest quality impedances (Rank 3.5 and 4)
Representative of bottom 10% of this survey's soundings.
15
Zonge International Inc. Akutan MT 26 October 2012
Figure 6: Examples of typical quality impedances (Rank 2 and 3)
Representative of middle 70% of this survey's soundings.
16
Zonge International Inc. Akutan MT 26 October 2012
Figure 7: Examples of best quality impedances (rank 1 and 1.5)
Representative of top 20% of this survey's soundings.
17
Zonge International Inc. Akutan MT 26 October 2012
DATA PRESENTATION
The MT station locations are shown in Figure 1. This map shows the location of each
impedance tensor with its ID number. Appendix H contains plots for each impedance tensor
showing apparent resistivity, phase, coherence, and skew.
Plan maps of the determinant apparent resistivity and the determinant phase are shown
for selected frequencies in Appendix I. The maps of the determinant apparent resistivity also
show impedance polar diagrams and the Swift Strike. The maps of the determinant phase also
show symbols of the phase tensor ellipse and phase tensor direction.
Data and image files are included on the Data DVD. Impedance data are provided in the
Zonge AVG, ASCII CSV tables, and SEG EDI file formats. Zonge AVG file formats are
described in Appendix G.
SAFETY AND ENVIRONMENTAL ISSUES
No production related health or safety incidents or accidents occurred during the course of this
survey. No environmental damage was sustained as a direct result of the survey progress.
Respectfully submitted,
ZONGE INTERNATIONAL INC.
Curtis Caton Gary Oppliger, PhD
Geophysicist Sr. Geophysicist
18
Zonge International Inc. Akutan MT 26 October 2012
REFERENCES
Cagniard, L., 1953, Basic theory of the magnetotelluric method of geophysical prospecting,
Geophysics, 18, 605-635.
Gamble, T.D., Gaubau, W.M., and Clarke, J., 1985, Magnetotellurics with a remote magnetic
reference: Geophysics, 50, 2245-2260.
Ranganayaki, R.P., 1984, An interpretive analysis of magnetotelluric data, Geophysics, 49,
1730-1748.
Simpson, F., and Bahr, K., 2005, Practical Magnetotellurics, Cambridge University Press,
Cambridge, UK.
Vozoff, K., 1987, The magnetotelluric method, in Nabighian, M.N., ed. Electromagnetic
methods in applied geophysics, Vol. 2, Society of Exploration Geophysicists, 641-711.
Wannamaker, P.E., Hohmann, G.W., and Ward, S.H., 1984, Magnetotelluric responses of three-
dimensional bodies in layered earths, Geophysics, 49, 1517-1533.
19
Zonge International Inc. Akutan MT 26 October 2012
APPENDIX A: MT Station Array Types and Electrode Coordinates
Field
Station
Final
Station Array Type Ex
Azimuth Ex Station Ey Station
UTM_X
(m)
WGS84
UTM_Y
(m)
WGS84
CON_RES
(k-ohm)
Remote 99R Remote 0 Receiver 447776 5997562
Remote 99R Remote 0 Ex+50 447825 5997560 4
Remote 99R Remote 0 Ex-50 447725 5997560 6
Remote 99R Remote 0 Ey+50 447776 5997609 5
Remote 99R Remote 0 Ey-50 447778 5997509 6
110 101A 400m 40 Receiver 440306 6000299
110 101A 400m 40 Ex+100 440372 6000375 55
110 101A 400m 40 Ex+200 440437 6000450 44
110 101A 400m 40 Ex-100 440240 6000223 47
110 101A 400m 40 Ex-200 440181 6000139 37
110 101A 400m 40 Ey+100 440230 6000365 41
110 101A 400m 40 Ey-100 440382 6000234 30
115 102A 400m 40 Receiver 440044 5999996
115 102A 400m 40 Ex+100 440110 6000072 42
115 102A 400m 40 Ex+200 440382 6000234 48
115 102A 400m 40 Ex-100 439978 5999920 67
115 102A 400m 40 Ex-200 439913 5999844 53
115 102A 400m 40 Ey+100 439968 6000061 40
115 102A 400m 40 Ey-100 440113 5999927 32
111 103A 400m 40 Receiver 440397 5999783
111 103A 400m 40 Ex+100 440462 5999859 32
111 103A 400m 40 Ex+200 440527 5999935 35
111 103A 400m 40 Ex-100 440331 5999707 45
111 103A 400m 40 Ex-200 440265 5999631 44
111 103A 400m 40 Ey+100 440321 5999848 64
111 103A 400m 40 Ey-100 440473 5999718 61
112 104A 400m 40 Receiver 440618 5999378
112 104A 400m 40 Ex+100 440683 5999453 42
112 104A 400m 40 Ex+200 440748 5999529 37
112 104A 400m 40 Ex-100 440551 5999302 16
112 104A 400m 40 Ex-200 440486 5999226 11
112 104A 400m 40 Ey+100 440541 5999443 42
112 104A 400m 40 Ey-100 440692 5999312 23
20
Zonge International Inc. Akutan MT 26 October 2012
Field
Station
Final
Station Array Type Ex
Azimuth Ex Station Ey Station
UTM_X
(m)
WGS84
UTM_Y
(m)
WGS84
CON_RES
(k-ohm)
103 105L Double L 130 Receiver 440909 5998897
103 105L Double L 130 Ex+50 440947 5998865 25
103 105L Double L 130 Ex-50 440870 5998930 15
103 105L Double L 130 Ey+50 440941 5998935 18
103 105L Double L 130 Ey-50 440875 5998860 17
106 106L Double L 130 Receiver 440775 6000077
106 106L Double L 130 Ex+50 440813 6000047 5
106 106L Double L 130 Ex-50 440735 6000109 34
106 106L Double L 130 Ey+50 440806 6000116 7
106 106L Double L 130 Ey-50 440738 6000042 4
107 107L Double L 90 Receiver 440910 5999753
107 107L Double L 90 Ex+50 440960 5999752 15
107 107L Double L 90 Ex-50 440861 5999754 20
107 107L Double L 90 Ey+50 440911 5999803 12
107 107L Double L 90 Ey-50 440911 5999711 10
104 108L Double L 130 Receiver 440497 5998643
104 108L Double L 130 Ex+50 440535 5998610 38
104 108L Double L 130 Ex-50 440459 5998676 24
104 108L Double L 130 Ey+50 440530 5998681 27
104 108L Double L 130 Ey-50 440464 5998605 44
108 109L Double L 90 Receiver 440726 6000849
108 109L Double L 90 Ex+50 440777 6000848 36
108 109L Double L 90 Ex-50 440676 6000849 54
108 109L Double L 90 Ey+50 440727 6000898 34
108 109L Double L 90 Ey-50 440726 6000799 50
113 110L Double L 130 Receiver 440110 5999048
113 110L Double L 130 Ex+50 440147 5999016 9
113 110L Double L 130 Ex-50 440072 5999081 13
113 110L Double L 130 Ey+50 440143 5999086 16
113 110L Double L 130 Ey-50 440077 5999010 20
101 111A 400m 130 Receiver 442740 5999636
101 111A 400m 130 Ex+100 442816 5999571 6
101 111A 400m 130 Ex+200 442893 5999506 7
101 111A 400m 130 Ex-100 442664 5999702 8
101 111A 400m 130 Ex-200 442585 5999775 8
101 111A 400m 130 Ey+100 442805 5999712 6
101 111A 400m 130 Ey-100 442675 5999560 7
21
Zonge International Inc. Akutan MT 26 October 2012
Field
Station
Final
Station Array Type Ex
Azimuth Ex Station Ey Station
UTM_X
(m)
WGS84
UTM_Y
(m)
WGS84
CON_RES
(k-ohm)
102 112A 400m 130 Receiver 443041 5999374
102 112A 400m 130 Ex+100 443120 5999310 4
102 112A 400m 130 Ex+200 443196 5999245 12
102 112A 400m 130 Ex-100 442965 5999439 2
102 112A 400m 130 Ex-200 442893 5999506 5
102 112A 400m 130 Ey+100 443097 5999449 2
102 112A 400m 130 Ey-100 442974 5999303 2
114 113L Double L 130 Receiver 439556 5998849
114 113L Double L 130 Ex+50 439594 5998816 46
114 113L Double L 130 Ex-50 439518 5998882 44
114 113L Double L 130 Ey+50 439589 5998887 40
114 113L Double L 130 Ey-50 439524 5998810 47
109 114L Double L 90 Receiver 440220 6001015
109 114L Double L 90 Ex+50 440270 6001014 48
109 114L Double L 90 Ex-50 440169 6001015 47
109 114L Double L 90 Ey+50 440221 6001064 87
109 114L Double L 90 Ey-50 440220 6000965 43
116 115L Double L 130 Receiver 439250 5999966
116 115L Double L 130 Ex+50 439288 5999933 36
116 115L Double L 130 Ex-50 439212 5999998 41
116 115L Double L 130 Ey+50 439283 6000003 37
116 115L Double L 130 Ey-50 439207 5999919 46
105 116LT Telluric 130 Receiver 439273 6001446
105 116LT Telluric 130 Ex+50 439311 6001413 25
105 116LT Telluric 130 Ex-50 439234 6001478 34
105 116LT Telluric 130 Ey+50 439306 6001483 24
105 116LT Telluric 130 Ey-50 439239 6001404 39
X03 117A 400m 105 Receiver 443734 6001034
X03 117A 400m 105 Ex+100 443830 6001008 11
X03 117A 400m 105 Ex+200 443927 6000980 10
X03 117A 400m 105 Ex-100 443707 6000939 11
X03 117A 400m 105 Ex-200 443541 6001089 14
X03 117A 400m 105 Ey+100 443759 6001132 11
X03 117A 400m 105 Ey-100 443707 6000939 10
118 118L Double L 130 Receiver 441102 6001802
118 118L Double L 130 Ex+50 441139 6001770 18
118 118L Double L 130 Ex-50 441064 6001834 26
118 118L Double L 130 Ey+50 441134 6001839 21
118 118L Double L 130 Ey-50 441069 6001764 28
22
Zonge International Inc. Akutan MT 26 October 2012
Field
Station
Final
Station Array Type Ex
Azimuth Ex Station Ey Station
UTM_X
(m)
WGS84
UTM_Y
(m)
WGS84
CON_RES
(k-ohm)
X05 119A 400m 65 Receiver 443533 6000332
X05 119A 400m 65 Ex+100 443624 6000373 1
X05 119A 400m 65 Ex+200 443715 6000414 1
X05 119A 400m 65 Ex-100 443441 6000291 5
X05 119A 400m 65 Ex-200 443350 6000250 7
X05 119A 400m 65 Ey+100 443490 6000423 3
X05 119A 400m 65 Ey-100 443569 6000258 2
M 120L Double L 130 Receiver 439577 6000003
M 120L Double L 130 Ex+50 439615 5999970 100
M 120L Double L 130 Ex-50 439538 6000035 95
M 120L Double L 130 Ey+50 439610 6000040 105
M 120L Double L 130 Ey-50 439544 5999965 108
117 121L Double L 90 Receiver 441926 6000719
117 121L Double L 90 Ex+50 441977 6000718 29
117 121L Double L 90 Ex-50 441876 6000720 17
117 121L Double L 90 Ey+50 441928 6000769 20
117 121L Double L 90 Ey-50 441926 6000670 28
L 122L Double L 90 Receiver 442390 6000791
L 122L Double L 90 Ex+50 442440 6000790 64
L 122L Double L 90 Ex-50 442339 6000792 62
L 122L Double L 90 Ey+50 442390 6000840 73
L 122L Double L 90 Ey-50 442389 6000741 79
23
Zonge International Inc. Akutan MT 26 October 2012
APPENDIX B: Merged Surveys Impedance Tensor Coordinates
ZZI-12 102A 1050 439972.6 5999882.3 451.4 40 2
ZZI-12 102A 1150 440038.6 5999958.3 470.6 40 1.5
ZZI-12 102A 1250 440104.6 6000034.3 475 40 2
ZZI-12 102A 1350 440172.6 6000105.8 474.4 40 2
ZZI-12 101A 1450 440237.6 6000181.3 462.8 40 3
ZZI-12 101A 1550 440300.6 6000261.3 443.9 40 3
ZZI-12 101A 1650 440366.6 6000337.3 429.6 40 4
ZZI-12 101A 1750 440431.5 6000413.3 414.6 40 4
ZZI-12 103A 3050 440325.6 5999669.3 436.8 40 2
ZZI-12 103A 3150 440391.6 5999745.3 452.5 40 2
ZZI-12 103A 3250 440457.1 5999821.3 454.5 40 3
ZZI-12 103A 3350 440522.1 5999897.3 443.6 40 2
ZZI-12 104A 4050 440546.1 5999264.3 370.6 40 3
ZZI-12 104A 4150 440612.1 5999340.3 421.1 40 3
ZZI-12 104A 4250 440677.6 5999415.8 439 40 3
ZZI-12 104A 4350 440742.5 5999491.3 414.4 40 3
ZZI-12 105L 5050 440936.1 5998898.3 387.2 130 2
ZZI-12 106L 6050 440802 6000077.3 282.9 130 3
ZZI-12 107L 7050 440938 5999753.3 321.3 90 2
ZZI-12 108L 8050 440524.1 5998643.3 281.7 130 2
ZZI-12 109L 9050 440753 6000849.3 510.7 90 3
ZZI-12 110L 10050 440137.1 5999049.3 308.7 130 2
ZZI-12 111A 11050 442651.8 5999738.8 60.9 130 1.5
ZZI-12 111A 11150 442729.8 5999669.3 34 130 1.5
ZZI-12 111A 11250 442805.3 5999603.8 31.6 130 1.5
ZZI-12 111A 11350 442881.3 5999538.8 30.2 130 1.5
ZZI-12 112A 11450 442955.8 5999473.3 28.7 130 1.5
ZZI-12 112A 11550 443029.8 5999407.3 27.7 130 1.5
ZZI-12 112A 11650 443107.3 5999342.3 30.6 130 1
ZZI-12 112A 11750 443185.3 5999277.8 51.2 130 1
Azim
Ex
deg.
E of N
Z
quality
1 to 5
Impedance Tensor IDs and Coordinates for Merged 2009 & 2012 Surveys
ZZI-12: Zonge Int. Inc. 2012 Survey Qty: 46 Z Tensors
GSI-09: Geosystem 2009 Survey Qty: 52 Z Tensors
1 = near prefect; 2= fully usable; 3=mostly usable;4=partly usable;5=not usable
Impedance quality (1 to 4 )
Survey
Receiver
Array
Name
MT station
( Tensor ID)
Merged
Surveys
Easting
meters
E_W84Uz3
Northing
meters
N_w84Uz3
Elevation
meters
(amsl)
24
Zonge International Inc. Akutan MT 26 October 2012
ZZI-12 113L 13050 439584.2 5998849 357 130 3.5
ZZI-12 114L 14050 440248.1 6001015 572 90 3
ZZI-12 115L 15050 439277.2 5999965 464.1 130 3
ZZI-12 116L 16050 439300.2 6001446 435.5 130 4
ZZI-12 117A 17050 443615.7 6001076 121.2 105 2
ZZI-12 117A 17150 443712.7 6001049 94.7 105 2
ZZI-12 117A 17250 443809.2 6001022 75.5 105 2
ZZI-12 117A 17350 443905.1 6000994 59.1 105 2
ZZI-12 118L 18050 441128.9 6001802 272.4 130 3
ZZI-12 119A 19050 443422.2 6000271 18.2 65 2.5
ZZI-12 119A 19150 443513.7 6000312 16.7 65 2.5
ZZI-12 119A 19250 443605.7 6000353 16.1 65 2.5
ZZI-12 119A 19350 443697.2 6000394 15.1 65 2.5
ZZI-12 120L 20050 439605.2 6000003 457.5 130 3.5
ZZI-12 121L 21050 441952.8 6000719 457.5 90 3
ZZI-12 122L 22050 442416.8 6000791 574.5 90 3
GSI-09 1 10 443112.4 6002266 21.1
GSI-09 2 20 443497 6001734 128.8
GSI-09 3 30 444126 6001708 45.5
GSI-09 4 40 444580.7 6001875 59.6
GSI-09 6 60 442404.6 6002260 25.1
GSI-09 7 70 441530.9 6001243 400.9
GSI-09 8 80 442176.7 6001365 313.2
GSI-09 9 90 442672.8 6001435 184.9
GSI-09 10 100 443129.8 6001420 166.8
GSI-09 11 110 443521.2 6001270 136.4
GSI-09 12 120 443967.3 6001252 71.9
GSI-09 13 130 444488.5 6001263 6
GSI-09 14 140 444916.2 6001300 7.3
GSI-09 15 150 445425.4 6001389 10.7
GSI-09 16 160 443562 6000609 47.9
GSI-09 17 170 444005.1 6000851 19
Azim
Ex
deg.
E of N
Z
quality
1 to 5
Survey
Receiver
Array
Name
MT station
( Tensor ID)
Merged
Surveys
Easting
meters
E_W84Uz3
Northing
meters
N_w84Uz3
Elevation
meters
(amsl)
25
Zonge International Inc. Akutan MT 26 October 2012
GSI-09 18 180 444480.4 6000771 8.5
GSI-09 19 190 445102.6 6000886 84.7
GSI-09 20 200 441299.4 6000262 316.8
GSI-09 21 210 443184.6 6000186 43.9
GSI-09 22 220 443727.9 6000094 16.5
GSI-09 23 230 444109 6000475 12
GSI-09 24 240 444509.5 6000192 68.4
GSI-09 25 250 444964.9 6000215 193.8
GSI-09 26 260 441161.7 5999466 350
GSI-09 27 270 441893.3 5999807 75.3
GSI-09 28 280 442482.8 5999666 53.5
GSI-09 29 290 443008.6 5999701 27.8
GSI-09 30 300 443613.4 5999591 38.7
GSI-09 31 310 444050.3 5999853 70.1
GSI-09 32 320 444514.7 5999730 87.7
GSI-09 33 330 445052.7 5999720 74.9
GSI-09 34 340 441587.3 5999317 238.6
GSI-09 35 350 442051 5999145 134.4
GSI-09 36 360 442680 5999265 35.1
GSI-09 37 370 443184.5 5999305 41
GSI-09 38 380 444069.8 5999345 277.2
GSI-09 39 390 444644.2 5999357 108.5
GSI-09 40 400 445030.3 5999277 19.9
GSI-09 41 410 445502.1 5999251 14
GSI-09 42 420 446001.8 5999240 93.9
GSI-09 43 430 442632.5 5998529 280.5
GSI-09 44 440 443136.5 5998694 281.9
GSI-09 45 450 443847.7 5998915 327.2
GSI-09 46 460 444633.1 5998618 203.3
GSI-09 47 470 445127.6 5998514 204.2
GSI-09 48 480 445605.8 5998656 12.8
GSI-09 49 490 446122.1 5998788 17.8
GSI-09 50 500 446014.5 5998325 3.5
GSI-09 51 510 442125.1 5998276 399.1
GSI-09 52 520 441344.8 5998476 416.4
GSI-09 53 530 441825.7 6002207 61.3
Z
quality
1 to 5
Receiver
Array
Name
MT station
( Tensor ID)
Merged
Surveys
Easting
meters
E_W84Uz3
Northing
meters
N_w84Uz3
Elevation
meters
(amsl)
Azim
Ex
deg.
E of N
Survey
26
Zonge International Inc. Akutan MT 26 October 2012
APPENDIX C: Production Log
Date Station Front Panel Antenna (Hx/Hy)Comments
8/9/2012 Mobilize to Anchorage
8/10/2012 Mobilize to Dutch Harbor
8/11/2012 Mobilize to Akutan and prepped MT gear
8/12/2012 Scouted and Installed Remote
8/13/2012 99R 299
1846/1796
1184/1114
Moved equipment into project area and deployed MT gear.
Cloudy/Wet/Windy conditions.
8/13/2012 101A 175 1554/1464
8/13/2012 102A 297 1564/1534
8/14/2012 99R 299
1846/1796
1184/1114
No helicopter support in the higher terrain due to inclement
weather. MT gear moved by crew.
8/14/2012 103A 297 1564/1534
8/14/2012 104A 175 1554/1464
8/15/2012 99R 299
1846/1796
1184/1114
No helicopter support in the higher terrain due to inclement
weather.
8/15/2012 105L 175 1554/1464 MT gear moved by crew.
8/15/2012 106L 255 1544/1474 MT gear moved by crew.
8/16/2012 99R 299
1846/1796
1184/1114
Crew on standby in camp due to weather ~3hrs. Helicopter
available for one sling load due to inclement weather
8/16/2012 107L 255 1564/1534 Helicopter available for one sling load due to inclement weather
8/17/2012 99R 299
1846/1796
1184/1114
No helicopter support in the higher terrain due to inclement
weather.
8/17/2012 108L 175 1554/1464 MT gear moved by crew.
8/18/2012 99R 299
1846/1796
1184/1114
Helicopter able to sling gear. Moved additional equipment from
camp to Hot Springs Valley to 111A
8/18/2012 109L 297 1564/1534 Helicopter supported move
8/18/2012 110L 175 1554/1464 Helicopter supported move
Crew noted high contact resistance on potential electrodes,
minimal topsoil. Receiver wires buried because of high wind.
Crew noted high contact resistance on potential electrodes,
minimal topsoil. Receiver wires buried because of high wind.
27
Zonge International Inc. Akutan MT 26 October 2012
Date Station Front Panel Antenna (Hx/Hy)Comments
8/18/2012 111A 114 1544/1474
Multiple creek crossings on array 111. Contact resistances dropped
in the valley with better soil condition. Dense root structures and
waist high to chest high vegetation.
8/19/2012 99R 299 Time Schedule not Activated at Remote.
8/19/2012 112A 114 1544/1474 Helicopter supported move
8/19/2012 113L 175 1554/1464 Helicopter supported move
8/19/2012 114L 297 1564/1534 Helicopter supported move
8/20/2012 99R 299
1846/1796
1184/1114
Clear weather conditions. Helicopter support available for all
stations.
8/20/2012 115L 175 1554/1464 Adjusted station locations due to terrain
8/20/2012 116LT 255 Telluric Telluric station
8/20/2012 117A 114 1544/1474 One receiver wire accidentally disconnected from receiver.
8/20/2012 118L 297 1564/1534 Adjust station location due to terrain
8/21/2012 99R 299
Time Schedule not Activated at remote. Extreme weather
prevented insertion of remote operator.
8/21/2012 119A 114 1544/1474
119A located in extremely dense vegetation, potential electrode
placement very poor due to dense root structure and swampy
conditions.
8/21/2012 120L 175 1554/1464 No helicopter supported moves. Crew hike out to harbor for
extraction by skiff.
8/22/2012 99R 299
1846/1796
1184/1114
8/22/2012 121L 175 1554/1464 Helicopter supported move. Transported excess MT gear back to
camp.
8/23/2012 99R 299
1846/1796
1184/1114 Helicopter supported move.
8/23/2012 122L 175 1554/1464 Helicopter supported move
8/24/2012 All field equipment back to camp, clean and organize for shipment.
8/25/2012 Demobilize to Anchorage
8/26/2012 Demobilize to Reno
28
Zonge International Inc. Akutan MT 26 October 2012
APPENDIX D: Geomagnetic Indices
Below are the geomagnetic A Indices for the earth (Ap=planetary) and for the three
individual reporting stations closest to the survey. College Station, Fairbanks, Alaska is 1540
km from Akutan. The daily and three hour Ap (planetary) index are often referenced in
Magnetotelluric survey logs as a measure of potential geomagnetic signal strengths below 0.1
Hz. These signals have their source in the Solar wind interaction with the Earth's
magnetosphere. The Ap index can increase or decrease hourly daily, but also follows 27 day,
seasonal, 11 year and ~50 year cycles. During the last quarter of 2009 and first quarter of 2010, it
reached the lowest recorded averaged monthly values (Ap=3) for the last 165 years. These lower
signal levels increased the susceptibility of MT surveys to any site-specific noise sources such as
microseism related ground motion. During the current Akutan survey, A index daily levels at
College Station, Fairbanks were 8 to 27. The planetary index, Ap was 7 compare to about 4 to 5
for during the Sept 2009 Akutan survey.
The A-index is a daily average level for geomagnetic activity. Larger values usually
related to better signal levels during magnetotelluric surveys.
29
Zonge International Inc. Akutan MT 26 October 2012
A 13 year record of average monthly planetary Ap index levels. The Sept 2009 and Aug 2012
MT survey periods Are record low monthly Ap = 3 are indicated with arrows.
The table below contains 3 hour A indices over the period of the August 2012 survey. The
report source is the U.S. Dept. of Commerce, NOAA, Space Weather Prediction Center.
30
Zonge International Inc. Akutan MT 26 October 2012
:Product: Daily Geomagnetic Data quar_DGD.txt
:Issued: 0030 UT 24 Aug 2012
# Prepared by the U.S. Dept. of Commerce, NOAA, Space Weather Prediction Center
# Please send comment and suggestions to SWPC.Webmaster@noaa.gov
#
# Current Quarter Daily Geomagnetic Data
#
# Middle Latitude High Latitude Estimated
# - Fredericksburg - ---- College ---- --- Planetary ---
# Date A K-indices A K-indices A K-indices
2012 08 13 8 1 2 2 2 2 2 3 2 10 1 2 2 4 1 3 3 1 9 1 2 2 2 2 3 3 2
2012 08 14 7 2 2 1 2 3 1 2 2 9 2 2 3 3 2 3 1 1 7 2 3 2 2 2 1 1 2
2012 08 15 7 2 1 1 2 3 2 2 2 8 2 1 2 3 3 2 1 1 6 2 1 1 1 1 2 2 2
2012 08 16 10 2 2 2 2 3 3 3 2 21 3 2 3 1 5 5 4 1 11 2 2 2 2 3 3 3 3
2012 08 17 11 4 3 3 2 3 1 2 0 14 3 4 2 5 2 1 1 1 10 4 3 2 1 2 2 1 1
2012 08 18 11 0 3 2 2 2 2 4 3 18 0 3 3 5 5 1 2 2 11 1 2 2 2 2 2 4 3
2012 08 19 10 2 2 2 3 3 2 1 3 21 2 2 1 6 5 2 2 2 12 2 2 2 3 3 2 2 4
2012 08 20 11 3 3 3 2 3 2 2 2 27 3 4 4 5 6 3 1 1 12 3 3 3 2 3 2 2 2
2012 08 21 7 2 1 1 2 2 3 2 2 13 2 1 1 5 4 2 1 2 7 2 1 1 2 2 2 1 3
2012 08 22 6 1 2 1 2 3 1 1 1 8 2 1 2 2 4 1 1 1 6 1 2 2 1 2 1 1 2
2012 08 23 11 3 2 3 3 3 2 2 1 27 2 4 5 5 5 4 2 1 10 3 2 3 2 3 2 3 1
2012 08 24 9 2 3 1 3 3 2 2 1 16 2 1 4 4 5 1 2 1 9 2 3 2 2 2 2 2 2
31
Zonge International Inc. Akutan MT 26 October 2012
APPENDIX E: Discussion of Magnetotellurics
Electric currents within the earth produce the magnetotelluric signals that are measured
by the Zonge AMT/MT System. The earth currents are induced by two types of natural
electromagnetic activity above the earth’s surface. Atmospheric electrical discharge, i.e.,
lightning from distant continents and nearby are the primary sources electromagnetic fields at
frequencies above 3 Hz. These signals are channeled globally in the “waveguide” between the
earth’s surface and ionosphere. Below 3 Hz the electromagnetic sources are associated with the
interaction between the earth’s magnetosphere (several thousand kilometers above the earth’s
ionosphere) and the solar wind plasma. These magnetic fluctuations, in turn, induce horizontal
electrical current circulation in the ionosphere and in the earth’s crust and oceans.
These natural sources produce unpredictable daily variation in the magnitude of the
source field used for magnetotelluric (MT) and audiomagnetotelluric (AMT) measurements,
which occasionally make it desirable to re-occupy or extend the recording time at a station. For
most areas and applications, a single over-night observation period is sufficient. (MT refers to
measurements below approximately 300 Hz, AMT to measurements at frequencies from 10 to
10,000 Hz.)
32
Zonge International Inc. Akutan MT 26 October 2012
Figure E1: Natural Source AMT and MT, power spectra.
The theoretical foundations the MT/AMT methods and the practical interpretation of the
field observation rest securely on several assumptions about the geometry of the electromagnetic
source fields and the earth materials electrical properties over the range of frequencies employed.
The natural EM (electromagnetic) fields are generated by large-scale current systems in
the ionosphere or electrical discharges in the ionosphere-earth wave guide. These sources
are typically far enough away from the Earth’s surface that the observed fields can be
treated as uniform, EM plane waves which are refracted into vertically propagating,
horizontally uniform, electric and magnetic fields in the earth. Any locally uniform
magnetic field variations originating above the surface will meet the plane wave
assumption. This includes any natural or artificial electrical or magnetic source located at
least six to eight horizontal earth skin-depths distance for the frequencies of interest.
These source assumptions may breakdown for ionosphere sources near the equator and
poles and very near lightning strikes.
The measured magnetic fields exhibit multiple azimuths or polarizations. For full solution
of the MT/AMT impedance Tensor, the direction of the surface horizontal magnetic field
vector must exhibit at least two distinct azimuths or polarizations over the observation
period in each frequency band. A change in source or direction to the source is not
required, just a change in the azimuth or polarization of the observed field. Observing
0.001 0.01 0.1 1 10 100 1000 10000 100000 1000000
FREQUENCY (Hz)
1E-005
0.0001
0.001
0.01
0.1
1
10
GAMMASMICROPULSATIONS
IP FREQUENCIES SCHUMANNFREQUENCIES60 Hz180 HzVLF
AM RADIOCONTROLLED SOURCE
LIGHTNINGSOLAR WHISTLERS
Pc
0.1
1
10
100
ELECTRIC FIELD IN MV/KM (BANDWIDTH)1/2
33
Zonge International Inc. Akutan MT 26 October 2012
multiple polarizations in measurements during a standard length occupation period is
rarely a problem.
Earth materials behave as ohmic conductors rather than dielectrics over the frequency
ranges employed. In other words, time-varying electric currents (displacement currents)
arising from earth material dielectric permittivity and macroscopic polarization effects
(graphite coated fractures) are negligible in comparison with conduction currents at a
given frequency and can be assumed to be zero in modeling. This assumption has two
implications: the electric and magnetic fields induced in the earth are controlled by
diffusion rather than electromagnetic wave propagation theory; and the earth can be
modeled as an ohmic conductor, i.e. Ohm’s law is obeyed: j = E where j is current
density in amps per square meter, is conductivity in siemens per meter, and E is the
electrical field in volts per meter.
Magnetic permeability’s of the earth materials are well approximated by free-space
permeability. This is a good approximation except over the most highly magnetic
deposits.
Surface-normal or vertical electric current flow and electric fields at the earth’s surface
are zero. This is a very good approximation. Due to the high air-earth resistivity contrast,
induced currents must flow surface parallel, so vertical currents and the vertical electric
field are zero at the surface.
Local measured electric fields are divergence free, which is equivalent to requiring that
there are no electric current sources or drains (active electrodes) contributing to the
magnetic and electric field measurements. This assumption can breakdown momentarily
during lightning strikes within a few km of the measurements or near points of artificial
current injection.
In the MT and AMT methods, the electric and magnetic fields generated by the
electromagnetic sources discussed above are measured over a range of frequencies. The
common recording range is 8000 Hz to 10-3 Hz. Regardless of the frequency the MT source field
geometry is assumed to be planar. The electric field signal is sensed as a potential difference
between two separated non-polarizable porous electrodes, connected by insulated wire to the
receiver. The magnetic fields are detected using mu-metal cored induction magnetic field
antennas. Different arrays can be used in the field to collect MT data, two examples of which are
presented below in Figure E2. These include tensor and scalar setups:
34
Zonge International Inc. Akutan MT 26 October 2012
Figure E2. Tensor and scalar MT sensor setups.
The first step in the use of magnetotelluric information is to calculate impedance
estimates for each pair if electric and magnetic fields. The scalar impedance, Z, is defined by
E/H. This impedance can then be used to calculate the apparent resistivity:
meters)-(ohm 5
1
2
xy Hy
Ex
f
and phase:
)real(Z
)imag(Zarctan
xy
xyxy ,
where Ex is measured in mV/km and Hy is in nT.
The full impedance tensor Z relates horizontal electric and magnetic field components as
Y
x
yyyx
xyxx
y
x
H
H
ZZ
ZZ
E
E
or equivalently
yyyxyxyyxyxxxxHZHZEHZHZE and .
The MT method is well suited for studying complicated geological environments because
the electric and magnetic relations are sensitive to vertical and horizontal variations in resistivity.
The method is capable of establishing whether the EM fields are responding to subsurface
35
Zonge International Inc. Akutan MT 26 October 2012
terranes of effectively 1-, 2-, or 3-dimensions. In a horizontally layered (1-D) earth diagonal
impedance elements Zxx and Zyy are zero and the off diagonal terms are equal in magnitude and
are related by Zxy = -Zyx. The impedance tensor Z is directionally independent in a 1-D earth. If
resistivity decreases with depth, phase measurements will increase over 45; if resistivity
increases with depth, phase measurements will dip below 45. An additional indicator of a 1-D
earth is when the ratio Hz/Hx (Tipper) is a small number. Hz measured at the earth’s surface
should be zero for a 1-D earth.
For a 2-D earth, the diagonal elements of the impedance tensor are zero if sensors are
aligned parallel and perpendicular to the structural strike, and in that case the off-diagonal
elements correspond to the impedances of the two principal coupling modes, which are known as
transverse electric (TE) and transverse magnetic (TM). For a y-directed strike direction, Zyx
(electric field parallel and magnetic field perpendicular to strike) would be the TE mode, and Zxy
(electric field perpendicular and magnetic field parallel to strike) would be the TM mode. If the
dominant strike direction is unclear or unknown, Zxx and Zyy will be non-zero, but it is possible
to apply mathematical rotation to make them zero (or nearly zero) and thus define the TE and
TM directions. Mode identification is important to direct interpretational processing. Both 1-D
and 2-D inversion algorithms require the identification and input of data in the TE and TM
modes.
In a 2-D environment, inversion on the TM mode is usually superior to TE mode
inversion, since the TM mode is more sensitive to lateral conductivity changes than is the TE
mode. Wannamaker et al. (1984) has demonstrated that inversion of the 2-D TM mode in many
circumstances can be a suitable approximation to model data in a 3-D environment. However, 2-
D inversions of 3-D electrical structures can sometimes produce conductivity artifacts. In
particular, conductivities can be estimated too low, and conductive blobs or dike-like bodies may
appear in the 2-D inversion model. This same effect can be caused by electrical anisotropy. In
addition, charges can build up on vertical conductivity contacts, and this can produce significant
frequency independent static shifts, which appear as offsets in the apparent resistivity curves. If
not identified and corrected, static shifts can lead to order-of-magnitude inaccuracies in the
inverted model.
36
Zonge International Inc. Akutan MT 26 October 2012
Data are often collected in an orientation that is not aligned to geological strike of the
target structure. For a 3-D earth, pure TE and TM modes do not exist, and the simplicity of
analysis by mode separation is reduced. In this case, all components of Z are present, and can
have similar magnitudes.
Tipper (Not measured for this survey)
The vertical component of the magnetic field Hz is linearly related to the horizontal
components by the relation
Hz = Tx Hx + Ty Hy.
In general, all quantities in the above equation are complex and frequency dependent.
The quantities Tx and Ty are called tippers because they have the effect of tipping the horizontal
components of the H fields slightly into the vertical plane, creating a usually small Hz
component.
Hz is generated by lateral gradients in conductivity or sharp conductivity discontinuities,
and quantities derived from Hz are therefore important in assessing 2-D or 3-D MT
environments. Tipper magnitude |T| is defined as sqrt(|Tx|2 + |Ty|2), and is directly proportional
to the lateral conductivity gradient. In a purely layered earth, there are no lateral gradients, thus
Hz = 0, as are the tipper components and tipper magnitude.
In a 2-D earth, the tipper is associated with only the TE mode, and Ty and Tx have the
same phases, so that and Ty/Tx is a real number. In this case, it is possible to determine the strike
direction of a 2-D contact by computing where
= arctan (Ty/Tx) .
This quantity is referred to as tipper strike.
It is also possible to infer the conductive vs the resistive side of a contact by analyzing
the tipper magnitude and phase across the contact.
37
Zonge International Inc. Akutan MT 26 October 2012
In a 3-D earth, the phases of Tx and Ty are different, and more complicated definitions for
are required. One definition involves finding the value of that maximizes the cross-power of
horizontal and vertical components. In this way a dominant strike direction may be found under
favorable conditions.
Tipper magnitudes and directions can be combined to make induction arrow maps.
Induction arrows can be used to infer the presence or absence of lateral conductivity gradients,
and can be used to identify 3-D conductors in the earth as they will point toward the conductor
(or away from it, depending on convention used).
38
Zonge International Inc. Akutan MT 26 October 2012
APPENDIX F: Instrument Specifications
GDP-32 Receiver
The Zonge GDP-3224TM is an integrated, 24-bit multi-channel receiver for acquisition of
controlled and natural-source geoelectric and EM data.
• 24-bit analog system
• Expanded keyboard
• ½-VGA graphics display
• 100BaseT Ethernet port
• GPS timing, plus high-accuracy quartz clock
• Multiple, selectable data storage modes in
a single data cache
• Remote control operation
• Broadband time-series recording
• High-speed data transfer
FEATURES
• 1 to 16 channels, user expandable
• 133 MHz 586 CPU
• Alphanumeric keypad
• Real-time data and statistics display
• Easy to use menu-driven software
• Resistivity, Time/Frequency Domain IP, CR, CSAMT, Harmonic analysis CSAMT (HACSAMT), AMT,
• Screen graphics: plots of time-domain decay, resistivity and phase, complex plane plots, etc., on
a 480x320 ½-VGA, sunlight readable LCD
• Internal humidity and temperature sensors
• Time schedule program for remote operation with Zonge XMT-32S transmitter controller
• Optional GPS time synchronization with transmitter Use as a data logger for analog data,
borehole data, etc.
• Full compatibility with GDP-32 series receivers.
• 0.015625 Hz to 8 KHz frequency range standard, 0.0007 Hz minimum for MT
• One 24-bit A/D per channel for maximum speed and phase accuracy
• 512 MB Compact Flash Card (up to 4 GB) for program and data storage, sufficient to hold many
days’ worth of data
• 128 MB dRAM (up to 256 MB) for program execution
• Optional data storage device (up to 40 GB)
Anti-alias, powerline notch, and telluric filtering
• Automatic SP buckout, gain setting, and calibration
• Rugged, environmentally sealed
• Modular design for upgrades and board replacement
• Complete support, field peripherals, service network, software, and training
39
Zonge International Inc. Akutan MT 26 October 2012
Signal Path in the GDP-3224 Receiver:
40
Zonge International Inc. Akutan MT 26 October 2012
ANT/6 CSAMT/MT Antenna
Power: 2 internal 9 volt alkaline batteries.
Battery Life at 12 hours per day:
o Alkaline: 10 days
o Lithium: 20 days
o Carbon Zinc: 4 days (temporary use only)
Sensitivity in Passband: 250 millivolts/gamma (250 mV/nT)
Frequency Range: 0.1 to 10,000 Hz
Noise Level:
o 200 microgamma (200 fT) per squareroot_Hz at 1 Hz
o 1 microgamma (1 fT) per squareroot_Hz nominal > 200 Hz
Tube diameter: 4.8 cm (1.875 in)
Length: 91 cm (36.0 in)
Weight: 3.2 kg (7.0 lb)
41
Zonge International Inc. Akutan MT 26 October 2012
ANT/4 MT Antenna
: 2 internal 9 volt alkaline batteries.
Battery Life at 12 hours per day:
o Alkaline: 10 days
o Lithium: 20 days
o Carbon Zinc: 4 days (temporary use only)
Sensitivity in Passband: 100 millivolts/gamma (100 mV/nT)
Frequency Range: 0.0005 to 1000 Hz
Noise Level:
o 100 microgamma (100 fT) per squareroot_Hz at 1 Hz
o 20 microgamma (20 fT) per squareroot_Hz nominal > 1 Hz
Tube diameter: 4.8 cm (1.875 in)
Length: 138 cm (54.0 in)
Weight: 6.2 kg (13.5 lb)
42
Zonge International Inc. Akutan MT 26 October 2012
APPENDIX G: File Structures
$Rx.GdpStn=450
$Rx.Stn=450
$Rx.Length=100 m
$Rx.Cmp=Zxy
Z.mwgt, Z.pwgt, Freq, Tx.Amp, Z.mag, Z.phz, ARes.mag, SRes, Z.%err, Z.perr, ARes.%err, Coher
1, 1, 0.125, *, 14.674, -2323.3, 344.5, 344.5, 8.2, 166.2, 16.5, 0.89
1, 1, 0.1875, *, 16.713, -2377.5, 297.95, 297.95, 8.2, 165.3, 16.4, 0.85
1, 1, 0.25, *, 16.649, -2460.8, 221.74, 221.74, 5.9, 118.5, 11.8, 0.9
1, 1, 0.375, *, 19.129, -2573.2, 195.16, 195.16, 2.7, 53.4, 5.3, 0.95
1, 1, 0.5, *, 21.656, -2677.9, 187.59, 187.59, 2.7, 53.9, 5.4, 0.98
1, 1, 0.75, *, 23.043, -2649.2, 141.6, 141.6, 9.8, 196.9, 19.6, 0.98
1, 1, 1, *, 23.739, -2729.4, 112.71, 112.71, 1.4, 27.1, 2.7, 0.99
1, 1, 1.5, *, 27.166, -2725.9, 98.396, 98.396, 2.6, 52.5, 5.2, 0.97
1, 1, 2, *, 27.301, -2709.9, 74.536, 74.536, 2.5, 50.2, 5, 0.97
1, 1, 3, *, 28.885, -2659, 55.624, 55.624, 9.4, 188.9, 18.8, 0.97
1, 1, 4, *, 31.581, -2600.7, 49.869, 49.869, 12.8, 257.5, 25.5, 0.98
1, 1, 6, *, 36.457, -2476.8, 44.305, 44.305, 0.8, 15.7, 1.6, 0.99
1, 1, 8, *, 42.918, -2458.7, 46.05, 46.05, 0.4, 9, 0.9, 1
Sample AMT/MT .AVG File
Z.mwgt: Electric field weight, -1 indicates polarity flip.
Z.pwgt: Magnetic field weight, -1 indicates polarity flip.
Freq: Frequency (Hertz).
Tx.Amp: Square wave current (amperes), none in AMT/MT.
Z.mag: Impedance magnitude (km/sec).
Ares.mag: Cagniard apparent resistivity magnitude (ohm-m).
SRes: Static-corrected apparent resistivity added by Astatic (ohm-m).
Z%err: Relative |Z| error (%).
Z.perr: Phase(Z) error (mrad).
Ares.%err: Relative apparent resistivity error (%).
Coher: Coherence, or spectral ratio, of cross-correlated electric and magnetic fields (dimensionless),
where 1 denotes perfect signal coherence between the fields.
43
Zonge International Inc. Akutan MT 26 October 2012
APPENDIX H: Impedance Data Plots
RHOXY (ρxy), RHOYX (ρyx), Resistivity, Phase, Predicted Electric Coherence, and Skew
vs. Frequency Plots for all Zonge 2012 Impedance Tensors. 46 Figures
44
Zonge International Inc. Akutan MT 26 October 2012
45
Zonge International Inc. Akutan MT 26 October 2012
E'
E: .c 100 .s. z.
:~ u;
"iii
Ql
0:::
c
e? 10 l1l a. a.
<{
1
100
o; 50
Ql
~ 0
Ql
(/)
l1l -50 .c a.
N -100
-150
1
0.8
0.6
0.4
0 .2
0
A kutan MT 1250
Apparent Resistivity
c.>--<>-<> XY
~vx
0 0 o XY Coherence . . • Skew
0.001 0.01 0.1
. · ..... .
1
Period (sec)
10 100 1000
A ku ta n MT 1350
1000 ~------------------------------------------~
E'
E: .c 100 .s. z.
:~ u;
"iii
Ql
0:::
c
e? 10 l1l a. a.
<{
1
100
o; 50
Ql 0 ~
Ql
(/) -50
l1l -g_ -100
N -150
-200
1
0.8
0.6
0.4
0.2
0
Apparent Res istivity
<>--<>--<> XY
~vx
69898 ~seeeeeeseeseeeseeeee~ sse eee-e
o o o XY Coherence
• Skew
0 .001 0.01 0.1 1
Pe riod (sec)
10
•
100 1000
46
Zonge International Inc. Akutan MT 26 October 2012
Akutan MT 1450
1000 ~-------------------------------------------.
E'
E: .s::: 100 ~
::-·:;;
~ ·c;;
Q) a::: c
~ 10 ro a. a.
<(
1
50
c;
Q)
0
:s -50
Q)
(/)
~ -100
a.
N -150
-200
1
0.8
0.6
0.4
0.2
0
• • • Skew
0.001 0.01 0.1
1~ i/
.. ...
1
Period (sec)
.
10 100 1000
Akutan MT 1550
1000 ~--------------------------------------------~
E'
E: .s::: 100
~
::-
:~
iii ·c;;
Q) a::: c
~ 10 ro a. a.
<(
1
100
c;
Q) 0 :s
Q)
(/) ro -a. -100
N
-200
1
0.8
0.6
0.4
0.2
0
0 .001 0.01 0.1 1
Period (sec)
10
00 0
0
••
•
100 1000
47
Zonge International Inc. Akutan MT 26 October 2012
Akutan MT 1650
1000 ~--------------------------------------------,
E'
E
.<: 100
~
~ ·:;;
~ ·u;
Ql
0::
c
~ 10
a. a. <
o;
Ql
:3.
Ql
"' co .<: a.
1
200
100
0
N -100
-200
0.8
0.6
0.4
0.2
0
Apparent Resistivity
<>--<>--<> XY
G---+--<> YX
• Skew
•
0 -o 0 ' oo 80 . . . .....
0.001 0.01 0 .1 1
Pe ri od (sec)
...
10
Oo 0 0
.... .....
100 1000
Akutan MT 1750
1000 ~----------------------------------------------,
E'
E
.<: 100 ~
~
-~ u; ·u;
Ql
0::
c
~ 10 co a. a. <
1
200
o;
Ql
100
:3.
Ql 0 "' co .<: a.
N -100
-200
0 .8
0 .6
0 .4
0 .2
0
Apparent Resistivity
<>--<>--<> XY
<>---+---& YX
aO o 8" 8.,.
C)
0 8-~
\: 0 o o XY Coherence
• Cl kew • .. . ... • ¢.•
0 .001 0 .01 0.1
r9
~
ocP
~
•• 0 •
8o• •
1
Period (sec)
• 1
c~o
0 .. •
10
0
.. .. . .. . .
100 1000
48
Zonge International Inc. Akutan MT 26 October 2012
Akutan MT 3050
1000 ~----------------------------------------------,
E'
E 100 .c
.£.
2:-
:~
iii
'iii
<ll a::
c
~ 10 ro a. a. <t:
1
200
o; 100
<ll
~
<ll
(/) 0
ro .c a.
N -100
-200
1
0 .8
0.6
0.4
0.2
0
Apparent Resistiv ity
G---<>--<J XY
.,............. YX
<> 9 oif ou'~ ? o ~~"\~Jo <>$~1oo
0 0 \ 0
o o o XY Coherence b 0
00 00 0
• • • Skew
0 .001 0.01 0.1 1
Period (sec)
10 100 1000
Akutan MT 3150
1000 ~----------------------------------------------,
E'
E 100 .c
.£.
2:-
:~
iii
'iii
<ll a::
c
~ 10 ro a. a. <t:
1
100
o; 50
<ll 0 ~
<ll
(/) -50
ro
'3.-100
N -150
-200
1
0.8
0.6
0 .4
0.2
0
Apparent Resistivity
G---<>--<J XY
.,............. YX
• Skew
0 .001 0 .01 0 .1
. . . ... . .
1
Period (sec)
10
0
0
. . . ...... . ...
100 1000
49
Zonge International Inc. Akutan MT 26 October 2012
Akutan MT 3250
1000 ~------------------------------------------~
E'
E: .c 100
.£.
i?:'
:~ en "iii
Ql a:
c
~ 10 ro c. c.
<1:
1
100
o; 50
Ql 0 ~
Ql
(/) -50
ro .g_ -100
N -150
-200
1
0.8
0.6
0.4
0.2
0
• Skew
0.001 0 .01 0.1
..
1
Period (sec)
10 100 1000
Aku tan MT 3350
1000 ~------------------------------------------~
E'
E: .c 100
.£.
i?:'
:~ en "iii
Ql a:
c
~ 10 ro c. c.
<1:
1
100
o; 50
Ql
~ 0
Ql
(/)
ro -50 .c c.
N -1 00
-150
1
0.8
0.6
0.4
0.2
0
o o o XV Coherence
0 .001 0.01 0.1 1
Period (sec)
0
10 100 1000
50
Zonge International Inc. Akutan MT 26 October 2012
Akutan MT 4050
10~0 ~----------------------------------------------,
10000
1000
100
10
1
100 -.----------------------------------------------,
50
0
5l -50
ro
"3_-100
N -150
Z Phase
::~00~
-200 ~----------------------------------------------~
0.8
0.6
0.4
0.2
0.001
\
0 eooo
o XY Coherence00 o, o
YX Coherence 0
• Skew .. ... ... ... ... ..
0.01 0 .1
..
1
Period (sec)
. ..
10 100 1000
e 1 000
E
.I::.
.2.
z. ·:;
~ 100 ·u;
Q)
0::
c
~ ro a. a. 10 <(
1
100
50 o;
Q) 0 ~
Q) -50 (/) ro -g_ -100
N -150
-200
1
0 .8
0 .6
0.4
0.2
0
0.001
Akutan MT 4 150
0 0
.. ...... ... ...
0.0 1 0.1
<> 0 0
.......... · ...
1
Period (sec)
10
..
0
0
.... • ....
100 1000
51
Zonge International Inc. Akutan MT 26 October 2012
Akutan MT 4250
10000 ~----------------------------------------------,
'E 1000
E:
.<:
.2.
;:.
:~ u; 100 ·u;
Q)
0:: c
[!?
ro a. a. 10 <(
1
100
o; 50
Q) 0 ~
Q) -50 en ro
-§. -100
N -150
-200
1
0.8
0.6
0.4
0.2
0
0 .001
• Skew
0.01
0
"
0 0 9 0 •• ~o 'i o_~ ~ •• ••• • • •• •• • •
0.1
. . .. -... · ..... .
1
Period (sec)
10 100 1000
Akutan MT 4350
10000 ~----------------------------------------------,
'E 1000
E:
.<:
.2.
;:.
"> ~ 100 ·u;
Q)
0:: c
[!?
ro a. a. 10 <(
1
100
o; 50
Q) 0 ~
Q) -50 en ro
-§. -100
N -150
-200
1
0.8
0.6
0.4
0.2
0
o o o XY Coherence 1
~ YX Coherence 600
• • • Skew
0
0
.............. ·.
0.001 0.01 0.1
.......... ........
·~ 0 o6 • ••• ••• • •
1
P eri od (sec)
10
.
100 1000
52
Zonge International Inc. Akutan MT 26 October 2012
Akutan MT 5050
1000 ~----------------------------------------------,
E'
E:
.<: 100
_£.
.?:-
:~ u;
"iii
Ql a::
"E
~ 10 ro a. a. <
1
100
o;
Ql 0 2-
Ql
(/) ro .g_ -100
N
-200
1
0.8
0.6
0.4
0.2
0
Apparent Resistivity
<>--<>--<> XY
~vx
o o o XY Coherence
• • •. Skew • • • • • • • . . ..
0.001 0 .01 0.1
. .
1
Period (sec)
. . . . .
10
• .
100
..
1000
Akutan MT 6050
10000 ~----------------------------------------------,
Apparent Resistivity
<>--<>--<> XY
~vx
1
50 ~--~~~~~~~~~~~~~~~~
0
Ol
Ql
2--50
Ql
(/)
~ -100
a.
N -150
-200 ~--------------------------------------------~
1
0.8 0 0
0.6
o o o XY Coherence
0.4 0
• • • Skew
0.2
0 .001 0 .01
53
Zonge International Inc. Akutan MT 26 October 2012
E'
E:
.<: 100
_£.
.?:-
:~ u;
"iii
Ql a::
"E
~ 10 ro a. a. <
1
100
o; 50
Ql 0 2-
Ql -50 (/) ro .g_ -100
N -150
-200
1
0.8
0.6
0.4
0.2
0
Akutan MT 7050
Apparent Resistivity
<>---<>-<> XY
~vx
o o o XY Coherence
• • ~Skew
0.001 0 .01
~Q
\o r. ~\o o 0
~ ..
•
0.1
.. . . .
1
Period (sec)
•
10
. . . .....
100 1000
Akutan MT 8050
10000 ~----------------------------------------------,
1
Apparent Resistiv ity
<>---<>-<> XY
~vx
100 -,------------------------------------------~~--~
Ol
~ 0
Ql
(/)
ro
"3.-100
N
-200 ~--------------------------------------------~
1
0.8 ~ V.i \ 0
0 ~ 0 0 ~ o]'
o o o XY Coherence 0 \ 0
$j
.
• . • Skew .. • • . . •
0.6
0.4
0.2
•
0
0 .001 0 .01 0 .1 1 10 100 1000
Period (sec)
54
Zonge International Inc. Akutan MT 26 October 2012
Akutan MT 9050
1000 ~----------------------------------------------,
E'
E:
.<: 100
_£.
.?;>
:~ u;
"iii
Ql a::
"E
~ 10 ro a. a. <
1
100
o; 50
Ql 0 2-
Ql -50 (/) ro .g_ -100
N -150
-200
1
0.8
0.6
0.4
0.2
0
Apparent Resistivity
<>--<>--<> XY
~vx
o XY Coherern:e . ~ • Skew
0
0 0
. . . . . . . . . . .
0.001 0 .01 0.1
0
Q o" o ~ \ 1~' <>
0 ~
0 .. . . .. ...... •
1 10 100 1000
Pe riod (sec )
Akutan MT 10050
10000 ~----------------------------------------------,
1
200
o; 100
Ql
2-
Ql
(/) 0
ro .<: a.
N -100
-200
1
0.8
0.6
0.4
0.2
0
Apparent Resistivity
<>--<>--<> XY
~vx
a s a &--e...e>-es-<a>----<38,..,-a-e a a e e e -&..s-.e e a e e s a a e a c
" \o '? <>-o ~~
o o o xv co herence [
1 _;\r"
o------&----0 YX Coherence \j
• • • Skew
•" •
0 .001 0 .01 0 .1 1
Period (sec)
..
10
... .
. •
100
0
1000
55
Zonge International Inc. Akutan MT 26 October 2012
Akutan MT 11050
10000 ~----------------------------------------------,
'E 1000
E:
.<:
.2.
;:.
:~ u; 100 ·u;
Q)
0:: c
[!?
ro a. a. 10 <(
1
100
o; 50
Q) 0 ~
Q) -50 en ro
-§. -100
N -150
-200
1
0.8
0.6
0.4
0.2
0
o o o XY Coherence
• .s.ke•w•• ••• ••• ••• .•• •• • •• • ... ••• ••• •• + ...............
0 .001 0.01 0.1 1
Period (sec)
10 100 1000
Akutan MT 11150
10000 ~----------------------------------------------,
'E 1000
E:
.<:
.2.
;:.
"> ~ 100 ·u;
Q)
0:: c
[!?
ro a. a. 10 <(
1
100
o; 50
Q) 0 ~
Q) -50 en ro
-§. -100
N -150
-200
1
0.8
0.6
0.4
0.2
0
+Skew
0.001 0.01 0.1 1
Period (sec)
10 100 1000
56
Zonge International Inc. Akutan MT 26 October 2012
1000
E'
E:
.I: 100 ~
::-
:~ u; ·u;
Ql a::
"E
~ 10 ro
Cl.
Cl.
<t:
1
100
o; 50
Ql 0 ~
Ql -50 en ro
.g_-1 00
N -150
-200
1
0.8
0.6
0.4
0 .2
0
Apparent Resistivity
<>--<>-<> XY
~vx
Akutan MT 11 250
-e&ee-eee~aeeeeeeee eeeeeeeeee ea 9686 ~
Z Phase
o o o XY Coherence
~ YX Coherence
•• • Skew
0
6 . ................... . .. ... ... ... ... .. . .......... .
0.001 0.01 0.1 1
Period (sec)
10 100 1000
Akutan MT 11350
1000 ~----------------------------------------------,
E'
E:
.I: 100 ~
::-
:~ u; ·u;
Ql a::
"E
~ 10 ro
Cl.
Cl.
<t:
1
100
o; 50
Ql 0 ~
Ql -50 en ro
.g_-100
N -150
-200
1
0.8
0.6
0.4
0.2
0
Apparent Resistivity
<>--<>-<> XY
~vx
~eeee-eeee~aeeeeeeeeeeeeeeeeeeeae$&$~
0.001 0.01 0.1 1
Period (sec)
10 100 1000
57
Zonge International Inc. Akutan MT 26 October 2012
Akutan MT 11450
1000 -=-------------------------,
E'
E:
.I: 100 ~
::-
:~ u; ·u;
Ql a::
"E
~ 10 ro
Cl.
Cl.
<t:
1
100
o; 50
Ql 0 ~
Ql -50 en ro
.g_-100
N -150
-200
1
0.8
0.6
0.4
0 .2
0
Apparent Resistivity
<>--<>-<> XY
~vx
• Skew
0.001 0.01 0.1
Ooo o
Oo
1
Period (sec)
10 100 1000
Akutan MT 11 550
1000 -=------------------------,
E'
E:
.I: 100 ~
::-
:~ u; ·u;
Ql a::
"E
~ 10 ro
Cl.
Cl.
<t:
1
100
o; 50
Ql 0 ~
Ql -50 en ro
.g_-100
N -150
-200
1
0.8
0.6
0.4
0.2
0
Apparent Resistivity
<>--<>-<> XY
~vx
Z Phase
o o o XY Coherence
<)----&----() YX Coherence
• • • Skew
• .. ... ... ... ... ... . .
0.001 0.01 0.1 1
Period (sec)
10 100 1000
58
Zonge International Inc. Akutan MT 26 October 2012
Ak utan MT 11 650
10000 ~----------------------------------------------,
'E 1000
E:
.<:
.2.
;:.
:~ u; 100 ·u;
Q)
0:: c
[!?
ro a. a. 10 <(
1
100
o; 50
Q) 0 ~
Q) -50 en ro
-§. -100
N -150
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1
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59
Zonge International Inc. Akutan MT 26 October 2012
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60
Zonge International Inc. Akutan MT 26 October 2012
Akutan MT 15050
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61
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Akutan MT 17050
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62
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Akutan MT 17250
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63
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Akutan MT 18050
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64
Zonge International Inc. Akutan MT 26 October 2012
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65
Zonge International Inc. Akutan MT 26 October 2012
Akutan MT 19350
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66
Zonge International Inc. Akutan MT 26 October 2012
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67
Zonge International Inc. Akutan MT 26 October 2012
APPENDIX I: IMPEDANCE RESISTIVTY AND PHASE PLAN MAPS
FIGURES:
MT Determinant Resistivity (color grid) with Z Polar Diagrams at 128 Hz
MT Determinant Phase (color grid) with Phase Tensor Ellipse at 128 Hz
MT Determinant Resistivity (color grid) with Z Polar Diagrams at 8 Hz
MT Determinant Phase (color grid) with Phase Tensor Ellipse at 8 Hz
MT Determinant Resistivity (color grid) with Z Polar Diagrams at 1 Hz
MT Determinant Phase (color grid) with Phase Tensor Ellipse at 1 Hz
MT Determinant Resistivity (color grid) with Z Polar Diagrams at 8 Seconds
MT Determinant Phase (color grid) with Phase Tensor Ellipse at 8 Seconds
MT Determinant Resistivity (color grid) with Z Polar Diagrams at 64 Seconds
MT Determinant Phase (color grid) with Phase Tensor Ellipse at 64 Seconds
68
Zonge International Inc. Akutan MT 26 October 2012
Figure 8: MT Determinant Resistivity (color grid) with Z Polar Diagrams at 128 Hz
69
Zonge International Inc. Akutan MT 26 October 2012
Figure 9: MT Determinant Phase (color grid) with Phase Tensor Ellipse at 128 Hz
70
Zonge International Inc. Akutan MT 26 October 2012
Figure 10: MT Determinant Resistivity (color grid) with Z Polar Diagrams at 8 Hz
71
Zonge International Inc. Akutan MT 26 October 2012
Figure 11: MT Determinant Phase (color grid) with Phase Tensor Ellipse at 8 Hz
72
Zonge International Inc. Akutan MT 26 October 2012
Figure 12: MT Determinant Resistivity (color grid) with Z Polar Diagrams at 1 Hz
73
Zonge International Inc. Akutan MT 26 October 2012
Figure 13: MT Determinant Phase (color grid) with Phase Tensor Ellipse at 1 Hz
74
Zonge International Inc. Akutan MT 26 October 2012
Figure 14: MT Determinant Resistivity (color grid) with Z Polar Diagrams at 8 secs
75
Zonge International Inc. Akutan MT 26 October 2012
Figure 15: MT Determinant Phase (color grid) with Phase Tensor Ellipse at 8 secs
76
Zonge International Inc. Akutan MT 26 October 2012
Figure 16: MT Determinant Resistivity (color grid) with Z Polar Diagrams at 64 secs
77
Zonge International Inc. Akutan MT 26 October 2012
Figure 17: MT Determinant Phase (color grid) with Phase Tensor Ellipse at 64 secs
174
Appendix 5
FINAL REPORT
STRATIGRAPHIC AND STRUCTURAL CONTROLS OF THE HOT SPRINGS BAY
VALLEY GEOTHERMAL SYSTEM, AKUTAN ISLAND, ALASKA
Nicholas H. Hinz and Gregory Dering
University of Nevada, Reno, NV 89557
2
Introduction
Detailed geologic mapping and structural analyses were carried out to develop a conceptual
structural model for the ~240-300ºC Hot Springs Bay Valley (HSBV) geothermal system on
Akutan Island, Alaska. Approximately 25 km2 were field mapped in August, 2012 and include
all of HSBV, the upper half of Long Valley, and along the northeastern flank of Akutan volcano
(Figures 1 and 2, Plate 1B). Approximately 5 km2 of areas with hydrothermal alteration and
active surficial geothermal manifestations in upper HSBV were mapped in greater detail (Plate
1C, inset from Plate 1B). The locations of the hot springs in lower HSBV were also mapped in
detail (Plate 1D, inset from Plate 1B). Faults were mapped island-wide through analysis of black
and white 1:24,000 scale stereo air photos, streaming online 2006 and 2010 Google Earth
imagery, and streaming online Bing Maps imagery (1:55,000 scale; Plate 1A). Faults and
fractures were analyzed to constrain kinematic evolution of the region and evaluate the local
strain and stress fields. This report summarizes the stratigraphic and structural framework and
extent of hydrothermal alteration and surface manifestations for the purposes of defining a
structural model for this geothermal system and facilitating selection of future drill targets.
Geologic Setting
Akutan Island, ~29 km long by 21 km wide, resides in the eastern part of the Aleutian
volcanic arc. The angle of plate convergence between the North American and Pacific plates
varies steadily across this arcuate shaped arc from near-perpendicular convergence in the east
(~Longitude 159° W) to a pure transform boundary at the west end of the arc (~Longitude 166°
E). As a result of oblique subduction, deformation of the arc is partitioned into two primary
components: 1) trench-perpendicular shortening accommodated by folds and thrust faults, and 2)
translocation of the entire arc along arc-parallel strike-slip faults in concert with arc-parallel
extension accommodated by a complex system of normal and strike-slip faults (Ave Lallemant,
1996; Ave Lallemont and Oldow, 2000). Obliquity of subduction increases from east to west
along the arc yielding greater magnitudes of dextral shear along intra-arc dextral strike-slip
faults. Located in the eastern part of the Aleutian arc and well back from the trench, Akutan
Island sits in a region primarily undergoing arc-parallel extension.
The western half of Akutan Island is dominated by the Akutan volcano, a composite
andesitic stratovolcano this is one of the most active volcanoes in the Aleutian arc (Figure 1).
The rest of the island is composed of older Plio-Pleistocene volcanic rocks that have
subsequently been deeply eroded by valley glaciers into a network of U-shaped valleys. The
most recent eruption of Akutan volcano was in 1992 (McGimsey et al., 1995). The most recent
major seismic event occurred in 1996 and was associated with ground fissures on the northwest
flank of the modern Akutan volcano and possible dike intrusions at depth (Powers et al., 1997;
Lu et al., 2000). Akutan Island has two known active geothermal systems and one ancestral
geothermal system. One of the active systems resides in the summit caldera of Akutan volcano
and the other is located immediately east of the volcano in HSBV. The ancestral geothermal
system occupies upper Long Valley (LV) northeast of the volcano and was newly identified in
this study.
3
Figure 1. Map of Akutan Island showing the location of key geographical features refered to in this report.
The hachered polygon corresponds to the area of map detail in Plate 1B and Figure 2.
Stratigraphic Framework
In the Hot Springs Bay Valley – Long Valley (HSBV-LV) area four principle late Tertiary
and Quaternary stratigraphic units were distinguished and include from oldest to youngest:
~1.4-3.3+ Ma basalt and basaltic andesite volcaniclastic deposits, lava flows (QTv), and
dikes (QTbai)
~0.3-0.6 Ma andesite plugs (Qbai)
~0.5 Ma to present deposits of the Akutan volcano (Qv)
Holocene post-glacial surficial deposits (Qa, Qls, Qab, Qb, Qe)
The oldest unit (QTv) is the most extensive unit in the HSBV-LV area (Plate 1B, Figure 2)
and also on the island, essentially acting as basement relative to the modern day Akutan volcano.
This unit is composed of a heterogeneous mix of mafic lava flows, volcaniclastic deposits, tuffs,
and dikes that were erupted and intruded from various on and off island sources. In upper HSBV
and upper LV volcaniclastics dominate QTv while the walls of lowermost HSBV are dominated
by lava flows. Age constraints for this unit consist of four previous 40Ar/39Ar dates on samples
from around the island, none of which were located within the HSBV-LV map area (Richter et
al., 1998).
The present Mt Akutan volcanic edifice is estimated to have started forming ~0.5 Ma and has
been very active throughout the Holocene and historic time with extensive Holocene deposits up
Hot Springs
Bay
Akutan Harbor
Akutan Volcano
Fumaroles
Map Detail in
Plate 1B and Figure 2
Flat Bight
4
to 40 to 60 m deep prograding out over the late Pleistocene glacial erosion surface (Richter et al.,
1998). Several ~0.3-0.6 Ma basaltic plugs and at least one sill intrude upper LV and the drainage
divide area between LV and HSBV (Figures 3 and 4). These plugs range in size from 10s of
meters to nearly 1 km across and intrude an area 1-2 km-wide by nearly 4 km-long, elongate
northwest-southeast. Based on age and proximity, these intrusions are probably a satellite center
of the Akutan volcano (Richter et al., 1998).
The most recent glaciation on Akutan Island retreated ~10,000 years ago (Wythomas, 1999).
This glaciation effectively stripped the island bare of surficial deposits, eroded the bedrock and
delivered all the sediment to the ocean leaving a very clean bedrock surface behind. Subsequent
to the glaciation, HSBV has aggraded with alluvium to the ocean level and Akutan volcano has
deposited a blanket of tephra across much of the map area. The tephra deposits range from <1 to
>3m-thick deposited on QTv and up to 10m thick on the slopes of the volcano (Richter et al.,
1998). For the purposes of mapping stratigraphy and structure, this study “looked through” the
Holocene tephra deposits and thus tephra was not included as one of the surficial geologic map
units.
Figure 2. Geologic map of the HSBV geothermal area (adapted from Plate 1B). Red lines correspond to
cross-sections A-A’ and B-B’ in Plate 2.
A
B
B’
A’
5
Figure 3. View looking southwest at a sill and plug associated with ~0.3-0.6 Ma Qbai intrusive suite that
intrudes the ~1.4-3.3 Ma QTv unit in upper HSBV and LV. Yellow star on inset map shows location of
this outcrop.
Figure 4. View looking at the north side of the largest exposed intrusion associated with the ~0.3-0.6 Ma
intrusive suite (Qbai). This plug is located directly up slope from the HSBV fumarole field on drainage
divide between HSBV and LV. Yellow star on inset map shows to location of this outcrop.
Structural Framework
Akutan Island is cut by a widely distributed array of moderately to steeply dipping normal,
oblique-slip, and strike-slip faults. Strike lengths of faults range from 10s of meters up to ~5-6
km-long. At map scale these faults have three primary general orientations including E-W,
WNW, and NE-strikes (Plates 1A and 1B, Figures 2 and 5). The WNW-striking faults are the
most pervasive set and are exposed over much of the island. East-west-striking faults are
primarily found on the peninsula north of Akutan Harbor and extending west from the peninsula
6
Figure 5. Fault map of the western three quarters of Akutan Island. Faults with possible or probable
Holocene fault scarps are highlighted red. Adapted from Plate 1A.
into upper HSBV. Both of the WNW and E-W-striking faults are characterized by overlapping
arrays of fault segments only partially connected by linkages. The NE-striking faults cut across
the middle of the island from Flat Bight on the south side through HSBV on the north side and is
much less pervasive at map scale than either the WNW or E-W-striking faults. All three of these
fault orientations, E-W, WNW, and NE-strikes come together and intersect in the HSBV area.
The majority of the mapped faults in the HSBV-LV area were interpreted as southward
dipping with down-to-the-south dip-slip or oblique-slip motion based in a large part by apparent
“Vs” across topography. However, limited exposure dictated that dip-direction and displacement
sense was not constrained for nearly one third of the mapped faults including a number of faults
inferred to intersect the centers of the active geothermal surface manifestations (Figure 2, Plate
2). Cumulative offset was identified for two separate, 5 km-long faults, one within HSBV and
one south of Akutan Harbor with maximum observed vertical stratigraphic offset only reaching a
few 10s of meters of offset along each fault (Figure 6).
Hot Springs
Bay
Akutan Harbor
Akutan
Volcano
Fumaroles
7
Figure 6. Stratigraphic offset across faults. A) Photo looking WNW along one of the main faults crossing
lower HSBV with down-to-south displacement. The ~15º west dip of the QTv strata displaced across this
fault reflects primary dips rather than tilting of fault blocks. B) Photo looking SE across Akutan Harbor
from the city of Akutan at a WNW-striking down-to-south fault displacing flat lying strata. Yellow and
orange lines highlight stratigraphic horizons offset across the faults.
Figure 7. Equal-area stereographic projections of poles to planes with Kamb contours for A) faults, N=55
B) fractures, N=57 and C) QTbai dikes, N=30. Rose diagrams are plotted with right hand rule in 10° bins,
with lengths of the petals drawn to the percentage of the data within a given bin, radius of the stereonet is
equal to 50% of the total data for each plot.
Within the HSBV-LV map area, the orientations of 55 fault surfaces were measured, with
most of these measured surfaces located in upper HSBV. A few of the measurements were made
on faults large enough to be mapped (Plate 1B, Figure 2) while the rest were on small faults that
were only traceable for several meters or less across a single outcrop (Plate 1C). Fault strikes
plotted on a rose diagram shows three data clusters listed here in decreasing order of abundance:
1) a group with a near continuous spread of strikes from ENE-WSW to ESE-WNW, 2) a group
with NNE to NE-strikes, and 3) a small group striking NNW (Figure 7A). Stereoplots of poles
to fault planes indicate nearly bimodal north and south dips ranging from ~50° to nearly vertical.
Within the HSBV-LV area, the orientations of 57 tensile fractures were measured to help
constrain the orientation of the maximum tensile stress and evaluate possible paleo-stress
orientations (Figure 7B). Strikes of fracture planes plotted on a rose diagram shows three data
A B C
A B
8
clusters that are consistent with the 3 primary fault orientations across the island, E-W, WNW,
and NE-striking. Stereoplots of poles to fracture planes indicate bimodal dips for each of the 3
primary strike orientations ranging from ~50° to nearly vertical. Assuming a transtensional
tectonic setting with the principal stress (σ1) oriented vertically, these three groups of tensile
fractures indicate a complex stress history with least principal stresses (σ3) oriented N-S, NNE-
SSW, and NW-SE. Tensile fractures with WNW-strikes opened up during a series of seismic
events in 1996 (Lu et al., 2000) suggesting that in the modern day stress field σ3 is oriented
NNE-SSW.
Stereoplots of poles to QTbai dikes measured in HSBV show that the dikes intruded two
dominate orientations, E-W and WNW (Figure 7C). These two orientations match with the two
most pervasive fault orientations mapped in HSBV. An inferred historic intrusion of one or
more dikes during a series of seismic events in 1996 may have also intruded with a WNW-strike
(Lu et al., 2000).
While attitudes of the QTbai dikes were easily collected, attitudes of the primary map unit,
QTv were difficult to obtain within the HSBV-LV map area (Figure 2). Most of this mapping
effort was focused in upper HSBV around the areas with hydrothermal alteration and active
geothermal surface manifestations. In this part of HSBV most of the QTv bedrock exposures
were of massive to poorly stratified volcaniclastic rocks from which bedding attitudes could not
easily be determined. A couple lava flows exposed in an otherwise volcaniclastic-dominated
section of QTv in the E-W ridge bounding the north side of HSBV exhibit ~15º west dips (Figure
6A). These dips are interpreted to reflect primary dips rather than tilting of fault blocks based on
small magnitudes of stratigraphic offset (10s of meters) observed along faults in this study.
Previous mapping of Akutan Island by Richter et al. (1998) documented that the age-equivalent
units to the Plio-Pleistocene QTv strata delineated in this mapping effort range from flat lying
(e.g. Figure 6B) to gently dipping and also inferred that the localized gentle dips reflected
primary dips.
Timing of Faulting
Most of the faults exposed across Akutan Island are only observed cutting the ~1.4-3.3 Ma
QTv “basement” rocks (Plate 1A, Figure 5). Akutan volcano and its associated satellite eruptive
and intrusive centers are significantly less faulted than the older (QTv) basement rock that makes
up much of the island. Even though the Akutan volcano started forming ~0.5 Ma, its flanks are
covered by late Pleistocene and Holocene units that would obscure any middle Pleistocene faults
if present. Holocene scarps were identified on a half dozen WNW-striking faults on the eastern
half of Akutan Island. Google Earth images of some of these fault scarps are shown in Figure 8.
The westernmost fault with a Holocene scarp is located on the eastern edge of the Akutan
volcanic edifice. Only one fault is associated with a Holocene scarp in the HSBV-LV area and it
is located in the southernmost corner of the HSBV drainage area (Figure 2).
Fault Kinematics
Fault surfaces exposed in the HSBV-LV area were analyzed to elucidate the kinematics of
the fault systems and determine the orientations of principal strain and stress axes. The stress
field can be derived from fault slip data assuming a linear and direct relationship between strain
and stress. The PBT method used here is a kinematic analysis that calculates the three
theoretical principal strain axes for each individual fault-slip datum, i.e. a compressional axis P, a
neutral axis B (which lies in the fault plane), and an extensional axis T (e.g. Sippel et al., 2009).
9
Figure 8. Views of probable Holocene age fault scarps. A) View is looking west-northwest in 2010 Google
Earth imagery of a west-northwest-striking fault that exhibits apparent down-to-south displacement. B and C)
Views are looking west-northwest in 2006 Google Earth imagery of fault strands that strike west-northwest,
exhibit apparent down-to-south displacement, and are part of a ~5-6 km-long series of nearly continuous fault
scarps. Red arrows on map indicate approximate locations of the Google Earth images. Red arrows on images
point two respective fault scarps in each image. Note that these fault scarps were not field checked as part of
this study.
10
Figure 9. A) Lower hemisphere stereographic projection of great circles of exposed faults in the map area.
Arrows indicate slip direction inferred from striae and other kinematic indicators (i.e., Riedel shears). N =
number of data points. B) PBT-lower hemisphere stereographic projection showing the orientations of
principal strain axes for each measured fault. Large symbols are mean vectors to all P, B, and T axes. C)
Fluctuation histogram of the dihedral angle between the measured lineation and the stress vector (the
resolved shear stress direction) for each fault plane. The lower the angle, the better the data fit to the
calculated stress field. An error of more than 20-30° indicates another stress field (Sippel et al., 2009). This
fluctuation histogram implies a heterogeneous stress field, based on stress field simulation.
Employing the Mohr-Coulomb failure criterion, the method incorporates a defined fracture angle
between P and the slip surface. For this study, a fracture angle of 30° was applied. The
application of PBT to the entire fault population of a particular location results in a
comprehensive pattern of kinematic axes. This cumulative plot permits detection of kinematic
consistencies as clusters of P, B, T axes in a heterogeneous data set.
Nineteen fault surfaces with kinematic data were observed in the HSBV-LV area. The
strikes of these fault surfaces span an ~90° range from NE-SW to ESE-WNW and include all 3
of the major strike orientations identified through field mapping, E-W, WNW, and NE (Figure
9A). Motion accommodated on these fault surfaces includes normal, oblique, or strike-slip, with
the dominant motion left-lateral normal-oblique regardless of strike orientation. Cumulative
results from stress analyses of all 19 fault surfaces indicate that one or more stress states have
been present on Akutan Island to produce the strain on these faults (Figure 9B and 9C).
Individual stress regimes were detected from evaluating the kinematic consistencies across
all 19 fault surface measurements by looking at clusters of P, B, and T axes from which
kinematically consistent subsets of faults can be separated from a heterogeneous fault
population. The results of separating these clusters of kinematic axes were then plotted on
individual plots to derive P, B, and T axes for each subset. The 19 fault surface measurements
were separated into 3 subsets based on this process and plotted individually in Figure 10. In
subsets A and B, the σ1-axes are all clustered near vertical while, the σ2- and σ3-axes are oriented
near horizontal. In subset C, axes are similarly grouped between vertical (σ1) and horizontal (σ2
and σ3), but are much more scattered. It is possible that the data in group C still represents a
heterogeneous kinematic data set, however the limited number of data points restrict further
separation of kinematic axes.
The results from kinematic analyses indicate 3 separate extension directions have influenced
faulting on Akutan through time, including N-S, NW-SE, and NE-SW-oriented extension
directions. This result agrees well with the 3 dominant fault orientations identified in this study,
E-W, WNW, and NE. Each of these fault orientations lies roughly perpendicular to one of the
three extension directions. Unfortunately, cross-cutting relationships were not identified in this
N = 19 A B C
11
Figure 10. Separation of kinematic analyses displayed in Figure 9 into three homogenous stress fields.
These stereographic and fluctuation histogram plots are the same type of plots as described in Figure 9.
Given the assumption that the strain is homogenous, without volume loss, and linearly related to stress,
P=σ1, B=σ2, and T=σ3. A) N=6, P=σ1=286°/73°, B=σ2=095°/22°, and T=σ3=183°/04° (trend/plunge). B)
N=7, P=σ1=004°/63°, B=σ2=236°/20°, and T=σ3=135°/17° (trend/plunge). C) N=6, P=σ1=322°/55°,
B=σ2=131°/47°, and T=σ3=044°/06° (trend/plunge). The T-axis is the extension direction. These limited
data suggest that Akutan Island has experienced 3 extension directions in the last ~3 Ma including N-S,
WNW, and NE-oriented directions. However, these data alone cannot distinguish the chronology of
changes in stress orientations, including which if any of these stress orientations reflects the current stress
field.
study and all the measured fault surfaces came from one unit (QTv) such that stratigraphic ages
cannot be used to distinguish the order of the 3 (or more) stress regimes. Of the 3 fault
populations identified from mapping, only WNW-striking faults have been active in the
Holocene. We assume that faults with Holocene rupture are most favorably oriented for failure in
the modern stress field and thus reflect the orientation of that stress field. Measured fault surfaces
A1
A2
A3
B1
B2
B3
C1
C2
C3
12
with WNW strikes fit within stress regime A and stress regime C. Based on the possible
correlation between orientations of measured fault surfaces and orientations of faults known to
be active in the Holocene, the modern day orientation of σ3-axes can be inferred as N-S to NE-
SW. Thus, steeply dipping faults and fractures oriented near perpendicular to σ3-axes would be
most likely to dilate (E-W to NW-SE strikes).
Tectonic stresses driving faulting on Akutan include arc-parallel extension (e.g., Ave
Lallemant, 1996), possible arc-perpendicular extension due to slab roll-back, and local uplift and
subsidence due to magmatic intrusion and withdrawal (e.g., Lu et al., 2000). The large number
of oblique slip faults probably represents recent slip on older normal faults that initially
developed perpendicular to the extension direction. Reorientation of the extension direction in
response to a shift in the regional stress field produced oblique slip on these preexisting planes of
weakness which were not oriented too far out of kinematic favorability.
Hydrothermal Alteration and Geothermal Surface Manifestations
Detailed mapping of the hydrothermal alteration and active geothermal surface
manifestations in the HSBV-LV map area resulted in the discovery of a fossil geothermal system
in upper LV, directly adjacent to the active geothermal system in HSBV (Figure 11). Since the
focus of this study was to establish a conceptual structural model of the HSBV system for the
purpose of selecting future drill target areas, the extent of hydrothermal alteration and surface
manifestations was mapped in detail for the HSBV geothermal system while the extent of
ancestral LV system was only briefly investigated. Investigating the ancestral LV geothermal
system fulfilled three vital purposes: 1) to investigate whether it is separate from the modern day
HSBV system, 2) to define the boundary between the HSBV and LV systems, and 3) to evaluate
whether characteristics of the better exposed LV system could be applied to the HSBV system.
The following two sections of this report characterize the alteration and surficial manifestations
for each system.
Figure 11. Spatial extent of alteration and active surface manifestations related to the modern geothermal
system in HSBV (semi-transparent pink overlay) and spatial extent of alteration associated with the
ancestral geothermal system in upper LV (semi-transparent green overlay).
13
HSBV-modern system
It has long been known that the HSBV geothermal system consists of two primary surface
manifestations, an ~4 km-long NE alignment of hot springs in lower HSBV and a tight cluster of
fumaroles in upper HSBV (Motyka and Nye, 1988). In addition to the main cluster of fumaroles,
a series of discontinuous hydrothermally altered bedrock outcrops and active surface
manifestations were distinguished in this study across upper HSBV within an area ~1.5 km-wide
by 3.5 km-long, elongate N-S (Figures 11 and 12). Alteration and active surface manifestations
are conspicuously absent throughout a ~1.5-2 km wide swath in the central part of HSBV.
All of the alteration and surficial manifestation features show preferential concentration
within the three primary tributary valleys of upper HSBV, delineated as areas A, B, and C in
Figure 12. Areas A and B are elongate WNW-ESE and area C is elongate E-W. The overall
density of faults and fault intersections generally increases within each of the areas (A, B, and C)
of concentrated alteration and surface manifestations in upper HSBV (Figure 12). Specific
mapped features include areas of moderate and strong intensity argillic alteration (detailed in
Figure 14), ferricrete spring deposits (detailed in Figure 15), native sulfur and sulfate deposits
(detailed in Figure 16), hot and warm springs, fumaroles, and boiling mud pots (detailed in
Figure 17). Of particular note, silica in any form was not identified in association with any of the
areas of alteration in upper HSBV, except for up slope from the fumaroles near the drainage
divide with LV where the silica was interpreted as being associated with ancestral LV
geothermal system. Each of these geothermal indicators is described in detail to better illustrate
the characteristics of each feature as they pertain to defining the geothermal system.
The points and polygons representing areas of bedrock alteration in Figures 12 and 13
represent areas of nearly 100% exposure. Elsewhere, the extent of Holocene tephra and
vegetation covering the bedrock in HSBV is considerable. Mapped areas of alteration were not
extrapolated under tephra, vegetation, or snow cover for two reasons. One reason was that the
areas of strong and moderate intensity argillic alteration were less resistant than the surrounding
of low intensity or no alteration and are prone to frequent hillslope and stream bank failures
leaving the altered areas exposed. In addition to preferential exposure of altered bedrock due to
erosion processes, substantial numbers of small ≤10 m2 well-exposed outcrops of unaltered
bedrock located adjacent and/or near the altered outcrops also limit extrapolation of altered areas.
Snow cover did limit mapping of some altered areas, particularly along the stream channels.
However, the extent of snow cover encountered during field mapping in August 2012 was very
similar to the snow cover in the 2011 base imagery (as visible in Plate 1C) and thus it is easy to
observe where the altered areas are bounded by snow cover. Based on the extent and density of
actual bedrock outcrop observed through windows in the Holocene tephra, vegetation, and snow
cover, we estimate that at least 75% of the area of strong to moderate intensity argillic alteration
was recorded by field mapping of HSBV during the 2012 field season.
The bedrock alteration in HSBV is characterized by a complex distribution of areas of strong
or moderate intensity argillic alteration that measure from <1m across to nearly 400m long
(Figures 12 and 14). The strong intensity argillic alteration areas are characterized by nearly
complete alteration of the bedrock to soft, light to medium blue-gray colored clay, easily
deformable by hand, and that contain ubiquitous disseminated pyrite (Figures 13A and 13B.
Areas of moderate intensity argillic alteration are characterized by having pockets 1-2 cm and up
to 10cm across of pure clay, but most of the bedrock is overall hard and brittle. Both the strong
and moderate intensity argillic alteration is associated with precipitated iron oxides along
fracture planes; however iron oxide is most ubiquitous with the areas of moderate intensity
14
Figure 12. Detail of bedrock alteration, surficial geothermal manifestations, faults, and dikes mapped in
upper HSBV. Dashed black ovals outline areas of concentrated alteration and surficial manifestations.
Primary
Fumarole
Field
B
A
C
15
Figure 13. Examples of alteration and surficial manifestations of the HSBV geothermal system in upper
HSBV. A) Blue clay representative of typical strong intensity argillic altered volcaniclastic rocks (QTv).
B) Hand sample of marbled blue and white colored clay with disseminated pyrite from an outcrop of strong
intensity argillic altered volcaniclastic rocks (QTv). C) Active sulfate (?) precipitate at a hot spring. D)
Native sulfur depositing in the throat of an active fumarole. F) Native sulfur exposed in strong intensity
argillic altered volcaniclastic rocks (QTv). F) Active ferricrete precipitate at a spring.
A B
C D
F
E
F
F
F
16
Figure 14. Detail of bedrock alteration in upper HSBV.
Primary
Fumarole
Field
17
Figure 15. Detail of ferricrete spring deposits in upper HSBV.
Primary
Fumarole
Field
18
Figure 16. Detail of sulfur and sulfate deposits in upper HSBV.
Primary
Fumarole
Field
19
Figure 17. Detail of fumaroles, hot springs, and boiling mud pots in upper HSBV.
Primary
Fumarole
Field
20
Figure 18. Detail of bedrock alteration, surficial geothermal manifestations, faults, and dikes mapped in
vicinity of the primary fumarole field. The dashed red oval outlines the extent of most robust and active
surface manifestations of the HSBV geothermal system.
Primary
Fumarole
Field
21
alteration. Many of the areas of alteration are elongate E-W, NW-SE, or N-S, locally following
traces. At outcrop scale the alteration preferentially follows faults, fractures, joints-including
columnar joints, and along the dikes. We interpret this pattern of alteration resulting from the
steam and fluids having ascended along whatever bedrock discontinuities are locally present.
The most extensively altered area of upper HSBV is area B (Figure 12), followed by areas A and
C. Much of the areas of alteration located away from the fumaroles are locally covered with a
layer of unaltered Holocene tephra and peat. We infer that these areas underwent alteration prior
to the most recent glaciation and imply that the surface manifestations have migrated through
time, and/or the overall intensity of the system has waned through time, and/or many of the
altered areas are now shielded from the main reservoir by an intervening clay cap.
The distribution of native sulfur deposits (Figures 13D and 13E) and springs actively
precipitating ferricrete (Figure 13 F), sulfates (Figure 13C), and travertine correlates remarkably
with cores of each of the three primary tributary drainages in upper HSBV (Figures 15 and 16)
and the general extent of areas with moderate and strong intensity argillic alteration (areas A, B,
and C in Figure 12). With high precipitation rates on Akutan Island, groundwater springs seem
to emanate from every little crack in the bedrock. However, outside of the hot springs in lower
HSBV, the only springs associated with active sulfate precipitation were observed in upper
HSBV, and although a few springs were found with active ferricrete precipitation elsewhere,
>90% were identified upper HSBV. Native sulfur deposits were only found associated within
areas of strong intensity argillic alteration, particularly around the active fumaroles.
In upper HSBV, nearly all the hot springs, fumaroles, and boiling mud pots are concentrated
in area B (primary fumarole area, Figure 12; Figure 17). Only one warm spring was found in
area A and one other in area B (Figure 12). Fumaroles and hot springs in area B are aligned on
trends consistent with regional structural grain defined by E-W, WNW, and NE-striking faults.
The primary fumarole field including one large boiling mud pot is aligned on a NE trend 125 m
in length. The fumaroles located downstream from the primary fumarole cluster align along a
WNW trend over 400 m distant. Lastly, several hot springs extending east from the primary
fumarole cluster align on an E-W trend nearly 400 m long. Faults are inferred along the NE and
WNW alignments of robust fumaroles, however these active surficial manifestations are not only
associated with simply two or three intersecting faults, numerous small faults and fractures with
E-W, WNW, NE and other orientations were noted in outcrops immediately surrounding these
features (Figure 18).
In lower HSBV over 3 dozen hot and warm springs emanate through Holocene alluvium in
an ~4km-long NE-trending alignment that stretches from near the midpoint of lower HSBV all
the way to the ocean and that follows within ~100 m of the north side of the valley (Plates 1B
and 1D). Most of the hot springs cluster within a ~1.5 km-long zone near the middle of the
greater ~4 km-long alignment. Locally, springs align along linear E-W and NNE trends over 10-
100 m distances within the broader 100m-wide by 4 km-long NE-trending alignment of springs
in lower HSBV suggesting upflow may be focused along a system of intersecting structures with
the dominant structure striking NE. Small amounts of silica sinter associated with some of these
springs have been noted by previous researchers (e.g., Kolker, 2011). In contrast to upper
HSBV, fumaroles, native sulfur deposits, or argillic alteration of the bedrock have not been
identified in lower HSBV.
In summary, the style of alteration and associated sulfidation observed in upper HSBV is
consistent with Motyka and Nye’s (1988) interpretation that the surface manifestations are
directly fed by gases and steam boiling off a reservoir at depth. The boiling of a reservoir a
22
depth also fits the pattern of there being no silica in the upper HSBV area as in this model the
silica gets left behind when the fluids boil to steam. The presence of silica sinter locally
associated with some of the hot springs in lower HSBV and lack of observed fumaroles imply
that these hot springs are fed by outflow and/or upflow that is sufficiently moderated to avoid
boiling until the fluids reach the surface. Although separated by an area devoid of alteration and
active surface manifestations in the central part HSBV, similarities in geothermal fluid chemistry
between upper and lower HSBV imply that both areas are connected to the same reservoir and
are part of a single geothermal system (Kolker, 2011).
LV – fossil system
The ancestral geothermal system located in upper LV is consists of two primary areas of
continuous moderate and strong intensity argillic alteration 1-1.5 km across (Figures 2 and 19,
Plate 1B). Small areas of alteration (meters to 10s of meters in lateral extent) are found around
the larger contiguous areas of alteration. Disseminated pyrite and silica vein deposits are locally
associated with these altered areas (Figure 20). The overall distribution of alteration in upper LV
is closely associated spatially with the Qbai plugs. Loci of strong intensity argillic alteration
Figure 19. Photo looking north at argillic alteration exposed in the north wall of Long Valley
23
Figure 20. Outcrop of volcaniclastic QTv exposed along the drainage divide between LV and HSBV that has
undergone moderate intensity argillic alteration and strongly silicified . The silicified volcaniclastics are very
hard and difficult to break with only a rock hammer.
correspond to zones, generally <10 m-across of highly fractured country rock (QTv) directly
adjacent to the plugs (Figure 21). The fracturing in the QTv country rock is comprised of a
combination of regional tectonic faults and fractures, and fractures directly related to the
emplacement of the Qbai plugs. Locally, the outer 1-2 meters of the plugs are moderately to
strongly altered, but the cores of the plugs only exhibit moderate to weak alteration intensity.
Fossil surface manifestations such as spring deposits or fumaroles were not observed in
association LV system and thus we interpret that erosion has exposed the deeper part of a paleo-
system.
Discussion and Conclusions
The detailed geologic mapping and structural analysis provide important insights into
developing a conceptual model for the geothermal system in HSBV and selecting drill targeting
strategies for future exploration efforts. The HSBV geothermal area is located where 3 distinct
structural grains come together, yielding numerous along-strike and up and down-dip
intersections between sets of E-W, WNW, and NE-striking faults and fractures. Each of these
24
Figure 21. View of hydrothermal alteration spatially associated with Qbai plugs in Long Valley.
Alteration intensity is focused in highly fractured country rock (QTv) along the plug margins. Plug
interiors are also altered, but much less than the immediately surrounding country rock.
fault sets are made up of broadly distributed fault arrays characterized by overlapping and locally
oppositely steeply dipping faults with cumulative offset on individual faults reaching only 10s of
meters. Based on the broad distribution of fault networks and the broad distribution of
geothermal surface manifestations the HSBV geothermal system is probably not defined by a
more prominent mature fault zone typical of a setting like the Basin and Range where faults
typically have 100s of meters of offset and fluid flow associated with a Basin and Range
geothermal system may be focused along a single fault.
In upper HSBV, hydrothermal alteration and geothermal surface manifestations are related to
gases and steam boiling off a reservoir at depth and cluster in three areas (Areas A, B, and C in
25
Figure 12) within a ~1.5 km-wide by 3.5 km-long area. The clustering of alteration and surface
manifestations in Areas A, B, and C may reflect lateral variations in permeability resulting from
localized concentrations of fault and fracture intersections or concentrations of faults and
fractures oriented preferentially for dilation, and thus yielding multiple upflow pathways above a
single reservoir. Or alternatively, the clustering of alteration and surface manifestations may
primarily reflect a complex of 3 closely spaced smaller reservoirs each feeding a separate cluster
of surface manifestations. In each of the Areas A, B, and C, the extent of alteration in upper
HSBV is considerably greater than the area punctuated by active features such as fumaroles and
hot springs (Figure 12, 17, and 18), suggesting that either the surface manifestations have
migrated around through time and/or the system has waned overall in intensity through time
and/or much of the altered areas are now shielded from the main reservoir by an impermeable
clay cap.
The hot springs in lower HSBV differ from the manifestations in upper HSBV by the
presence of silica and absence of fumarolic activity. Therefore these features are probably fed
directly by geothermal fluids either as upflow or outflow from the reservoir rather than by steam
and gases as interpreted for the features in upper HSBV. Previous researchers have pointed out
that the hot springs in lower HSBV emanate from points north of a prominent WNW-striking
fault zone that crosses the valley (e.g. Kolker, 2011; Figure 2). The fact that the springs come up
along inferred NNE to NE-striking faults only in the footwall of the cross-valley fault rather than
directly along the trace of the cross-valley fault suggests that at least this section of WNW-
striking cross-valley fault may be a barrier to fluid flow rather than an open conduit. Thus this
steep south dipping WNW-striking cross-valley fault may intersect part an inferred reservoir at
depth and direct some fluid upflow to the north.
Fault slip data indicate N-S to NE-SW modern day extension direction which favors dilation
on either the E-W or WNW-striking faults. However, throughout upper and lower HSBV, active
surface manifestations coincide with structures representative of all three of the major
orientations identified in the study area including E-W, WNW, and NE-striking faults. In
extensional terranes, steeply plunging fault and fracture intersections can also exhibit enhanced
dilation regardless of extension direction which may explain why active surface manifestations
are found along faults representing all 3 major fault orientations.
A broad (4 km2) area of hydrothermally altered QTv and Qbai sits in upper LV directly
adjacent to the HSBV geothermal system and likely represents the roots of a fossil geothermal
system. The hydrothermal alteration in upper LV is spatially, and possibly temporally,
associated with a WNW-trending alignment of ~0.3-0.6 Ma Qbai plugs. The presence of silica
veinlets and spotty silicification in volcaniclastic rock in the ancestral LV system distinguishes it
from the modern HSBV system (Figures 11 and 20). We suggest that silica alteration mapped in
the LV system represents deeper parts of a fossil geothermal system subsequently exposed by
erosion. Therefore, exposures of the LV system may provide us virtual windows into what may
lie below the surface manifestations in upper HSBV.
Conceptual models
Based on the results of this study, we propose three conceptual reservoir models for the
HSBV geothermal system:
1) Fault Intersection Model. The geothermal reservoir is controlled by the intersection of 3
separate E-W, WNW, and NE-striking fault zones, each defined by a broadly distributed
system of normal, oblique-slip, and strike-slip faults. Numerous steeply plunging
26
intersections of faults and fractures may generate high permeability and promote the deep
circulation of fluids. Overall greater density of faults were observed to conside with each
of the three clusters of alteration and surface manifestations in upper HSBV.
2) Intrusive Body Model. The geothermal reservoir is controlled by dense fracture networks
associated with the emplacement of one or more hypothetical intrusions at depth which
promotes geothermal fluid flow at the intrusion margins in a style similar to the ancestral
LV geothermal system.
3) Hybrid Intrusive Body-Fault Intersection Model. A combination of both models 1 and 2
whereby the geothermal reservoir is controlled both by fracture networks directly
surrounding and resulting from hypothetical intrusions at depth and by abundant
intersections of the regional fault and fracture networks.
For each of these conceptual models, the reservoir or cluster of reservoirs must be located
suffciently deep enough and/or be broad enough to feed active surface manifestations across the
upper reaches of HSBV and also lower HSBV extending all the way to the beach, while at the
same time avoiding delivering any fluids to surface areas in central HSBV. Based on the
presumed upflow pathways along steeply dipping (>60°) faults and fractures, a hypothetical
reservoir measuring <0.5 km across would probably need to be situated at ~1.5-2+km depth
under central HSBV with radial upflow pathways feeding the active surface manifestations (e.g.
Plate 2). However, a wider reservoir or cluster of reservoirs located at <1 km depth could also
feed the respective surface manifestations. The potential for a cluster of interconnected smaller
reservoirs is also fitting with the conceptual reservoir models described above. Each of these
conceptual models accommodates local reservoir-sized pockets of increased permeability that
either reflect areas of greater concentrations of intersecting faults or reflecting variations in
fracture density of country rock surrounding one or more hypothetical inrusive bodies at depth.
Drill Targeting
Of all the surface manifestations, the fumaroles and hot springs in Area B in upper HSBV are
the most robust and are probably the most well connected to the heart of the reservoir at depth
and should be considered as the primary target area (Figures 12 and 18). The overall dense
network of intersecting and steeply dipping faults and fractures indentified within this target area
provides abundant steeply plunging conduits for fluid flow. Within Area B the active surface
manifestations are focused within ~500 m long by ~100 m wide zone sourrounding an inferred
WNW-striking fault (Figure 18). The distribution of fumaroles and hot springs are further
concentrated into two primary fields along a WNW-striking fault, specifically where the WNW-
striking fault separately intersects with an inferred E-W-striking fault and a NE-striking fault.
The sweet spot for geothermal exploration might be to place a production well such that it
intersects the steeply plunging NE/WNW fault intersection at depth. Unfortunately, we weren’t
able to measure the dips of either of these three inferred faults directly. Regional map patterns
show predominantly southward dips for the E-W and WNW-striking fault systems, but so few
NE-sriking faults were identified at map scale that regional dip patterns were not identified for
this set of faults. Small outcrop-scale faults measured around the fumaroles support that the
WNW-striking structure might dip north (Figure 18). However, without being able to constrain
the dips of these faults we are left with four possible plunge directions for each of the resultant
fault intersections and thus intersecting one of these fault intersections at depth may take
multiple holes to constrain the subsurface geometry. Ultimately, the overall dense network of
27
intersecting and steeply dipping faults and fractures in this area may provide sufficient
permeability without having to target specific fault intersections.
A conceptual target not readily identifiable from geologic mapping alone is the densely
fractured margin of one or more intrusions at depth. Fracture densities and associated
permeability of an intrusion-related reservoir is probably greatest in the country rock directly
adjacent to an intrusive body, as we observed around the margins of Qbai plugs in LV, which we
interpret to be the roots of a fossil geothermal system. The drill target would be the fractured
and possibly silicified margin of an intrusion at depth proximal to the active geothermal system
in HSBV, thereby exploiting to the permeability associated with compounded tectonic and
intrusion-related fracture density. Based on exposures in LV, the scale of such a target would be
<10-100+ m across. However in the case of HSBV, geologic mapping cannot constrain the
location or geometry of intrusion(s) that have not breached the surface. It is important to note
that intrusive bodies can have irregular geometries. From geologic mapping we don’t know if
there is one intrusion, are no intrusions, or are several intrusions. Defining an intrusion-related
target depends on the integration of geologic mapping with geophysical data and probably
multiple drill holes to constrain the position and geometry of the intrusive body.
Any drilling strategy should take into account the character of the volcanic lithologies
present within the geothermal field and the fracture-controlled permeability associated with these
rock types. Within the area of detailed mapping unit QTv is dominated by fine grained
volcaniclastics. Based on field observations, these volcaniclastics do not maintain good fracture
permeability and are probably not a viable target for geothermal production. Finely crystalline
basaltic andesite lava flows, dikes, and sills exhibit much more favorable fracture-controlled
permeability, both along cooling joints and tectonic fractures. Thus crystalline volcanic rocks
are much more suitable targets for geothermal production. The overall lateral and vertical
composition of QTv across the island is highly variable and includes thick sections of lavas
intercalated with volcaniclastics. The proportion of crystalline volcanics to volcaniclastics in
QTv beneath the geothermal field is unknown. Finding suitable reservoir rocks may be critical
for geothermal development of HSBV. It would appear that such rocks are present, and where
combined with favorable fracture networks may yield a viable geothermal reservoir.
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Akutan, 34 p.
Lu, Z., Wicks Jr., C., Power, J.A., and Dzurisin, D., 2000, Ground deformation associated with
the March 1996 earthquake swarm at Akutan volcano, Alaska, revealed by satellite radar
interferometry: Journal of Geophysical Research, v. 105, no. B9, p. 21,483-21,495.
McGimsey, R.G., Neal, C.A., and Doukas, M.P., 1995, 1992 volcanic activity in Alaska:
summary of events and response of the Alaska Volcano Observatory: USGS Open-File
Report 95-83, 26 p.
28
Motyka, R., and Nye, C., eds., 1988, A geological, geochemical, and geophysical survey of the
geothermal resources at Hot Springs Bay Valley, Akutan Island, Alaska: Alaska Division of
Geological and Geophysical Surveys (ADGGS), Report of Investigations 88-3, 28 p.
Powers, J.A., Paskievitch, J.A., Richter, D.H., McGimsey, R.G., Stelling, P., Jolly, A.D., and
Fletcher, H.J., 1996, 1996 seismicity and ground deformation at Akutan Volocano, Alaska:
EOS, Transactions, American Geophysical Union, v. 77, no. 36, p. F514.
Richter, D.H., Waythomas, C.F., McGimsey, R.G., and Stelling, P.L., 1998, Geology of Akutan
Island, Alaska: USGS Open-File Report 98-135, 1 sheet, 1:63,360 scale, 22 pages.
Sippel, J., Scheck-Wenderoth, M., Reicherter, K., and Mazur, S., 2009, Paleostress states at the
south-western margin of the Central European Basin System — Application of fault-slip
analysis to unravel a polyphase deformation pattern: Tectonophysics, v. 470, p. 129-146.
Waythomas, C.F., 1999, Stratigraphic framework of Holocene volcaniclastic deposits, Akutan
volcano, east-central Aleutian Islands, Alaska: Bulletin of Volcanology, v. 61, p. 141-161.
203
Appendix 6
MT AND GRAVITY MODELING REPORT,
AKUTAN, ALASKA, 2012
for
CITY OF AKUTAN, ALASKA
ZONGE JOB # 12140
ISSUE DATE: 30 November 2012
Revised 5 Dec 2012
ZONGE INTERNATIONAL, INC.
9595 Prototype Court
Reno, Nevada 89521
Phone: (775) 355-7707, Fax: (775) 355-9144
ii
Zonge International Inc. Akutan MT 30 November 2012
TABLE OF CONTENTS
INTRODUCTION ..................................................................................................................................... 2
MT MODELING OBJECTIVE ............................................................................................................... 2
DATA PREPARATION, EDITING, AND CONVERSIONS ............................................................... 3
2D INVERSION ....................................................................................................................................... 4
2D INVERSION TO 3D VOLUME MODEL ......................................................................................... 7
MODEL RESOLUTION .......................................................................................................................... 8
MODEL RESULTS ................................................................................................................................... 9
REFERENCES ........................................................................................................................................ 25
APPENDIX A: 3D RESISTIVITY MODEL SECTIONS AND PLAN LEVEL MAPS .................. 26
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Zonge International Inc. Akutan MT 30 November 2012
LIST OF FIGURES
Figure 1 MT Model Viewed in Delivered Geosoft 3D Workspace. ................................. 1
Figure 2 Resistivity fence model from 2D inversions ....................................................... 6
Figure 3 Fence model showing well resolved shallow conductive layers ......................... 6
Figure 4 3D resistivity voxel model view. ......................................................................... 7
Figure 5 Detail view up HSBV of the 3D voxel model . ................................................... 8
Figure 6 3D resistivity model area and section index. ..................................................... 10
Figure 7 Section AA' with 3D model resistivities ........................................................... 12
Figure 8 Section BB' with 3D model resistivities ............................................................ 13
Figure 9 EW section 56 with 3D model resistivities ....................................................... 14
Figure 10 3D model resistivity at -400m elevation ......................................................... 15
Figure 11 Near surface 3D model resistivities. ............................................................... 15
Figure 12 Elevation top of 40 ohm-m conductive layer from 3D resistivity model ........ 17
Figure 13 Conductivity-Thickness to 1 km depth computed from 3D resistivity model. 18
Figure 14 Elevation of base of 40 ohm-m conductive layer. ............................................ 19
Figure 15 Elevation contours on the gravity-based pseudo-basement interface. ............. 22
Figure 16 -1000 m elev. plan resistivities in color w/ gravity basement elev. contours .. 23
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Zonge International Inc. Akutan MT 5 December 2012
Figure 1 MT Model Viewed in Delivered Geosoft 3D Workspace.
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Zonge International Inc. Akutan MT 5 December 2012
INTRODUCTION
Zonge International, Inc. performed gravity and magnetotelluric (MT) surveys on the
Akutan Project, located in the Aleutian Islands, Alaska for the City of Akutan. These surveys
were conducted during the period of 12 August 2012 to 24 August 2012. The survey area is
located in Township 70 South and Range 112 West, and lies within the Unimak (A-6), 15-minute
topographic map. MT data were acquired in and around Hot Springs Bay Valley (HSBV) at 22
receiver sites employing standard orthogonal and multiple orthogonal electric dipole lines
(arrays) which yielded 46 MT impedance tensor soundings at spacing's of 100 meters to 800
meters.
The MT survey was designed to augment and expand coverage to the west of the 52
station 2009 MT survey conducted by Geosystem (GSI).
This report covers data integration, modeling and provides a general description of the
3D model and results. Figure 1 shows the delivered MT model viewed with the Free Geosoft
Viewer in its Geosoft 3D workspace. The mesh surfaces are iso-resistivity outlines interpreted as
representing 1) possible outflow through shallow aquifers, in light orange, defined by 32 ohm-m
(1.5 in Log10); a possible intrusive complex, in dark blue, defined by 180 Ohm-m (2.25 in
Log10). Profile AA from the geologic report and a pair of concept 6000 foot drill trajectories
(red) are shown for reference.
This report is part of Zonge Job number 12140, and refers to the Zonge 12140, 26 Oct
2012, Gravity Survey, Data Acquisition and Processing Report, and Zonge 12140, 26 Oct
2012, Magnetotelluric Survey, Data Acquisition and Processing Report.
MT MODELING OBJECTIVE
The Akutan 2012 MT field survey (46 tensor soundings) and the 2009 MT survey (52
tensor soundings) were designed to support resistivity modeling for geothermal exploration. The
investigation area is delimited by the coverage of the 98 soundings to an area of 4 km by 7 km.
Stations, model extent and section locations are shown in Figure 6
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Zonge International Inc. Akutan MT 5 December 2012
The resistivity analysis objectives are: 1) determination of the extent and conductivity
pattern of a previously identified capping, conductive layer, 2) location of the peak of a 1 km
deep, broad high-resistivity core found at the western limit of the previous survey, 3) definition
of possible hydrothermal outflow plumes in shallow aquifers to differentiate lateral vs. vertical
feeds to hot springs.
The objective of the MT modeling is to provide a 3D map of the morphology, intensity
and spatial relations of the resistivity features of the survey area. This model, accompanying
figures and discussions, are intended to illuminate resistivity morphologic relations as one
component of the integrated resource evaluation conducted by project geoscientist consultants.
Because the modeled resistivities represent volume-averages of in-place formation
resistivities which are controlled by lithology, alteration, fracturing, cementation, porosity, fluid
salinity and temperature, the interpretations can be expected to evolve with the inclusion of
addition data.
DATA PREPARATION, EDITING, AND CONVERSIONS
Data processing, data quality and the merging of impedances from the 2012 and 2009
surveys are discussed in the Zonge 12140, 26 Oct 2012, Magnetotelluric Survey, Data
Acquisition and Processing Report. Merging of the two datasets for use in the Zonge 2D MT
modeling software (program SCS2D), required impedances from both surveys be rotated to a
common x-axis azimuth and interpolated to a common set of frequencies.
For 2D modeling of station fences, x-axis azimuths were aligned to the apparent
geoelectric strike of N60E. TM+TE mode 2D inversions were used to build the 3D voxel model.
The EDI impedance files from the 2009 survey were loaded to the Zonge software for
review of the tensor sounding elements vs. frequency. Impedance elements exhibiting high
frequency to frequency variations were deleted. This type of editing was generally only
necessary in the low-signal-level band between 0.1 Hz to 2 Hz. To allow interpolation to
specific needed frequencies, the Zxy and Zyx curves were individually fit to D-plus layered
responses, and the Zxx and Zyy real and imaginary terms were fit to splines. This produced
modest data smoothing. After this interpolation, the tensors could be rotated to a selected
4
Zonge International Inc. Akutan MT 5 December 2012
orientation with fidelity maintained on all 4 elements. As a quality control measure on the
interpolation process, Z consistency over frequency and between stations was verified by review
of polarization ellipse and phase tensor diagrams on plan maps which are included in the Zonge
Data Acquisition and Processing Report. A similar editing process was applied to Zonge 2012
impedance tensors.
2D INVERSION
The applied hybrid modeling approach constructs a 3D resistivity model by assembling
2D fence inversions which include all of the MT stations. The 2D fence inversions are tuned to
preferentially fit the observed data using a geoelectric environment in which moderately dipping
to horizontal lens or layers may be either continuous or discontinuous between stations.
Adequately representing a moderately layered 3D geologic environment, using 2D inversions
requires staying within the limitations of the data and programs and applying appropriate
geologic and hydrogeologic constraints on the model's structure.
It is advantageous to this study's 2D modeling approach that impedance skews are very
low within the most diagnostic frequency ranges. In one- and two-dimensional geology, the
skew is near zero. Skew values less than 0.2 are generally accepted as indicating soundings
which can be modeled using 2D or 1D methods, while areas with skew values greater than 0.3
indicate measurements near the corners of three-dimensional structure where 2D modeling is not
appropriate. Simpson and Bahr, (2005) argue that while values of 0.2 and greater are indicators
of influence of 3D structure, values of less than 0.2 cannot be used to infer that the geologic
structure is two-dimensional. In this study, the impedance skews remain below 0.2 for
frequencies greater than 0.05 Hz with few exceptions. Only a few stations show skews higher
than 0.2 above 10 Hz.
Uncorrected static offsets have the potential to produce localized, false 3D features from
one or two stations which may not be geologically impossible, but are incongruent with their
surroundings. These artificial features may have the appearance of resistivity pedestals or
conductive diapers. The 2012 and 2009 surveys did not employ separate electrical field
measurements to gauge statics, but instead depended on accommodating static effects within the
data modeling process. This practice is most effective in surveys, like this study, employing
5
Zonge International Inc. Akutan MT 5 December 2012
closely spaced stations. The Zonge 2D inversion code has the ability to assess and correct statics
effects within the inversion process. Testing shows this process capable of identifying and
correcting large static effects produced, for example, from terrain ridges, isolated surface
resistivity patches and even wire-length or gain errors. The Zonge static fitting identified a
small number of soundings which needed significant corrections, these included stations 3050,
3150, 520, and 300.
The model's depth of exploration is limited by the adjacent deep ocean which distorts
the low frequency data. This effect begins to be visible in the N30W directed impedance as a
phase decline initiated at 0.5 Hz to 0.2 Hz and a steady increase in apparent resistivity below 0.1
Hz. Both effects increase in strength with decreasing frequency. Since resistivities in the upper 1
km of the investigation area average between 28 to 125 ohm-m, frequencies of 1 Hz will yield a
model depth of investigation of 2 km, or more, without resistivity distortions from the ocean
effect.
The 2D fence model utilized cell dimensions of 100 meters horizontal and 25 meters
vertical at the surface. The vertical cell dimensions increase at rate of 1.2 per cell downward to a
total active model depth of 13 km. Frequency range used was 0.1 Hz to 10000 Hz. The data
were fit to the combined TM+TE modes with a static adjustment. Relatively strong horizontal
smoothing was imposed on the model to identify layering as opposed to single station resistivity
features. Results from this hybrid TM+TE with static fitting were compared with the 2009 3D
model, a determinant fence, and standard TM and TE sections, and were found to be more
effective at the task of generating a 2D fence system with the desired resolution and stability of
resistivity values across modeling area. The synthesis of a 3D volume model from 2D inversions
is both science and art. All modeling approaches produce visible differences in the models'
details. However, major features in well-executed models should look similar. The 2009 MT
3D model is included in the delivered 3D Geosoft Model for reference and comparison.
Figure 2 shows the fence inversion model results. The fence is clipped at 2 km depth to
remove the deeper, less reliable results from the inversion. Figure 3 shows a detailed view of the
fence model looking southwest, up HSBV, past drill hole TG-2. Vertical exaggeration is unity.
The assembled 2D fence model has defined semi-continuous 3D geologic and hydrogeologic-
like resistivity features in its profiles.
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Zonge International Inc. Akutan MT 5 December 2012
Figure 2 Resistivity fence model from 2D inversions
1.16 1.28 1.40 1.52 1.64 1.76 1.88 2.00 2.12 2.24
MT Model Resistivity
Log10 Ohm-meters
Figure 3 Fence model showing well resolved shallow conductive layers
1.16 1.28 1.40 1.52 1.64 1.76 1.88 2.00 2.12 2.24
MT Model Resistivity
Log10 Ohm-meters
7
Zonge International Inc. Akutan MT 5 December 2012
2D INVERSION TO 3D VOLUME MODEL
Refined spatial interpolation procedures were used to extend the 2D fence models to
continuous 3D voxel models. The procedures maintain the resolution of the shallow layers
between stations and blend the deep resistivities based on the constraining physics. These
procedures were used to create the delivered 3D model. Figure 4 shows the delivered 3D
resistivity voxel produced from interpolation of the 2D fence resistivity inversions. The corner-
cut view looks northwest and shows the yellow-green 32 ohm-m iso-surface which seems to
trace an outflow from the upper hot spring area to the lower valley Figure 5 shows the detail of
shallow horizontal low resistivity layers preserved in the 3D voxel model. Resistivity color
bars are consistent across all report 3D model views, sections, and plan level maps.
Figure 4 3D resistivity voxel model view.
1.16 1.28 1.40 1.52 1.64 1.76 1.88 2.00 2.12 2.24
MT Model Resistivity
Log10 Ohm-meters
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Zonge International Inc. Akutan MT 5 December 2012
Figure 5 Detail view up HSBV of the 3D voxel model .
MODEL RESOLUTION
The delivered 3D resistivity model is provided in Geosoft voxel and ASCII voxel xyz,
value formats, in which the source 2D fence inversion cells have been uniformly resampled to a
spatial resolution of 25 meter vertical and 50 meters horizontal. Elevations are in meters above
mean sea level and horizontal coordinates are in WGS84 UTM Zone 3 meters. Values are
defined from the topographic surface to a depth of 2 km below sea level in the units of Log10
ohm-m. Standard resistivity values in ohm-m are obtained by applying the voxel value as the
exponent of the number 10. Example: Rho = 10**(Log10Res) .
The useful spatial resolution of this model is less than the voxel size due to influences
including: electromagnetic wave physics, survey design, applied inversion constraints.
Electromagnetic physics constrains the vertical resolution for the shallowest and deepest
features in different ways. Just below the surface, vertical resolution depends primarily on the
vertical average of the shallow conductivity (1/resistivity) and the highest frequency impedances
1.16 1.28 1.40 1.52 1.64 1.76 1.88 2.00 2.12 2.24
MT Model Resistivity
Log10 Ohm-meters
9
Zonge International Inc. Akutan MT 5 December 2012
are modeled. Here, survey impedances were recovered at upper frequencies of 250 to 10,000 Hz.
In the low resistivity sediments of the Hot Springs Bay Valley, (10 ohm-m) model resolution will
range between 11 m and 70 meters. And on 40 ohm-m terrain, the minimum vertical resolution
doubles to between 22 and 140 m. At depth, resistivity layers are resolved as bulk conductivity
(1/ resistivity) averages over minimum thicknesses that are proportional to depth. At any depth,
a thin layer with a low resistivity will, at best, be represented in the model with a minimum
thickness of at least one-quarter of the depth, with a corresponding thickness-diluted resistivity.
Features such as domes, ridges and furrows in the deep resistivity structure (conductive
or resistive) of this model will have minimum radii of curvature in any horizontal direction equal
to the depth to the top of the feature.
Horizontal resolution in the model is limited to the station-to-station spacing as defined
over the survey area (100 to 800 meters). This limitation impacts resistivity sensing at depths
shallower than the station spacing. Below that depth, the overlap in sounding data from
neighboring stations begins to bridge the gap. For example, station gaps will under sample the
detail of shallow upflow features.
An assumption of horizontal smoothness of the resistivity structure is applied in the 2D
inversions as the most likely case. This helps the inversion model predict the most likely pattern
of shallow resistivities between stations several hundred meters apart. Where a significant
change in the shallow resistivity structure exists between stations, the model seeks to fit this as a
gradational resistivity change in the shallow unmeasured zone between the stations, regardless of
the presence of a vertical contact.
The interpreter should be considering the possibility of shallow high-angle contracts
between MT stations showing significant change.
MODEL RESULTS
Figure 6 shows the section index map for the 3D resistivity model. This report contains
representative sections from this index. Appendix A contains 21 E-W sections and 18 N-S
sections as enumerated in the figure 6 section index, plus 21 level plan slices at the following
elevations: 400, 350, 300, 250, 200, 150, 100, 50, -10, -25, -50, -100, -200, -300, -400, -500, -
10
Zonge International Inc. Akutan MT 5 December 2012
600, -800, -1000, -1500, and -2000 meters amsl. These maps and sections were sliced from the
delivered 3D resistivity model (ID J4).
Figure 6 3D resistivity model area and section index.
11
Zonge International Inc. Akutan MT 5 December 2012
Figure 9 shows model resistivities for E-W section number 56. This section shows the
shallow and intermediate depth conductive horizons near drill hole TG-2. These are associated
with shades of yellow and orange representing resistivities below 20 ohm-m. The lower
elevation conductive anomaly forms part of the continuous 10 to 40 ohm-m mantle overlying a
zone of intermediate resistivities 50 to 100 ohm-m, which in-turn overlie a resistive core zone
defined by the 160 ohm-m ( ~2.2 log10) contour.
Figure 7 and Figure 8 show the geology report sections AA' and BB' with 3D model
resistivities, respectively. These two sections are identified on the section index map Figure 6 but
appear only in the report as figures.
Note the resistivity color bars are intended to be consistent across all 3D model views,
sections, and plan level maps in this report.
Figure 10 provides a plan view of the sliced 3D model resistivity – 400 meters elevation
(amsl). The intermediate depth resistivity features seen in the Figure 9 section are apparent in
this plan slice. Shown by yellow and green the 40 ohm-m layer (1.6 Log10) is present only in
the northeast, the remainder being above the -400 meter elevation. In the central map area, the
top of intermediate resistivity 50-100 ohm-m above the core resistivity high is exposed. The
strongly expressed northeast trend bounding intermediate and core resistivity features is also
apparent.
Figures 11 through 14 are resistivity theme maps produced from a moderately filtered
voxel model identified in the figure as J2. The filtering allowed a smoother rendering of the 40
ohm-m layer. Figure 11 shows near surface resistivities; Figure 12 shows the elevation of the top
of the 40 Ohm-m layer; Figure 13 shows the conductivity thickness of the upper 1 km below the
surface; and Figure 14 shows the elevation on the bottom of the 40 ohm-m layer.
12
Zonge International Inc. Akutan MT 5 December 2012
Figure 7 Section AA' with 3D model resistivities
13
Zonge International Inc. Akutan MT 5 December 2012
Figure 8 Section BB' with 3D model resistivities
14
Zonge International Inc. Akutan MT 5 December 2012
Figure 9 EW section 56 with 3D model resistivities
15
Zonge International Inc. Akutan MT 5 December 2012
Figure 10 3D model resistivity at -400m elevation
16
Zonge International Inc. Akutan MT 5 December 2012
Figure 11 Near surface 3D model resistivities.
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Zonge International Inc. Akutan MT 5 December 2012
Figure 12 Elevation top of 40 ohm-m conductive layer from 3D resistivity model
18
Zonge International Inc. Akutan MT 5 December 2012
Figure 13 Conductivity-Thickness to 1 km depth computed from 3D resistivity model.
19
Zonge International Inc. Akutan MT 5 December 2012
Figure 14 Elevation of base of 40 ohm-m conductive layer.
20
Zonge International Inc. Akutan MT 5 December 2012
The following are the salient features recognized from the resistivity model.
• Surface and shallow layers:
Bedrock - non-alluvial and non-clay altered surface layers: 60 to 200 ohm-m.
Alluvium saturated with meteoric water: 100 to 200 ohm-m.
Shallow saline saturated alluvium and sediments: 5 to 10 ohm-meters.
Hydrothermal clay alteration: 10 to 30 ohm-m
• Low-resistivity layer:
A 10 to 40 ohm-m low-resistivity unit is present as a nearly continuous layer over the
northwestern 70 percent of the MT model area. Its maximum thickness is ~500 meters and it
averages about 250 m. Its top reaches its maximum elevation of ~520 m at the northwestern
extreme of the MT survey area where it is defined by three MT sites. The layer has rough
antiformal attributes along an east-plunging axis with several irregularities. Northeast of the
main fumarole by 500 to 1000m, it thins or is absent. Generally the 40 ohm-m iso-resistivity
surface lies just 50 to 200 meters beneath the surface but crops-out, exposing lower resistivities,
over about 25 percent of its domain. In the drainages exposing the fumaroles, the low resistivity
40 ohm-m iso-surface is incised, exposing 20 to 30 ohm-m surface resistivities. Similar surface
exposures of the low resistivity unit extend to the west of the fumarole area, outline the hot
springs in the lower valley, and follow the lower elevation contours to a distance of 1300 meters
northwest of eastern lower hot springs.
Conductive outflow plume
This shallow plume is defined by the 30 ohm-m iso surface and appears to travel from the upper
thermal area to the bay.
Intermediate resistivity zone:
Beneath the conductive mantle is a 50 to 100 ohm-m layer, 500 to 1500 meters thick.
Among other possibilities, this is compatible with the represent bulk average resistivity of
unaltered, volcano clastics and flows with low salinity pore waters. Perhaps this is a
temperature controlled resistivity transition.
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Zonge International Inc. Akutan MT 5 December 2012
High resistivity core:
A 1500 meter deep, core area of relatively high resitivities is defined by values exceeding 160
ohm-m ( ~2.2 log10 contour ) .
This core feature is consistent with a transition toward increased percentages of intrusive units
and may represent a dike or sill complex.
Figure 15 shows elevation contours on the gravity-based pseudo-basement interface. The
blue dots are gravity stations and the darker multicolored dots are USGS horizontal-density-sheet
edge solutions. The elevation contour interval is 25 meters, which is linked to an assumed
density contrast of 0.145 g/cc across the pseudo basement interface. The complete Bouguer
reduction density for the source gravity anomaly is 2.3 g/cc, which implies a basement density of
only 2.445 g/cc. To make the basement density more compatible with an intrusive it can be
increased to 2.60 g/cc, if the corresponding elevation relief in pseudo-basement is halved. The
absolute elevation shown on the contours of the density interface is estimated by the depths of
the USGS horizontal-density-sheet edge solutions.
The exploration value of the pseudo-basement resides in its shape and its gradient as
defined edges as indicators of significant scale variations in the basement's relief and/or density.
Figure 16 shows the 3D model resistivities at -1000 m elevation. The high resistivity basement
is in dark blue. The contours of the gravity pseudo-basement elevation are overlain on this map.
The gross match of the shapes of the high resistivity basement and the pseudo-basement
elevation contours increase the odds of that the deep thermal source is associated with intrusive
complex contained within the study area. Additional, the gravity and MT data together indicate
an intrusive feature, perhaps a dike and sill complex, may be encountered in drilling to the -1000
meters elevation (msl).
22
Zonge International Inc. Akutan MT 5 December 2012
WGS 84 / UTM zone 3N250 0 250 500 750 1000 1250
(meters)
City of Akutan Geothermal Project, Alaska
Gravity Survey, August 2012
Contour interval: 25, 100 meters
Zonge International Inc.
Gravity stations
Gravity Pseudo Basement Elevation Pattern Model 1
Assumed densities: Lower Unit 2.445 g/cc, Upper Unit 2.30 g/cc
-1275 -1100 -925 -775 -650 -500 -350 -225 -75 50
Elevation in meters (amsl)599800060000006002000599800060000006002000440000 442000 444000 446000
440000 442000 444000 446000
54°07'30"54°07'30"-165°52'30"
-165°52'30"-1200-1200-1100-1100-1000-1000-900-900-900-800-800-800007--700-700006--600-600005-0
0
5
--500-500-400-400-400004-004--300-3 0 0 -300-300-300-200-200-200-1 0 0 -1000 0
Figure 15 Elevation contours on the gravity-based pseudo-basement interface.
23
Zonge International Inc. Akutan MT 5 December 2012
City of Akutan Geothermal Project, Alaska
Magnetotelluric Survey, August 2012
Zonge International Inc.
WGS 84 / UTM zone 3N
250 0 250 500 750 1000 1250
(meters)
City of Akutan Geothermal Project, Alaska
Zonge International Inc.
MT Resistivity - Model J4 - Plan Level -1000 m (amsl)
Mopdel J4 Resistivty - CI: 0.05 Log10(Ohm-m)
1.2 1.3 1.4 1.5 1.6 1.8 1.9 2.0 2.1 2.2599800060000006002000 599800060000006002000440000 442000 444000 446000
440000 442000 444000 446000
54°07'30"54°07'30"-165°52'30"
-165°52'30"
530
1050 Zonge 2012
MT Station with Tensor ID
GSI 2009
10
20 30
40
60
70
80 90 100
110 120 130 140
150
160
170 180
190
200 210 220
230
240 250
260
270
280
290
300
310
320 330
340
350 360 370
380 390 400 410 420
430
440
450
460
470
480
490
500510
520
530
10501150125013501450155016501750
3050315032503350
4050415042504350
5050
6050
7050
8050
9050
10050
1105011150112501135011450115501165011750
13050
14050
15050
16050
17050171501725017350
18050
19050191501925019350
20050
21050 22050
0.0001--800.0-600.0-600.0-400 .0 0.004--400.0-2 0 0 .0 0.002-0.0
Figure 16 -1000 m elev. plan resistivities in color w/ gravity basement elev. contours
24
Zonge International Inc. Akutan MT 5 December 2012
Submitted,
ZONGE INTERNATIONAL INC.
Gary Oppliger
Sr. Geophysicist
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Zonge International Inc. Akutan MT 5 December 2012
REFERENCES
Ranganayaki, R.P., 1984, An interpretive analysis of magnetotelluric data, Geophysics, 49,
1730-1748.
Simpson, F., and Bahr, K., 2005, Practical Magnetotellurics, Cambridge University Press,
Cambridge, UK.
Zonge Job 12140, 26Oct2012, Gravity Survey on the City Of Akutan Geothermal Project,
Akutan, Alaska for the City of Akutan, Alaska: Data Acquisition and Processing Report,
Zonge International Inc. Reno, Nevada.
Zonge Job 12140, 26Oct2012, Magnetotelluric Survey on the City Of Akutan Geothermal
Project Akutan, Alaska for the City of Akutan, Alaska: Data Acquisition and Processing
Report, Zonge International Inc. Reno, Nevada.
26
Zonge International Inc. Akutan MT 5 December 2012
APPENDIX A: 3D RESISTIVITY MODEL SECTIONS AND PLAN LEVEL MAPS
-1 -----'
MT St ation with Tensor ID
1050 o Zonge 2012
530 l!l GS I 2009
440000
+
10050
0
8050
0
+ 250 0
440000
442000
530
1!1
18050
0
200
[']
270
0
260
[']
340
l!l
5050
0
250 500 750 1000 1250
(meter s )
+
80
+
1!1
350
[']
+
442000
60
90
11050 0
-165'52'30.
10
[']
100
[']
444000
+
20
[']
30
110 120
['] l!l
17q1~1~fu o o
17350 °
160
[']
19350
170 [']
230
19~500 0 0
210 0
['] 220
1!1
31 o+-
290 l!l
280 ill 1150 0 11fffi 8 0
300
[']
11450 0
11550 0
360 ['] 11ffp5g ~
440
430
370
450
•
380
[']
1!1
+
444000
MT Model Resistivity -Plan Level 400 m (amsl)
Model J 4 Res is tivt y -C l : 0 .0 5 Log 10(0 hm -m )
1 .2 1.3 1.4 1.5 1.6 1.8 1 .9 2 .0 2 .1 2 .2
•
446000
40 +
150
130 140 [']
['] l!l
190
180 1!1
[']
•
240 250
l!l
+
320 330
['] 1!1
390
l!l 400 410 420
['] ['] [']
490
480 l!l
460
l!l 470
[']
1!1
500
[']
+
WGS 84 I UTM zone 3N
446000
C ity of Akutan Geotherma l Project, Alaska
Magnetot elluric Survey, A ugust 2012
plotted 26 Nov 12
Zonge International Inc.
440000
+
2 / 10050
0
•
8050 • ~ ~
+ 250 0
440000
MT Sta t io n with Tensor 10
1050 • Zo nge 2012
530 ['] GS12009
-165°52'30.
442000 444000
530 60 10
1':1 eJ
I!]
+ + 18050 • 20 30
eJ [']
~l 90 100 80 ['] [']
eJ 110 120
['] l!l
7~ 17 ~7~50
·• ~'V I__ 172 §b e~ ••
17350
170 ['] 050 ~ ~.~ 160
['] 230 19350
------~ 'If 50 [']
1 ~cf~ 0 8 II
['] 210 &
['] 220
• I!] + 31 o+-270 290 I'J [']
1'1050 . [']
280 ill 1150 0 300
1 ~~?§58 •
[']
340 11550 . 380
['] 11650 . ~ 370 [']
350 360 ['] 11750
[']
450
440
I'J
430
[']
250 500 750 1000 1250 + +
!me!ers j
442000 444000
·165'52'30"
MT Model Resistivity-Plan Level 350 m (amsl)
Model J4 Res istiv ity-C l : 0 .05 Log1 0(0hm-m)
1 .2 1.3 1.4 1 .5 1 .6 1 .8 1 .9 2 .0 2 .1 2 .2
446000
40 +
eJ
150
130 140 I'J
['] l!l
190
180 [']
[']
I
240 250
['] [']
+
320 330
['] I'J
390
I'J 400 410 420 ['] ['] [']
490
480 I'J
460 ['] I'J 470
I'J
500
[']
+
WGS 84 I U TM zone 3N II:
cl
446000 ~
C ity of A kuta n Geothermal Project , Alaska
Magne totellu ric Surve y, August 2012
plotted 26 Nov 12
Zonge lntemationallnc.
\
MT Station with ·r ensor 10
1050 e Zo nge 2012
530 C!l GSI 2009
440000
+
8050 •
+ 250 0
440000
18050
~
()
·165'52'30'
442000 444000
530 60 10
CJ CJ
(!)
+ +
20 30
[!)
90 100
(!] (!]
110 120
(!] (!] 70 ~
~ &'?l]l!!IJ • •• 17350
0
21050 L 9 . ~ --170 Cl ~ 160 ~~ 193~0 230
+ 270
-co." 1a&~5o •• •
210 •
1'1050 .
280 11l1150 e
1 1.fl!£M
290
CJ
[!) 220
300
380
(!]
CJ
(!]
11458 0
11550 •
11650 e
360 (!] 11750 Ill 370 (!]
350
(!]
440
430
CJ
250 500 750 1000 1250
(mete rs)
+ +
t
442000 444000
·165 '52'30'
MT Resistivity -Model J4 -Plan Level 300 m (amsl}
Resis tivity -Cl : 0 .05 Log 10(0 hm -m)
1.2 1.3 1.4 1.5 1.6 1.8 1.9 2.0 2.1 2.2
446000
40 +
l!l
150
140 CJ 130 (!] l!l
190
180 (!)
l!l
240 250
CJ l!l
+
320 330
I!J
390
(!) 400 410 420 (!] [~ I!J
490
480 (!]
[!) 470 CJ
(!)
500
(!)
+
WGS 84 I UTM zone 3N
446000
City of A ku tan Geothermal Project, Alaska
Magneto telluric Survey , Augu st 2012
plotted 26 Nov 12
Zonge International Inc .
440000
+
~
+ 250
440000
MT Station wit h ·r ensor 10
1050 e Zo nge 2012
530 l!l GSI 2009
0
18050
'-..__.)>
530
l!l
270
442000
+
+
l!l
350
250 500 750 1000 1250
(mete rs)
I!J
+
442000
·165 '52'30'
60 10
l!l l!l
90 100
l!l l!l
290
1 '1050 . l!l
280 11J1150 fl
11{-,~<t~
11458 .
11550 •
360 l!l
11650 e ~
11750
t
·165 '52'30'
444000
+
20 30
l!l
110 120
230
l!l
310+
l!l
300
l!l
370
+
444000
MT Resistivity -Model J4 -Plan Level 250 m (amsl}
Re sistivity -C l: 0.05 Log10(0hm-m)
1.2 1.3 1.4 1.5 1.6 1.8 1.9 2.0 2.1 2.2
446000
40 +
l!l
150
140 l!l 130 l!l
190
180
240 250
+
320 330
l!l I!J
400 [!) 410 420 l!l [~ l!l
490
480 l!l
470 l!l
500
+
WGS 84 I UTM zone 3N
446000
City o f A ku t an Geothermal Project, Alaska
Magne t o tellu ri c Survey, August 2 0 12
plotted 26 Nov 12
Z onge Internation al I nc.
440000
+
14050 ,,
+ 250
440000
MT Statio n with Tensor 10
1050 • Zonge 2012
530 [!] GS1 2009
0
530
I!]
18050
250 500 750 1000 1250
(meters)
442000
+
+
442000
-16 5°52 '30.
60 10
lei eJ
290
1'1050 .
280 ill 11500
1~~~§5g .
11550 .
11650 • 360 [!] 11750 ~
·165'52'30"
444000
+
20 30
[!]
300
370
+
444000
MT Resistivity-Model J4 -Plan Level 200 m (amsl)
Resistivity-C l: 0.05 Log10(0hm-m)
1 .2 1.3 1.4 1.5 1.6 1.8 1 .9 2 .0 2 .1 2 .2
446000
40 +
150
130 140 [!]
Cl Cl
180
Cl
240 250
[!]
+
320
410 4:-1
490
480
500
+
WGS 84 I UTM zone 3N
446000
City of Akutan Geothermal Project , Alaska
Magnetotelluric Survey , August 2012
plotted 26 Nov 12
Zonge lntemationallnc.
440000
+
-f -~+ 250
440000
MT Statio n with ·r e nsor 10
1050 e Zo nge 2012
530 C!l GSI 2009
0
530
(!)
18050
270
250 500 750 1000 1250
(meters)
442000
+
(!]
+
442000
·165 '52'30"
60 10
(!J (!J
290
1'1050 .
280 11J1150 tl 1111~'M
(!J
11458 0
11550 •
11650 C!l
360 (!] 11750 Ill
t
·165 '52'30"
444000
+
20 30
(!]
120
(!]
1~go e o 0 17350
170 (!J
230
(!J
• 220
(!)
310+
(!]
300
(!]
370
+
444000
MT Resistivity -Model J4 -Plan Level 150 m (amsl}
Res istivity -C l: 0.05 Log10(0hm-m)
1.2 1.3 1.4 1.5 1.6 1.8 1.9 2.0 2.1 2.2
446000
40 +
l!l
150
130 140 (!J
(!]
180
240
(!J
+
320
[~
490
480 (!]
(!J
500
+
WGS 84 I UTM zone 3N
446000
City of A kutan Geothe rm al P roj ect , Alaska
Mag ne t o t e llur ic S u rvey, Aug u st 20 12
plotted 26 Nov 12
Zon g e International Inc .
440000
+
16~
4050
+ 250
440000
MT Statio n with Te nsor 10
1050 • Zonge 2012
530 CJ GSI 2009
18050
0
70
90 0
0 250 500 750 1000 1250
(meters)
442000
60
lei
+
442000
444000
10
+
30
[']
0
[']
230 19350
~50 [']
1 ~Jl~ • Cl
2 0 Q
0
['] 220
[']
31 ot
290 CJ.
['] 300
[']
370
+
444000
·165'52'30"
MT Resistivity-Mod e l J4 -Pla n Leve l 1 00 m (a ms l)
Res istivity-Cl: 0.05 Log10(0hm-m)
1 .2 1.3 1 A 1.5 1 .6 1 .8 1 .9 2 .0 2 .1 2.2
446000
40 +
[']
150
130 140 [!]
['] [']
190
180 [']
[']
240
+
+
WGS 84 I UTM zone 3N
446000
C ity of Akutan Geothermal Project, Alaska
Magnet ot e llur i c Survey , Aug u s t 20 12
plotted 26 Nov 12
Zonge lntem ationallnc.
0
0
0
N
0
0 co
0 0
0
0
0
0 co
0
0
0 o:>
"' "' "'
440000
+
"Wi -~ -----
-165 '52'30'
442 000
~ ~ 18050
""-J
\0 ....
7()
0)
90~
2 050
520
0 250 500 750 1000 1250 +
60 10
~
22050 ....
<I
(]'
l!l
l!l
19350
_/ l!l 220 1!l
f ---. ~ 290
1 '1050
280 11l1150 e l!l 300
11f.,~'t8
11458 0
11550 •
C,'~"
440 0)
444000
40
l!l -.......
130
l!l
180
i!J
230
l!l
240
l!l
310+
+
150
140 l!l
l!l
190
446000
+
+
490
l!l
500
l!l
+
"' 0
0
"' 0
0
0
"' 0
0
0
0
0
0
"' co co ,
0
0
L-~~==~~:~~=-~44tcooWcoo~----------------------~~---------±~------------~~n-------~W~G~S~8~4~/~U~T~M~z~~o~n:e~3~N~ ____ _do ~.-
MT St atio n with ·r e nsor 10
442000
_165 •52•30•
4440
00 446000 "'
(mete rs)
1050 • Zo nge 2012 MT Resistivity -Model J4 -Plan Level 50 m (amsl)
530 l!l GSI 2009
City of A k utan Geothe rm al P roj ect , Alaska Re sistivi ty -C l : 0.05 Log 10(0hm-m )
Magnetot e llur ic S u rvey , A ug u st 201 2
1.2 1.3 1.4 1.5 1.6 1.8 1.9 2.0 2.1 2.2
plotted 26 Nov 12
Zon g e International Inc .
440000
+
"f[)
/'-....__ ('
1
\
15050
+ 250
440000
MT Statio n wit h ·r ensor 10
1050 e Zonge 2012
530 ['] GSI 2009
18050
~ \
7
'.>
gD
0
Cl
6' 5050
520
250 500 750 1000 1250
(mete rs)
-165'52'30"
442000 444000
0
+ +
t
442000 444000
-165 '52'30"
MT Resistivity-Model J4 -Plan Level -10 m (amsl)
Res istivity -C l: 0 .. 05 Log10(0hm-m)
1.2 1.3 1.4 1.5 1.6 1.8 1.9 2.0 2.1 2.2
446000
+
+
+
WGS 84 I U TM zone 3N
446000
City of A kutan Geothermal Project, A laska
Mag ne t o t e llur ic Survey, A ug u st 20 12
plotted 26 Nov 12
Z onge Internation al I nc.
---
1 6~~ 0 ~~
---/ -----
----.....___,
15050 2065°6' __ _)
M T Station wit h ·r ensor 10
1050 e Zo nge 2012
530 ['] GSI 2009
440000
+
>4050 ' ~
050
·" ~4350
4250
150 ' 40 0
5050
050
520
+ 250 0 250 500 750 1000 1250
(meters)
440000
442000
+
442000
-165'52'30"
444000 446000
+
+
430
I -------
+ +
t WGS 84 I UTM zone 3N
444000 446000
-165 '52'30"
MT Resistivity -Model J4 -Plan Level -25 m (amsl)
City of A kutan Geothermal Proj ect, Alaska Res istiv ity -C l: 0.05 Log10(0hm-m)
Mag ne t o t e llur ic S u rvey, A ug u st 20 12
1.2 1.3 1.4 1.5 1.6 1.8 1.9 2.0 2.1 2.2
plotted 26 Nov 12
Zonge Internation al Inc .
-165'52'30"
440000 442000 444000
+
18050
15050 20050
4350 2
42r.\
4150
1005r:
50
,
~ 5050 ].305 v ~
-f -~
MT St atio n with ·r ensor 10
1050 e Zo nge 20 12
530 ['] GSI 2009
eo 5o[)
520
+ 250
440000
0 250 500 750 1000 1250
(mete rs)
)
+ +
t
442000 444000
-165 '52'30"
MT Resistivity -Model J4 -Plan Level -50 m (amsl)
Re sistivi ty -C l: 0.05 Log1 0(0hm-m )
1.2 1.3 1.4 1.5 1.6 1.8 1.9 2.0 2.1 2.2
446000
+
+
+
WGS 84 I UTM zone 3N
446000
City of A kutan Geothermal P roj ect , Alaska
Magneto t e llur ic S u rvey , Aug u st 2 0 12
plotted 26 Nov 12
Zonge Internation al Inc .
) 0
-f -~
MT Station with ·rensor 10
1050 e Zonge 2012
530 ['] GSI 2009
440000
+
18050
7050
41 0 u 4050 l 260
4050 '
(
+ 250 0
440000
520
()
250 500 750 1000 1250
(meters)
-165'52'30"
442000 444000
430
510 ( \
+ +
t
442000 444000
-165 '52'30"
MT Resistivity-Model J4 -Plan Level -100 m (amsl)
Resistivity-C l: 0..05 Log10(0hm-m )
1.2 1.3 1.4 1.5 1.6 1.8 1.9 2.0 2.1 2.2
460
446000
+
+
' 2 .0 -
48 1~
/;~
+
WG S 84 I UTM zone 3N
446000
City of Akutan Geothermal Project, Alaska
Magnetotelluric Survey, August 2012
plotted 26 Nov 12
Zonge International Inc.
440000
+ 250
440000
MT Statio n with ·r ensor 10
1050 • Zo nge 2012
530 ['] GSI 2009
0 250 500 750 1000 1250
(mete rs)
442000
+
442000
-165'52'30"
444000
+
t
444000
-165 '52'30"
MT Resistivity -Model J4 -Plan Level -200 m (ams l)
Resi stivity-C l: 0 .. 05 Log10(0hm-m )
1.2 1.3 1.4 1.5 1.6 1.8 1.9 2.0 2.1 2.2
446000
+
WG S 84 I UTM z one 3N
446000
City of A kutan Geothe rm al P roj ect , Alaska
Magnetot e llur ic S u rvey , Aug u st 201 2
plotted 26 Nov 12
Zonge Internation al Inc .
440000
+
16050
0
10050
0
13050 0 0
+ 250
440000
MT Station with Te nsor 10
1050 o Zonge 2012
530 ('] GSI 2009
)
0
18050
0
5050
0
250 500 750 1000 1250
(meters )
442000 444000
60
430
510 \
+ +
442000 4440 00
-165"52'30'
MT 3D Resistivity Model -Plan Level -300 m (amsl)
Model J4 Resistiv ity-C l : 0.05 Log10(0hm-m)
1.2 1.3 1.4 1 .5 1.6 1.8 1.9 2 .0 2 .1 2.2
446000
+
+
+
WGS 84 I UTM zone 3N
446000
p lotted 26 Nov 12
City of Akutan Geothermal Project, Alaska
Magnetotelluric Survey, Augus t 2012
Zonge lntemationallnc.
1 6 ~
0
J
440000
+
1750
50
155 (.,
1450 0
1350 0)0 605
2 1A5 1250 0
15050 )r-(l 1150..L '£1
0 7050 ,,, 3350 3250 (;
______./ 3150 C>
30 50 c
~ 10050
1~1050
0
4350
4250 w
4150 0
4050
"
18050
0
" 20
-f--/ /
+ 250 0 250 500 750
(mete rs)
440000
MT St atio n with ·r ensor 10
-165'52'30'
442000 444000
10
430
" ~) 510
1000 1250 + +
t
442000 444000
-165 "52'30'
1050 e Zo nge 20 12 MT 3D Resistivity Model -Plan Level -400 m (amsl)
530 ~ GSI 2009
Model J4 Resistivity-Cl : 0.05 Log1 0(0hm-m)
1.2 1.3 1.4 1.5 1.6 1.8 1.9 2.0 2.1 2.2
460
[!) 470
446000
+
150
+
'110
[~
490
48 1~
0 ( " 500 "
\
+
WG S 84 I UTM z one 3N
446000
City of A kutan Geothermal P roj ect , Alaska
Magnetotellu ric S u rvey , Aug us t 201 2
plotted 26 Nov 12
Zonge Internation al Inc .
~
?
::\
440000
+
16050
0
0
14050
150}0 0050 ' '-
10050
"' __J
4350
4250
4150
4050
18050
0
442000
22050 ~ :
~0 1
444000
+
30
230
+ 250 0 250 500 750 1000 1250
(meters)
+ +
440000 442000 444000
MT S tatio n with Te nsor 10 ·165'52'30"
1050 • Zonge 2012 MT Resistivity -Model J4 -Plan Level -500 m (ams l}
530 CJ GSI 2009
Res istivity-Cl: 0.05 Log10(0hm-m)
1 .2 1.3 1 A 1 .5 1 .6 1 .8 1 .9 2 .0 2 .1 2 .2
446000
40 ~ 6 _) +
150
130 140
190 ~~ 180
240 250
_/' +
420
490
460
1!1
480~
\ 500 !:l'
470
);)
+
WGS 84 I UTM zone 3N
446000
City of A kutan Geothermal Proj ect , Alaska
Ma gne t o te ll uric Surve y , Aug u s t 2 0 12
plotted 26 Nov 12
Zonge lntem ationallnc.
440000
+
16050
<?) 14 0
I
1:il050
-I -:=.....-.j~ / rv
+ 250
440000
MT St atio n with ·r ensor 10
1050 e Zonge 2012
530 ~ GSI 2009
-165'52'30"
442000 444000
--------10 --------~
18050
0 250 500 750 1000 1250
(meters)
+
442000
440
t
-165 '52'30"
+
30
120
300
370
'0
" 450
!
(!)'
+
444000
MT Resistivity -Model J4 -Plan Level -600 m (ams l)
Res istivity-C l: 0 .. 05 Log10(0hm-m)
1.2 1.3 1.4 1.5 1.6 1.8 1.9 2.0 2.1 2.2
446000
40 +
1 0
130
180
240 250
490
481)
t!l' 460
470
Cl
500
(!]
+
WG S 84 I UTM zone 3N
446000
City of A kutan Geothermal Project , A laska
Magnetotellur ic Survey, A ugust 201 2
plotted 26 Nov 12
Z onge Internation al I nc.
440000
+
050
0
1 50 21~050
1:il050
-f -"'-----!~+ 250 0
440000
MT St atio n with ·r ensor 10
442000
60
530 ~
~ 18050
"" (
250 500 750 1000 1250
(mete rs)
510
+
442 000
-165'52'30"
44 4000
+
30
370
450
44
+
t
444000
-165 '52'30"
1050 e Zo nge 20 12 MT Resistivity -Mode l J4 -Plan Level -800 m (ams l)
530 ~ GSI 2009
Re sistiv ity -C l: 0.05 Log1 0(0hm-m )
1.2 1.3 1.4 1.5 1.6 1.8 1.9 2.0 2.1 2.2
446000
40 +
150
190
250
+
500 f!TI
+
WG S 84 I UTM zone 3N
446000
City of A k utan Geothermal P roj ect , Alaska
Magnetotellur ic S u rvey , A ug u st 201 2
plotted 26 Nov 12
Zonge Internation al Inc .
·165'52'30"
440000 442000 444000
+
18050
16050
/
)
380
1•050
450
() 440
430
/\ r-1
r:J:
-I--=-----...._____ + 250 0 250 500 750 1000 1250
(mete rs)
510
(!!
+ +
t
440000
MT St atio n with ·r ensor 10
1050 • Zonge 2012
530 ['] GSI 2009
442000 444000
·165 '52'30"
MT Resistivity-Model J4 -Plan Level -1000 m (amsl)
Model J4 Resi sti vity -C l: 0.05 Log 10(0hm-m )
1.2 1.3 1.4 1.5 1.6 1.8 1.9 2.0 2.1 2.2
40
l!/
18"tr
240
320
390
460
50
330
400
I:J
470
:cJ
150 co
~
\
410
481)
446000
+
+
420
490
500
(!!
+
WG S 84 I UTM zone 3N
446000
City of A kutan Geothermal Project , A laska
Magnetot e llur ic S u rvey , A ug u st 201 2
Z onge Internation al I nc.
-1 65.52'30~
440000 442000 444000 446000
..........
530
+ + + 40 ) +
30 C)
rv
150
120 130 140 'I!!
ll
0
180 1!1
230
l!t
240 250
[!J
+
320 330
300 'I!! Cl
~
380 390
l:!t 400 410 420 [!J 1!1
450
1!1 490
440 480 1!1
C) 460 Cl ·430 t!J 470
'\ C'IJ Cl
~-j_ 500
!510
+ 250 0 250 500 750 1000 1250 + + +
(meters) WGS 84 I UTM zone 3N \1:
cl
440000 442000 444000 446000 ~
MT Statio n with Te n s or 10 -165'52'30"
MT 3D Resistivity Model -Plan Level -1500 m (amsl) plon e<J26 NoY 12 1050 • Zonge 2012
530 CJ GSI 2009
Model J4 Res istivity-C l : 0.05 Log 10(0hm-m) City of Akuta n Geothe rma l Proj ect , A laska
Magnet ot e lluric Survey , Aug u s t 20 12
1 .2 1 .3 1.4 1 .5 1 .6 1 .8 1 .9 2 .0 2 .1 2 .2
Z onge International Inc .
-165'52'30"
440000 442000 444000
+
80
50
44(
-I _L...--...1~-----""-------'----------'+.___ _._+
+ 250 0 250 500 750 1000 1250
(mete rs) ±
MT Statio n wit h ·r ensor 10
1050 • Zo nge 2012
530 ['] GSI 2009
440000 442000
-165 '52'30"
MT Model Resistivity -Plan Level -2000 m (amsl)
Model J4 Resi stivi ty -C l: 0.05 Log 10(0hm-m )
1.2 1.3 1.4 1.5 1.6 1.8 1.9 2.0 2.1 2.2
444000
446000
7 +
150
140
190
/
~50
+
320 33
390
~ 400 410 420 1!1 ~
490
460
t!) 470
481)
'(!!
CJ
500
l!l
+
WG S 84 I UTM zone 3N
446000
C ity of A kutan Geothermal Project , A laska
Magnetotellur ic Survey , A ugust 201 2
Z onge Internation al I nc.
0
0 ..,.
N
0
0
CD
8
CD ,--
0
0
CD
0
0 co
0
0
0
CD
0
0 0
0
0
0 CD
0
0
N
Ol Ol
Ol
l{)
0
0 ..,.
co
Ol
Ol
l{)
-439200 440000 440800 441600 442400 443200 444000 444800 445600 446400
'""
,, ~= ~·' ...... ~~ 2' ~ ~ .. .~ !:!,!
~ ") I, 60 1U [2] bo ~~ .. -a. .. ~~ 530 m l 0 ~; N 80 ~ 0
«;> ' ' ~" ~~ ~76 ~6 --~ ~ v 14U [2]
72 " ,...., ' / ?n 172 / [2] [2] ,..,.. "' u~l 05 ~ Vo 90 [!] 100 fT1
64 0
70 / ICJ 110 120 13(
""'
~ [2] [2] c: [!] 60
1 05~ / 17~7~~~1 56 ~On ~
\ 9050 1 350 ~ .. ,.. ' 0 L _.,, '"" 22050 47n b 180
'OJ~ ~ / 0 160
L:.J
48 1\ ~ n-
h\ v .• w193s
iJ
44 1N~0 / 50 [2]
1550 ,C \ /0 21ri
9
l
15o
00 P"'
450 0 CJ ' 24
40 125 ov bU!>\ v ... '--'
"AS 50 210050 [!] 1150 n'
>JV <.) 'IUOU ( ~ 32so lr.o /o~ I~ 270 310
31-so 290 [2] 'l? 32
28
24
"'"' .....
16 1~1050
12 c
-f ~-8 ..
'1'
0
C) C»
439200
MT Sta ti o n with ·re nsor 10
1050 O Zonge 2012
530 [!] GSI 2009
m
3050 'c / ~ 1 1oso q 9 . 0 280 ID150 c:J 300
~ 425ou o ,·.;n r-.· 26~ 'n{(~ S...~Qr-3Rn
/40 0 0 c 11650 0 ~ 370 CJ
0 1\ 3!in 360 CJ 11750
100 lP"o \ CJ
'"'
0,/
~u.
0 \ I CJ \
805 Po \ •u
CJ
l'ln
:>; fU CJ \ CJ
\ !>' IU CJ
c:~ N ~~ ~ ~~ ~ ~ ·~ ...... ~~ C» ~~ ~ 0 N C»
\
-y
i
440000 440800 441600 442 400 443200 444000
MT 3D Resistivity Model Index Map for EW and NS Sections
Red outline shows model extent
[2]
390
460
..
0
~
.,.. r I
flU /· 150 ~4 LJ
[2]
~0
' ~6 1liU [2] ,,.. ~· ~~-~· ~8
~4
2p0 m ~0 ,..
•v
~~1\ ~2 ..
c:J ~
2
2 ICJ 14UU CJ 410 420
[~ rh ,., ....
1
49p
4810 '--' 1
cb m .... 8 CJ
<:nn .. p I""
0
-a. .. ~~ ..
~; N w
0 i:~ 0') ~
444800 445600 446400
WG S 84 I U TIIA zone 3N
plotte d 2:6 Nov 12
Ol
0
0
"' ~
0
0
Ol
0
~
Ol
0
0
Ol
0
0
0
CX>
0
0
Ol
0
0
0
0
0 0
()l
CD
CD
CD "' 0
0
()l
CD
CD
CX>
~
0
0
City o f Akutan Geot herma l Proj e ct , A la s ka
250 0 250 500 750 1000 1250
(meters )
Mag netote llu ric S urv ey, Au g ust 201 2
Z onge Int erna tio n al Inc.
w
440000 .
Vert1cal/ Horizontal : 1
2f>O 2f>O 50(1
440500 «=1~s~oo~--~4~42~p~oo~--~4~4~~sooa-__ ~4=43~
MT Resistivity -EW Section 0
Model J4 Resistivity-Cl: 0 .05 Log10(0hm-m)
1.2 1 .3 1.4 1.5 1.6 1.8 1.9 2 .0 2 .1 2.2
44400Q
E
City of Akuta n Geothe rm a l Project, Al ask a
MT Survey, August 2012
Zonge International Inc.
plotted 26 Nov 12
w
439000 439500 440000
Vertica l / Horizontal : 1
250 250 500
,__,.)
~S .. IUf~L'OM»>
440500 441000 441500 442000 442500 443000 443500
+
MT Resistivity -EW Section 4
Model J4 Resistivity-Cl : 0.05 Log10(0hm-m)
1.2 1.3 1.4 1.5 1.6 1.8 1.9 2 .0 2.1 2 .2 .,
444000
E
444500 445000 445500 446000
+ +
'-I 9 ---+
+ ~-
2 0 ---
Northing : 6008400 m
City of Akutan Geothermal Project, Alaska
MT Survey, August 2012
Z onge International Inc.
plotted 26 Nov 12
w E
439000 439500 440000 440500 441000 441500 442000 442500 443000 443500 444000 444500 445000 445500 446000
+ + + + + -t--t
19 ~
2 0----
Northing : 6008600 m Essting in Meters
~~----~~r---.m~r---~~w----~o~4~4~1&N~----~44'~2~~----~44"<2~~Mr--~4~43mbNoor---•4D4~~~oow----4n4m~M~·~44~&N~--~4°4~~~----4~4~~~o~o----T44~ro·ow---~
Vertica l / Horizontal : 1
250 250 500
~S .. IUT~lOM!H
MT Resistivity -EW Sec tion 8
Model J4 Resistivity-Cl: 0 .05 Log10(0hm-m)
1.2 1.3 1.4 1.5 1.6 1.8 1.9 2.0 2.1 2 .2 .,
City of Akuta n G eothe rm a l Proj ect , Alas ka
MT Survey, August 2012
Z onge Intern ational Inc.
plotted 26 Nov 12
w
439000 439500 440000
+ +
Vertica l / Horizontal : 1
250 250 500
~
(-...)
m1S .. 1Uf~L'OM~
440500
+
441000
+
441500 442000 442500 443000 443500
+ + +
MT Resistivity -EW Section 12
Model J4 Resistivity-Cl : 0 .05 Log10(0hm-m)
1.2 1.3 1.4 1.5 1.6 1.8 1.9 2 .0 2.1 2 .2 .,
444000
+
E
444500 445000 445500 446000
+ -t--t
2 .o -
Northing : 6008800 m
City of Akutan Geothermal Project, Alaska
MT Survey, August 2012
Z onge International Inc.
plotted 26 Nov 12
w
440000 440500 441000
Vertical / Horizo ntal : 1
E
441500 442000 442500 443000 443500 444000 444500 445000 445500 446000
..
---1 .9 --
.. +
1 9::::::------------------------------------2 .0 ~
~
~~
r
MT Resistivity -EW Section 16
Model J4 Resistivity -Cl : 0 .05 Log10(0hm-m)
1.2 1.3 1.4 1.5 1.6 1.8 1.9 2.0 2.1 2.2 .,
+
1 .9 -
o -
City of Akuta n Geothe rm a l Proj ect , Alas ka
MT Survey, August 2012
Zonge International Inc.
plotted 26 Nov 12
w
439000 439500 440000
+
Vertica l / Horizontal : 1
250 250 500
~S .. IUT~lOM»>
440500 441000
+ +
441500 442000 442500 443000 443500
t +
MT Resistivity -EW Section 20
Model J4 Resistivity-Cl: 0.05 Log10(0hm-m)
1.2 1.3 1.4 1.5 1.6 1.8 1.9 2 .0 2.1 2 .2 .,
444000
E
444500 445000 445500 446000
+ t t
+
1 .9--
City of Akuta n G eothe rm a l Proj ect , Alas ka
MT Survey, August 2012
Z onge Intern ational Inc.
plotted 26 Nov 12
w
439000 439500 440000
+
Vertica l / Horizontal : 1
250 250 500
~S .. IUT~l'OI'It!H
440500
+
441000
+
441500 442000 442500 443000 443500
+
MT Resistivity -EW Section 24
Model J4 Resistivity -Cl: 0 .05 Log10(0hm-m)
1.2 1.3 1.4 1.5 1.6 1.8 1.9 2.0 2.1 2 .2 .,
444000
E
444500 445000 445500 446000
-j-t t
~
(ii
E
~
(/)
(j;
Q)
~
c + -j-.Q
~
Ill w
2 0---
City of Akuta n G eothe rm a l Proj ect , Alas ka
MT Survey, August 2012
Zonge International Inc.
plotted 26 Nov 12
w
4~"""--_:!.4.39~00 440000 .
Vert1cal/ Horizontal : 1
2f>O 2f>O 500
440500 44100Q 441500 442000
'
MT Resistivity -EW Section 28
Model J4 Res istivity-Cl: 0 .05 Log10(0hm-m)
1.2 1 .3 1.4 1.5 1.6 1.8 1.9 2 .0 2 .1 2 .2
.;
. I
E
~ ti)
E
2-
~
Q)
~ (i)
::2:
c ... t 0
~
1 . 9 ---..,__ Q)
~ w
~ 2 -0-------
\
City of Akutan Ge othermal Project, Alas ka
MT Survey, August 2012
Zonge International Inc.
plotted 26 Nov 12
w
4~.,.,._ __ ""4.39~00 440000 .
Vert1cal/ Horizontal : 1
2f>O 2f>O 50(1
440500 44 100Q
'T"
E
«Jsoo,__"""'4""42,..9""oo"-_ _;4""4"'~~soo __ 443,.000..,._ _ ____.4~4~.,.soo""'-----'4"'4~400Q....,_ ___ ._.
+ .,. t + +
MT Resistivity -EW Section 32
Model J4 Res istivity-Cl: 0.05 Log10(0hm-m)
1.2 1 .3 1.4 1.5 1.6 1.8 1.9 2 .0 2 .1 2 .2
+ T 't
~
~ ti)
E
~ 2-
~
Q)
~ Q)
::2
c 0
~
Q)
~ w
City of Akutan Ge othermal Project, Alas ka
MT Survey, August 2012
Zonge International Inc.
plotted 26 Nov 12
w
440000 . 440500
MT Resistivity -EW Section 36
Model J4 Resistivity-Cl: 0.05 Log 10(0hm-m)
1.2 1 .3 1.4 1.5 1.6 1.8 1.9 2 .0 2 .1 2 .2
E
City of Akuta n Geotherm a l Project, Al ask a
MT Survey, August 2012
Zonge International Inc.
plotted 26 Nov 12
w
439000 439500 440000
Vertica l / Horizontal : 1
250 250 500
~S .. IUT~lOM!H
440500 441000 441500 442000 442500 443000 443500
+ +
MT Resistivity -EW Section 40
Model J4 Resistivity-Cl : 0.05 Log10(0hm-m)
1.2 1.3 1.4 1.5 1.6 1.8 1.9 2 .0 2 .1 2.2 .,
444000
E
444500 445000 445500 446000
+ +
t
+
+ +
Northing : 6000200 m
City of Akuta n G eothe rm a l Proj ect , Alas ka
MT Survey, August 2012
Zonge International Inc.
plotted 26 Nov 12
w E
439000 439500 440000 440500 44 1000 4 41500 442000 442500 443000 443500 444000 444500 44 5000 445500 446000
+ + + + -t-+ -t-
-----
t
1 .7 ......___
+
' 1 •
"\ +
Northing : 6000400 m WGS 84 1 UTM zone 3N casting in Meters
~nMr---•4M3~9~~----74~4o~~Nr----4n4~d~~0~4"4~1~~----74~4·2m~~----r.44'~2~~~----4D4~wmoo~--~44~3~so~o~--~4"~~oom~'~44~~~----~44~s~~Mr--~4"4~~~o~o----74~4ro~,o~--_J
Vertica l / Horizontal : 1
(-...)
m1S .. IUfMlOM»>
MT Resistivity -EW Section 44
Model J4 Resistivity -Cl : 0 .05 Log10(0hm-m)
1.2 1.3 1.4 1.5 1.6 1.8 1.9 2.0 2.1 2.2 .,
City of Akutan G eotherm a l Project , Alaska
MT Survey, August 2012
Z onge Intern ational Inc.
plotted 26 Nov 12
w
47='"""---""4_.39~00 44Q,OOO 440500 441 00Q 441500 442000
'
442500
'
443QOO 443500
MT Resistivity -EW Section 52
Model J4 Resistivity-Cl: 0 .05 Log 10(0hm-m)
1.2 1 .3 1.4 1.5 1.6 1.8 1.9 2 .0 2 .1 2.2
44400Q
E
City of Akut a n Geotherm a l Project, Al ask a
MT Survey, August 2012
Zonge International Inc.
plotted 26 Nov 12
w E
439000 439500 440000 440500 44 1000 4 41500 442000 442500 443000 443500 444000 444500 445000 445500 446000
+ -t-+
I 9 ~
(ij
t
E
~
(/)
(j;
Q)
~
c
.Q
~
Ill w
+
WGS 84 I UTM zone 3N £esting in Meters Northing : 6001000 m
~~----~4~39~~Mr---.40040~ooo~--~4~4d~~0~4~4~1~~----~44'~2ooo~----~44"<2~~Mr---,4~43muNoo~--·4~4~~~oo~---4n4~~M~'T.44~~~--~4°4~~~----4~4~~~o~o----~44~ro,o~--~
Vertica l / Horizontal : 1
250 250 500
~ ,_...)
m1S .. 1Uf~L'OM»>
MT Resistivity -EW Section 56
Model J4 Resistivity-Cl : 0 .05 Log10(0 hm-m)
1.2 1.3 1.4 1.5 1.6 1.8 1.9 2 .0 2 .1 2.2 .,
City of Akutan G eotherm a l Project , Alaska
MT Survey, August 2012
Z onge Intern ational Inc.
plotted 26 Nov 12
w
439000 439500 440000
Vertica l / Horizontal : 1
250 250 500
~
(-...)
m1S .. 1Uf~L'OM»>
440500 441000 441500 442000 442500 443000 443500
+ +
MT Resistivity -EW Section 60
Model J4 Resistivity-Cl : 0.05 Log10(0hm-m)
1.2 1.3 1.4 1.5 1.6 1.8 1.9 2 .0 2.1 2 .2 .,
444000
E
444500 445000 445500 446000
+
t
+
+
Northing : 6001200m
City of Akuta n G eothe rm a l Proj ect , Alas ka
MT Survey, August 2012
Z onge Intern ational Inc.
plotted 26 Nov 12
w E
439000 439500 440000 440500 441000 441500 442000 442500 443000 443500 444000 444500 445000 445500 446000
"t" + + "t" +
~
(ij
t
E
~
(/) a;
Q)
~
c
.Q
~
Ill w
\ +
WGS 84 I UTM zone 3N Eesting in Meters Northing : 6001400 m
~~----~4~39~~Mr---.40040~~~--~4~4d~~0~4~4~1&N~----~44'~~~----~44"<2~·&NMr--~4~43muNoo~--·4~4~~~oo~---4n4~~M~'T.44~&N~--~4°4~~~----4~4~~~o~o----T44~ro,o~--~
Vertica l / Horizontal : 1
250 250 500
~
m1S .. IUWlOMIH
MT Resistivity -EW Sec tion 64
Model J4 Resistivity-Cl : 0.05 Log10(0hm-m)
1.2 1.3 1.4 1.5 1.6 1.8 1.9 2 .0 2 .1 2 .2 .,
City of Akuta n G eothe rm a l Proj ect , Alas ka
MT Survey, August 2012
Zonge Internat ional Inc.
plotted 26 Nov 12
w
440000 . 440500 441 00Q 441500 442000 .
MT Resistivity -EW Section 68
Model J4 Resistivity-Cl: 0 .05 Log10(0hm-m)
1.2 1 .3 1.4 1.5 1.6 1.8 1.9 2 .0 2 .1 2.2
E
City of Akuta n Geotherm a l Project, Al ask a
MT Survey, August 2012
Zonge International Inc.
plotted 26 Nov 12
w
439000 439500 440000
Vertica l / Horizontal : 1
250 250 500
~
440500 441000 441500 442000 442500 443000 443500
+ +
MT Resistivity -EW Section 72
Model J4 Resistivity-Cl : 0.05 Log10(0hm-m)
1.2 1.3 1.4 1.5 1.6 1.8 1.9 2 .0 2.1 2.2 .,
444000
E
444500 445000 445500 446000
-t--t-
+
Northing : 6001800 m
City of Akutan Geothe rm a l Proj ect , Alaska
MT Survey, August 2012
Zonge International Inc.
plotted 26 Nov 12
w E
~'---'4""40'1l',Q""00<--_.....:4u4,.,050Q..,._ _ __,4ft000 __ 441500 __ 4429,90 ___ 442,.500 __ 443000 __ ~443.,.50Q,.,._ _ __;,4..._44,.9QO""'------'
t t 't
t t
t +
+ +
WGS 84 1 UTM zone 3N Eestlng In Meters Northmg : 6002000 m
~~r--~•9~5~our--~4~4moo•"oo~-~4~4~o~~--~44"1N~~-~44~1~5w~r--~4~42~o~oo~-~4~4~25'"wrr--~44nJN~n--~44~3~~mr-~4~«moornor--•«~45~o~o--=«®Wr--~4~45~~~-~«n=&NO~--~
Vertical/ Honzonlal : 1
:M :M «>0
MT Resistivity -EW Section 76
Model J4 Resistivity-Cl: 0 .05 Log 10(0hm-m)
1.2 1.3 1.4 1.5 1.6 1.8 1.9 2 .0 2 .1 2.2
City of Akutan Geotherm a l Project, Al ask a
MT Survey, August 2012
Zonge International Inc.
plotted 26 Nov 12
w
Vert1cal/ Horizontal : 1
2f>O 2f>O 500
'--""44:u..~,soo __ 443000 443500
MT Resistivity -EW Section 80
Model J4 Resistivity-Cl: 0 .05 Log10(0hm-m)
1.2 1 .3 1.4 1.5 1.6 1.8 1.9 2 .0 2 .1 2.2
444QOO
E
tii
E
+ ~
~
Q)
(i)
~
c + t 0
·~
Q)
jjj
City of Akuta n Geothe rm a l Project, Al ask a
MT Survey, August 2012
Zonge International Inc.
plotted 26 Nov 12
439200 440000 440800 441600 442400 443200 444000 444800 445600 446400
~ , ~ '-~ ~~)~~ ~ :~ ~~' S!;l ~,) d ,_::. ~~ ~ g
§r--....:...~~-t---=_~~-----':-'.)-=--i:l•-==-,~-:;=:;~'1.:;-;;/.~-t--;--=-8'-0---ri-_,+-:5_~3-0--l~m-=-:~-=--:_6~0-=flllF•l-=---=--=-::-=-~,,~,u--=[']:~-=---=-,--=:l<y~'-~8-_0-_-:~f-~-------~~~----_-_-=~:;~~~:~:~~~~-...,·-j;~~---~--t---t'-l§
1' ~" _ .," ~ ~., ~76 l -t---+---tv-r-~-'---t----'--1 -+--''+---+---tt14u"[']--+---+--~6 r
g ~~f-~~~~=:~~"~~~~==~~~~~::::~====~~~~.~~~~~~=~--+.--~~~--~=7/~ _____ ,_ ____ +-~?)~10[']~----~~f-[']--~-----+--~-r----_ ~~ c ~
8 u ~, 05 ""' I / 90 [1] 100 _ {'so ~g ;:::~"1[1 ?3
<O 64 0 ""' 18'0 ,,, u ~4 r'J; o I 70['] / l..CJ 110 120 13( , .. ['] f"' ~F 6o ~·~------~---+~~--+---~~~~~~r-+-+----~----~---~==~[']+-~~~---~~---~---~--~o ~m
1 05~ / 1 17~7~1~'~1 ·' f" ~~~
8 1 ~6 \ 9050 , 1 3~{1·1-~ b l ~U ~ ; . ~6 ~~ ?3 ~~---~ .. 9Y·""~-----~'-----+----~--~O~----+---~~-M·~~·~n·~·~n .~_2_2o_5~·~~~----+-~--~----~~·~ll4"~~1~~~r---~---4~~~~-HY'""~-~A~.~~~~8
g ~;l-+--~1~~~·~--~~~\--+---~~/-+---04--'~+-~4----+--16_orr~--~~~--~~~.~~-·4-~{~ ;; ~ g
44 1H~o h \ L/ ~ .• w1~35 ,... lJ ~~ ~ ' ~4 '5)·~~~-
1550 ,C \ ,1.10 ['] 19 rou 0 P'"' 2 ~~0 I"'' 450 0 / , 210 0 24 ~· m
o 40 125 O_v t>U:>l\v "~I!! :e.. ....., ~0 OJ
o .-.A S 50 20050 1150 n" '< II• .-I .._C!J -~ o 8~--~~~~--~~~~~~~----~----+-----~--~~~_,·-----+----~----~----~----+-----~--~~----H~A~----~--~8 8 -.IV U "JU::>U (~ 32S~ Q /1\ 270 290 -' 310 ' •u 8
<0 3f5o 1'1""'1' hodo\ m , ['] 'l? .,.,,. . !). ..a. 0 32 •~---~-+----~J~o~so~0~_c~~ /~~~~--+---=-~--~~----~1 '~1 o~5Aoo~~~~~~-+-----r~~~[']----~~~~----.,.~_.2 __ ~~~~
0
0
N
8l
OJ
10
L _ --"' 280 [Jt 15~9, ['] 3oo 28 ·~----4~--~~~~4-~~---+~--4--~+~~~--~---~----+--~~--+----+---~--~--2
24 ~--·---+--~4---~~~~~2~5~0~~v~~04--2-6~~~~~-----+----4---~-~~4}1~~j~~2~Q·,~~~---+---~31:R~n~--+3~9~0 ~~mr--+-----~--_,--
/40 0 00 n 11650 0 n 370 ['] L.:..l 14UU ['] 410·•[n _ 420 rh ,.,.. : 1\ ~ 350 360 ['] 11750 ~ "'" •
•v 100 lit o \ [']
16 ~-----4---~~~-r--~ffi----+~--+----+----4----4--~~~--~ .. ~·--~-'--+----+--'--+----~----4---1 ~1050 ( (>o, / ~v, 0 \ i ['], ( 49p
2 ~
<0
.... <0 ... ~
1
0
0
12 ~-----~~~----80-)5rp----~---+--~\~--~+-~-+----4-~.vr~~~--r---~~--+----+--=~+-~48ci~0 4---~-R-~ 1
o --f~--8 ~----~-----+-----r~o~_,----~~J~r~----~~~~~·'lw_n __ +-----~----~--_,--~-+4_6_o ~m -r~·~~"~----,cb~---+--8 ~ ~ :><fU ['] \ ['] ['] 1 <0
~ .t o:nn .t g; ~ .,. \ :,· 1u ~ I ' I p ~ g
o c:;» oo c:~ ~ ~~ ~ ~~ ~ ~ ·~ jj -'~~· : ~~
439200
MT St atio n wit h Tensor ID
1050 0 Zonge 2012
530 [I] GSI 2009
""'?
\ ' I
440000 440800 441600 442400 443200 444000
MT 3D Resistivity Model Index Map for EW and NS Sections
Red outline shows model extent
250 0 25 0 500 750 1000 1250
(meters)
..a.
N
0
444800 445600
WGS 84 1 UTI'vf zone 3N
plotted 2:6 Nov 12
446400
Ci t of Akutan Geo t hE~rma l Proj ect, A laska
Mag netotelluric Survey, Aug ust 2012
Zonge In ternational Inc.
s
5998000 59918 500 5999000 5999500 6000000 6000500 6001000
+
~
6001 ~i00
+
WGS 84 1 UTM zone 3N Northing in Meters
Vertica l / Ho rizontal: 1
250
!iijji!
0 250
(meters)
V\IGS 84 I UTIW zone 3N
500
5999000 5999500 bOOCIOOO 6006500 6001000 bOOb
MT Resistiivity -NS Section 16
Model J4 Resistivity -Cl: 0 .05 Log1 O(Ohm-m )
1.2 1.3 1.4 1.5 1.6 1.8 1.9 2 .0 2.1 2.2
I I
6002000
+
+
+
+
+
+
N
6002500
+ ~ 0
+ 0
,......,.
Cii
E ca '-+ ~ ~
0 (!) a:;
~
c:
0
·~ + 0 > 8..92
UJ
' + c:n 0
0
' N + 0 0
0
City of Akutan Geothermal Project, Alaska
MT Survey , August 2012
Zonge International Inc .
p lotted 26 Nov 12
s
5998000 59918500 5999000 5999500 6000000 6000500 60 01000 6001 ~;oo
1 .a___..-
+'----
WGS 84 / UTM zone 3N Northing in Meters
Vertical/ Horizontal: 1
250
ijiijji!
0 250
(meters)
Y\IGS 84 I UTIW zone 3N
500
5999000 5999500 600CIOOO 6006500 6001000 6001 t>
MT Resistiivity -NS Section 16
Model J4 Resistivity -Cl: 0 .05 Log1 O(Ohm-m )
1.2 1.3 1.4 1.5 1.6 1.8 1.9 2 .0 2.1 2.2
I I
6002000
+
+
+
+
+
+
N
6002500
+ ~ 0
+ 0
,......,.
Cii
E ca '-+ ~ ~
0 Q) a:;
:::2:
c:
0
~~ + 0 > 8..92
UJ
' + c:n 0
0
' N + 0 0
0
City of Akutan Geothermal Project, Alaska
MT Survey , August 2012
Zonge International Inc .
p lotted 26 Nov 12
s
0
0
0
U')
I
0
~
I
0
0
U')
0
0
0
N
I
5998000 5998500 5999000 5999500 6000000 6000500 6001000 6001500
WGS 841 UTM zone 3N Northing in Meters
Verti cal I Horizontal: 1
250 250 500
(meters)
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Appendix 7
Akutan Geothermal Resource Assessment
Commissioned by
City of Akutan, Alaska
As part of its
Geothermal Development Project
June 2011
Principal Investigator:
Amanda Kolker, AK Geothermal
Other Investigators:
Bill Cumming, Cumming Geoscience
Pete Stelling, Western Washington University
David Rohrs, Rohrs Consulting
Akutan Geothermal Resource Assessment
2
Contents
Summary ........................................................................................................................................................................ 3
Objectives of Study ........................................................................................................................................................ 3
Introduction ................................................................................................................................................................... 4
Background and Previous Studies ................................................................................................................................. 4
Geologic Setting ........................................................................................................................................................ 5
Geothermics .............................................................................................................................................................. 7
MT Resistivity ............................................................................................................................................................ 7
New Data 2011 .............................................................................................................................................................. 8
1. Temperature Gradient Data ............................................................................................................................. 8
1a. Core Hole Drilling ............................................................................................................................................ 8
1b. End-of-Well Logs ............................................................................................................................................. 9
1c. Equilibrated TG Logs ...................................................................................................................................... 10
1d. P/T Data Analysis .......................................................................................................................................... 13
2. New fluid chemistry and geothermometry .................................................................................................... 15
2a. Sample Collection and Data Sources ............................................................................................................. 15
2b. Chemistry ...................................................................................................................................................... 15
2c. Geothermometry .......................................................................................................................................... 15
2d. Geochemical Model ...................................................................................................................................... 16
3. Core data ........................................................................................................................................................ 16
3a. Overview ....................................................................................................................................................... 16
3b. Rock Types and Primary Mineralogy ............................................................................................................. 17
3c. Secondary Mineralogy, Mineral Paragenesis, and Hydrothermal History .................................................... 17
3d. Permeability and Porosity of Well Rocks ...................................................................................................... 20
Resource Conceptual Models ...................................................................................................................................... 21
Future Drilling Targets ................................................................................................................................................. 25
Capacity Assessment ................................................................................................................................................... 27
Resource Existence and Size ................................................................................................................................... 27
Confidence in Resource Existence ........................................................................................................................... 27
Probable Resource Capacity .................................................................................................................................... 28
An alternative approach .......................................................................................................................................... 28
Monte Carlo Heat-in-Place Option .......................................................................................................................... 28
Resource Risks ............................................................................................................................................................. 29
Upflow Development Risks ..................................................................................................................................... 29
Outflow Development Risks .................................................................................................................................... 30
Conclusions .................................................................................................................................................................. 30
Recommendations ....................................................................................................................................................... 31
References and Bibliography ....................................................................................................................................... 32
Akutan Geothermal Resource Assessment
3
Summary
The Akutan geothermal resource can be conceptualized as containing two major zones: an upflow zone
and an outflow zone. While the outflow and upflow zones likely represent one interconnected field,
they are distinguished here for the purposes of development. The upflow zone temperatures could
approach 572 °F (300 °C), and the reservoir probably consists of a brine liquid overlain by a small steam
cap. The outflow zone temperatures are lower, decreasing as the brine flows eastward. Fluids produced
by corehole TG-2 show evidence of chemical re-equilibration to lower temperatures, with cation
geothermometry providing a range from 392-464 °F (200-240 °C). The outflow fluids become
extensively mixed with cooler meteoric waters near the surface hot springs.
Alteration mineralogy in exploratory coreholes suggests two disappointing conclusions about the
outflow system: (1) the rocks in both TG-2 and TG-4 were at temperatures greater than 469 °F (250 °C)
in the geological past and have cooled to present temperatures; and (2) the part of the outflow
encountered by the wells appears to lack sufficient thickness and permeability to support commercial
development. Additionally, development of the shallow outflow would entail significant risk of rapid
cooling during exploitation as a result of either cold water influx from near-surface aquifers or injection
breakthrough. Exploratory corehole drilling encountered the outflow zone with fluid temperatures of
359 °F (182 °C) at shallow depths of 585’ (187 m). Recent data suggests that the 359 °F (182 °C) zone
produced in TG-2 is drawn from a nearby fault zone not located directly below the well. Although it is
possible that a hotter resource may exist slightly deeper than either of the current wells, this is unlikely
to be the lowest risk target for development.
Although TG-2 encountered the outflow predicted near its location, the two exploration coreholes did
not demonstrate an outflow resource that would be suitable for development. Given these drilling
outcomes and results of new gas geothermometry from the fumaroles, a well targeted to cross the 1500
ft2 (0.5 km2) fumarole field would have the highest probability of encountering commercial production
at Akutan. This target is likely to be >428 °F(>220 °C) and could be as hot as 572 °F (300 C). The depth to
the target will depend on the elevation of the drill pad but it is likely to be greater than 4000’ (1300m).
An important issue is the trade-off between the cost and practicality of constructing a pad closer to the
fumarole and drilling further directionally. A 380-428 °F (180-200 °C) outflow resource target about
2200’ (800 m) to the northwest of TG-2 might be preferred if its higher targeting risk and lower
generation per well were sufficiently offset by lower drilling and access cost.
Objectives of Study
This study has three primary objectives: (1) to report on data collection efforts for the Akutan
geothermal resource to date; (2) to provide the technical parameters needed when assessing the
feasibility of developing the geothermal resource for a combined heat-and-power project envisioned by
the City of Akutan and other stakeholders; and (3) to provide well targets for future drilling efforts. This
report synthesizes all the datasets collected on the Akutan geothermal field to date (listed on p. 4), and
provides an updated assessment of the Akutan geothermal resource based on all available data. Well
targets and recommendations for mitigating resource risks are given.
Akutan Geothermal Resource Assessment
4
Introduction
Akutan Island is located 790 mi (1271 km) southwest of Anchorage and 30 mi (48 km) east of Dutch
Harbor. The Island is home to North America’s largest seafood processing plant. The City of Akutan and
the fishing industry have a combined peak demand of ~7-8 MWe which is currently supplied by diesel
fuel. In 2008, the base cost of power in the City of Akutan was $0.323/kWh (Kolker and Mann, 2009).
Since 2008, the City of Akutan has led exploration and other assessment activities in an effort to
determine the feasibility of geothermal development on the island. The 2009 exploration program
included practical access assessments, a geologic reconnaissance field study, soil and soil gas
geochemical surveys, a remote sensing study using satellite data, a review of existing hot springs
geochemistry data, a magnetotelluric (MT) survey, and a conceptual model analysis. The 2010
exploratory drilling program included the drilling of slim-hole temperature gradient (TG) wells, fumarole
sampling, and chemical analysis of well and fumarole fluids. Follow-up production-size wells are being
planned for the near future.
Background and Previous Studies
As a volcanic island with accessible hot springs, Akutan has been the subject of geothermal resource
studies since 1979. The original exploration effort was limited to the immediate hot springs area and
included geologic mapping, shallow (<500’ / 150m) geophysical surveys, and fluid geochemical studies.
In summer 2009, the COA executed a follow-on exploration program including a geologic
reconnaissance field study, soil and soil gas geochemical surveys, a remote sensing study using satellite
data, a review of existing hot springs geochemistry data, a magnetotelluric (MT) survey, and a
conceptual model analysis. In summer 2010, an exploration drilling program was carried out with two
temperature gradient (TG) wells drilled in Hot Springs Bay Valley (HSBV). The following reports have
been written on the Akutan geothermal resource, in chronologic order:
1. Motyka, R., and C. Nye, eds., 1988. A geological, geochemical, and geophysical survey of the
geothermal resources at Hot Springs Bay Valley, Akutan Island, Alaska. Alaska Division of
Geological and Geophysical Surveys (ADGGS), Report of Investigations 88-3.
2. Motyka, R.J., S. Liss, C. Nye, and M. Moorman, 1993. “Geothermal Resources of the Aleutian
Arc.” Alaska Division of Geological and Geophysical Surveys (ADGGS) Professional Paper 114.
3. Kolker and Mann, 2009. “Heating up the Economy with Geothermal Energy: A Multi-Component
Sustainable Development Project at Akutan, AK.” Transactions, Geothermal Resources Council
Annual Meeting 2009. *Both paper and poster format available.
4. Kolker, Cumming, Stelling, Prakash, and Kleinholtz, 2009. “Akutan Geothermal Project: Report
on 2009 Exploration Activities.” Unpublished report to City of Akutan and the Alaska Energy
Authority, 37p.
5. WesternGeco, 2009. Magnetotelluric Survey at HSBV, Akutan, Alaska: Final Report – 3D
Resistivity Inversion Modeling. Unpublished report prepared for the City of Akutan, Alaska,
GEOSYSTEM/WesternGeco EM, Milan, Italy, 27p.
Akutan Geothermal Resource Assessment
5
6. Kolker, Stelling, and Cumming, 2010. “Akutan Geothermal Project: Preliminary Technical
Feasibility Report.” Unpublished report to City of Akutan and the Alaska Energy Authority, 31p.
7. Kolker, Bailey, and Howard, 2010. “ Preliminary Summary of Findings: Akutan Exploratory
Drilling Program.” Unpublished report to City of Akutan and the Alaska Energy Authority, 32p.
8. Kolker, Cumming, and Stelling, 2010. Geothermal Exploration at Akutan, AK: Favorable
Indications for a High-Enthalpy Hydrothermal Resource Near a Remote Market.” Transactions,
Geothermal Resources Council Annual Meeting 2010. *Both paper and poster format available.
9. Rohrs, 2011. “Geochemistry of the Akutan Geothermal Prospect, Alaska.” Unpublished report to
City of Akutan, 36p.
10. Stelling and Kent, 2011. “Akutan Geothermal Exploration Project: Geological Analysis of Drill
Core from Geothermal Gradient Wells TG-2 and TG-4.” Unpublished report to City of Akutan,
24p.
11. Kolker, Bailey, and Howard, 2011. “The 2010 Akutan Exploratory Drilling Program- Preliminary
Findings.” Draft paper submitted to the Geothermal Resources Council for
publication/presentation at the GRC Annual Meeting, October 2011.
12. Kolker et al, 2011. “Akutan Geothermal Project: Summary of Findings from the 2010 Drilling
Program.” Unpublished report to the City of Akutan, 33p.
Geologic Setting
Akutan volcano is part of the Aleutian Volcanic Arc, which is Alaska’s most promising setting for
geothermal energy. Akutan volcano is one of the most active volcanoes in the Aleutians, with 32 historic
eruptions (Simkin and Siebert, 1994). Akutan volcano is a composite stratovolcano with a summit
caldera ~1 ¼ mi (2 km) across and 200-1200’ deep (60-365 m; Newhall and Dzurisin, 1988; Miller et al.,
1998). Most of the reported eruptions included small-to-moderate explosions from the active
intracaldera cone. An initial volcanic hazard review indicated that the proposed geothermal
development area was unlikely to be directly impacted by eruption activity consistent with the previous
1500 years, excepting ash fall that might cause temporary closure.
The HSBV walls are composed of ~1.4 Ma lava flows, with the SE wall being slightly older and containing
numerous dikes. The valley was glacially carved, perhaps during the last major glaciation ending ~8,000
years ago, and potentially reworked during a minor glacial event ending ~3,000 BP (Black, 1975). The
HSBV is composed of two linear valleys (the SE-trending Fumarole Valley and the NE-trending valley that
contains the hot springs; Fig. 1) joined at right angles, suggesting structural control of glacial flow. Soil
geochemical anomalies (Arsenic (As), mercury (Hg), and carbon dioxide (CO2) at the junction of the
Fumarole Valley and the HSBV also suggest that the valley junction is structurally controlled and could
be an important fluid conduit (Kolker et al, 2010). Hg, As and CO2 occur in anomalously high
concentrations near the hot springs as well.
Akutan Geothermal Resource Assessment
6
In March 1996, a swarm of volcano-tectonic earthquakes (>3000 felt by local residents, Mmax = 5.1)
beneath Akutan Island produced extensive ground cracks but no eruption of Akutan volcano. InSAR
images that span the time of the swarm reveal complex island-wide deformation, suggesting inflation of
the western part of the island and relative subsidence of the eastern part. The axis of the deformation
approximately aligns with new ground cracks on the western part of the island and with Holocene
normal faults that were reactivated during the swarm on the eastern part of the island. The deformation
is thought to result from the emplacement of a shallow, east-west-trending, north-dipping dike plus
inflation of a deep magma body beneath the volcano (Lu et al., 2000). Studies of 3He/4He ratios in
Akutan fumarole gasses indicate degassing of relatively fresh near-surface magma (>6 RC/RA; Symonds
et al., 2003). This implies that unlike many other composite stratovolcanoes, Akutan’s magmatic
plumbing system includes two lateral rift zones, similar to the classic rift zones at Hawaiian volcanoes
and elsewhere. These rift zones are aligned along the regional least-compressive-stress axis (John
Power, pers. comm.), and serve as active magmatic conduits at shallow crustal depths (Fig. 1). NW-
trending rifting appears to be providing the large-scale permeability as well as the magmatic heat source
- crucial for the development of an extensive hydrothermal reservoir beneath the volcano.
Figure 1. Topographic map of Akutan Island, showing the geothermal project area and pertinent geologic features.
Hot Springs Bay Valley (HSBV) is an L-shaped topographic low that lies at the center of the geothermal project area.
Akutan village
Akutan Geothermal Resource Assessment
7
Geothermics
Several thermal springs are located in HSBV, about 6 km from Akutan village (Fig. 1). Five groups of hot
springs with about ten vents have been identified, including tidewater springs on Hot Springs Bay beach
that are only exposed at low tide. Temperatures range from 129 to 205 °F (54 to 96 °C); and some have
been reported as boiling. A fumarole complex (often called the “fumarole field”) exists at the head of
HSBV to the west of the hot springs and covers an area of approximately 1600 ft2 (500m2).
Motyka and Nye (1988) concluded that the fumaroles are likely fed directly by gases and steam boiling
off the deep hot reservoir and that these fluids then mix with cool groundwaters to produce the hot
spring waters further down the valley, forming a geothermal system that is at least 2.4 mi (4 km) long.
Recent studies of the chemical composition of the fluids confirm that they become extensively mixed
with cooler meteoric waters near the surface. Fumarole gas geothermometry indicates that the
reservoir fluids attained a temperature of at least 518 °F (270 °C). Cation chemistry from the hot springs
and produced fluids indicates that the fluids are re-equilibrating to lower temperatures along the
outflow path, with cation geothermometry from the fluids produced by corehole TG-2 providing
temperatures of 211-232 °C. The silica geothermometry of 320 °F (~160 °C) indicates that the resource
close to the hot springs (probably <1500’ / <500m distance and depth) is likely to be 320-358 °F (160 to
180 °C) (Rohrs, 2011). This is consistent with active silica sinter deposition at the hottest springs. Based
on the springs, well TG-2 was expected to encounter a permeable zone with 320-358 °F (160-180 °C)
fluid, which it did.
The structure(s) controlling upflow of hydrothermal fluids is probably one or more NW-trending normal
fault(s). One such mapped fault cuts near-perpendicularly across HSBV (Fig. 2). All of the hot springs are
topographically lower than the fault’s surface trace, consistent with geochemical indications that they
outflow from an upflow near the fumarole. The fumarole field lies along a parallel linear feature, but no
fault has been mapped there. A perpendicular NE-trending fault may control the linear shape of the hot
spring locations, but that fault has not been conclusively identified with available data.
MT Resistivity
The resistivity pattern of the Akutan geothermal prospect has an overall geometry similar to that of
most geothermal reservoirs where a low resistivity, low permeability smectite clay caps a higher
resistivity, higher temperature, permeable geothermal reservoir. However, the resistivity values of >20
ohm-m within the low resistivity zone at Akutan are much higher than in the smectite zone of most
developed geothermal fields. Several models can explain such a pattern, including an unusually high
fraction of dense lavas causing weak alteration, or relict alteration that formed at higher temperatures.
A localized pattern of alteration near the hot springs is more conventional, with a <600’ (<200 m) thick,
5-15 ohm-m zone that represents a smectite clay cap overlying a higher resistivity geothermal outflow
(Figs. 10-13). Therefore, the overall resistivity geometry is consistent with the geochemistry. A tongue of
high resistivity at -300 m elevation in Figs. 10-13 trends from the fumarole to the hot springs. This is
consistent with a relatively resistive flow path that originates from a >428 °F (>220 °C) upflow near the
fumarole to a 358–428 °F (180–220 °C) outflow extending to the hot springs.
Unfortunately, steep topography and high winds prevented the MT from accessing much of the
prospective area. There are no MT stations over the fumarole area and so a well that targets an
interpreted upflow in its vicinity might target only the surface extent of the altered ground and
Akutan Geothermal Resource Assessment
8
fumaroles. There are also no MT stations over a large part of the likely outflow path, making it difficult
to assess the risk of targeting a well on the accessible part of the likely north flank of the outflow.
New Data 2011
1. Temperature Gradient Data
1a. Core Hole Drilling
In 2010, two small-diameter temperature gradient (“TG”) core holes were drilled at locations given in
Fig. 2. Since the Akutan Geothermal area is roadless, the drilling operations were supported by
helicopter. Due to budget constraints, only two of the four planned holes were actually drilled; these are
marked with black arrows in Fig. 2. The 2010 exploratory drilling plan was designed to test whether the
shallow resource was potentially commercial. Within narrow budget constraints, the wells were
designed for long-term monitoring as well as a test of the shallow, accessible targets at the Akutan
geothermal field. The hole(s) were completed as temperature gradient wells and available for future
monitoring. A detailed report on the drilling operations, P/T survey results, and other data is provided in
Kolker et al. (2011).
Well “TG-2” was drilled to a TVD (total vertical depth) of 833’ (254 m). It was sited to test the outflow
aquifer(s). Between 585 and 587’(178 and 179 m), a highly permeable zone was encountered that
flowed geothermal fluid at 182 °C (359 °F). This productive zone was cased and cemented, sealing it off,
at which point it cooled to about 329 °F (165 °C). The structure hosting the flowing fluid appeared to be
a fractured, highly vesicular, flow margin. Due to the temperature and permeability of the formation at
relatively shallow depths, drilling this well was challenging. Although targeted to 1500’ (457 m), the well
was terminated due to drilling problems.
Well “TG-4” was drilled to the planned TVD of 1500’. It was sited at the southern part of the junction
between the two perpendicular valleys, to test the size and extent of the outflow zone. Since well TG-4
did not encounter substantial fluid flow, its location appears to be outside the margins of the outflow
zone, vertically or horizontally (or both). However, well TG-4 did encounter an anomalously high shallow
temperature gradient, implying close proximity to a geothermal source.
Akutan Geothermal Resource Assessment
9
Figure 2. Map of the Akutan Geothermal area, showing the four candidate exploration well locations that were
considered for the 2010 program. The two holes drilled in 2010 are marked with black arrows.
1b. End-of-Well Logs
After TD was reached, three P/T logs were recorded at 12, 24, and 36 hours after circulation ended for
each well. For every run, stops were made at 20 foot stations. Because these surveys were taken so
soon after the well was drilled, the temperature readings were still influenced by the cooling effects of
fluid circulation. Therefore these represent “unequilibrated” downhole temperatures. In order to
predict the equilibrated downhole reservoir temperature, we used the Horner method to extrapolate
the measured values to a longer period. The end-of-well elevation vs. temperature plot generated from
extrapolated Horner values is shown in Fig. 3. The MRT reading from the flowing zone is also shown as a
purple dot.
Akutan Geothermal Resource Assessment
10
Figure 3. Estimated equilibrated Temp v. depth plot for TG-2, based on Horner extrapolations of downhole survey
data (see Fig. 11 and text for details).
Both wells show very high shallow temperature gradients, which is consistent with their proximity to the
shallow outflow zone. Following production, TG-2 shows a drop in temperature occurring just above the
casing shoe at 603’ (181 m), corresponding to the hot fracture zone between 585 and 587' (178 and 179
m) that was cemented in. The apparent cooling is likely the result of drilling fluid and cement injected
across that entire area. TG-4 shows a relatively rapidly increasing temperature gradient until ~900’ (274
m), transitioning to a slowly increasing temperature gradient from 900’-1500’. The fact that there was
no temperature reversal and that the gradient continues to increase suggest there could be a deeper,
hotter aquifer below 1500’ (457 m) that was not penetrated by drilling. An injection test performed on
well TG-4 suggested that the well has generally poor permeability.
1c. Equilibrated TG Logs
While the end-of-well surveys were conducted with a memory tool, it was not possible to use a memory
type tool for the post-completion logging due to the small inner diameter of the Akutan TG wells (inner
diameter > 1.5 inches / 3.81 cm). Due to this and other unique conditions of Akutan TG wells (high
-1600
-1400
-1200
-1000
-800
-600
-400
-200
0
200
0 100 200 300 400 500 Elevation (feet ASL) Akutan TG Well Temperatures (F)
TG2
TG4
Boiling Curve 0.1% NCG
Flowing MRT TG2
Akutan Geothermal Resource Assessment
11
temperatures at shallow depths, remoteness of the wellsites, among others), thermistor-type
temperature logging equipment was used, and downhole pressures were not recorded.
The survey for TG-2 was completed on May 22, 2011. Results from the equilibrated survey are shown
with the three build-up surveys in Fig. 4.
Figure 4. Equilibrated temperature profile for TG-2, plotted with the three end-of-well heat-up surveys. The end-of-
well surveys were taken 12 hours. 24 hours, and 36 hours after circulation; the equilibrated profile was obtained 9
months later in May 2011.
The new temperature profile shows a distinctly different shape from the end-of-well temperature build-
up profiles. Among the new features to note are: (1) The well was bleeding while the log was run,
resulting in a minor steam or two-phase section in the upper 60-70’ (20-25 m). (2) Apparent cooling of
the well since shut-in is noticeable in the upper 400’ (122m). This probably reflects a trickle of water
downflowing from around 200’ (61m) MD and exiting into the formation at about 415’ (126m) MD. It
can only be a trickle of water because the water is heating up as it flows down behind the casing. (3)
The highest temperatures occur in the permeable zone near 585’ MD (415’ / 126m elevation), with a
temperature reversal of about 9 °F (5 °C) below the permeable zone to the bottom of the well.
0
100
200
300
400
500
600
700
800
900
0 50 100 150 200 250 300 350 400 Elevation (feet ASL) Akutan TG2 Temperatures, oF
12h 08-2010
24h 08-2011
36h 08-2011
May-11
Akutan Geothermal Resource Assessment
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The new data also shows that the permeable zone at 585’ MD (415’/ 126 m elevation) has fully
recovered in temperature. Notably, the static temperatures measured in this permeable zone are about
338 °F (170 °C), which is lower than the 359 °F (182 °C) temperature measured in this zone when the
well was flowing. Since the MRT reading does appear to be correct based on silica geothermometry, this
implies that the well was drawing in higher temperature fluids when it was producing.
A temperature survey was run in TG-4 by the City of Akutan crew on May 10, 2011 (Fig. 5).
Figure 5. Equilibrated temperature profile for TG-4, plotted with the three end-of-well heat-up surveys.
The temperature profile from the equilibrated survey differs very slightly from the end-of-well
temperature profile. The new profile shows that the top 800’ (244 m) of well TG-4 heated up slightly,
but the bottom temperatures remained extremely close to those measured during the end-of-well
surveys. This is not surprising in light of the fact that that well was relatively impermeable and exhibits a
temperature profile that shows heating primarily from conduction for the upper 800’ (244 m). By
contrast, the bottom of the hole is approaching an isothermal gradient. This suggests that the
conductive heating is from the side (i.e., from a shallow outflow zone at some lateral distance), not from
a hot aquifer below.
-200
0
200
400
600
800
1000
1200
1400
1600
0 50 100 150 200 250 300 350 Elevation (feet ASL) Akutan TG4 Temperatures, oF
12h 08-2011
24h 08-2011
36h 08-2011
May-11
Akutan Geothermal Resource Assessment
13
1d. P/T Data Analysis
Although TG-2 flowed geothermal fluid at 359 °F (182 °C) during drilling, the equilibrated temperature
logs show a maximum temperature of 338 °F (165 °C) with a reversal at the bottom of the hole. This
implies that the 359 F fluid was not circulating in the immediate vicinity of TG-2 but rather was “pulled
in” from elsewhere due to the pressure drop caused by flowing the well. A likely scenario is that the
productive subhorizontal fracture at 585’ (178m) in TG-2 is connected to a subvertical fracture dipping
west (see Fig 11). When the subhorizontal fracture was produced, the subvertical one became a
temporary conduit for fluids in the outflow zone. It is unlikely that the source of the 359 °F (182 °C) fluid
is directly below Well TG-2 because of the temperature reversal recorded in the most recent log.
A comparison of the static temperature profiles in TG-2 and TG-4 shows the difference between the
shape of a convectively heated outflow profile in TG-2, and a conductively heated temperature profile in
TG-4 (Figure 6). Also, the temperatures in the upper 800’ (254 m) of TG-4 are generally lower than in TG-
2, indicating that TG-4 is further from the shallow outflow path. No strong conclusions can be drawn
from the temperature profiles as to whether additional high temperature permeable zones underlie
either well, but it appears unlikely based on the shape of the bottom of both well profiles.
Figure 6. Equilibrated well profiles for both Akutan TG wells, shown with a boiling point with depth curve for water
with 0.1% non-condensable gas content.
-200
0
200
400
600
800
1000
1200
1400
1600
150 200 250 300 350 400 450 Elevation (feet ASL) Akutan TG Well Temperatures May 2011 (oF)
TG2-05-2011
TG4-05-2011
Boiling 0.1% NCG
Akutan Geothermal Resource Assessment
14
The 359 °F (182 °C) temperature measurement during drilling of TG-2 does appear to be correct based
on silica geothermometry. If the well was drawing in higher temperature fluids when it was producing,
this suggests that TG-2 was drilled on the margins of a more permeable and hotter outflow path. The
higher temperature fluids drawn into TG-2 during the flow test suggest that the production zone is in
proximity to the higher temperature zone but that it has a relatively low permeability connection to this
zone. The slight temperature reversal of about 9 °F (5 °C) below the permeable zone is consistent with
the geologic model that the thermal features in HSBV represent a confined lateral outflow from a
geothermal reservoir located further west, or possibly north.
The temperature gradients for Akutan wells TG2 and TG4 vary widely, but compared to the continental
average of 1.65 °F/ft (30 °C / km; Fig. 7) they are very high above 600’ (244 m). This suggests that both
are within proximity of a very shallow outflowing resource.
Figure 7. Temperature gradients, in degrees Fahrenheit per foot, for both Akutan TG wells. The average continental
geothermal gradient of 1.65 °F/ ft is shown for comparison. The outlier data point at ~410’ depth can be ignored as
it reflects a transition between the part of the well affected by a small amount of downflowing water and the part
unaffected, and thus does not represent an accurate temperature gradient.
0
200
400
600
800
1000
1200
1400
1600
0 5 10 15 20 Elevation (feet ASL) Akutan Well Temperature Gradients (T2-T1, oF)
TG2
TG4
Average continental
Akutan Geothermal Resource Assessment
15
2. New fluid chemistry and geothermometry
2a. Sample Collection and Data Sources
Two fluid samples were obtained from well TG-2, with the first obtained from the entry zone at 585’-
587’ (178-189m) measured depth (MD) during a well discharge. This production zone was subsequently
cased off. A second flow test of the well obtained samples from production zones between 603’ (184m)
and 833’ (245m) MD, which was the completion depth of the core hole. Because TG-4 encountered
poor permeability conditions, a sample of the fluids in the wellbore was obtained by flowing with an air
assist. The MD of TG-4 was 1500’ (457m) and had a cemented casing at 596’ (182m) MD. Therefore the
data obtained during the discharge of the wells vary in quality.
New gas chemical data are from samples obtained from the fumaroles in 2010. All other analyses used
in geothermometry calculations and chemical modeling were obtained from past reports (fluid analyses
from the hot springs, non-condensable gas analyses from the summit fumarole and from the hot springs
– see p. 4 for sources).
2b. Chemistry
Geochemical data were interpreted using a combination of binary and ternary diagrams and
geothermometry gas plots. The data set used to interpret the reservoir conditions consists of all of the
data obtained at Akutan. Data and interpretations are provided in full in Rohrs (2011).
The chemical analyses of the hot springs water shows that they are derived from a dilute, near-neutral
Na-Cl reservoir brine. The Akutan hot springs show slightly elevated HCO3- and SO4 concentrations,
suggesting mixing along the outflow path with dilute, steam-heated near-surface waters. Hydrogen and
oxygen isotopic data shows that the hot spring waters are derived from local meteoric water.
The chemistry of the fumarole gasses demonstrates a strong magmatic affiliation. There is no evidence
from the gasses that the reservoir water has mixed with air-saturated fluids along an outflow path.
Compared to many geothermal systems, Akutan displays enriched N2 concentrations, which in some
cases would raise concerns with regards to acid or vapor-cored conditions in the reservoir. However, the
other gas plots show that the gases are well-equilibrated and likely to be derived from a high
temperature neutral chloride reservoir. In addition, the gas concentrations in the flank fumaroles imply
that some fraction of gas is derived from equilibrated steam, indicating the presence of a localized
steam cap in the reservoir. The chemistry of the fumaroles are consistent with an equilibrated
geothermal system associated with an andesitic stratovolcano (Giggenbach, 1991). In comparison, the
gas from the summit fumarole originates from a more oxidizing environment and exhibits high H2S
concentrations. These all suggest a magmatic affiliation for the summit fumarole steam.
2c. Geothermometry
The hot spring and well discharge samples are well suited to chemical geothermometry using the silica
and Na, K, Ca, and Mg concentrations of the fluids. The estimated temperature of last equilibration
along the outflow path suggests that the fluids have equilibrated at ~338 °F (~170 °C) and ~392 °F (200
Akutan Geothermal Resource Assessment
16
°C) for the two samples from TG-2. This temperature is similar to the estimated entry temperature of
359 °F (182 °C) at 585-587’ (178-179m) MD in well TG-2 (Kolker et al, 2010). Cation concentrations in hot
spring and well discharge analyses show that the springs and well fluids are mixed or partially
equilibrated fluids. This is commonly observed along outflow paths where the fluids are re-equilibrating
to lower temperatures and mixing with near surface waters with elevated Mg concentrations (Rohrs,
2011).
The data from hot spring HS-A3 and the entry at 585’ (178 m) MD in core hole TG-2 suggest that the
fluids originate in a deeper reservoir with temperatures in the range of 428-464 °F (220-240 °C). This
compares to a temperature of 412 °F (211 °C) estimated from the Na-K-Ca geothermometer for the well
discharge. Geothermometers that apply Na, K, Ca, and Mg concentrations tend to partially re-equilibrate
to lower temperatures in the outflow zone, and so the deep reservoir temperature is likely to exceed
464 °F (240 °C; Rohrs, 2011).
Geothermometry estimates from flank fumarole gasses exhibits very good consistency, indicating an
origin from a mature, equilibrated neutral chloride reservoir. The gas geothermometry consistently
suggests reservoir temperatures of 518-572 °F (270-300 °C; Rohrs, 2011).
2d. Geochemical Model
The new geochemical data set confirms the previous interpretations of the resource distribution in
HSBV. The hot springs represent a shallow outflow from a high temperature neutral chloride reservoir
that exists further west. The chemistry of the hot springs indicates that they have experienced
significant mixing with cooler, dilute near surface meteoric waters. Because the fumarole gases show
little evidence of mixing with air-saturated waters, the upflow zone is likely to lie near the fumaroles.
Also, gas grid plots indicate that the fumaroles contain a component of equilibrated steam, suggesting
the possibility that a localized steam cap overlies the deeper geothermal reservoir. Geothermometry of
the well discharges and the fumarole gases indicate a likely deep reservoir temperature of at least 464
°F (240 °C) based on Na/K geothermometry, with temperatures possibly as high as 572 °F (300 °C) in the
upflow based on gas geothermometry. The geochemical data do not provide any constraints on the
reservoir boundaries to the west nor on the reservoir volume within the outflow area (Rohrs, 2011).
The non-condensable gas data from the fumaroles suggest that a steam cap may overlie the deep brine
reservoir. The chloride hot springs in HSBV represent shallow outflow from the reservoir. The outflow
becomes diluted by mixing with cool meteoric waters, especially in the near surface environment
(Rohrs, 2011). Thus, the geochemical data are very consistent with the geochemical outflow models
suggested by Kolker et al. (2009).
3. Core data
3a. Overview
Composite logs from Akutan TG wells TG-2 and TG-4 of the bulk lithologies, alteration mineralogy, and
temperature data are provided in Appendix A. Full lithologic logs were recorded at the wellsite during
drilling and are provided as an Appendix in the Summary of Drilling Findings (Kolker et al., 2011). The
core was then sent to Western Washington University (Bellingham, WA) for detailed laboratory analysis.
Akutan Geothermal Resource Assessment
17
The goal of the laboratory analysis was to determine the hydrothermal history of the HSBV. Core
samples were selected based on zones of interest from drilling records, core photographs, and complete
coverage of the depth of core. Determination of specific mineral species was conducted through X-ray
Diffraction (XRD) analysis, Scanning Electron Microscopy (SEM), and petrographic observations.
Quantitative permeability studies of the core were not conducted, however qualitative observations
about the permeability of the field by visual observations of the core were recorded. Finally,
compositional analysis of 19 bulk rock samples were conducted by X-ray Fluorescence (XRF) at the
Geoanalytical lab at Washington State University in Pullman, WA. Methodologies for above studies,
detailed results, and discussions are provided in Stelling and Kent (2011).
3b. Rock Types and Primary Mineralogy
There are four main lithologies present in the Akutan core: basalt, andesite, ash tuff, and “lithic basalt.”
The most common lithology in the core is basalt lava. These flows appear to be subareal in nature and
contain plagioclase, clinopyroxene, rare olivine and primary apatite. The ash tuffs are fine grained rocks
lacking phenocrysts. Groundmass phases are plagioclase microlites, glass, and alteration minerals (see
below). In TG-2, these units are <3’ (1 m) thick. In TG-4, which is ~2 miles (3.2 km) closer to Akutan
Volcano, similar units are as thick as 60’ (18 m).
The rocks provisionally named “lithic basalt” were a puzzle during the on-site evaluation, and remain
enigmatic. The lithic basalt is composed of multiple different rock types, suggestive of some sort of
debris flow deposit, yet the matrix between the clasts is crystalline, indicating a magmatic origin. At this
time, the origin of this lithology is unknown.
3c. Secondary Mineralogy, Mineral Paragenesis, and Hydrothermal History
A graphical representation of secondary mineralization and clay replacement is presented in Stelling and
Kent (2011). The rocks in general appear to be only weakly altered. As a result of the increased porosity
near lava flow tops, these regions tend to be more altered and more readily brecciated than the main
body of the lava. Heavy Fe-oxidation was observed between flow layers. Alteration minerals occurred
interstitially, in fractures, in vesicles, and in contact zones.
Alteration assemblages in both wells are dominated by chlorite, zeolites, epidote, prehnite and calcite,
and this alteration appears to have happened multiple times in both wells. The presence of adularia in
specific locations in both wells indicates higher temperature and permeability conditions existed at
some point in the past. The presence of kaolinite in TG-2 indicates argillic alteration with lesser extent
and intensity. Illite was identified in both wells, although much more sparsely in TG-2.
Within the most recent propylitic alteration event in TG-2, the sequence of zeolite formation shows a
classic trend toward higher temperatures with depth. It is likely that this trend will continue below the
base of the well (833’ / 254 m). Figure 8 shows that some higher-temperature minerals (illite, epidote,
prehnite, wairakite and adularia) occur in regions that are currently much colder than expected for
these minerals. This suggests that the TG-2 region underwent higher temperature alteration in the past.
The presence of these higher-temperature minerals at unexpectedly shallow depths further suggests
that a significant portion of this older alteration sequence has been removed through erosion, possibly
Akutan Geothermal Resource Assessment
18
glacial. Overprinting of these minerals by lower-temperature alteration assemblages indicates the
sampled region has since returned to a lower-temperature alteration regime with reduced permeability.
Figure 8. First occurrence of indicator minerals with depth in core from Akutan well TG-2. Horizontal arrows
indicate formation temperature ranges for each mineral. Dashed lines indicate published values; solid lines indicate
the most commonly reported minimum temperatures.
The pattern of alteration in TG-4 is more complex than TG-2 (Fig. 9). That the depositional history of TG-
4 includes multiple alteration events does not necessarily mean that the two wells have had significantly
different thermal histories; rather, the most recent alteration event may have been stronger in the TG-2
region, overprinting more completely the alteration sequence observed in TG-4.
Akutan Geothermal Resource Assessment
19
Figure 9. First occurrence of indicator minerals with depth in core from Akutan well TG-4. Horizontal arrows
indicate formation temperature ranges for each mineral. Dashed lines indicate published values; solid lines indicate
the most commonly reported minimum temperatures.
HSB 4 W e ll Te mp era ture (0 C)
50 Smectite
u
200
Smectite
100
400
1'1 Heu
150 1'1 Ana l
Smectite
Smectite 600
2 Y," casing shoe
200
I
-= Smectite a. .. c 800
250
350
Adu: Adularia 1200
Anal: Analcime
Chab: Chabazite
Cptl: Clinoptil olite --+--Temp heating 12 hrs
400 Ep: Epidote ......,... Temp heating 24 hrs
Heu: H eulandit e -+-Temp heating 36 hrs
Lau: Laumontite
Mord: Mordenite -o Temp heating 260 days 1400
Pre: Prehn ite ~BPDTemp
Yug: Yugawaril ite
Akutan Geothermal Resource Assessment
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Comparing the observations made in the two drill cores provides a basic sequence of alteration for the
HSBV geothermal outflow zone. Both cores show an alteration sequence progressing from an early
propylitic event, a narrow band of adularia-bearing propylitic alteration, followed by a later propylitic
event. The trend from moderate propylitic to high-temperature adularia-forming alteration and back to
moderate propylitic indicates that the shallow portion of the HSB field has reached its thermal peak and
has cooled moderately. Additionally, many of the higher temperature minerals occur at depths much
shallower than reported in other geothermal fields. Thus is it likely that 1) this region was hotter than it
is currently, and 2) the uppermost portion of the rock column has been removed and these rocks have
risen to their modern shallow depths.
3d. Permeability and Porosity of Well Rocks
The primary lithologies do not lend themselves to high primary permeability. The abundance of isolated
vugs filled with secondary minerals indicates that fluid flow through microscopic intergranular networks
has been important, but flow rates are likely very low. Vug filling is especially common in fine-grained,
detrital deposits (e.g., ash tuff), but clay alteration and fracture mineralization by carbonates and
zeolites reduces permeability in these rocks.
The primary fluid pathways appear to be associated with brittle fracturing and lithologic contacts, based
on the abundance and degree of alteration and secondary mineralization. Very fine grained deposits
(ash tuff) lack large crystals that would add structural control over the fracture patterns. As a result,
these rocks are prone to planar fractures at prescribed orientations (30o, 45o, 60o and 90o). The majority
of these fractures have some sort of secondary mineralization associated with them, and some of the
larger fractures are deeply altered to a variety of clays and other minerals. Because the tuff is more
susceptible to clay alteration, these fractures can seal before major secondary mineralization becomes
intense. However, these units are not very thick in the wells, so may not have extensive control over the
overall fluid flow in the reservoir. Permeability may locally increase at the top of lava flows where vugs
in vesicle-rich flow tops may collapse, but this was not observed in the core. Some heterogeneous
lithologies (e.g., “lithic basalt,” see below) contain entrained clasts of older material. Fluid flow within
these lithologies are concentrated and directed around the entrained clasts, which would likely result in
moderately increased permeability compared to intergranular flow.
The occurrence of the mineral adularia helps to elucidate the permeability. Although adularia occurs in
all lithologies in the HSB cores, the restriction of adularia to fractures highlights the importance of
secondary permeability, as it does in many fields worldwide. Adularia is strongly associated with zones
that once had high permeability but each occurrence of adularia in the core is in veins that are now
thoroughly sealed by mineralization. Therefore, the waxing of a higher temperature system and
subsequent waning has apparently reduced the permeability in the HSBV outflow system.
No evidence for large scale structures were encountered in Akutan geothermal wells. A number of
brecciated zones were observed in TG-4, but most were “sealed” with secondary mineral deposits and
therefore probably do not represent active faults. Minor slickensides observed in cores could be related
to a possible normal fault on the SW side of the valley near TG-4.
Akutan Geothermal Resource Assessment
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Resource Conceptual Models
Several conceptual models of the Akutan Geothermal Resource were presented in an earlier report
(Kolker et al, 2009). The acquisition of new data in 2010-2011 are consistent with the same basic upflow
– outflow model. The most important change in the conceptual model assessment is the increase in
confidence in the fumarole as the locus of a benign reservoir upflow that would be a suitable target,
based on the promising new gas geochemistry analyses. The new data significantly reduces the
probability that an economic reservoir would be found in HSBV.
The location of the high permeability outflow path is still only loosely characterized by two models,
although the upflow seems more closely connected to TG-2 than to TG-4. The alternative outflow
pathways continue to be either along the HSBV or along a northern trajectory from the fumaroles to the
hot springs. These two alternatives are explored in Figs. 10-13. Both conceptual models are based on the
notion that the Akutan geothermal system is a single resource comprised of two distinct features: a
high-temperature (>500 °F / >240 °C) upflow zone, and a lower-temperature outflow aquifer (~360-390
°F / 180-200 °C), as suggested by chemical data.
Figure 10. Map view of “Conceptual model 1,” showing outflow along HSBV. Isotherm contour placement is based
on downhole temperature data, chemical geothermometry, hot springs and fumarole locations, and MT resistivity
data (shown here at -300 m (~984 ft) depth). Profile line “CM1” corresponds to Fig. 11.
Akutan Geothermal Resource Assessment
22
Figure 11. Profile “CM1,” as shown in Fig. 10. This model shows outflow along HSBV. Isotherm contours based on
downhole temperature data, chemical geothermometry, hot springs and fumarole locations, and MT resistivity
data (shown here as 3D inversion model).
Conceptual model ‘CM1’(Figs. 10 and 11) shows a high temperature resource upflowing beneath the
fumarole field, and cooling along an outflow path that follows the L-shaped path of HSBV. The upflow
must be some lateral and vertical distance from well TG-4, since no trace of conductive heating from a
deep source was observed in the temperature profile of TG-4. Also, for this model to fit the observed
downhole temperature profiles in both wells, the outflow along HSBV can only be very thin (vertically
constrained low-permeability) and restricted to the shallow subsurface.
Because the 359 °F (182 °C) flow during the flow test completed while drilling is higher than the
measured static temperature, because there does not appear to be a downflow that would reduce the
temperature of this zone from 359 °F (182 °C) to 338 °F (165 °C) when the well is static, and because the
temperature reversal in TG-2 below this zone makes upflow from below the well unlikely, the produced
higher temperature fluid appears to have been “pulled in” laterally from a nearby source. This could be
related to the westward-dipping fault near TG-2 shown in Figs. 11 and 13. The rapidity with which this
hotter fluid was drawn in during such a short test implies that the 338 °F (165 °C) permeable zone in TG-
2 must be restricted in volume and at a higher natural state pressure than the 359 °F (182 °C) adjacent
reservoir.
Akutan Geothermal Resource Assessment
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Figure 12. Map view of “Conceptual model 2,” showing outflow beneath the mountain to the north of HSBV.
Isotherm contour placement is based on downhole temperature data, chemical geothermometry, hot springs and
fumarole locations, and MT resistivity data (shown here at -300 m (~984 ft) depth). Profile line “CM2” corresponds
to Fig. 13.
Conceptual model 1 ‘CM1’ does not resolve the location of a hotter outflow resource of 360-392 °F (180-
200 °C), for which there is a substantial amount of geochemical evidence. Therefore, an alternative
model is proposed called ‘CM2’ (Figs. 12 and 13). In CM2, the shallow outflow path takes a northerly
trajectory from the fumarole to the ENE towards the hot springs, circumventing HSBV altogether. This
model appears more likely based on several lines of reasoning: 1) the temperature profile for TG-4
shows no evidence for being along an outflow path, implying that outflow feeding the hot springs is
laterally distal; 2) a low-resistivity clay cap appears to form a dome pattern around the northerly outflow
path, which is consistent with the interpretation that the HSBV is near, but not in, the main outflow path
of geothermal fluids (Figs. 10 and 12); and 3) the isotherm contours on the CM2 profile (Fig. 13) are
slightly more typical of an outflowing geothermal system.
Akutan Geothermal Resource Assessment
24
Figure 13. Cross section of profile line “CM2,” as shown in Fig. 12. This model shows outflow beneath the mountain
north of HSBV. Isotherm contours based on downhole temperature data and chemical geothermometry, MT
resistivity data, hot springs and fumarole locations, and fault lines.
Both models suggest that producing the outflow resource would be very risky, both because of the
generally low permeability expected based on several lines of reasoning and also because there is no
well-developed clay cap to indicate that a large-very permeable reservoir volume at ~360-390 °F (180-
220 °C) exists under HSBV. The lack of widespread surface alteration, geochemical, and ground
temperature anomalies (Kolker et al., 2009) in HSBV are consistent with this interpretation. Additionally,
the chemical composition of the hot springs fluids suggests that outflow fluids become extensively
mixed with cooler meteoric waters near the surface, raising concerns about cold water influx into the
outflow system with production.
While the conceptual models of the outflow resource have downgraded its potential for development,
geochemical data from the fumaroles significantly upgrades the fumarolic area as a drilling target . The
fumarole data suggest that the flank fumarole field lies in fairly close proximity to an upflow zone from
the reservoir, that a steam cap may overlie the upflow, and that reservoir temperatures could approach
570 °F (300 °C) within the upflow. The deep reservoir probably consists of a brine liquid capped by a
small two-phase region (steam cap) (Rohrs, 2011). Resistivity data suggest that the upflow reservoir is
situated in brittle rocks, implying propylitic alteration regime and a good possibility of high permeability.
Akutan Geothermal Resource Assessment
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Future Drilling Targets
The two types of resource targets represent two development options for the AGP:
1) The upflow resource has a high exploration risk, but potentially low development risk and high
output capacity.
2) The outflow resource has potentially easier access but higher exploration risk, and presents
several development risks, including a lower output capacity per well.
The highest priority target for future drilling is at the upflow resource, due to the following factors:
(1) Evidence of commercial-grade permeability due to the presence of fumaroles that are
chemically connected to a neutral-chloride reservoir with a steam cap.
(2) The fumarole gas geothermometry indicates that the source fluids are likely to be being
equilibrated to a temperature of >572 °F (>270 °C). These temperatures could exist directly
beneath the fumaroles. Alternatively, if the upflow originates further west, the fumaroles may
mark the location where the outflow first encounters boiling conditions. In this case,
temperatures beneath the fumaroles could be in the range of 428-464 °C (220—240 °C).
(3) As the highest-enthalpy target, the fumarole area could be expanded to meet additional
power demand if it should ever present itself (e.g., new industrial capacity, larger scale
secondary use, etc.)
The exploration data suggests that the likely upflow location is in the general vicinity of the fumarole.
However, the fumarole is located ~1150’ (~350 m) up a very steep hillside, posing access limitations.
Hence, an important issue is the trade-off between the cost and practicality of constructing a pad closer
to the fumarole and drilling further directionally. A small rig may only be able to achieve a very modest
directional offset, but a rig capable of greater directional offset would be several times more expensive.
Two alternative surface locations have been sited to target the high priority upflow zone. Regardless of
the pad location, the well should be targeted to at least 4500’ (1350m) MD and preferably to 6000’
(1800 m) MD.
The first, preferred alternative “A” is located near the fumarole field (Fig. 14). This is closest to the high
temperature upflow zone. Access to this location could be via a road running east-west which skirts the
mountain to the north of HSBV. Such a road appears to be buildable at less than 3 miles (6 km) from Hot
Springs Bay access, but would require dock facilities to be built at the beach. However, this access
option raises the question of transmission to Trident and Akutan Village. It also raises questions about
volcanic hazards (see ‘Risks’ section, below).
The second, less preferable alternative is located in the Fumarole Valley >2/3 mi (~1 km) southeast of
the fumarole field (Fig. 14). A directional well drilled from this location (called “Well-1” in past reports)
beneath the fumarolic features or to the north beneath the local resistivity high may intersect the
upflow zone. From pad location “B”, the margins of the upflow resource would be targeted via
directional drilling and the outflow resource could be targeted via vertical drilling. However, limitations
Akutan Geothermal Resource Assessment
26
on directional drilling may not allow the target to be reached from pad location “B.” Hence, this wellpad
location is riskier than alternative “A.” A determination on this issue should be solicited from a qualified
geothermal drilling engineer before a final decision is made.
Figure 14. Map showing possible wellpad locations to target the Akutan upflow resource (blue triangles). Each pad
could host two wells – a directional well aimed towards the fumarole field (knobbed black line), and a vertical well.
Possible road alignments to the wellpads are shown in red. Also shown are the three sections recently selected by
the Akutan Corporation for subsurface ownership rights in a land swap agreement with the Aleut Corporation.
Should drilling at sites A and B fail, or if developing the upflow resource is not possible, subsequent pad
locations could be sited to target other parts of the outflow zone. A 380 to 428 °F (180 to 220 °C)
outflow resource target about 2200’ (800 m) to the northwest of TG-2 might be preferred if its higher
targeting risk and lower generation per well due to its lower temperature were sufficiently offset by
lower drilling and access cost.
Prior to generating outflow targets, additional exploration activities should be undertaken to target the
hottest and most permeable part of the outflow resource. This would likely require additional
subsurface imaging work using one or several geophysical techniques. It would also probably require
additional slimhole drilling and/or deepening of TG-2. Since the casing in TG-2 was not cemented in
place, it could be retrieved it and the well deepened by 1000’ (300m) or more.
Akutan
Corporation
Subsurface
Selections
1
36
12
A
A
A
B
Akutan Geothermal Resource Assessment
27
Capacity Assessment
As summarized by Glassley (2008), different approaches and methodologies to geothermal resource
capacity assessment have given rise to a broad range of results that are not directly comparable. Hence,
the outcomes of resource assessments are sensitive to the methodology and assumptions employed in
the analysis, and different studies often produce widely different estimates of a resource. The
assessment of reserves by analogy used in previous Akutan reports is updated here based on the results
of the wells and the fumarole gas geochemistry. The approach applied here is to use analogies to a few
published examples in order to highlight important similarities and differences with respect to Akutan.
Resource Existence and Size
Resource risk assessment approaches commonly divide the assessment into two parts; 1) an assessment
of confidence in the existence of a resource as a percent probability, and, 2) assuming the resource
exists, an assessment of its size, usually as a statistical distribution (e.g. Newendorp and Schuyler, 2000).
The probability of existence is sometimes restated as the probability of exploration success; i.e., the
probability that an exploration drilling program would discover at least one economically productive
well. In many published geothermal resource assessments, the assessment of existence is often not
explicitly evaluated but nominally included in the size distribution, for example, in the Western
Governors’ Association (2006), Clean and Diversified Energy Initiative Geothermal Task Force Report. In
this report, many geothermal prospects in the western USA with poorer indications of temperature over
220 °C and much lower surface heat flow than Akutan are assessed as having over 20 MW potential.
Confidence in Resource Existence
The most common method of estimating the probability of existence for a resource is to have a group of
experts review the available data and, based on analogous experience with other geothermal prospect
areas, estimate the confidence (as a probability) that the necessary components of a resource exist
together. For volcanic prospects that have hot springs with cation geothermometry similar to Akutan’s
and a non-magmatic fumarole, few failure cases exist in which the most attractive target was drilled.
With respect to the earlier assessments of Akutan, the very minor magmatic indications and excellent
gas geothermometer estimates of resource temperature have increased confidence in the existence of a
high enthalpy resource.
The numerous geothermal success cases differ in detail, particularly with respect to the geology and
very dilute chemistry characteristic of Akutan. For example, in the Americas there are several developed
geothermal fields in volcanic systems with different geologic settings but broadly similar liquid and/or
gas geochemistry. The 572 °F (270 °C) San Jacinto Field in Nicaragua has 10 MW installed and 72 MW
under development. It has roughly analogous fumarole gas geochemistry and a similar area of intense
alteration, although the resistivity of its clay cap is much lower, more like a conventional geothermal
field. The 320 to 350 °F (160 to 175 °C), ~40 MW Casa Diablo field at Long Valley (Sorey et al., 1991) and
the 320 to 360 °F (160 to 180 °C), 45 MW Steamboat Springs Field near Reno (Mariner and Janik, 1995)
have liquid chemistry similar to Akutan, but again, a lower resistivity clay cap. At Akutan, the
combination of a non-magmatic flank fumarole with excellent gas geothermometry over 518 °F (275 °C),
a trend in cation geothermometry to >428 °F (>220 °C), silica geothermometry over 320 °F (160 °C) with
Akutan Geothermal Resource Assessment
28
sinter deposition proven to exist in the subsurface by a well support the existence of a convecting
geothermal resource on Akutan with a high confidence of 80%.
Probable Resource Capacity
The capacity of the geothermal resource at Akutan in terms of electrical power can be assessed using
analogies, both the rough comparisons to the prospect estimates provided in the Western Governors’
Association report and the analogs to the 20 to 72 MW San Jacinto development and the 40 to 45 MW
Casa Diablo and Steamboat Springs developments. Because of the dilute outflow chemistry and low
permeability relict alteration at Akutan, handicapping the Akutan likely 320 to 358 °F (160 to 180 °C)
outflow resource by 75% relative to these developed reservoirs would be reasonable, giving an
analogous low temperature resource capacity estimate of 15 MW with a 66% probability. Because a
high temperature resource very likely exists, a more optimistic capacity estimate for the entire system
would be like San Jacinto, 10 to 72 MW with a 66% handicap because of its high clay cap resistivity and
difficult access, this results in a risk weighted estimate of about 20 MW. Using the Western Governors’
Association report assessments as analogs, an assessment as high as 100 MW seems reasonable.
An alternative approach
An output capacity for the 359 °F (182 °C) fluids produced by TG-2 was estimated in 2010 based on the
flowing temperature of the well and assumptions about flow rate (Kolker et al., 2011). Based on then-
available information, it was estimated that a production well drilled at or near the TG-2 site could
produce 1.34 MW up to a maximum of 2.38 MW. However, recent data and analyses including the
stabilized temperature curves, alteration mineralogy from cored rocks, have supported revisions of the
earlier assumptions used for estimating wellhead flow capacity were optimistic.
Monte Carlo Heat-in-Place Option
In previous reports on Akutan (Kolker, 2010), the heat-in-place method has been outlined but it has not
been formally applied. Initially developed by the USGS for rough regional estimates (Muffler, 1979),
more elaborate Monte Carlo versions of the method have recently been adopted by stock exchange
regulators in Australia and Canada as a standard for publishing geothermal reserves (Lawless, J., 2010).
Despite its common use by geothermal investors, as detailed by Garg and Combs (2010) and more
generally considered in the context of other methods by Grant and Bixley (2011), Monte Carlo heat-in-
place approaches are commonly misleading and difficult to validate. If such an analysis is needed to
meet a reviewer’s request, the City of Akutan could consider employing a large consultancy that
routinely provides such analyses to meet regulatory needs, like GeothermEx or SKM.
Akutan Geothermal Resource Assessment
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Resource Risks
The major volcanic hazard posed to a geothermal development on Akutan is ash fall. The modern
volcanic complex forms the western half of the island, and future eruptions are unlikely to affect the
eastern portions of the island (Ancestral Akutan), including HSBV. Destabilization of the fumarole area,
at an elevation of ~1150’ (350 m; see Fig. 1), may generate debris flows, and such deposits are seen in
the valley floor. According to the hazards map of Akutan, it is possible that the entire HSBV could be
inundated by cohesive lahars associated with small-scale slope failure(s), but not likely. Another possible
but unlikely hazard is a pyroclastic flow near the fumarole field (Waythomas et. al., 1998).
Re-injection beneath the surface is the most environmentally responsible means of disposing of the
produced fluids. Re-injection also supports reservoir pressure. Normally the fluids would be injected
back into the reservoir because this is where adequate permeability exists. Because of the possibility
that HSBV is fault-controlled, reinjecting the fluids into the shallow aquifer incurs a high risk of pre-
mature thermal breakthrough to the producing wells. This risk can be investigated through pressure-
interference and possibly tracer testing as additional delineation wells are drilled.
Calcite scaling in the production wells and silica scaling of the production pipeline system and injection
wells are both possibilities for the Akutan system. However, gas levels are likely to be moderate and
calcium and silica concentrations are low, suggesting that scaling risks should be low. Silica is unlikely to
achieve a significant level of supersaturation should the fluids be produced to a binary power plant.
Also, the risks of silica precipitation can be mitigated through pH modification of the produced waters .
In addition, the potential for producing acid corrosive fluids at Akutan is very low . The hot springs and
discharge waters demonstrate that the reservoir hosts a near neutral chloride reservoir.
Further assessment of the risks would require the acquisition of additional well performance data, such
as interference testing, and additional geochemical samples from the production and injection zones.
Upflow Development Risks
Two key questions remain unresolved concerning the deep high temperature (“upflow”) resource. The
first is the location of the upflow to the system. The second is the volume of the high temperature
resource. Resolution of these questions would require additional drilling and possibly the acquisition of
additional resistivity data near and to the west of the fumaroles. A reasonable minimum size at a 10%
confidence level would be the area covered by the fumaroles and gassy alteration. If this is taken to be
roughly 1500 ft2 (0.5 km2) then using the base case numbers for power density of 15 MW/km2, a
reasonable minimum for expected capacity of the upflow is 8 MW after Grant and Bixley (2011).
The risk of volcanic hazards should be carefully investigated if the wellpad were to be sited near the
fumarole field on the flank of Akutan volcano, as that location lies on a possible path of pyroclastic flows
from Akutan volcano (Waythomas et al, 1998).
Fumarole gas contents are 3.5 to 4 wt. %, which do not present an obstacle to development. A well
drilled beneath the fumarolic complex is also at a low risk of encountering acid fluids, because the gas
chemistry is indicative of a neutral chloride upflow to the geothermal system.
Akutan Geothermal Resource Assessment
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Outflow Development Risks
Other than the general risks mentioned above (injection breakthrough, scaling/corrosion, etc), the most
important development risk of the outflow resource discovered by core hole TG-2 is permeability and/or
volume limitations. Neither of the coreholes encountered clay alteration characteristic of a well-
developed smectite-rich argillic caprock. Perhaps the argillic alteration did not develop because of
dense lavas, or higher rank alteration that does not retrograde, or it never became well-developed, or
perhaps it was eroded off. Resistivity profiles of the HSBV also suggest only a very thin conventional clay
has formed over the outflow system, close to the hot springs (Kolker et al., 2009). The rocks in general
appear to be only weakly altered. This implies that HSBV does not host a substantial volume of hot fluid,
making it a risky development target.
Mineralogical studies of TG-2 core rocks suggest that secondary mineralization of permeable fractures
has “sealed” the outflow area, restricting flow. Analysis of the temperature profiles and flow behavior of
TG-2 suggests that the produced fluid of 359 °F (182 °C) was “pulled” into the system from elsewhere.
The subsurface source of the 359 °F (182 °C) fluid is unknown and therefore targeting this resource is
highly risky.
Additionally, there is a high risk that exploitation of the shallow reservoir could result in rapid enthalpy
declines during exploitation. The risk arises from any of the following: recharge of the reservoir by sea
water; cold meteoric water influx from near surface aquifers; and breakthrough from injection wells.
Groundwater influx probably poses the most significant risk. There are also a large number of
connection points between the shallow thermal aquifer and the surface along the outflow path. Any
pressure decline as a result of exploitation would likely allow these colder waters to descend into the
reservoir and cool the production wells.
Conclusions
The Akutan geothermal resource can be divided into an upflow zone and one or more outflow zones.
Studies of alteration minerals in the core suggest that the outflow resource discovered by TG-2 is likely
to have significant permeability limitations (Stelling and Kent, 2011). The outflow resource of 359 °F (182
°C) discovered by slimhole exploratory drilling in 2010 appears to have migrated from a more distal
source and may not be commercially developable. A temperature reversal at the bottom of the
stabilized TG-2 profile reduces the possibility that a hotter or more voluminous reservoir would be
encountered by drilling deeper at that location. These conclusions indicate that earlier estimates of
production capacity of the outflow resource discovered by slimholes are inaccurate, because the flow
assumptions for this estimate appear to have been overly optimistic.
The hottest modern zone in the TG-2 core is at 585-590’ (178-180 m), with a static measured
temperature of 338 °F (165 °C). The occurrence of wairakite, epidote and prehnite suggest that this
zone was permeable during the earlier higher temperature alteration event. The outflow zone
penetrated by the exploratory slimholes shows evidence of “self-sealing” through mineralization of
primary and secondary permeability channels. The chlorite- and zeolite-dominated hydrothermal
mineralogy of wells TG-2 and TG-4 indicate that a lower temperature alteration assemblage has been
overprinted on a higher temperature assemblage. The higher temperature alteration assemblage
contains illite, epidote, prehnite, and adularia. The mixed layer clays, illite-smectite and chlorite-
Akutan Geothermal Resource Assessment
31
smectite, and zeolites are part of the lower temperature retrograde assemblage corresponding to
temperatures of 300 – 430 °F (150-220 °C). The overprinting observations can be explained by a
vertically-limited outflow system in a waning phase after attaining higher temperatures, which suggests
that a deeper well drilled at or near the location of TG-2 is not likely to encounter the hotter fluids
predicted by chemical geothermometry. Since those fluids appear to be flowing laterally along the water
table towards Hot Springs Bay, the source fluids are probably westward up the valley towards the
fumaroles and/or southwest to the valley junction and then northwest (Figs 10 and 12).
Future drilling should target the upflow resource as the highest-grade, lowest-risk part of the system.
The upflow source fluids are likely to be within the range of 428-572 °F (220-300 °C), and are chemically
benign. The estimated output capacity of the upflow target is 15-100 MW by analog analysis, with a
minimum output of 8 MW based on pessimistic volume considerations.
If developing the upflow resource is not possible, the hottest part of the lower-grade outflow zone could
be targeted but with greater risk.
Recommendations
The unresolved resource properties and risks can only be addressed by additional characterization of the
resource through drilling. An evaluation of the access, drilling and financial issues involved in targeting a
well on the upflow resource below the fumarole should be top prioirity at this time. Future well(s)
should be directed west beneath the fumaroles or north to a postulated upflow beneath a local
resistivity high. The well should attain a minimum depth of 4500’ (1350m) to insure that it penetrates
through the reservoir top, although a target depth of 6000’ (1800m) would be preferable in order to
better establish reservoir thickness. As evaluation of the resource potential continues, obtaining
samples of separated steam and water from the production wells will be valuable for further assessing
the geochemical risks related to scaling and cold water influx. Pressure-interference and possibly tracer
testing will need to be conducted as additional wells are drilled.
The risks associated with drilling the upflow target could be mitigated by additional exploration work.
One focus of additional exploration work could be the identification of controlling structures (likely
faults). Since hot fluids are constantly plugging up the "plumbing" channels by depositing minerals in
open fractures, large-scale activity on faults is required to keep the system permeable. These faults must
exist, but none have been conclusively identified in HSBV or near the fumaroles. Hence, mapping large-
scale active structures controlling permeability in the Akutan geothermal field could reduce well
targeting risks. Aerial photography survey planned for summer 2011 may provide useful data.
Additional studies (LIDAR, seismic, or other geophysical methods) could supplement this investigation
after the initial review of the aerial photography.
Another exploration activity that could mitigate the upflow target drilling risk is extending coverage of
the MT survey further to the north and west of HSBV. This could be done with very limited additional
stations (possibly 10-20) with or without a helicopter, limited to the area in the direct vicinity of the
fumaroles. However, this additional data may not have a significant effect on the project risk assessment
of the outflow, especially along the north rim of the outflow. Because it is likely to be relatively
expensive and prone to severe wind noise, this activity is not top priority at this time.
Akutan Geothermal Resource Assessment
32
If the shallow outflow is deemed to be more suitable for geothermal development, an important risk
mitigation measure would be analog studies of more shallow outflows that have been developed for
power generation and/or space heating. Analog studies of similar geologic environments in Iceland
could be particularly useful for assessing risks associated with cold water influx and injection
breakthrough. Additionally, several low-cost additional studies could help characterize the outflow
resource. These include: (1) Fluid inclusion analysis of hydrothermal minerals in deposited in fractures in
core rocks; (2) Analysis of the fracture orientation within well cores; (3) Detailed clay analysis identifying
the percentages of illite in illite-smectite and chlorite-smectite. As with the upflow target, pressure-
interference and possibly tracer testing should be conducted as additional wells are drilled.
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