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HomeMy WebLinkAboutREFG Round III AES_Geophysical_Report Report on the Sub-surface Geology from Site Investigations in the King Salmon – Pikes Ridge Area to Locate an Exploration Drill-hole to Assess Geothermal Energy Potential Prepared for: Naknek Electric Association Geothermal Exploration Project King Salmon – Naknek, Alaska Peninsula Prepared by: Alaska Earth Sciences, Inc. 11401 Olive Lane, Anchorage, Alaska 99515 Summary Naknek Electric Association (NEA) has embarked on an exploration program to identify a source of geothermal energy that can supply the local community with a renewable source of power. To date the community has been entirely dependent on diesel power generation; the associated cost threatens the future of the community through higher domestic fuel costs. Efforts have focused on an area centered around the community of King Salmon on the Alaska Peninsula. For the last ten years NEA has been investigating the potential for developing a geothermal resource within acceptable transmission line distance to the local communities. To this end NEA contracted Alaska Earth Sciences (AES) in December 2008 to assist with ongoing geologic and geophysical investigations with an aim of locating NEA’s first geothermal exploration well. Geologic and geophysical studies prior to AES involvement were principally regional, focusing on publicly available regional geology, aeromagnetic and gravity data from state and federal government sources. Lapp Resources Inc. provided an interpretation based upon a remote sensing study of the area. This study was the first that targeted the King Salmon – Pikes Ridge area. The first of many recent surface site and non-invasive investigations began with a seismic survey conducted by CGG Veritas over the NEA selected target area. The survey was conducted in 2007 but the results produced no interpretable data despite considerable expense and effort. Quality control with respect to data collection appears to have been poor. The next phase of investigations involved soil sampling and sample boring to test trace element geochemistry and thermal gradients for indications of an active hydrothermal system . The surveys, analysis and interpretation were conducted by HDL but the results did not indicate an active geothermal system within the target area around King Salmon. Aeromagnetic interpretation by Centennial Geoscience Inc. gave an indication of the underlying structural complexities associated with the target zone. This specifically pertains to the northeast-southwest Lake Clark Fault zone and related fault zones, and a number of cross-cutting, conjugate northwest-southeast trending faults. AES directed a program of further geologic interpretation based upon the results of non-invasive ground magnetic, controlled and natural source audio-magnetotelluric geophysical surveys conducted jointly by AES and Zonge Geoscience. The results suggest the potential for uplifted basement rocks that may provide a higher than normal geothermal gradient suitable for an advanced EGS geothermal development program. Based upon the available geologic and geophysical evidence provided by a number of sources NEA has opted to cite the proposed geothermal exploration well G#1 on its allotment and drilling will begin in the late summer or early fall of 2009. i Table of Contents 1 Introduction ................................................................................................................ 1 2 Accessibility, Climate, Infrastructure & Physiography .............................................. 2 2.1 Accessibility ....................................................................................................... 2 2.2 Climate ............................................................................................................... 2 2.3 Local Resources ................................................................................................. 3 2.4 Infrastructure ...................................................................................................... 3 2.5 Physiography ...................................................................................................... 3 3 Geology ...................................................................................................................... 6 3.1 Regional Geology ............................................................................................... 6 3.2 Local Geology .................................................................................................... 7 4 History ...................................................................................................................... 10 5 Geophysics ............................................................................................................... 13 5.1 Aeromagnetic Surveys ..................................................................................... 13 5.2 Regional Gravity Surveys ................................................................................ 18 5.3 Seismic Reflection Studies ............................................................................... 18 5.4 Ground Magnetic Surveys ................................................................................ 23 5.5 Magnetotelluric Studies .................................................................................... 25 6 Conclusions .............................................................................................................. 29 7 References and Bibliography ................................................................................... 31 List of Figures Figure 1: Naknek Electric Association (NEA) allotment Regional Location Map ............ 4 Figure 2: Naknek Electric Association Location map - local area ..................................... 5 Figure 3: Regional Geologic Interpretation ........................................................................ 8 Figure 4: Remote Sensing Interpretation (Lapp 2006) ..................................................... 12 Figure 5: Aeromagnetic Survey Interpretation – reduce-to -pole map. ............................. 15 Figure 6: Aeromagnetic Survey Interpretation – Vertical gradient map. ......................... 16 Figure 7: Aeromagnetic Survey Interpretation with nort-northwest conjugate fault sets illustrated. ................................................................................................................. 17 Figure 8: Regional Gravity Anomaly Map ....................................................................... 19 Figure 9: Local Gravity Anomaly Map ............................................................................ 20 Figure 10: Detailed Gravity Anomaly Map covering the proposed geophysical survey areas. ......................................................................................................................... 21 Figure 11: Seismic Survey Shot Point and Receiver Station Location Map .................... 22 Figure 12: Ground Magnetic and CSAMT/NSAMT Survey Lines.................................. 24 Figure 13: 3-Dimensional CSAMT-NSAMT smoothed resistivity profiles .................... 26 ii List of Appendices Appendix 1: Aeromagnetic Survey Methodology Appendix 2: Geometrics Ground Magnetometer Specifications Appendix 3: CSAMT -NSAMT Survey Methodology Appendix 4: Seismic Reflection Survey Methodology Appendix 5: Location Maps Appendix 6: Aeromagnetic Maps Appendix 7: Gravity Maps Appendix 8: Ground Magnetics & CSAMT/NSAMT sections Appendix 9: DVD data disk with additional historical data compilation 1 1 Introduction This report intends to detail the geologic and geophysical site investigations carried out at the request of Naknek Electric Association (NEA) on its geothermal exploration project in the King Salmon – Pikes Ridge Area on the Alaska Peninsula. The following paragraph is taken from NEA’s website and summarizes the difficult energy situation faced by the local community: “Naknek Electric Association, Inc. (NEA) is exploring geothermal power production in an effort to improve its ability to provide reliable and affordable electricity. The cooperative faces an urgent need to identify sound alternatives to diesel generation due to the increasing and unpredictable costs of using fossil fuels. These costs threaten the economic health and sustainability of the Bristol Bay and Lake Region of western Alaska. The diesel fuel surcharge to NEA consumers necessary to cover jumps in the price of oil recently spiked the effective electric rate by about 68%, or more than $73.00 per month for a typical residential customer. New information indicates that development of geothermal power production will stabilize and lower electric rates in Naknek and in the region where approximately 6,500 people live in 25 isolated rural communities.” During the first half of 2009 Alaska Earth Sciences, Inc., on behalf of NEA, managed and conducted site investigations using a number of geophysical survey tools. The surveys would allow modeling of the sub-surface electrical and magnetic properties of the rocks underlying the King Salmon – Pikes Ridge area adjacent to NEA’s surface allotment. Interpretation of the sub-surface model would give a more clearly defined target and refine the location of a drill hole for a geothermal exploration well. AES was tasked with managing the geophysical surveys that were carried out across the target area and to integrate the results with other geologic data compiled from regional mapping, published reports and from deep drill holes carried out throughout the region. The geophysical surveys comprised: a controlled source audio-magnetotelluric (CSAMT) survey conducted by Zonge Geosciences, Inc. of Anchorage, Alaska; a natural source audio-magnetotelluric (NSAMT) survey also conducted by Zonge, and a ground-magnetometer survey coincident with the CSAMT-NSAMT survey was conducted by AES . This work was carried out in February and March 2009. The work outlined above was carried out in addition to earlier work carried out by other geophysical contractors which included desk-top analysis of publicly available satellite and photo imagery, and gravity by HDL and Lapp Resources Inc. of Anchorage, Alaska; aeromagnetic and geophysical data by Centennial Geoscience Inc. of Littleton, Colorado. NEA also contracted Veritas DGC Land of Houston, Texas to conduct a seismic reflection geophysical survey in December 2007. The results from that survey will be incorporated here though little information about the survey is available. 2 2 Accessibility, Climate, Infrastructure & Physiography 2.1 Accessibility The proposed Naknek geothermal well is sited near the town of King Salmon, Alaska. King Salmon is located on the Alaska Peninsula, about 290 air miles southwest of Anchorage, Alaska. King Salmon Airport, formerly King Salmon Air Force Base has two paved runways, one 8,500 ft long and the other 4,015 ft long, allowing for sizeable air traffic operations. Five daily flights from Anchorage to King Salmon are available on PenAir, a regional carrier. Air cargo operations are provided by Northern Air Cargo and Everts Air Cargo, two major statewide cargo carriers. Egli Air provides helicopter services at the King Salmon airport. Naknek is also accessible by barge in the summer months. The proposed site for the geothermal well is 5.3 miles northeast of King Salmon. It is accessible over the current main road and 1.9 miles of new road scheduled to be built to provide access to the well site, a distance of 7.2 miles from King Salmon. 2.2 Climate Climate at King Salmon is generally categorized as maritime, characterized by cool, humid and windy weather. Weather records from a climate station in King Salmon, covering a combination of 38 years, from 1955 to 2005, summarize the climate data as follows: summer temperatures, (June-July-August) range from 45.0° to 61.8° F; winter temperatures (Dec-Jan-Feb) range from 8.2° to 23.7° F. Precipitation averages 19.59 inches over the year, which includes 45.1 inches of snow. Skies are cloudy approximately 80% of the time and the area is frequently foggy in the summer months. Average wind speed is 9 knots, with occasional winds up to 70 knots. Ice break-up in the bay averages about April 6, with the break-up on the river averaging about April 18. The average date of the last freeze is late May and the average date of the first freeze is early September. Ice is usually solidified by November 11 in the bay, and on the Naknek River by November 25. 3 2.3 Local Resources The village of King Salmon and Naknek has a year-round population of 426+ people, as estimated in 2007. In the summer fishing season the population swells to several thousand. The village is a major transportation point for salmon from Bristol Bay, so many domestic and commercial services are available in both King Salmon and Naknek. The villages have a post office, an AC grocery store, a Wells Fargo bank branch, and various other services. There is a local health clinic in King Salmon, and another in Naknek. BC Contractors and Bristol Bay Contractors are located in King Salmon and heavy equipment and operators are available through both companies. 2.4 Infrastructure Commercial air travel between Anchorage and King Salmon is available several times per day. Daily air freight service is also available. During the summer months, barge deliveries are available for heavy equipment and supplies. King Salmon and Naknek have lodging, restaurants, cell phone service through a local provider, land-line phone service, internet service, cable TV, gas stations, a grocery store, public schools in Naknek, power plants, water and sewer service, and a health clinic. 2.5 Physiography The general topography of the area consists of an undulating, hummocky plain due to the glacial morainal upland physiography of the area. Relief at the proposed well site is approximately 70 ft above sea level. The vegetation consists mainly of tundra species; there are sparse forests of dwarf birch and small black spruce. Kodiak Egegik Akhiok Chignik Ouzinkie King Cove Port Lions Larsen Bay Old Harbor Sand Point Pilot Point Port Heiden Ekuk Naknek KarlukUgashik PerryvilleIvanof Bay King Salmon Nelson Lagoon 156°W158°W160°W162°W 154°W 59°N58°N58°N57°N57°N56°N56°N55°N55°NNEA Regional Location Map Figure 1 B r i s t o l B a y NEA Project Location K v i c h a k B a yS h e l i k o f S t r a i tAlaska Earth SciencesA Angel, R Young12 May 2009 0 25 50 75 100MilesNAD'83 Alaska Albers1:2,500,000 NEA Allotment632000 634000 636000 638000 640000 642000 644000 646000 648000 6504000650400065060006506000650800065080006510000651000065120006512000Naknek Electric Association, Inc.Geothermal Exploration Project Location Map NAD'83 UTM z4N 0 1 2 3 4 5 Km1:63,360 AlaskaEarthSciencesRyan Young1 Apr 2009 6 3 Geology 3.1 Regional Geology Several tectonic elements are present in this portion of the peninsula that forms the basis of the geologic framework. The Bristol Bay basin lies in a back-arc tectonic setting bounded on the south by the Alaska Peninsula and associated active volcanic arc, the result of the active subduction along the northeast-trending Aleutian trench. Two major northeast-trending and bounding fault zones are present: the Bruin Bay Fault system along the crest of the Aleutian Range to the southeast and the Lake Clark Fault system to the northwest. Both of these regional faults converge in Upper Cook Inlet where they are named the Castle Mountain Fault syste m. The Lake Clark is a graben-like extensional structure which has documented vertical of approximately 3000 feet and up to 16 miles of right-lateral displacement. The Bruin Bay has had up to 2 miles reverse (up-to -northwest) and 12 miles of left-lateral displacement. The Bruin Bay Fault is a major thrust fault that dips to the northwest. It can be mapped for over 300 miles from near Anchorage to the south shore of Becharof Lake. The fault parallels the Alaska Peninsula and divides it into two distinct geologic terrains. The two sub-terranes of the Alaska Peninsula terrane are characterized by radically different structural and metamorphic styles. The rocks of the Iliamna sub-terrane, which lie west of the Bruin Bay Fault, are characterized by metamorphism up to amphibolite-facies grade and intense folding. West of the fault there are metamorphic and volcanic rocks of Triassic and Early Jurassic age that are intruded by the Aleutian Range Batholith of Jurassic age and small stocks of Tertiary age. These rocks are locally overlain by non-marine Tertiary clastic rocks. East of the faults is a sedimentary province containing Late Jurassic and Cretaceous marine clastic rocks with abundant fossils and few small areas of non-marine Tertiary clastic rocks. This area also contains all the Quaternary and Recent volcanoes. The geothermal province associated with Katmai is east of the Bruin Bay Fault. Note that the study area is located west of the main fault. However, several significant volcanoes are located east of the Bruin Bay Fault. These include all of the volcanoes in Katmai National Park and Preserve, except for the Augustine volcano which is north of the park. The Lake Clark Fault is a 125 miles long, northeast-striking reverse fault that is along strike from the Castle Mountain Fault on the northwest side of Cook Inlet. The Lake Clark Fault is a linear magnetic low on regional aeromagnetic data. Other faults, such as the Telaquana Fault, have been inferred on the basis of parallel magnetic lineaments. The Mulchatna Fault runs parallel to these features and is well documented to the northwest. These lineaments also appear to show significant lateral offset. 7 North-vectored convergence throughout the Tertiary has created a series of northwest fault controlled arc-perpendicular structural zones that have accommodated differential subsidence that created sub-basins and Eocene-Oligocene (Meshik age) volcanic highs. In the Ugashik Lake area 65 miles south of the Naknek Lake area seismic studies and the nearest 3 wells document the development of a 1.87 miles thick sub-basin flanked on the south by a thick Meshik volcanic center (Decker et al., 2008). The sub-basin is filled with marine to non-marine locally coaly sediments of the Stepovak Formation (Meshik volcanics age-equivalent) sitting on Jurassic or older crystalline basement overlain by younger Bear Lake and Milky River Formation. A series of northwest trending fault structures and very young Holocene dikes are present south of a sharp bend in the Bruin Bay Fault in the Valley of Ten Thousand Smokes (the center of the famous eruptive event at the beginning of the last century). This northwest trend strikes into the Pikes Ridge study area and is strikingly analogous to the Ugashik Lake study area which predicted basinal development to the south and uplift to the north at a northwest bend in the Bruin Bay Fault; complete with recent volcanoes. The regional interpretation of the geology is summarized in Figure 3. 3.2 Local Geology In the Naknek Lake area the geology mapped immediately north and east of the glacial moraine covered project area is Meshik age mafic volcanics. Along the east side of Naknek Lake they are sitting unconformably on lower Jurassic Talkeetna formation sediments and volcanoclastic s. The Talkeetna is considered the basement along with some older Triassic metamorphic rocks. The basement has been intruded extensively by mid -Jurassic granitic bodies and locally by mid-Tertiary granitic bodies. 155°W 155°W 156°W 156°W 157°W 157°W 59°N59°N58°30'N58°30'N58°N58°N57°30'N57°30'NNaknek Lake Area - Regional Geology Lake Illiamna U D DU U USGS Geology with additional structural interpretation by AES NEA AllotmentGeophysical Survey LinesLake Clark Fault ZoneGraben Structure D 26km offset BasementUp Uplift Valley of 10,000 SmokesBruin Bay Fault Zone19km offset UgashikSub-Basin Block FaultedDown Be c h a r o f D i s c o n t i n u i t y RecentVolcanoes Block FaultedDown RecentVolcanoes Alaska Earth SciencesMarch 2009Interp by WTEDrafting by RPY 0 10 20 30 40 50KmNAD'83 Albers Equal Area Figure 3 Lake Illiamna Dillingham-Illiamna Geology Unconsolidated Qs Igneous rocks Tpg Tmv Tmf Tmba TKgd Alaska Aleutian Range - Sedimentary rocks Tcl - Talkeetna Formation Jtk - Kamishak Formation Trku Trkm - Structural assemblages JPk - Igneous rocks Togd Jqd Jmu Naknek-MtKatmai-Ugashik Geology Sedimentary Sedimentary Rocks Qa Qaf Qb Qmt Qm - Tertiary Tbl Th - Cretaceous Kc Kk Kp Khe Kst- Jurassic Jn Jni Jnk Jns Jnn Jnc Js Jk Jt- Triassic Trk Igneous - Volcanic - Quaternary Qv Qpd QTv - Volcanic - Tertiary Tv Tm - Volcanic - Triassic Trv - Intrusive - Tertiary Ti Tiu Tgd Tqd - Intrusive - Jurassic Jgd Jgr Jqd Jgb Metamorphic - Mesozoic JPk Trcother Water Glacier Altered zones Hornflesed zonesGeologic Line Symbols Contacts - certain - approx - inferred - concealed Faults - certain - approx - inferred - concealed Thrust Faults - certain - approx - inferred - concealed Other Shoreline or Riverbank Caldera or Crater Rim Ice Contact Naknek Lake Area - Regional Geology - USGS List of Symbols Figure 3a 10 4 History There appears to be relatively little history of any exploration work carried out in the proposed area surrounding the allotment prior to the recent interest by NEA. There are no deep exploration wells for oil and gas and there is no history of adjacent geothermal testing or drilling; there are no adjacent seismic survey lines. However NEA has been investigating geothermal energy sources for the last ten years in geothermal energy. The previous sentence makes no sense. NEA states that they have “…found considerable data to justify further exploration. That research was within the Katmai National Park and Preserve.” Three drill holes were completed approximately 65 miles south-west of the proposed geothermal exploration well site (G#1). Detailed well log information is available for the closest of these wells: Becharof #1, Great Basin #1 and Great Basin #2. These three wells form the basis of the litho-tectonic interpretation of the Ugashik area in the Decker et al., 2008, paper “Structural linkage of major tectonic elements in the Ugashik-Becharof Lakes region, northeastern Alaska Peninsula”. The State of Alaska, Department of Natural Resources – Oil and Gas Division published a map sheet in 2004 entitled “Oil and Gas Well Data for the Alaska Peninsula and Bristol Bay Region” and part of a four map series covering the oil and gas potential of the Alaska Peninsula and Bristol Bay Region. Copies of the well logs for the three drill-holes are included in the appendices on the digital DVD copy of this report. In 2007 NEA contracted HDL to conduct a preliminary geologic evaluation of Naknek’s geothermal sources. A number of studies were conducted prior to June 2007, these included: • Soil sampling at several locations throughout the Naknek area. • Advancing three borings to between 255 feet and 400 feet. A full and comprehensive write up of the investigations is included in the report: Preliminary Geological Evaluation, Naknek Geothermal Sources, Alaska; HDL 07-302 (Dillie , 2007). A remote sensing study was undertaken by Lapp Resources Inc. and is included as part of the report. The interpretation of this study proved critical to the location of the drill hole as it identified the final chosen location as target #3 and is highlighted in Figure 4. 11 The HDL (Dillie, 2007) report concluded the following: “At this time (September 2007) a resource has not been confirmed. An exploration followed by a confirmation phase needs to be conducted prior to any decisions about type of power plant and number of wells. We would recommend that two or more of the following exploration methods be conducted for assessing the local thermal and hydrological gradient: • Accurately and uniformly characterize the chemistry of local springs and river • waters; • Conduct shallow temperature probes in select areas to develop a refined picture of • the local geothermal gradient; • Conduct seismic an/or gravity studies to identify local faults and bedrock structures; • Conduct CO2 gas surveys to identify potential faults in select areas; • Conduct electric and/or magnetotelluric methods to identify argillic alteration locations • Target potential well locations • Develop a hydrological model of the region by reviewing existing literature and studying deeper wells in the area • Consider potential locations closer to the volcanoes in the region.” 13 5 Geophysics 5.1 Aeromagnetic Surveys The principal purpose for undertaking the aeromagnetic investigation was to examine deep-seated faults and fractures, which may provide conduits for geothermal fluids. Within the target area, there are several major structural features. The most important of these features are the Lake Clark Fault and structures parallel and perpendicular to the Lake Clark Fault. While mapped as single features, it is most likely that these faults are part of much bigger systems. In Figure 5 the color image is the starting point for processing - the reduced to pole magnetics. Interpreted contacts have been overlain on the data, with NEA’s primary focus area highlighted by the red box just to the left of center. Magnetic highs are red and lows are blue. A pronounced magnetic lineament cuts across the study area. This lineament is associated with the Lake Clark Fault trend and is probably an unmapped sub-parallel fault which makes up this major system. Note the prevalence of this trend in the study area. When the reduced to pole field is replaced with the vertical gradient of the magnetics as shown in Figure 6 the magnetic contacts become even more evident. A third trend in the immediate area has not been drawn in on the interpretation. It may be significant, however. Several of the magnetic contacts and the magnetic highs that they bound appear “broken”. A conjugate north-northwest trending lineament system can be drawn between these breaks, shown in Figure 7. This trend fits well with the existing hypothesis that there is significant right-lateral movement along the Lake Creek Fault system. In this system, the yellow contacts would have left-lateral movement associated with them. Hence, this break may be another component of an unrecognized right-lateral wrench fault system. In the NEA primary focus area the following points are interpreted from the aeromagnetic survey data: • Across the data set as a whole, many magnetic trends can be observed. However, there are few areas where lineaments are spaced so closely and trend parallel to the extent seen in the immediate area around the study area. (Another area is located approximately twenty miles to the southwest.) • The orientation of these lineaments runs sub-parallel to the Lake Clark Fault. These contacts have been interpreted as subsidiary faults. In all likelihood each of these lineaments is not a single fault but zones of closely spaced faults or fractures. 14 • These trends appear more closely related to geothermal processes in the Katmai National Park. Despite the proximity of the Mount Katmai geothermal system, its location to the east of the bounding Bruin Bay Fault indicates that it may be a separate system altogether. • There is a fault and fractures system present in the study area that would allow for geothermal resource exploration. It must be noted that the presence of these faults and fractures does not guarantee the presence of geothermal fluids, nor the amount of heat contained in them if they do exist. 18 5.2 Regional Gravity Surveys A desktop review of the publicly available data has been conducted by HDL (Dillie, 2007) and Centennial Geophysics report. Both reports conclude that there is a large Bouguer gravity anomaly high in the King Salmon area but that the resolution of the regional gravity data is too coarse to provide more information. 5.3 Seismic Reflection Studies Veritas DGC Land of Houston, Texas conducted a 3-dimensional seismic reflection study of the area containing the proposed drill site and allotment for the geothermal exploration well. The seismic survey was conducted in December of 2007 by Veritas for NEA directly and the data tapes were handed over once the survey was complete. It is unclear what processing has been conducted on the data contained in the tapes as no formal reports have been written . A map showing the location of the shot and receiver stations is shown in Figure 10. The survey consisted of 5 shot lines with a total of 312 source points and 11 receiver lines with a total of 599 receiver stations. Attempts were made by various groups to work with the data. The following paragraphs represent the final attempt to extract useful information from the 3D datasets by Excel Geophysical Services of Greenwood Village, Colorado. “The biggest problem with the datasets after analysis was the sparse shooting and receiving parameters. The long offsets and lack of near offset information in the zones of interest, along with low stack fold, proved to be the big problem.” Excel is preparing a list of the processing approaches implemented to extract meaning from the data. Though at the time of writing this report that list had not been received. “Excel also does not believe there is any usable 'structural' information in the 3D seismic data. The most that can be gained from the data is a depth to the top of the volcanic layer, or thickness of the glacial deposits. This 'depth' or 'thickness' is also unqualified because of an assumption on the velocity. Excel looked closely at velocities that make perfect sense for the Qt deposits, so that interpretation may be relatively correct. It appears the deepest/thickest glacial deposits are under the topographic high. Assumedly, the scoured channel was backfilled w/ morainal deposits upon retreat of the ice. This information only relates to the upper ~100m or ~200-300 feet and was clearly not the reason that a 3-D survey was conducted.” Kodiak Egegik Akhiok Chignik Ouzinkie King Cove Port Lions Larsen Bay Old Harbor Sand Point Pilot Point Port Heiden Ekuk Naknek KarlukUgashik PerryvilleIvanof Bay King Salmon Nelson Lagoon 156°W158°W160°W162°W 154°W 59°N58°N58°N57°N57°N56°N56°N55°N55°NRegional Gravity Anomaly Figure 8 B r i s t o l B a y NEA Project Location K v i c h a k B a yS h e l i k o f S t r a i tAlaska Earth SciencesA Angel, R Young12 May 2009 0 25 50 75 100MilesNAD'83 Alaska Albers1:2,500,000 Egegik Pilot Point Ekuk Naknek KarlukUgashik King Salmon 155°W156°W157°W158°W 59°N58°30'N58°30'N58°N58°N57°30'N57°30'NLocal Gravity Anomaly Figure 9 B r i s t o l B a y K v i c h a k B a yS h e l i k o f S t r a i tAlaska Earth SciencesA Angel, R Young12 May 2009 0 10 20 30MilesNAD'83 Alaska Albers1:1,000,000 NEA Project Location Naknek King Salmon 156°W156°30'W157°W 58°45'N58°45'N58°30'N58°30'NDetailed Gravity Anomaly Figure 10 NEA Project LocationK v i c h a k B a yAlaska Earth SciencesA Angel, R Young12 May 2009 0 5 10 15MilesNAD'83 Alaska Albers1:400,000 23 “There is no useful seismic data "in" the volcanic section, and less than none below that. The latest reflections are on the order of 200-300 ms (two-way travel time), in your 5 second records. At best, 300ms of data represents <1,500 feet of penetration or subsurface layer/structural information. Even the 300ms reflection "event" is mostly incoherent. The rest of the section is completely incoherent, and no reflections stack into the bins/section.” Excel recommended that no further processing be carried out on this data set. Excel sincerely hoped that they could have achieved more with the data set, but it has too long of offsets, and too wide of field set-up to make use of the coverage. Excel stated “We do believe, however, that high-fold tight coverage 2D data would have potentially been more useful than all of the 3D data; and, as recommended - collect 2D post-haste.” Excel had confirmed what previous groups had identified and that was that the 3-D data collected by Veritas DGC Land is of no practical use in assessing the sub-surface geology in any way. 5.4 Ground Magnetic Surveys AES conducted a ground magnetic survey in conjunction with the CSAMT / NSAMT survey being conducted by Zonge Geoscience Inc. A Geometrix 859 Roving magnetometer with integrated GPS was used and the results data sent to Zonge for processing and interpretation. A map showing the combined ground magnetic and CSAMT -NSAMT survey is shown in Figure 11 Detailed ground magnetic profiles acquired along the five Naknek MT lines show details missing from regional aeromagnetic coverage. Naknek ground magnetic profiles have rapidly varying anomaly patterns that could only be produced by shallow sources. The most likely sources for much of the character in the ground magnetic data are shallow Tertiary volcanics, which outcrop just east of the survey area (Riehle et al., 1993). It is highly likely that the volcanics extend westward across the survey area below a relatively thin layer of glacial till overburden. Additional interpretation of the ground magnetic data is incorporated with the CSAMT / NSAMT discussion below in Section 5.5. 25 5.5 Magnetotelluric Studies Zonge Geosciences Inc. of Anchorage, Alaska was contracted by AES on behalf of NEA to carry out a controlled source audio-magnetotelluric/natural source audio magnetotelluric survey across the project area, focusing on the area surrounding NEA’s allotment. The survey lines and CSAMT transmitter station location are shown in Figure 11. A detailed report on the CSAMT-NSAMT methodology applied to this survey is included in Appendix 3. The following paragraphs here represent only the conclusions reached by Zonge’s geophysicists. A state -wide aeromagnetic data compilation (Saltus et al., 1999) provides a good overview of regional geologic structure. Near Naknek, the compilation is primarily based on a 1955 USGS survey with north-south flight lines spaced 1.87 miles apart (Andreasen et al., 1963). A more recent 1976 USGS aeromagnetic survey also overlaps the Naknek - King Salmon area, but its east-west flight lines are spaced 6.24 miles apart and only two of them cross the project area. The 0.62 mile grid spacing of the aeromagnetic data compilation is consistent with the wide flight line spacing in the original data set, but does not include the detail possible from a high resolution survey. Naknek's aeromagnetic coverage is smoothed by the wide source-data spacing, simplifying the resulting magnetic anomaly patterns. Even with smoothing, the regional aeromagnetic data is very helpful and is interpreted in a report by Centennial Geoscience (Fagan, 2008). The report interprets a N60E-S60E magnetic low across the Naknek project area as an expression of the Lake Clark Fault trend. It also interprets edges in magnetic anomaly patterns as an indication of cross-cutting wrench faults that intersect near the center of the Naknek project area. Detailed ground magnetic profiles acquired along the five Naknek MT lines show details missing from regional aeromagnetic coverage. Naknek ground magnetic profiles have rapidly varying anomaly patterns that could only be produced by shallow sources. The most likely source for much of the character in the ground magnetic data are shallow Tertiary volcanics, which outcrop just east of the survey area (Riehle et al., 1993). It is highly likely that the volcanics extend westward across the survey area below a relatively thin layer of glacial till overburden. While magnetic surveys map the geology's magnetic susceptibility, CSAMT and MT measure a different physical property, resistivity. The three-dimensional resistivity coverage from Naknek CSAMT/MT surveys can be characterized by both its vertical and lateral variation. Fence plots of the CSAMT/NSAMT survey have been organized to give a 3-dimensional diagram shown in Figure 12. In terms of vertical structure, Naknek CSAMT/MT smooth-model resistivity sections have a layered pattern over most of the survey area. A complete set of the section plots is included as fold-outs in Appendix 8. Glacial till overburden is resolved as a thin, high 27 resistivity layer on lines 1, 2 and 3. The glacial till layer is 160 – 320 feet thick and has a resistivity of 200 to 300 ohm-m. The till overlies an irregular layer of 50 to 100 ohm-m that is up to 1300 feet thick, most likely the response from Tertiary volcanics. Below the resistive Tertiary volcanic layer, resistivities drop to form a low resistivity layer of 5 to 50 ohm-m, labeled as layer 3 on the annotated smooth-model sections. The rock type of layer 3 is not known. Geologic mapping by Riehle et al., 1993, shows interbedded marine sediments and volcanics of the Talkeetna Formation just below the Tertiary volcanics in outcrops near Naknek Lake. Reilhe et al., 1993, also show a small outcrop of Kakhonak Complex metamorphic rock outcropping south of Naknek River at the western end of Naknek Lake. Naknek CSAMT/MT sections have a fourth layer of 50 to 200 ohm-m at depths of 0.6 to 2 miles. The correspondence between resistive layer 4 and geologic rock type is currently unknown. In a project area 100 miles north of Naknek, resistivities from a CSAMT survey show a resistive 500 to 2500 ohm-m response from Cretaceous volcanoclastics and sedimentary rocks. Below the resistive layer 4, Naknek resistivities drop below 50 ohm-m in the southern and southeastern edges of the survey area, indicating a low resistivity area at a depth of 2 miles or more (Figure 12). The overall layered pattern is distorted by lateral resistivity variation, particularly on north-south line 4. Layers 2 and 3 appear to thin to the north along line 4 and are truncated near line 4 station 16500. North of line 4, station 16500, a resistive block with large depth extent is visible on the smooth-model resistivity section. The resistive block is coincident with a prominent high in the magnetic data. Both the MT and magnetic data show a geologic break at line 4 station 16500. South of line 4 station 16500, the layered resistivity pattern is offset by a series of steeply south dipping breaks, consistent with a sequence of cross-cutting faults. Another distinctive result from the CSAMT/MT survey is a narrow resistive feature, labeled Feature A on the annotated model sections. Feature A's pattern is distinct in every MT line except line 4. Feature A is a resistive ridge that appears to bow low-resistivity layer 3 upward and it is also associated with a ground magnetic profile peak. Feature A's morphology, high resistivity and high susceptibility is consistent with a mafic dike intruding upward through a layered sequence of basement rocks. Feature A trends N60E-S60W across lines 1, 2 and 3, but intersects line 5 at station 5000. Its intersection with line 5 is significantly displaced from the projection of its trend across lines 1, 2 and 3. Feature A either folds to the south or offsets by a cross-cutting fault between MT line 1 and line 5. In either case, the offset is a strong indication of secondary geologic structure cutting across the dominant regional structural trend of N60E-S60W. Although dominate trend in Naknek area geology is N60E-S60W, both the magnetic and MT surveys indicate that the Naknek area also has secondary cross-cutting structures like the NW-SE trending wrench faults interpreted by Centennial Geoscience's review of regional aeromagnetic data. The offset of Feature A between MT lines 1 and 5 is also evidence of cross-cutting structure. Although the correspondence between deep resistivity 28 units and geologic lithology is currently unknown, the geophysical data are providing information about geologic structure in the Naknek project area. Structural patterns from the geophysical data can be used to extrapolate future drilling results laterally from drill-hole points to the Naknek project area. If any future geophysical work is contemplated, there are gaps in the current geophysical coverage that could be filled. Regional aeromagnetic coverage provides a helpful overview of geologic structure, but it is based on widely spaced flight lines and is missing significant detail that would be revealed by a high resolution survey with closely spaced lines. The MT data indicate cross-cutting structure, but coverage could be extended to provide a more complete picture. Feature A is either folded or faulted in the unmapped wedge between the northwest end of MT line 1 and the west end of line 5. Tensor MT data indicate the most two-dimensional resistivity structure when components are rotated to N30W/N60E, which indicates that the optimal MT line orientations are either across or along the N60E strike of the dominant trend in Naknek area geology. Orienting MT lines to align with the dominant geologic fabric will improve results from surveys using contiguous along-line measurements, but more widely spaced across-line components. Naknek geology is sufficiently complex to require multiple lines to track offsets in the trace of distinctive resistivity anomalies like Feature A. In the Naknek Lake – King Salmon – Pikes Ridge study area it could be argued that a northwest trending bounding structure has uplifted basement to the south and down-dropped, and preserved, the Meshik volcanics to the north. That interpretation is corroborated by the apparent break in the geophysical signature along MT line 4 at ~ 9500N which can be interpreted as bringing the (radiogenic?) basement up on the south end of the line figure 12. The Meshik volcanics exhibit a noisy magnetic signature and have variable conductivity. They appear to thicken from 0 on line 4 & 5 to ~ 0.6 feet thick and dip gently to the south and east. Several conductive breaks are present on line 4 that could be interpreted as northwest trending block faults with a thickening basin (Stepovak sediments (?) below Meshik volcanics) developing to the north of 9500 on line 4. There is another distinctive north-northeast trending feature observed on several of the geophysical lines. It is a distinctive 91-122 foot wide, magnetic, shallow (<61 ft) resistive dike-like feature that extends through the entire survey area. At 5000W on line 5 it domes the conductive unit, on line 2 it is at 5000NW, and on line 4 it is at 12,400N. 29 6 Conclusions Naknek Electric Association (NEA) has conducted a number of studies in the northern Alaska Peninsula to identify a source of alternative energy. From a purely socio-economic standpoint the communities served by NEA are faced with an uncertain future. Both industrial and domestic customers are reliant on a single source of domestic energy, namely diesel, and this places the local communities at the mercy of markets over which they have no control. Last years (2008) spike in oil prices brought this reality home as utility bills rose. Standard economic models dictate that any business dependent upon a single source of income is relying on a status quo in its supply. Short-term variations in source parameters can be mitigated and long-term, slower changes can be addressed. However the increasingly fractious and unstable markets can have disastrous consequences. To this end it is simple to understand NEA’s decision to fund an extensive research program into diversifying its energy portfolio. Conventional hydrocarbon sources (e.g., coal, coal-bed methane, oil and natural gas) do not offer a cost effective option. The levels of investment required, combined with the distance to the consumers, to develop new hydrocarbon or coal fields are high and therefore prohibitive. In consideration of this, only alternative energy sources (e.g., wind, solar, wave and geothermal) offer a relatively low cost option in the vicinity of NEA’s distribution area. Of these, only geothermal power can offer a consistent and year round energy source when taking local climate variations into account. Factoring the variables into account NEA embarked on a program to identify a geothermal energy resource within reasonable transmission line distance of its principal consumer communities. NEA has conducted relatively comprehensive technical studies over the last few years. The results have, to date, been generated by a number of non-invasive surface geophysical, remote sensing and seismic studies, and limited near surface geochemical and geothermometric investigations at less than 500 foot depth. Results have been mixed. A 3-dimensional seismic study conducted across the area provided no useable information. Geochemical and sub-surface thermal studies do not indicate a near surface active hydrothermal system. Remote sensing and geophysical investigations (air- and ground- magnetics; CS and NSAMT) integrated with geologic study has given new insight into the structural fabric of the northern Alaskan Peninsula. This includes the potential for a crystalline basement structural high which may give an elevated geothermal gradient. None of these studies have indicated a near surface source of geothermal energy within “easy reach.” All the studies written to date, and made available at the time of writing of this report, conclude that the area falling within NEA’s selection criteria does not show any near surface expression of an economic geothermal resource. 30 It should be noted that the preceding statements pertain to the near surface. No deep sub-surface (>500ft) investigations have been conducted close enough to the target area to allow reasonable geologic correlation. Deep drill holes are a minimum 65 miles from the target area and current literature supports the idea that correlation in this terrane is difficult. NEA has therefore decided to undertake a full-bore deep drilling exploration program to investigate the potential for developing an enhanced geothermal system (EGS). The drill hole will target depths in excess of 10,000 feet and will test the concept of an elevated geothermal gradient in excess of 2ºF per 100ft. The combination of gradient and depth could produce sufficient heat to provide the 5-10 MW of power required. The exploration drill hole probably represents the lowest cost option available to NEA when taking all other factors into consideration. 31 7 References and Bibliography Allen, E.T. and Zies. E.G., 1923, A Chemical Study of the Fumaroles of the Katmai Region. National Geographic Society, Contrib. Tech. Pap., Katmai Series, 275155. Barnes, I. and McCoy, G.A., (1979) Possible role of mantle-derived C02 in causing two "phreatic" explosions in Alaska, Geology v. 7 Andreasen, G.E., et al., 1963, Aeromagnetic map of part of the Naknek quadrangle, Alaska; USGS Geophysical Investigations Map GP-353. Centennial Geoscience 2008, Summary report for Naknek-King Salmon Area Public Domain Aeromagnetics Processing. Unpublished report prepared for Naknek Electric Association. Cross, R., and Freymueller, J.T., 2007, Plate Coupling Variation and Block Translation in the Andreanof Segment of the Aleutian Arc Determined by Subduction Zone Modeling Using GPS data, Geophys. Res. Lett., 2006GL029073. Cross, R., and Freymueller, J.T., Evidence for and Implications of a Bering Plate Based on Geodetic Measurements from the Aleutians and Western Alaska, submitted to J. Geophys. Res., April 2007. 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