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HomeMy WebLinkAboutSeismic Refraction Survey Power Creek, Cordova AK 1981Alaska Power Authority LIBRARY COPY COR-P 001 Seismic Refraction Survey Power Creek, Cordova, Alaska For Department of the Army Alaska District, Corps of Engineers Contract No. DACW85-81-C-0012 4040 ‘B” Street pew: Anchorage, Alaska a aac August 1981 DOWL_ Engineers 4040 “B” Street Anchorage, Alaska 99503 Phone (907) 278-1551 ( Telecopier (907) 272-5742 ) August 12, 1981 W.O. #D13055 Department of the Army Alaska District Corps’ of Engineers P.O. Box 7002 Anchorage, Alaska 99510 Attention: Remy Williams Re: Seismic Refraction Survey Power Creek, Cordova, Alaska Dear Mr. Williams: Please find attached our completed interpretation of the seismic refraction survey conducted at the proposed Power Creek dam site near Cordova, Alaska, in June, 1981. While the survey was not as definitive as we had hoped in terms of mapping the bedrock inter- face, the data does put definite constraints on the nature of the material underlying the northern abutment. We trust that the results will assist you in planning further exploration work at the site. If you have any questions concern- ing the survey or the interpretation, please contact us. Sincerely yours, DOWL ENGINEERS wDhomoa Re WYitliarma Thomas R. Williams Engineering Geophysicist Approved by: Melvin R. Nichols, P.E. Partner TRW/mg Attachments ie E Miclinenn RAateinn BD Aeuald Vannath R Walch Maluin Do Nichole SEISMIC REFRACTION SURVEY POWER CREEK, CORDOVA, .ALASKA FOR DEPARTMENT OF THE ARMY ALASKA DISTRICT, CORPS OF ENGINEERS CONTRACT NO. DACW85-81-C-0012 Prepared for DEPARTMENT OF THE ARMY CORPS OF ENGINEERS Prepared by DOWL ENGINEERS August 1981 w.O. #D13055 SEISMIC REFRACTION SURVEY POWER CREEK, CORDOVA, ALASKA FOR DEPARTMENT OF THE ARMY ALASKA DISTRICT, CORPS OF ENGINEERS CONTRACT NO. DACW85-81-C-0012 INTRODUCTION A seismic refraction survey was conducted at the site of a pro- posed hydroelectric dam on Power Creek, approximately 6.5 miles northeast of Cordova, Alaska (see Figure 1). The purpose of the survey was to determine the configuration of bedrock beneath the northern abutment of the proposed dam. SITE GEOLOGY The proposed dam site lies at the southwest end of a northeast- southwest trending glaciated valley in the Chugach Mountains of “ southcentral Alaska. Most of the Power Creek Valley exhibits the classic "U-shaped" cross section of a mature glacial valley, but in the lower two miles this profile is obscured by the occurrence of a broad fan-shaped hummocky ridge. This fan-shaped ridge has been interpreted by previous investigators (Miller, 1951; Alaska Geological Consultants, 1967) to be a landslide of massive pro- portions which originated in the northwest valley wall. The landslide forced Power Creek over to the southeast canyon wall, where it has downcut a gorge through a bedrock spur. This obstruction of the normal gradient of Power Creek has led to the steep fall of the creek in its lower two miles and consequently to the interest in hydroelectric power development which has centered on the site for several decades now. Bedrock in the area is composed of interbedded argillite, slaty argillite, and graywacke, with some minor occurrences of metamor- DOWD ENGINEERS 4 vat Len at - eer a ae LOCATION... we © DN iH : eet 3, sof rey dt, SL A is Hum phack 32 EPROJECT. Wp OL 97 yh ¢ pie See ) Wg se 4 Ar, (Cm. Slough . ., |. h | i 45 > a i oe (\ im a : } : , | ese ae fo Vo Be LSA ya : Moats - \ i 7 yoy Pata were a LOCATION OF SEISMIC SURVEY NORTHEAST OF CORDOVA FIGURE | phosed volcanic rocks (Miller, 1951). The southeast canyon wall at the proposed dam site exhibits massive exposures of competent bedrock, and would compose the southern abutment of the proposed dam. The competence of the material underlying the northern abutment, however, is highly suspect. Miller (1951) interpreted the northwestern canyon wall to be com- posed‘ of bedrock consisting of intensely fractured argillite and graywacke, and he called on a fault zone in the vicinity of the gorge to explain the vast change in the character of the bedrock across the gorge (see Figure 2). The purpose of the seismic refraction survey was to help clarify the true configuration of the bedrock interface beneath the northern abutment of the proposed dam. SURVEY LOCATION The seismic refraction lines were located relative to the USGS benchmark installed at the Ohman Falls observation point in 1948, and were also tied to two photo panel points established near the 1980 drill holes. These control data are presented as Figure 3. Three seismic profiles were surveyed along the control lines: Line "A", a 200 foot line running northeast up the ridge from the bedrock bench just northeast of Ohman Falls; Line "B", a 500 foot line running up the same ridge to the crest of the block of sus- pect material; and Line "C", a 500 foot line running along the crest of the suspect feature and approximately perpendicular to the "B" line. The geophone locations for these profiles along the survey control lines are indicated in Figure 4. It should be noted that Line "A" closely parallels the northern abutment of the proposed dam. Permanent monuments in the form of 5/8 inch rebar with aluminum caps were left in three positions: the low (southwest) end cof DOWD ENGINEERS 600° Loose rock ond soll leoose rock and soil Intensely fractured argillite and groywocke Loose rock and soll Outcrops odjacent to mopped area indicote bedrock is intensely deformad and fractured 500'- 300 Horizontal and verticol scale: are equal SECTION ALONG THE LINE A—A CROSS-SECTION THROUGH POWER CREEK GORGE (FROM MILLER, 1951) FIGURE 2 Lon 5 +23.22 "Cc" Y = 2409121.0 AX 574509.7 5+01.38 "Cc" Y = 2409115.1 X = 574530.7 Y = 2408547.1 X= 574638.4 COORDINATES (X,Y) ARE ALASKA ° iG ZONE 2 STATE PLANE COORDINATES. 62 BEARINGS ARE TURE BEARINGS AND DISTANCES ARE TRUE DISTANCES. “20004” ae gaT-48 Y = 24094405 *S X= 576126.7 3%! 2». " . (SOURCE: NORTH eee Y = 2409555.1 PACIFIC AERIAL SURVEYS) ——— = 576395.0 CONTROL DIAGRAM SCALE: 200 400 Os 00 “oY ~ 2+ 48.46 "c" Y = 2408456.9 Y = 2408989.4 X= 5747349 X= 574750.1 e5a' in" 2 + 83.32 “B" S. 21° 28°15" W. Y = 2408600.! = 574849.4 3+ 21.71 “B" Y = 2408635.9 X= 574863.2 4+ 00.98 “B" Y = 2408706.1 wpe X= 574900.| Beg ten 8 Y = 2408848.9 . X = 574955.0 (SEE INSERT -"CONTROL DIAGRAM”) KEY @ 8Riss CAP MONUMENT @ 5/¢" REBAR W/ALCAP & TURNING POINT & PHOTO PANELS POWER CREEK SEISMIC REFRACTION SURVEY SURVEY CONTROL DATA W.O.NO: 013055 FIGURE 3 cle cil "EO cg c8 C7 SCALE 25 0 50 io0 —————— POWER CREEK SEISMIC REFRACTION SURVEY GEOPHONE LOCATIONS W.0. NO. 013055 FIGURE 4 700 600 E 500 300 ts WwW = t 3 =z . o 5 = <q > - ul us ad 4400 a | | 300 300 700 ; Ble 600 Al2 D8 01 : SHOTPOINT 1B Al, Bl | 7 400 200 peccd 5+00 4+00 3+00 2+00 1+00 0+00 1+00 2+00 3+00 4+00 5460 eiac DISTANCE (FEET) POWER CREEK SEISMIC REFRACTION SURVEY SCALE: 25 0 50 100 FIGURE 5 PROPOSED DAMSITE TOPOGRAPHIC PROFILE aaprermreaaee 90 SHOT APPROX. 125 FT. OFF-END “> —— moog ~AQ ~ BOF 8 Ke ere en . SS ~ ~ --" ~. FIGURE 6 70 APPARENT, VELOCITY IN FEET PER SECOND 60 50 TIME {MILLI SECONDS) TIME (MILLISECONDS) 30 LINE A HOT 42 FT. OFF-END TRAVEL TIME PLOTS OTPOINT POWER CREEK SEISMIC REFRACTION SURVEY DISTANCE (FEET) AG AT GEOPHONE tN 160 160 Ww e 3 SHOT APPROX. = 225 FT. OFF-END 140 140 1204) APPARENT VELOCITY ie IN FEET PER SECOND ta 3 J 3 > & 100 100 8 & wi w 3 [| a 2 = = za ~ oO Ww Ww ee = 80 80 z = n - e OF to a wo 7 T *° Ww = SHOT 42 FT. © = OFF-END = YO I 40 40 ig WJ n> y << a Wi 20 - oO a uJ o = oe 0 50 100 150 200 250 300 350 400 450 500 2 DISTANCE (FEET) ¢ BI B2 B3 B4 B5 B6 87 Bs B9 BIO Bil Bi2 GEOPHONE 200 SHOT APPROX. 300 FT. OFF-END SHOT APPROX. 350 FT. OFF-END APPARENT VELOCITY IN FEET PER SECOND 120 g g]> é s | 3 3 | = = 3 e a 2 = = 210 2 wu ls F = Sw 80 FE oOo q ea Le uy W cs OF = oi 40 iW > Od vx = w & lJ a oO a uJ = = SHOT POINT oO oO DISTANCE (FEET) cé c? GEOPHONE ELEVATION (FEET) 600 550 + 500 F 450 400 c3 C4 c5 v = 5100 v= 4100 MINIMUM BEDROCK DEPTH BASED ON CRITICAL DISTANCE CONSIDERATIONS ee EASRSREREBME Qeormeansee SAURnees Rg eneee serene —— MINIMUM BEDROCK DEPTH V= 11,000 BASED ON TIME - INTERCEPT PHANTOMED FROM OFF-END SHOT —_— Beene eeeeesisssidlien es EE 1*090 1+ 50 2+00 2+50 3+00 3+50 4+00 4+50 5+00 5+50 0+50 NOTE: SEISMIC VELOCITIES (V) ARE POWER CREEK SEISMIC REFRACTION SURVEY VELOCITY CROSS-SECTION FIGURE 9 SURFACE TOPOGRAPHY cl2 eal GEOPHONE WU Nine cé6 if cio cg DISTANCE (FEET) GIVEN IN FEET PER SECOND. LINE C (FEET) ELEVATION 650 soak 550 r 500 450 FOOT TRAIL 1+00 POWER CREEK SEISMIC REFRACTION SURVEY GEOPHONE LOCATIONS = BIO Bil SURFACE TOPOGRAPHY MINIMUM BEDROCK DEPTH MINIMUM BEDROCK DEPTH BASED ON CRITICAL DISTANCE USING APPARENT VELOCITY ~~ CONSIDERATIONS FOR LINE C = 2720 FROM OFF-END SHOT 1B —— V = 2700 a = MINIMUM BEDROCK DEPTH BASED ON _——— TIME - INTERCEPT PHANTOMED FROM — OFF-END SHOT ON LINE C v= 11,000 ——— Vv = 11,000 1+50 2+00 2+50 3+00 3+50 4+00 4+50 5+00 5+50 6+00 DISTANCE (FEET) NOTE: SEISMIC VELOCITIES (V) ARE GIVEN IN FEET PER SECOND * VELOCITY CROSS-SECTION FIGURE 10 LINES A&B the "A" and "B" lines (Al and Bl), the common point between the “"B" and "C"™ lines (Bl2 and Cl), and the northwest end of the "C" line (5 +01.03 "C", near C12). FIELD INVESTIGATION The field work was conducted on June 16 through 19, 1981, utiliz- ing helicopter support from Chisum Flying Service of Cordova. The field party consisted of a two-man seismic crew and a two-man survey crew. In addition to brushing the seismic lines, the sur- vey crew also brushed the existing foot trail from the vicinity of the 1980 drill holes back to the USGS benchmark in order to facilitate the transportation of the seismic equipment and sur- veying instruments. - In addition to surveying the topography along the seismic lines, the survey crew also projected Line "A" across the southwestern side of the gorge in order to provide additional topographic con- trol along the southern abutment of the proposed dam. The com- plete topographic profile is presented as Figure 5. It was not possible to survey points at the bottom of the gorge due to the mature of the sheer rock cliffs above Ohman Falls. Relatively accurate elevations could be most reasonably obtained for this portion of the profile by photogrammetric means using recent air photo pairs. All seismic recordings were made on a GeoMetrics ES-1210F Signal Enhancement Seismograph using a string of 12 GeoMetrics 14 Hz vertical geophones. The shot holes were hand augered using a shovel and iron bar. The seismic energy source consisted of the two-component explosive Kinestik, and shot size varied from 1/3 pound up to four pounds. A minimum of five shots were recorded into each seismic profile: two end shots, a mid-spread shot, and two off-end shots. In addition, ‘one-quarter and three-quarter spread shots’ were DOWD ENGINEERS recorded into Lines "B" and "C" for better overburden velocity definition. SEISMIC DATA The seismic data are presented as time-distance plots of first arrivals from the respective shots in Figure 6 for Line "A", Figure 7 for Line "B" and Figure 8 for Line "C", Interpreted apparent velocities in feet per second (fps) are also indicated on these figures. Line "A" was, first surveyed to give detailed coverage in the vicinity of the northern abutment of the proposed dam. Also, if the bedrock interface were just a few feet below the surface as interpreted in Miller's Plate 2 (Figure 2), then the refracted bedrock arrival should easily be seen on a 200 foot spread. From previous seismic refraction experience in the Cordova area we know that typical bedrock velocities in this region run from 10,000 to 12,000 fps. The apparent velocities given on Figure 6 indicate that nothing approaching expected bedrock velocities resulted from the shots fired into Line "A". Rather, there appears to be a two layer system consisting of a rather slow sur- face layer (1000 to 1300 fps) underlain by more saturated mater- ial (2600 to 3200 fps). The off-end shot 42 feet southwest of Al was-placed directly on competent bedrock on the bench above Ohman Falls, yet even this shot resulted in a maximum apparent velocity of only 4650 fps. The end-shot at Al2 and the off-end shot 125 feet above Al2 exhibit curious travel-time curves, in that geo- Phones most distant from the shots (Al, A2) show earlier arrivals than some of the intermediate geophones. Since it was known that Phone Al was only a foot or two above competent bedrock, these arrival times suggested large delay times through low velocity material below the intermediate geophones. Since field plotting of the "A" line arrival times indicated that bedrock-like velocities were not being seen along a 200 foot spread, it was decided to stretch out the "A" line to a "B" line, going from approximately a 20 foot geophone spacing to a 50 foot spacing. This extended the ridge line to the crest of the suspect topographic feature, and seven shots into this "B" line gave the travel-time data in Figure 7. Again, even with a 500 foot line, no bedrock-like velocities are seen on Figure 7. The same two layer system, with velocities similar to those on the shorter "A" line, is seen. The end-snot at Bl and the off-end shot 42 feet southwest of Bl give very similar arrival times, with apparent velocities around 4600 fps. The shots at 3/4 spread, station Bl2, and 225 feet off-end of B12 show the curious earlier arrivals at the more distant phones, again indicative of large delays at the intermediate geophones. The third seismic spread, Line "C", was then run along the crest of the topographic feature in question and approximately perpen- dicular to Line "B". The two lines had a common tie point at B12 and Cl. The topography along this line, as opposed to the steep "A" and "B" lines, was nearly flat, thus leading to fewer ambigu- ities in the interpretation. The travel-time plots for Line "C" are given in Figure 8. In spite of the blocky nature of the landslide debris through which we were shooting, the data are very consistent. Again, no bed- rock-like apparent velocities are seen for the five shots along the 500 foot spread. Reciprocal times for the end shots are very close, indicating that the same refraction path is being followed from either end. The data indicates a two layer system for the northwestern three-quarters of the line, a slow surface layer running from 1250 to 1550 fps, and a faster second layer running at 4000 to 5000 fps. The second layer appears to slow to the southeast, and at Cl a three layer system may be present (1500, 2600, and 4100 fps). The off-end shot approximately 350 feet to the southeast of sta- tion Cl was the only shot in the entire survey which resulted in apparent velocities close to those expected for competent bedrock in the Cordova area. As seen in Figure 8, the bedrock headwave was apparently sweeping across nearly the entire spread, with apparent velocities of 10,000 to 11,300 fps. The other off-end shot, approximately 300 feet northwest of Cl2, nearly parallel the end-shot at Cl2 until the last four geophones (Cl through C4), when higher apparent velocities (perhaps the bedrock arrival) began to occur. . INTERPRETATION The interpretation of the seismic data in terms of the subsurface velocity structure is presented in Figures 9 and 10. Since the travel-time plots for Line "C" are the least ambiguous, the interpretation for this data will be discussed first. As described in the last section, the travel-time data indicates a two layer system beneath the "C" line, as illustrated in Figure 9. The surface layer, running at 1250 to 1550 fps, is inter- preted to be unsaturated landslide debris, with forest litter (roots, organic silt, etc.) filling the intervals between blocks of graywacke. This interpretation is based on direct field observation, as the entire ridgeline is obviously slide debris. The higher velocity material at a depth of 10 feet to 25 feet is interpreted to be saturated landslide material, since it has a velocity around 5000 fps. The decrease in subsurface velocity from northwest to southeast along the "C" line is thought to be due to a change from gray- wacke blocks to highly fractured and sheared argillite. Our shotpoint at Cl revealed the argillite "bedrock" just a few feet beneath the surface, and much flyrock resulted from this shot. Since the velocity of this argillite appears to be only about 2600 fps, it is certain that it cannot be in-place bedrock. DOWD ENGINEERS Since bedrock velocities were not detected for the shots within the 500 foot "C" spread, a minimum depth to bedrock can be calcu- lated from critical distance considerations (Redpath, 1973). Assuming a Vj of 4900 fps and a V2 of 11,000 fps, the critical distance formula z= *c (1-"1/%2) 1 2 cos ysint = Y1/%2) tells us that competent bedrock must be at least 155 feet beneath the crest of the ridge, as indicated by the dashed line in Figure 9. The off-end shot 350 feet southeast of Cl provides us with an additional piece of information. We can use this bedrock arrival to phantom in a time-intercept for the end-shot at station Cl. Assuming that the arrival at the geophone at Cl2 is the earliest arrival of the bedrock headwave from the end shot at Cl, and using the off-end shot to phantom a time-intercept at Cl, we arrive at a minimum depth to bedrock of 205 feet beneath station Cl. This is the lower dashed line below Cl on Figure 9. The interpretation of the travel-time plots for Lines "A" and "B" ‘is complicated by the severe topography along these lines. Basically the data suggests a two layer system similar to that along the "C" line, however, and the velocity structure is given in Figure 10. The surface layer consists of a combination of unsaturated weathered argillite and organic soil, and the second layer at a depth of a few feet to 30 feet is interpreted to be saturated fractured argillite. A typical velocity for the argil- lite at depth appears to be about 3200 fps, again indicative that the highly fractured and sheared argillite cannot be in-place bedrock. It should be noted that we could easily penetrate the argillite to a depth of four feet with our iron bar when digging our shot holes along the "A" and "B" lines. A clue to the configuration of the true bedrock interface beneath the "A" and "B" lines may be provided by the off-end shot 42 feet southwest of Bl, the end shot at Bl, and the one-quarter spread shot between B3 and B4. In the field, it was hoped that by plac- ing an off-end shot directly on the bedrock bench above Ohman Falls, a bedrock headwave arrival would be measured along the "A" and "B" lines. If so, then the arrivals along the "A" and "B" lines from the off-end bedrock shot should yield a bedrock appar- ent velocity. This is believed to be the case, ‘and this inter- pretation is supported by the arrivals from the one-quarter spread shot. At station B8, although the travel-time curve is here complicated by topographic changes at the ridgecrest, it appears that a bedrock arrival may be represented by the change in slope. The fact that the travel-time curve for the one- quarter spread here parallels the arrivals from the end shot and the off-end shot suggests that the 4600 fps apparent velocity we see along the arrivals from these latter two shots is indeed a bedrock apparent velocity. Assuming that this is correct, we can then calculate the dip of the bedrock surface relative to the land surface (i.e. the geo- phone string) using the relationship (Redpath, 1973): V2a = Vi/sin ye iol ID where y =the dip angle of the interface relative to the surface, a = the critical angle of incidence, V, = velocity of the overburden, and V2q = the downdip apparent velocity. Then assuming that the V, = 2700 fps, V2 = 11,000 fps, and V2q = 4600 fps, we obtain a dip of about 22° for the bedrock beneath the topographic surface. This results in a minimum depth to the bedrock interface as indicated by the dashed line on Figure 10. DOWD ENGINEERS If the true velocity of the bedrock is greater than 11,000 fps, then the dip angle would be greater and the interface would have to be deeper. This minimum bedrock depth beneath the "A" and "B" lines agrees roughly with the minimum depths calculated beneath the "C" line (also drawn on Figure 10 beneath the tie point Bl2). As a check on the validity of this configuration, a ray tracing procedure was applied to the model (Musgrave, 1967), and the calculated travel times were found to be in reasonably close agreement with those actually observed. CONCLUSIONS AND RECOMMENDATIONS In conclusion, the seismic evidence indicates that the main mass of material to the northwest of Power Creek at the site of the proposed hydroelectric dam is not in-place bedrock. The seismic velocities of about 3200 fps suggest that the highly fractured argillite either is part of the landslide mass itself or was pushed out ahead of the landslide. Earlier investigators (Alaska Geological. Consultants, 1967) have suggested that the bedrock profile is.more likely that of a "U" shaped glacial valley, as indicated by the heavy dashed line in Figure 2. - The seismic data seems to support this interpretation. The conclusions drawn from the seismic velocity data are sup- ported by other evidence: l. The relatively easy penetration of our iron bar into the fractured argillite. 2. The difficulty of tunneling through the weak material in the ridge in 1915 (Miller, 1951). 3. The highly discordant nature of bedding attitudes in the zone of fractured argillite (Miller, 1951). 4. The numerous springs which issue from the fractured argil- lite at an elevation of about 350 feet downstream from the falls, and which have caused the washout of the old foot trail. 5. The occupation of the Power Creek Valley by ice during repeated glaciations, which should have long ago removed the highly friable argillite north of Power Creek. We recommend that if further exploration at the proposed dam site is conducted, a drill hole to bedrock at station Al2 (approxi- mately the northeast end of the northern abutment) would help to refine the seismic interpretation. REFERENCES Alaska Geological Consultants, 1967, Preliminary Geological Investigation of the Power Creek Hydroelectric Project, Cordova, Alaska, Phase I: Report to A & L Engineering and Construction Company, Anchorage, Alaska. Miller, Don J., 1951, Geology at the Site of a Proposed Dam and Reservoir on Power Creek Near Cordova, Alaska: U.S. Geolog- ical Survey Circular 136. Musgrave, Albert W., ed., 1967, Seismic Refraction Prospecting: Society of Exploration Geophysicists, Tulsa, Oklahoma. Redpath, Bruce B., 1973, Seismic Refraction Exploration for Engi- . neering Site Investigation: U.S. Army Engineer Waterways Experiment Station, Explosive Excavation Research Labora- tory, Livermore, California.