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2 GRC Transactions, Vol. 34, 2010 Field Investigations and Temperature-Gradient Drilling at Marine Corps Air-Ground Combat Center (MCAGCC), Twenty-Nine Palms, CA Christopher Page¹, Howard Ross², Jeff Unruh³, Michael Strane³, Wei-Chuang Huang¹, Michael Lazaro¹, David Meade¹, and Andrew Sabin¹ ¹USN Geothermal Program Office, China Lake, CA ²Energy & Geosciences Institute (EGI) University of Utah, Salt Lake City, UT ³Fugro William Lettis & Associates, Walnut Creek, CA Keywords Geophysics, self-potential, resource assessment ABSTRACT The U.S. Navy s Geothermal Program Office (GPO) has been conducting geothermal exploration activities in the Camp Wilson area of Marine Corps Air-Ground Combat Center (MCAGCC), Twenty-nine Palms, CA, for almost two years. Work has included self-potential (SP) surveys, fault structure analyses using LiDAR surveys, and drilling and assessment of five (5) temperature-gradient holes. For several decades the GPO has worked intermittently at MCAGCC at various sites. The objective of this project was to determine whether any utility-grade geothermal targets exist in this region. SP anomalies recently identified in Camp Wilson are of lower amplitude than similar anomalies recorded in previous SP studies conducted by the GPO. This additional SP work was designed to traverse mapped faults and lineaments identified in recently acquired LiDAR imagery. The objective of this work was to determine if SP anomalies coincide with faults and elevated temperatures from new or existing drilling. To date, three new anomalies have been identified in the Deadman Lake area of Camp Wilson. These are above local noise levels, are only partially defined, and do not coincide with existing temperature anomalies. The GPO has also collaborated with the Naval Construction Division (NCD) to utilize the Seabees to perform temperaturegradient drilling. The Seabees completed five temperature gradient test holes ranging from 600-1,000 feet. Each hole was completed with 3 steel tubing inserted to depth and filled with water to the surface. Baseline measurements show that at least two of the holes have elevated temperature gradients. Both the temperature-gradient drilling and the fault-structure analyses with the use of LiDAR mapping have proven to be extremely useful in assisting the GPO to define targets for intermediate to deep drilling in mid-to late Introduction This report explains the rationale, methods, and procedures that the Department of the Navy, Geothermal Program Office (GPO) used from 2008 to the present in conducting field tasks which were performed as part of an effort to define potential geothermal resources at Marine Corps Air-Ground Combat Center (MCAGCC) Twenty-Nine Palms, CA. This report presents the field-data evaluation based on our current understanding of exploration efforts to date. This effort consisted of conducting water sampling of groundwater monitoring wells, self-potential geophysical surveys, fault-structure analyses through use of LiDAR, and drilling shallow temperature-gradient test holes. The GPO began examining the geothermal potential of MCAGCC in the early 1980 s. Between 1981 and 1983, the GPO had conducted groundwater sampling and gravity and magnetic surveys in the Surprise Springs and Lavic Lake areas. Seven 1,000-foot temperature-gradient holes were drilled in 1984 in the Camp Wilson and Deadman Lake areas. In 1990, two additional temperature-gradient holes drilled to depths of 3,000 feet in the Surprise Springs basin showed temperature profiles of 2.2 F/100ft and 2.4 F/100ft, respectively. In 1996, three more test holes were drilled in the Camp Wilson area with depths ranging from 1,300ft to 1,488ft. Field Activities This section describes the field tasks which were performed during the exploration phase of the GPO s geothermal assessment of Twenty-Nine Palms. Field activities consisted of water sampling and analysis, self-potential (SP) surveys, fault mapping and analysis through the use of LiDAR imagery, and shallow temperature-gradient test hole drilling. Water Sampling The rationale to be used for selection of the wells to be sampled at Twenty-Nine Palms was based partly on the location of known 621

3 wells (both production and monitoring), and on new information gathered from the base. The water-sampling crew consisted of three personnel from the GPO and took about one week in April, 2008 to accomplish. Valley-fill sediments, buried playas and anhydrites can contribute a significant amount of sodium, chloride, sulfate, etc. to groundwater in desert valleys, and this character may simply be a function of water-rock interaction with the valley-fill sediments where they occur. All samples showed near-neutral to very slightly acidic ph. The presence of significant bicarbonate is usually thought to be an indication of shallow groundwater with a strong meteoric component. It is possible that these waters are a mixture of meteoric water and a deeper geothermal component. A full comparison of these analyses with those of other groundwater from the Twenty-Nine Palms/Joshua/Johnson Valley/Yucca Valley areas may indicate an enhanced mixing component, or it may show that these waters are simply consistent with most other groundwater in the region. Given the apparent gross immaturity of the waters sampled here, it is difficult to even estimate an order of magnitude of a geothermal component to these fluids, if one exists at all. Self-Potential (SP) Geophysical Survey Self-potential anomalies are generated by fluid, heat, or ion flows in the earth. Therefore, SP surveys have generally been used to help locate and delineate geothermal sources. A self-potential survey totaling about 23 line-miles of data was completed along a primary road network within MCAGCC in the early 1990 s (Figure 1), and these data are available but survey details, soil conditions, and interpretation are not known. The gradient or fixed dipole method, if used, was appropriate for such a reconnaissance survey of a large area, but may be subject to major noise or errors due to changing electrode hole resistance, drying of electrode locations, infiltration potentials if the electrode holes were watered, and time-varying telluric currents. If the survey was completed during a period of good soil moisture and watering of electrode holes was not necessary, the data would be of better quality. Inspection of the data finds that the SP values are reported to tenths (0.1) of a millivolt (mv) implying a high precision of measurement. Some profile segments show large areas of gradual (smooth) voltage change, and other areas of large station-to-station change. Such changes may be real, but generally indicate variable surface or infiltration conditions which could distort anomalies since these effects may be summed to subsequent values, rather than just single-station anomalies readily identified as noise. In this environment, some single-station anomalies could result from buried, oxidizing, metal objects. In a few cases where profiles intersect, the voltage values along the two profiles differ substantially, suggesting some form of reference problem (Ross, 2009). Surprise Springs SP Survey In our 2008 SP survey work, several profiles of ft ( m) were completed from each base station, using a station interval of 200 ft (60 m). Base station #1 (BS-#1) for this survey was established at the southeastern corner of the old SP work where a value of -4.5 mv was reported (Figure 2). Five additional stations to the north were repeated, excluding one erratic value (-18.8 mv difference). The average absolute difference was 3.5 mv, and the net average difference (for an assumed base station value of -4.0 mv) was -1.2 mv. This is considered acceptable for a tie-in to previous work. Figure 2. Surprise Springs SP contour map showing identified anomalies. Figure 1. Existing SP survey contour map showing major anomalies which have been identified. The SP values generally vary smoothly throughout the survey area, with a few single-station variations (possibly noise ) of +/- 10 mv or more. A small but coherent low of -13 to -22 mv (anomaly SS-A) occurs on the west end of Line SS-4W, and this is probably continuous with anomaly A of earlier SP work, west of TG-6. A second low-amplitude anomaly (SS-B; -10 to -20 mv) occurs in an area of about 5000 ft by 2500 ft, elongated north-south, perhaps 2000 ft east of a USGS monitor well. The anomaly is not large in amplitude, but may be related to fluid movement in a buried fault. Low-amplitude positive anomalies of 10 to 18 mv occur on the eastern ends of Lines SS-4E and SS-5E, and these may reflect 622

4 higher average surface resistivity in this area. The remainder of the survey data appears to be near background level. The SP data appear to be of good quality. Electrode-circuit resistance was typically in the range 1.5 to 3.0 K-ohm after watering, but one should always be suspect of some erratic infiltration potentials after watering (Ross, 2009). Southeast SP Survey A temporary base station was established at the southeast-most SP station of earlier work, and three stations to the west and an additional station to the north were used to establish a reference value of -12 mv for this station. A second base station was established approximately 100 ft. north of the southern base border, which is marked by a series of grounded fences. The contoured data (Figure 3) show only four local values greater than +20 mv or less than -20 mv. Two of these, -25 and -125 mv, could be associated with possible buried drill casing of a temperature-gradient hole drilled in the early 1990 s by the GPO. Two weak anomalies about 1000 ft long occur in the southern and eastern parts of the survey. Neither is considered a significant indication of geothermal fluid movement. Figure 3. Southeast SP contour map. Deadman Lake SP Survey Interest in the Deadman Lake area of MCAGCC is due in part to the presence of a fairly large, west-northwest-trending SP anomaly of -40 to -70 mv, identified by the GPO SP survey. It extends about 8,000 feet west-northwest of test wells CW-1 and CW-3. These wells may be up-gradient from the SP source area but appear to partially test the anomaly (Figure 4). Six features of the Deadman Lake SP survey have sufficient amplitude and extent to be considered anomalies. These are discussed below: Anomaly B: Four new SP profiles traverse the area of anomaly B from four base stations located outside of the anomalous SP area. These profiles typically record SP values of 0 to -25 mv, compared to values of -40 to -70 mv recorded earlier. Anomaly C: This anomaly of -40 to -60 mv on the earlier SP survey occurs along the west side of Deadman Lake. The anomaly Figure 4. Deadman Lake SP contour map. The dotted lines within the contours indicate the lines of data acquired. contours are reduced to +15 to -16 mv along the new line DL- 9SW. There is no correlation with other subsurface information and nearby well CW-1 exhibits a low temperature gradient. Anomaly DL-1N-A: This anomaly is defined by profiles DL- 1N and D2-2W, and occurs along the northern most edge of the Deadman Lake salt flat. A broad area displays a low of more than -50 mv, with minimum values of -76 and -88 mv. The anomaly may be due in part to soil-type changes at the end of Deadman Lake. The anomaly does align with a pronounced east-trending drainage and canyon 3 to 5 miles to the east. The SP source could result from fluid flow along a buried structure. Anomaly DL-1N-B: This is a small but smoothly contoured minimum of -40 to -74 mv located just east of Deadman Lake. There is no obvious correlation with soils, topography or faults, so a buried geologic source is likely. There does not seem to be any encouragement from resistivity or temperature-gradient wells, however, and the extent of the anomaly (observed on only one line) is limited. Anomaly DL-3-N: This anomaly, with minimum values of -30 to -53 mv, exhibits smoothly varying contours and ties nicely to older GPO SP contours. The anomaly is not fully defined (i.e. open to the west) but may extend 3500 ft north-south and at least 2000 ft east-west. There is no obvious correlation with topography, but a grounded fence is located across the road, ft west of the profile. Other grounded structures are present along line DL-8. Anomaly DL-7W: A somewhat noisy low of -30 to -50 mv amplitude occurs along a 2800 ft length of Profile DL-7W in the extreme northwest part of the survey area. There is some support for the presence of a real anomaly from the north end of Profile DL-3N. The feature is poorly defined but could extend to the north and west into the basin area. Summary of SP Results The SP anomalies identified in this work are of lower amplitude than earlier SP anomalies mapped in the Surprise Spring and Southeast areas. New SP surveys in the Deadman Lake 623

5 area tend to downgrade anomalies B and C defined by earlier SP surveys because of their reduced SP amplitude where new profiles cross into the area, and concerns over possible errors in the older data. Three new anomalies have been identified in the northern part of the Deadman Lake area, DL-1N-A, DL-3N, and DL-7-W. The anomalies occur at the limits of the present survey and consequently their extent is only partially defined. Further definition of these anomalies may be enhanced by additional SP work, especially if this could be completed during periods of adequate natural soil moisture rather than in the dry, summer months. All three areas are located north of the existing electrical resistivity data and temperature-gradient holes, so no support is available from these data bases. Regional USGS gravity data indicates that these anomalies occur further north in the basin, over deeper valley fill (USGS, 1984). This may reflect movement of hot water away from all existing temperature-gradient holes. The amplitudes of the anomalies are somewhat smaller than those reported for moderate and high-temperature geothermal areas identified in the Great Basin and elsewhere. mutual benefits to 1NCD and GPO working together allowed the Seabees to gain unparalleled operational experience with drilling operations and GPO to reduce contract funding for exploration. The purpose of the Seabees performing the drilling was: 1) provide invaluable training to Seabees pertaining to drilling and construction and; 2) minimize costs of drilling for GPO and provide a key component for DON geothermal exploration. From November 2008 to March 2009, Seabees from the Naval Construction Division (NCD) successfully completed fivetemperature gradient holes for the GPO. Samples taken from each hole were similar in nature; mixtures of sand and conglomerates with the occasional granite sections were typically encountered. Each hole varied slightly in depth, ranging from 600ft to 1,000ft; however, each hole has been completed to acceptable standards of the GPO. Upon completion of drilling, 3 metal tubing was inserted to bottom depth, filled with water to the surface, then capped and sealed (Figure 6). Temperature-Gradient Drilling The Geothermal Program Office (GPO) approached First Naval Construction Division (1NCD) in late fiscal year 2008 with an opportunity to drill five (5) holes at Marine Corps Air-Ground Combat Center (MCAGCC) 29 Palms, CA, to support geothermal exploration (Figure 5). This opportunity presented a mutually beneficial project to allow the GPO to collect data for exploration and potential production of future power plants while, at the same time, allowing 1NCD to gain critical well-drilling skills in an austere environment. The Figure 6. Temperature-Gradient Hole design. Figure 5. Topographic map showing the locations of the five temperature gradient holes. The inset (top left corner) shows the location of drilling with reference to the base boundaries. The last hole was drilled and completed on March 2, Each hole was ultimately given 90 days to equilibrate to the surrounding downhole soil conditions. However, the GPO returned to perform preliminary temperature logging approximately three (3) days after the last hole was complete. Preliminary results indicated that the water level had dropped in virtually every hole. Probable causes for such may be poor seals at the joints or an improper seal on the bottom cap. The following are preliminary results: 624

6 Site #1 showed the water level had dropped to 195ft, and at the bottom-hole depth of 859ft the temperature was F. Site #2 showed the water level had dropped to 304ft, and at the bottom depth of 807ft the temperature indicated F. Site #3 showed the water level dropped to 335ft, and the bottom-hole temperature was 110 F at 805ft. Site #4 indicated the water level had only dropped to 13ft, and the bottom-hole temperature was F at 877ft. Site #5 indicated the water level had dropped slightly to 5.5ft, and at a bottom-hole depth of 708ft, the temperature was F. In June 2009 the GPO performed final temperature-logging surveys on the five test holes. It was thought, based on preliminary data, hole #2 would have the highest gradient, in terms of heat; however, this was not the case. Final results indicated hole #5 showed the highest thermal gradient (Figure 7). Final results are as follows: Site#1 had a maximum temperature of F at 859ft, giving a temperature profile of 5 F/100ft. Site #2 had a maximum temperature of F at 807ft, giving a temperature profile of 8.7 F/100ft. Site#3 had a maximum temperature of F at 805ft, giving a temperature profile of 5.8 F/100ft. Site#4 had a maximum temperature of F at 877ft, giving a temperature profile of 7.6 F/100ft. Site#5 had a maximum temperature of 131 F at 708ft, giving a temperature profile of 8.9 F/100ft. Depth (ft) Temperature (F) D. Meade, GPO Site #1: Temp. Survey (6/30/09) Site #2: Temp. Survey (6/30/09) Site #3: Temp. Survey (6/30/09) Site #4: Temp. Survey (6/30/09) Site #5: Temp. Survey (6/30/09) Figure 7. Graph indicating temperature-gradient profiles for each test hole Figure 8. Fault trace map. Red dashed line indicates where fault may lie versus fault A (blue line) which was originally thought to be the location. Yellow dots indicate shallow TG holes Based on the final temperature-logging results, it was apparent the most prospective site was #5. Given the results from the shallow drilling program, it was determined there existed a need to further delineate the geologic and neotectonic structure of the training area. Further delineation may prove to be a pivot point to move forward with intermediate to deep drilling. Fault Mapping with Lidar LiDAR is a remote-sensing technology which measures properties of scattered light to determine distance, range, and other information regarding a distant target. For the purposes of neotectonic analysis, the most useful products of this technology are hill shade and first-derivative (gradient) maps, which can be used to map Quaternary geology and to identify subtle geologic features that are indicative of active faulting. A hill-shade map is artificially illuminated topography and used to identify features such as fault scarps, hillsides and vegetation lineaments. Using a GIS program, the first derivative of the elevation can be obtained from LiDAR topographic data and analyzed to identify subtle features that may be related to surface faulting. Primary LiDAR application to this project was Airborne Laser Swath Mapping (ALSM). This particular application was used to gather data over a specific land area then used to create a Digital Elevation Model (DEM) with a resolution of approximately 1m in the horizontal direction and 10cm in the vertical direction. The LiDAR data gathered for MCAGCC was analyzed in conjunction with other data, such as aerial photography and field reconnaissance, with assistance from Fugro William Lettis & Associates (FWLA), Inc.

7 Primary location for the LiDAR analysis and fault mapping was based off the results of shallow temperature-gradient drilling. Temperature-Gradient (TG) hole 5 is located at a discrete right step in the trace of fault A (Figure 8). The main trace of the fault crosses the southern base boundary approximately 800 m west of TG-5 and dies out as a recognizable photo lineament, LiDAR lineament and geomorphic feature several hundred meters north of the base boundary. This trace exhibits a component of east-sidedown separation and likely dips moderately to steeply east. Fault A is dominantly a right-lateral strike-slip fault and TG-5 appears to have been drilled in the hanging wall of the eastdipping trace of the fault south of the stepover. This could be a zone of locally elevated porosity and permeability which accommodates hydrothermal upwelling of hot fluids which could account for the elevated temperatures seen at site #5. If it is assumed the fault lies approximately 790 meters west of TG-5 and dips 70 or 60 degrees to the east, then the fault plane may be 2170 m or 1370 m, respectively, beneath TG-5 (Unruh, 2009). Summary Final SP reports indicate there may be low-amplitude anomalies in the Deadman Lake area. These low-amplitude anomalies, however, are not indicative of a production-size resource. The amplitudes of the anomalies are somewhat smaller than those reported for moderate and high-temperature geothermal areas identified in the Great Basin and elsewhere. Further definition of the southeastern area may be enhanced by performing deeper drilling. Both the temperature-gradient drilling and the fault-structure analyses with the use of LiDAR mapping have proven to be extremely useful in assisting the GPO to define targets for intermediate to deep drilling in mid-to late Potential targets for intermediate drilling have been selected based on the results of the field activities and exploration efforts. A primary target has been selected slightly northwest of TG-5 and two alternate targets, one to the northeast of TG-5 and one northwest of TG-4. These deeper (2,500-3,000ft) holes will allow the GPO to determine if there may be a resource large enough for power production. References Corwin, R. F., and Hoover, D. B., 1979, The Self-Potential method in geothermal exploration; Geophysics, v.44, pp Dibblee, T.W., Jr., 1967, Geologic map of the Deadman Lake quadrangle, San Bernardino County, California: U.S. Geological Survey Miscellaneous Geologic Investigations Map I-488, scale 1:62,500. Dibblee, T.W., Jr., 1967, Geologic map of the Twenty-Nine Palms quadrangle, San Bernardino County, California: U.S. Geological Survey Miscellaneous Geologic Investigations Map I-488, scale 1:62,500. Moyle Jr, W. R., 1984, Bouguer Gravity Anomaly Map, Twenty-Nine Palms area, California; U.S. Geological Survey Water Resource Investigations rept :62,500 scale map and text. Ross, H. P., 2009, Self-Potential (SP) Surveys at MCAGCC Marine Base, Twenty-Nine Palms, California; Final Report for GPO under contract #N P Ross, H. P., and Witcher, J. C., 1992, Self-potential expression of hydrothermal resources in the southern Rio Grande Rift, New Mexico; Geothermal Resources Council Transactions, v. 16, pp Ross, H. P., Blackett, R. E., and Witcher, J. C., 1995, The self-potential method: Cost-effective exploration for moderate-temperature geothermal resources; Proceedings, World Geothermal Congress, 1995; v. 2, pp Trexler, Dennis T., Flynn, Thomas, and Ghusn Jr, George, 1984, Drilling and Thermal Gradient Measurements at U.S. Marine Corps Air Ground Combat Center; Final Report for GPO under contract # DE-AC03-83SF Unruh, Jeffery R., and Strane, Michael, 2009, Results of Neotectonic Reconnaissance, Twenty-Nine Palms Marine Corps Air Ground Combat Center; Final report for GPO under contract# N D