Task 3.0 Geothermal Prospecting -- Subtask 3.2.1: SHALLOW ... · information on surface albedo, topographic slope orientation, and elevation to correct 2-meter temperatures for the

Task 3.0 Geothermal Prospecting -- Subtask 3.2.1: SHALLOW TEMPERATURE AND SOIL GAS SURVEYS

Preliminary Final Report

October 2009 - December 2010

Mark Coolbaugh1, Chris Sladek1, Paul Lechler2, Chris Kratt3, and Lisa Shevenell2

1Great Basin Center for Geothermal Energy, University of Nevada, Reno 2Nevada Bureau of Mines and Geology

3Desert Research Institute

January 17, 2011

PREPARED FOR: THE NEVADA RENEWABLE ENERGY CONSORTIUM

Funded by Award # DE-EE0000272 Recipient: DOE, Golden Field Office

1726 Cole Blvd., Bldg. 22, Golden, CO 80401

Subtask 3.2.1 Shallow Temperature and Soil Gas Surveys 17 January 2011 Preliminary Final Report, Nevada Renewable Energy Consortium Award # DE-EE0000272

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Contents Page Number Special Note .....................................................................................................................................4 Executive Summary .........................................................................................................................4 1.0 Justification ................................................................................................................................6 1.1. Shallow Temperature Surveys (Tasks 1-3) ........................................................................6 1.2. Soil Gases (Task 4) ............................................................................................................6 1.3. Shallow Water Samples (Task 5) ......................................................................................7 2.0 Objectives and Scope .................................................................................................................7 3.0 Task Descriptions.......................................................................................................................7 3.1. Shallow Temperature Measurements in Difficult Terrain .................................................8 3.2. Compensation for Solar Radiation and Surface Environmental Effects ...........................8 3.3. Compensation for Thermal Conductivities of Surface Materials ......................................8 3.4. Shallow Soil Gas Measurements .......................................................................................8 3.5. Shallow Water Sampling ...................................................................................................8 4.0 Work Completed/Results ...........................................................................................................8 4.1. Task 1: Drilling in Difficult Terrain ..................................................................................8 4.2. Task 2: Compensation for Solar Radiation Variations ....................................................11 4.2.1. Field Temperature Survey .....................................................................................11 4.2.2. Subtask A: Shallow Temperature Gradients .........................................................11 4.2.3. Subtask B: Temperature Processing Techniques ..................................................14 4.3. Task 3: Measurements of Thermal Diffusivity/Thermal Conductivity ...........................19 4.4. Task 4: Gas Chromatograph with Sulfur Detection ........................................................24 4.5. Task 5: Sampling of Shallow Groundwater .....................................................................24 5.0 Comparison of Accomplishments with Goals and Objectives ................................................25 5.1. Task 1: Drilling in Difficult Terrain ................................................................................25 5.2. Task 2: Solar Radiation/Environmental Compensations .................................................25 5.3. Task 3: Thermal Diffusivities/Thermal Conductivities ...................................................25 5.4. Task 4: Soil Gas Measurements ......................................................................................26 5.5. Task 5: Shallow Groundwater Sampling .........................................................................26 6.0 Project Summary: Conclusions, Comments and Future Plans.................................................26 6.1. Task 1: Drilling in Difficult Terrain ................................................................................26 6.2. Task 2: Digital Corrections ..............................................................................................26 6.3. Task 3: Measurements of Thermal Diffusivity/Thermal Conductivity ...........................27 6.4. Task 4: Gas Chromatograph with Sulfur Detection ........................................................27 6.5. Task 5: Sampling of Shallow Groundwater .....................................................................27 7.0 Products Developed, Milestones, and Deliverables .................................................................28 7.1. Tasks 1: Drilling Equipment/Techniques ........................................................................28 7.2. Task 2: Temperature Data Processing .............................................................................28 7.3. Task 3: Thermal Conductivity Device .............................................................................28 7.4. Task 4: Soil Gases ...........................................................................................................28 7.5. Technology Transfer ........................................................................................................28 7.5.1. Publications and Presentations ..............................................................................28 7.5.2. Web Technology Transfer .....................................................................................29 8.0 References ................................................................................................................................29


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Figures 1 One-inch-diameter masonry bit with extension 2-meter probes and spline drive. ...................9 2 R-thread auger drill steel in lengths up to 7 ft (top four steels), soil auger with a wedge-

shaped tungsten carbide bit and two auger extensions (lower three steels), and 42" high-lift jack with customized slot for pulling 2-meter probes (bottom) ..............................................10

3 Cross-point drill bits (1 1/2" and 1 3/16" diameter, tungsten carbide) ...................................10 4 Uncorrected 2-meter temperatures from the Desert Peak Field Survey .................................12 5 Example shallow temperature gradients from the Desert Queen area ....................................13 6 Shallow temperature gradients at the Desert Peak field site ...................................................14 7 2-Meter Temperatures at Desert Peak plotted against the solar radiation factor ....................15 8 Shallow temperature gradients at Desert Peak plotted against the solar radiation factor .......16 9 2-Meter Temperatures at Desert Peak plotted against elevation ............................................17 10 Temperature residuals from regression plotted against predicted background temperature ..18 11 Residual 2-meter temperatures from the Desert Peak Field Survey .......................................19 12 Thermal diffusivities for selected sites at the Desert Queen geothermal area ........................21 13 Laboratory equipment for calibrating thermal conductivity measurements ...........................22 14 Laboratory heating curves for the prototype device ...............................................................23 15 Linear relationship between heating curve and calibrated thermal conductivity for the

prototype down-probe device .................................................................................................23 16 Shimadzu GC-2014 gas chromatograph with flame photometric detector (on right) ............24


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Special Note Most, but not all, of the work associated with this project has been completed as of the end of 2010. However, in part because of time-delays in receiving initial funding, and in part due to time constraints of long-term field surveys, some portions of this project will be completed during 2011. Given that the original timetable called for project completion before of 2010, and in consideration that most of the goals and work plans have been completed, it is considered appropriate to document this progress a “near-final” or “preliminary final” report as described herein. Executive Summary This research has improved the ability of shallow temperature surveys and soil gas surveys to detect hidden geothermal resources that do not have surface manifestations such as hot springs. This improved ability has been accomplished by way of:

1) An improved capability to drill shallow temperature holes, to depths of 2 meters, in rougher terrain where rock outcrops may exist. This improved technology is accomplished through the use of auger steel to help prevent jamming of drill bits and steel during drilling.

2) Development of a down-hole sensor for measuring thermal conductivities, which in turn makes it possible to correct temperature measurements for the effects of thermal diffusivity variations. This makes it possible to better discriminate real geothermal anomalies from false anomalies produced by ground conductivity differences.

3) Development of a statistical approach (regression) that uses independently derived information on surface albedo, topographic slope orientation, and elevation to correct 2-meter temperatures for the effects of elevation and solar radiation. Where DEM and remote sensing data are not readily available to build heat energy models or estimate elevations, an alternative methodology has been demonstrated wherein three-zone temperature measurements at depths of 1 to 2 meters can be used to calculate a shallow temperature gradient, which in turn can be used to minimize the effects of solar radiation variations on shallow temperature measurements.

4) A Shimadzu GC-2014 gas chromatograph with flame photometric detector has been installed in the laboratories of the Nevada Bureau of Mines and Geology (NBMG). This equipment makes it possible to detect a number of sulfur gases in soils. Sulfur gases are often associated with geothermal activity and the ability to detect them with shallow soil probes provides an alternative method of detecting hidden geothermal resources.

The techniques and methodologies developed during this research project are relatively inexpensive to implement and easy to adopt by the geothermal industry. Earlier related research has proven the ability of this equipment and methodologies to detect hidden geothermal systems and a number of geothermal exploration companies have already begun using these techniques, and they are likely to have a vested interest in learning from this research project in order to improve their implementation of the methodology. No patents have been issued that would potentially impair the ability of the geothermal industry to employ these techniques, and the procedures and approaches are relatively simple, such that written papers and descriptions are all


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that is needed for experienced geologists and engineers to build their own equipment and run shallow surveys. This project will materially benefit the public by making it possible to discover additional geothermal resources using inexpensive exploration methods. These geothermal resources can then be used to expand our renewable energy electricity production, which in turn reduces our carbon footprint and our reliance on foreign sources of energy.


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1.0 Justification 1.1. Shallow Temperature Surveys (Tasks 1-3) The discovery in the last 5 years of approximately a dozen previously unknown blind geothermal systems in the Great Basin, particularly in Nevada, in spite of significant levels of geothermal exploration over the last 40 years, demonstrates a need for developing improved exploration tools for finding more of these hidden geothermal systems. Coolbaugh et al. (2007a) and Williams et al. (2009) have used spatial statistics to estimate that the undiscovered geothermal resource base in Nevada is several times greater than the known (discovered) resources. Preliminary field studies using equipment developed at the Great Basin Center for Geothermal Energy (GBCGE) has demonstrated the effectiveness of shallow temperature surveys (at a depth of two meters) for detecting blind geothermal systems. Seven (7) such blind geothermal systems have been discovered in Nevada using this equipment and techniques in the last 5 years and an additional 8 geothermal systems have had their blind thermal groundwater outflow plumes mapped more effectively than had been possible in the past. The shallow temperature equipment in use at the GBCGE consists of 2.1-meter-long, hollow steel pipes (probes) with tungsten-carbide tips that are inserted into the ground using electric impact hammers. After an equilibration time ranging from 1 hour to 1 day, temperatures at the bottom of the probes are measured by inserting a platinum resistance temperature device (RTD) into the bottom of the probe. The electric impact hammers are powered by a portable generator, and all of the equipment and personnel (2-3 people) are transported in an all-terrain-vehicle (ATV). The equipment and methodologies are described in detail by Coolbaugh et al. (2007b) and Sladek et al. (2007). In spite of this success, the shallow temperature equipment and techniques have limitations. For example, they are difficult and slow to use in areas of outcrop, and variations in soil albedo, topographic slope aspect, and thermal diffusivities can produce background temperature “noise” that can obscure contributions of heat from underlying geothermal systems. UNR (in conjunction with DRI) will develop improved shallow temperature measurement techniques and soil gas techniques for detecting heat flux anomalies associated with hidden geothermal systems. Earlier work employing these techniques has been highly successful, providing key information leading to the discovery of at least five geothermal systems. Under this subtask, researchers will develop techniques for filtering and compensating for solar radiation and surface environmental effects which influence shallow temperature measurements. In addition, surveys of soil gas anomalies, which may be indicative of geothermal activity, will be improved with the addition of new analytical equipment. A Shimadzu GC-2014 FPD gas chromatograph with flame photometric detector will augment the existing GC-2014 with thermal conductivity and flame ionization detectors, enabling characterization of sulfur gases important in these systems. 1.2. Soil Gases (Task 4) Lechler and Coolbaugh (2007) and Lechler et al. (2009) have demonstrated the ability of soil gas surveys to detect gas inputs from underlying geothermal activity. Current gas analytical equipment at the NBMG can detect many gases, but they can not currently detect sulfur species,


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which are a key component of geothermal gas emissions. Upgrading of laboratory equipment to detect sulfur gases would significantly improve the ability to detect geothermal contributions to soil gases. It is anticipated that soil gas surveys may be effective in detecting blind geothermal systems where shallow temperature surveys may not work as effectively, for example, in areas where shallow cold groundwater overlies thermal aquifers and where the groundwater table is very deep. 1.3. Shallow Water Samples (Task 5) Shallow thermal outflow aquifers are commonly located peripheral to blind geothermal systems. If these aquifers can be sampled using portable drilling equipment, the chemistry of those fluids could be used to estimate subsurface geothermal reservoir temperatures. 2.0 Objectives and Scope The primary objective of this research is to improve the ability to locate previously unidentified, blind geothermal resources that will be identified to the geothermal industry for development. The task-specific objectives of the shallow (2-meter) temperature survey research (tasks 1-3) are to 1) design and develop improved field equipment for detecting geothermal groundwater in areas where visible surface manifestations such as hot springs and steam vents do not exist, 2) develop improved methods of digital data interpretation and background noise reduction to better detect geothermal signals in shallow temperature surveys. This will be accomplished through 1) experimentation with drill bit and equipment designs for drilling into rock where soils are not present, 2) development of down-probe apparatus for measuring thermal diffusivities and/or conductivities to facilitate the use of temperature corrections for variations in thermal conductivity of the ground, and 3) development of computer processing techniques to correct raw shallow temperature data for the effects of solar radiation and thermal conduction. A field temperature survey will be used to evaluate the ability of the processing techniques to remove temperature noise from the data. The objective of the soil gas survey research (task 4) is to acquire improved laboratory analytical equipment and capabilities for detecting soil gases (particularly sulfur species) related to geothermal activity. Specifically, a gas chromatograph will be purchased, because sulfur gases are a key component of geothermal emissions, and existing laboratory equipment at the NBMG does not have the capability of detecting or measuring sulfur in gases. The gas chromatograph will be tested with soil gas samples taken from the field from both geothermal areas and non-geothermal areas to verify and quantify the ability of the equipment to detect sulfur gas anomalies associated with geothermal activity. The objective of task 5 is to develop methods of efficiently and unobtrusively drilling and sampling shallow groundwater so that such groundwater can be chemically analyzed for signs of geothermal contributions. The initial plans were to modify and adapt existing shallow drilling equipment owned by the Desert Research Institute (including a Giddings” drill rig) and/or the hydrogeology program at the University of Nevada, Reno.


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3.0 Task Descriptions The following task descriptions are taken verbatim from the original proposal. 3.1. Shallow Temperature Measurements in Difficult Terrain Research would continue the development of efficient means to drill through hard soils and rock. Funding will include $10,000 for additional equipment and upgrades to existing equipment. 3.2. Compensation for Solar Radiation and Surface Environmental Effects Research will develop techniques for filtering and compensating for solar radiation-related effects on 2-meter temperatures. These influences include albedo, topographic slope aspect, and thermal diffusivity. This task has two components. Subtask A involves the development of methods for accurately measuring shallow temperature gradients, which provide additional data for compensation of solar radiation effects. Subtask B involves development of temperature modeling processing techniques that use independent information on solar radiation effects to predict and model background temperatures at a 2-meter depth in order to better discern geothermal contributions. Funding for both subtasks will include $3,750 to upgrade instrumentation and data loggers and semi-consumables including 2-meter probes. 3.3. Compensation for Thermal Conductivities of Surface Materials Equipment and instrumentation would be developed for field measurements and estimation of thermal conductivities of shallow soils and rocks where shallow temperature measurements are conducted. Estimated cost of electrical and thermal devices and supporting equipment is $2,500. 3.4. Shallow Soil Gas Measurements Funding of $12,500 will be used for additional equipment more specific for sulfur gas analyses…this equipment may include a gas chromatograph with a flame photometric detector, but other options will be investigated. 3.5. Shallow Water Sampling Modest equipment purchases, in combination with rental of existing Giddings drill-rigs, should make it possible to obtain shallow groundwater samples. The equipment (drill bits, 2-meter probes, screens, casing, and valves) is estimated to cost $5,000. An additional $2,500 is requested for geochemical analyses of sampled waters. 4.0 Work Completed/Results 4.1. Task 1: Drilling in Difficult Terrain Prior to beginning this project, the GBCGE had acquired a nominal capability for drilling holes into bedrock. Equipment used for this purpose included a gas-powered portable drill, drill steel in multiple lengths up to 6 feet, and a small compressor and air hose for facilitating removal of cuttings from the holes drilled. This equipment was functional, but drill steel frequently became stuck, and several drill steels were unrecoverable, the standard probe depth of 2 meters was not always achieved, and drilling was particularly difficult in soft rock or when alternating layers of bedrock, clay and colluvium were encountered (drilling in homogenous hard rock is relatively easy).


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Several design changes were explored as a means of improving the efficiency and reliability of drilling into different materials. Equipment purchased for this purpose includes spline drive 1” dia. masonry bit with extension 2-meter probes (Fig. 1), a new Milwaukee 19.9 lb rotary impact hammer with spline drive (Fig. 1), a soil auger with a wedge-shaped tungsten carbide bit and two auger extensions (Fig. 2), and R-thread auger drill steel in lengths up to 7 ft for completing holes in bedrock (Fig. 2). R-thread cross-point drill bits (1 1/2" and 1 3/16" diameter, tungsten carbide, Fig. 3) and a 42" high-lift jack with customized slot for pulling 2-meter probes (Fig. 2) were also purchased.

Figure 1. One-inch-diameter masonry bit with extension 2-meter probes and spline drive.

The soil auger with a wedge shaped carbide tip was tested for the ability to penetrate soft rock more efficiently than standard rock drill bits. Airways in the standard drill bits tend to become plugged in soft rock. Unfortunately, it was found that the auger steel did not function well in soft rock, in part because of the high impact energy relative to rotation caused the wedge shaped tip to bind, and it proved difficult to extract the auger after it became stuck, because it is not possible to decouple the SDS Max rotary impact hammer used when it is in the normal vertical drilling position. Additional testing during year 2011 will experiment with alternate tip designs for the soil auger to eliminate binding caused by the wedge shaped tip, and changing the drive to match the spline drive rotary impact hammer. The masonry bit (Fig. 1) arrived after most of the locations requiring drilling in the study area had been completed, so this system will be tested in 2011 for its ability to penetrate soft rock, and also for the ability to penetrate outcrop, caliche, and sinter layers. Since the rotary impact hammer can also be use for driving the 2-meter probes


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into the ground, use of this system this can reduce equipment requirements, in areas that have only minor outcrop exposure, which may not warrant use of the heavier gas powered rock drill. The R-type threaded auger drill steel is functioning well. This drill steel is less prone to jamming in the hole and the longer steel lengths have made it possible to achieve full target depth of 2 meters in the majority of cases.

Figure 2. R-thread auger drill steel in lengths up to 7 ft (top four steels), soil auger with a wedge-shaped tungsten carbide bit and two auger extensions (lower three steels), and 42" high-lift jack with customized slot for pulling 2-meter probes (bottom).

Figure 3. Cross-point drill bits (1 1/2" and 1 3/16" diameter, tungsten carbide).


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In general, experimentation with the additional equipment and supplies described above has improved drilling success in terms of achieving target depth in difficult terrain. Some challenges remain, particularly when 0.5 to 1.5 meters of unconsolidated materials overlie bedrock. The R-thread auger drill steel (Fig. 2) helps minimize jamming of the bit caused by caving, but drilling is some what slower then when using conventional hexagonal drill steel. The use of PVC pipe as a casing in the upper 2 feet of a drill hole has been found to effective in most instances in preventing unconsolidated materials from caving in the upper part of the hole. Although dry sand infilling in bouldery or cobbly ground still can pose problems. The growing collection of equipment used for drilling in hard and soft rock is adding weight to the all-terrain vehicle (ATV) which is used to transport the equipment and personnel in the field. As a consequence, in hilly terrain under some circumstances, the ATV is underpowered and it is difficult to negotiate obstacles and climb slopes. A request was made late last year to use some of the remaining funds in this research project to purchase a new ATV with a more powerful engine that would be better able to negotiate hilly terrain with the additional drilling equipment. 4.2. Task 2: Compensation for Solar Radiation Variations 4.2.1. Field Temperature Survey: A field site was selected for testing equipment and testing and evaluating the processing techniques of tasks 1-3. The Desert Peak area of northwestern Churchill County was chosen as the test site because it consists of hilly terrain with a variety of slope aspects, elevations, and albedos, and contains a variety of near-surface rocks and soils, including unconsolidated coarse sands and gravels, sand dunes, soft bedrock (diatomite), and hard bedrock (basalt). A strong shallow temperature anomaly (associated with the Desert Peak geothermal field) is known to exist in this area based on earlier 2-meter orientation survey work conducted by the Great Basin Center for Geothermal Energy (unpublished data) and previous temperature gradient drilling (Benoit et al., 1982), which together approximately define the extent of a shallow thermal groundwater outflow plume. The field survey currently consists of 107 stations covering an approximately 13 km2 area surrounding the Desert Peak geothermal power plant (Fig. 4). The initial survey was completed in a two-week period between late September and early October, 2010. The 2-meter hollow steel temperature probes were left in the ground to make it possible to revisit the stations repeatedly over the course of a year to monitor seasonal temperature variations. Documentation of the seasonal temperature curve at a depth of 2 meters will ultimately make it possible to validate estimates of thermal conductivity/thermal diffusivity made with a prototype field device being designed in task 3. 4.2.2. Subtask A: Shallow Temperature Gradients: The measurement of shallow temperature gradients in 2-meter probes provides additional information on the character of shallow sub-surface temperatures. Shallow temperature gradients are calculated from temperature measurements at depths of 1.0, 1.5, and 2.0 meters. A number of 3-zone RTD’s to allow measurement of temperatures at 1, 1.5, and 2m positions in the 2-meter steel probes had previously been purchased and developed, however detailed studies and evaluation had not been conducted. Additional equipment purchased to make improvements in the accuracy and evaluation consist of 1) two 4-channel data loggers and materials to assemble two additional 4-zone RTD’s to allow for long term data collection of temperatures at the ground surface and at the 1, 1.5 and 2m depth in the 2-meter probe, and 2) a data logging meter with higher accuracy then the current meters used. The two data loggers were assembled in weather tight enclosures to


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allow them to be deployed in the field for extended periods from weeks to a year to collect continuous temperature data for studying the influences of short and long term surface temperature variations on 2-meter temperature gradient measurements. Temperature gradients between 1.5 and 2.0 meters correlate strongly with temperatures measured at a depth of 2.0 meters when surveys are performed in geothermal areas (Fig. 5). This correlation is a direct result of geothermal heat flux contributions, which under static conditions are proportional to the temperature gradient. Because of increased errors associated with calculating the shallow temperature gradient between 1.5 and 2.0 meter depths, single temperature measurements at a depth of 2.0 meters should provide a more sensitive indicator of the presence of geothermal heat flux. However, when solar radiation variations are present in a survey area, such as those produced by changes in ground albedo or topographic slope aspect, shallow temperature gradient data has the potential to more reliably map geothermal heat contributions than the temperature measured at a depth of 2 meters alone.

Figure 4. Uncorrected 2-meter temperatures from the Desert Peak Field Survey. Note that the solid curves lines represent temperature contours at a depth of 100 ft, derived from temperature gradient drilling (Benoit et al., 1982), and they do not represent contours of the 2-meter temperature data.


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The presence of dark soils (low albedo) or topographic slopes that more directly face the sun (south-facing slopes in the northern hemisphere) result in greater absorption of solar radiation in the form of heat, compared to soils with higher albedo or north-facing slopes. These solar radiation variations are readily detectable at a depth of 2 meters and can result in the false identification of geothermal anomalies without proper assessment and compensation (Kratt et al., 2009; Sladek et al., 2009). Shallow temperature gradients partially filter or reduce the effects of solar radiation variations because high solar heat flux works to reduce the shallow temperature gradient and low solar heat flux acts to increase the shallow temperature gradient. Thus, the value of the shallow temperature gradient moves in the opposite direction to that of the magnitude of solar heat flux, whereas temperatures at a depth of 2 meters move in the same direction as the magnitude of the heat flux.

Figure 5. Example shallow temperature gradients from the Desert Queen area. Geothermal anomalies are characterized by both anomalous temperatures at a depth of 2 meters and anomalous temperature gradients between 1.5 and 2.0 meters.

The completion of the Desert Peak field survey in hilly terrain with variable albedos provided the opportunity to quantitatively assess the ability of shallow temperature gradients to compensate for the effects of solar radiation variations. Three-zone temperature measurements were made at all 107 of the probe stations. Comparison of the shallow temperature gradient map of Desert Peak (Fig. 6) to the 2-meter temperature map (Fig. 4) provides qualitative evidence that the temperature gradient data is compensating for the effect of topographic slope-related variations in solar radiation input. This


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can be seen best near the center of the survey area shown by Figs. 4 and 6, where a profile of closely spaced temperature measurements cross a topographic ridge. Temperatures at a depth of 2 meters (Fig. 4) are consistently cooler on the north facing slope relative to the south facing slope, suggesting that these temperature differences are indeed related to topographic slope aspect. In contrast, the shallow temperature gradient is more similar on both sides of the topographic ridge, suggesting that the gradient is less influenced by topographic slope. The shallow temperature gradient does rise at the extreme south end of the profile, where it presumably is influenced by geothermal heat flux, as indicated by the location of the 100-ft-depth temperature contours. Further evidence of the ability of shallow temperature gradients to compensate for solar radiation variations will be presented in the following section on temperature processing techniques (Fig. 9).

Figure 6. Shallow temperature gradients at the Desert Peak field site.

4.2.3. Subtask B: Temperature Processing Techniques: Statistical processing of the temperature data was used to compensate (adjust) the 2-meter temperature data for the effects of solar radiation and elevation variations in the field survey area. Elevations for each probe location were derived from a 10-meter digital elevation model (DEM) using geographic information system (GIS) software (ArcGIS). An independent estimate of absorbed solar radiation at each probe location was constructed using an equation from Watson (1973) that quantitatively combines the effects of albedo and


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topographic slope. A simplified form of that equation for use in a geothermal environment is documented by Coolbaugh et al. (2007c) and is reproduced below: Q* = S0 (1 - A) M(Z) cosZ Q* is the net surface flux at the surface, S0 is a solar constant, A is the ground albedo, M(Z) equals the atmospheric transmission as a function of the zenith angle Z, and cosZ is the cosine of the angle between the surface normal and the sun’s rays. An atmospherically corrected ASTER satellite image was intersected with the DEM in order to calculate albedo independently of topographic slope using the approach described by Coolbaugh et al. (2007c). Values for cosZ and M(Z) at each probe location were calculated for a representative day approximately 1 month prior to the survey, in order to account for the time delay of temperature propagation from the surface down to 2 meters. The calculations were made using a hill shading routine available in the commercially available remote sensing software package ENVI. A plot of 2-meter temperatures versus the calculated combined solar radiation factor (a function of both albedo and topographic slope aspect) reveals a distinct correlation between 2-meter temperatures and solar radiation (Fig. 7), providing verification that temperatures at a depth of 2 meters are indeed influenced by solar radiation effects at the ground surface.

Figure 7. 2-Meter Temperatures at Desert Peak plotted against the solar radiation factor. A positive correlation between the two is apparent. The linear correlation line was calculated only using probe locations conservatively classified as “background” or “cold” based on knowledge of the field site.

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y = 4.9074x - 4.6231R2 = 0.0437

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A similar plot of the solar radiation factor against the shallow temperature gradient reveals a lower degree of positive correlation between the two (Fig. 8). This provides statistical evidence that the shallow temperature gradient is indeed providing a measure of compensation for solar radiation effects (a perfect compensation would yield a horizontal correlation line which would indicate that there is not a correlation between solar radiation and temperature gradient). Figure 8. Shallow temperature gradients at Desert Peak plotted against the solar radiation factor. A weak positive correlation between the two exists. The linear correlation line was calculated only using probe locations conservatively classified as “background” or “cold” based on knowledge of the field site. A plot of 2-meter temperatures against ground elevations reveals the expected inverse correlation caused by climate, since ground temperatures are typically cooler at higher elevations (Fig. 9). A regression of the independent variables of “elevation” and “solar radiation” was calculated against the dependent variable “2-meter temperatures” using the commercially available Systat software package. This regression was calculated only using probe locations conservatively believed to lie within areas of background temperatures outside of the influence of geothermal activity. The resulting linear equation was then applied to all of the probe temperature


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y = -0.0114x + 37.876R2 = 0.3167

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measurements, including those within the geothermal anomaly. The difference between the predicted temperature (based on solar radiation and elevation inputs) and the actual measured temperature is termed the “temperature residual” which represents an improved measure of temperature in which the effects of solar radiation and elevation have been minimized (Fig. 10).

Figure 9. 2-Meter Temperatures at Desert Peak plotted against elevation. An inverse correlation between the two is apparent. The linear correlation line was calculated only using probe locations conservatively classified as “background” or “cold” based on knowledge of the field site.

Contoured residual temperatures should provide an improved anomaly map from which to judge the location and strength of near-surface geothermal heat flux (Fig. 11), since these temperatures have been statistically treated to minimize the effects of elevation, slope aspect, and albedo on 2-meter temperatures.


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Figure 10. Temperature residuals from regression plotted against predicted background temperature. Differences between hot and cold probes are optimized.


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Figure 11. Residual 2-meter temperatures from the Desert Peak Field Survey. Residual temperatures are the 2-meter temperatures after correction for solar radiation (albedo and topographic slope) and elevation effects. Note that the solid curved lines represent temperature contours at a depth of 100 ft, derived from temperature gradient drilling (Benoit et al., 1982), and they do not represent contours of the 2-meter temperature data.

4.3. Task 3: Measurements of Thermal Diffusivity/Thermal Conductivity Seasonal temperature variations at the earth’s surface are transmitted into the subsurface at rates that depend on the thermal diffusivity of near-surface materials (winter temperature minima are experienced sooner at a 2-meter depth if the overlying rock and soil is relatively thermally diffusive). Under adverse field conditions, where thermal diffusivities vary from one portion of a survey area to another over a maximum range that includes contrasting areas of soil and bedrock, and when the survey is conducted near the time of peak summer surface temperatures or near minimum winter surface temperatures, temperature differences at a depth of 2 meters can be as great as 7°C (Coolbaugh et al., 2010). Consequently, it is important to monitor and estimate the temperature variations induced by thermal diffusivity differences, and to be able to compensate for these effects when their magnitudes are sufficiently great as to distort the results of a shallow temperature survey. A number of methods are available for estimating thermal diffusivities. They include:


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1) measurement a detailed vertical temperature profile from surface to depth at a specific period of time to determine the depth of the local temperature maxima (or minima) (Lange et al., 1982), which is a function of thermal diffusivity,

2) measuring the annual temperature amplitude at a specific depth (employed herein), 3) direct field measurements using transient heating techniques in current 2-meter probes

(under development herein), 4) measuring the lag time between maximum or minimum surface temperatures and

maximum or minimum temperatures at a specified depth (LeSchack and Lewis, 1983). Each of these methods has its drawbacks or challenges. These include the need for multiple measurements over the course of a year (methods 2 and 4), measurements at depths greater than 2 meters (method 1), or assumptions of representativeness with depth (method 3). To enable independent verification of a field device being developed herein for direct measurement of thermal conductivities, and indirect measurement of thermal diffusivities, method number 2) listed above was developed for independently measuring thermal diffusivities in the field using multiple measurements of 2-meter temperatures over the course of a year. The following equation, taken from Lange et al. (1982) was used as the basis for estimating thermal diffusivities:

Ts = Ta e-mz cos(ωΔt-mz) where: Ts = seasonal temperature deviation from mean temperature.

Ta = surface seasonal temperature amplitude m = (π/τ α)0.5

τ = wave period (one year) α = thermal diffusivity (= thermal conductivity/volumetric specific heat) z = depth below surface ω = 2π/τ Δt = time elapsed since surface summer temperature peak Differentiation of this equation with respect to time Δt makes it possible to solve for thermal diffusivity if the magnitude of the annual surface temperature wave is known at the given location at a depth of 2 meters and at a depth of 0 meters (ground surface). As part of the current research project, this approach was developed and applied to existing 2-meter temperature data from the Desert Queen geothermal area located approximately 10 km northeast of the Desert Peak field area. Multiple two-meter temperature measurements over the course of an entire year were available for six locations from the Desert Queen area (Sladek et al., 2009). The calculated thermal diffusivities range from 0.0028 cm2/s to 0.0055 cm2/s (Fig. 12). This range of thermal diffusivities has a relatively insignificant effect on temperatures at a depth of 2 meters, given the magnitude of the geothermal anomaly present at the Desert Queen site, as can be qualitatively determined from inspection of Fig. 12, because there is very little overlap of the temperature waves from different locations over the course of a year. An instrumentation device is being developed as part of the current task 3 that can be inserted down the 2-meter hollow steel probes to measure thermal conductivities. Once thermal conductivities can be measured, thermal diffusivity can be approximated based on the


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Figure 12. Thermal diffusivities for selected sites at the Desert Queen geothermal area. Temperature anomalies caused by geothermal heat flux (warmer colors) significantly out-weigh relatively minor temperature variations produced by differences in thermal diffusivities. Figure adapted from Sladek et al. (2009).

assumption that water content is the dominant factor influencing thermal conductivity variations. The methodology for measuring thermal conductivities with a down-probe heating element and digital thermometer is analogous to that described by Drury (1988) and employed in a thermal conductivity device manufactured by Hukseflux Thermal Sensors (model TP02). The Hukseflux thermal conductivity device is being used to calibrate the instrumentation being designed for the 2-meter probes. Despite the relatively high thermal mass of the current 2-meter probe design, laboratory tests are demonstrating that the prototype down-hole thermal conductivity device (Fig. 13) is easily capable of distinguishing thermal conductivity variations in the soil materials external to the probe (Fig. 14).


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Heat transfer theory (Drury, 1988) predicts that for a narrow, linear source, a linear relationship exists between the logarithm of the rate of temperature increase and the thermal conductivity of the material surrounding the linear probe. This linear relationship is confirmed by our laboratory measurements of the prototype device (Fig. 15), in which a linear relationship is observed between the log of the rate of temperature increase and the thermal conductivity measured with the Hukseflux thermal conductivity device. This linear relationship can be used to convert field measurements using this device in the 2-meter steel probes into thermal conductivity estimates. Additional laboratory and field testing, is planned during 2011 to refine the design and calibration of the down-hole thermal conductivity device, even though the device is functionally operative in its current condition. Published data, potentially augmented with additional laboratory measurements will be used to convert the thermal conductivity data into equivalent thermal diffusivity data based on certain assumption of water content, which is the primary variable that affects thermal conductivity and diffusivity variations.

Figure 13. Laboratory equipment for calibrating thermal conductivity measurements. The narrow stainless steel probe on the lower right is the down-hole combination heater and thermocouple device being designed as part of this project.


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y = -0.6456x + 2.5569

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Figure 15. Linear relationship between heating curve and calibrated thermal conductivity for the prototype down-probe device.

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4.4. Task 4: Gas Chromatograph with Sulfur Detection A Shimadzu GC-2014 gas chromatograph with flame photometric detector (Fig. 16) was purchased near the end of June and it arrived in July, 2010. The cost of this equipment was more than originally anticipated, and an external account was identified to cost-share the extra expense. The chromatograph has been installed and tested at the NBMG. This equipment will enable the detection of sulfur species, which are key components in geothermal gas emissions. Installation of a chromatographic column within the chromatograph will make it possible to discriminate between the sulfur species: H2S, SO2, SO3, COS (carbonyl sulfide), CS2 (carbon disulfide), SH (mercaptan), and S.

Figure 16. Shimadzu GC-2014 gas chromatograph with flame photometric detector (on right).

Testing of the gas chromatograph with soil gas samples from selected field sites is in progress and is now scheduled for completion during the spring of 2011. 4.5. Task 5: Sampling of Shallow Groundwater As described in a progress report presented as a poster at the 2nd annual meeting of the Nevada Renewable Energy Consortium at the University of Nevada, Las Vegas on Aug. 19th, 2010, we have recommended suspension of task 5 of the research project. After the grant was awarded, and after further discussion, it was realized that government permitting issues associated with the sampling of water from shallow wells would be complex and time consuming. In addition, an alternative water sampling technology has recently been tested at several geothermal sites in Nevada, and comprises an alternative, commercially available technology. This technology is named the “Hydroprobe” or “Direct Push Technology” and is operated by Pediment Gold, LLC.


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5.0 Comparison of Accomplishments with Goals and Objectives 5.1. Task 1: Shallow Temperature Measurements in Difficult Terrain The use of R-thread auger drill steel in place of conventional hexagonal drill steel has reduced the potential for getting drill steel stuck. The action of the auger better keeps debris behind the drill bit in motion reducing the potential for rocks to jam behind the bit. This has significantly increased the ability to drill holes in areas where soil overlies rock and in rubbly ground. Drilling with the auger drill steel is somewhat slower because the auger increases the amount of resistance drill chips experience while being blown out of the drill hole, but the lower potential for drill steel to become stuck improves efficiency. The custom 7 foot auger steel purchased has allowed reaching the target depth of 2 meters, Evaluation of a carbide tipped soil auger found that the wedge shaped tip can easily be impacted into soft rock causing the auger to bind up. The soil auger is supplied with and SDS Max adapter, and because the chuck of the SDS Max rotary impact hammer that was used can not be decoupled from the drill when in the vertical position, it was difficult to remove the auger or add extensions to drill deeper. Investigating different auger tips and an adapter to couple to a spline drive rotary impact hammer to the soil auger are planned for 2011. A masonry drill and extension system was purchased along with a spline rotary impact hammer that can be used both for drilling and driving the 2-meter probes into the ground. However all the probes that required the use of the rock drill had been emplaced in the study area when the equipment arrived. This equipment will be tested for use in both soft rock and hard rock areas in 2011. 5.2. Task 2: Solar Radiation/Environmental Compensations The objectives of the solar radiation/environmental compensation task have been well achieved. Results of the field survey at Desert Peak have demonstrated the ability of shallow temperature gradient measurements to appreciably compensate for the variable effects of solar radiation produced by local variations in albedo and topographic slope. A heat energy model was constructed using a DEM and visible light remote sensing imagery (ASTER), and this heat energy model was used in a linear regression model to correct for the effects of solar radiation on 2-meter temperature measurements as well as correct for the effects of elevation on 2-meter temperatures. Geothermal anomaly maps produced using the shallow temperature gradient data appear similar to the anomalies produced using the 2-meter data that has been statistically corrected to minimize solar radiation and elevation effects. This two-fold approach offers choices to the geothermal industry. If a geothermal company does not have the requisite GIS and statistical expertise necessary to integrate DEM data and remote sensing data together to produce statistical methods of calibrating 2-meter temperature data, they can employ the simpler approach of measuring shallow temperature gradients with three-zone RTD sensors to provide an approximate correction for solar radiation effects. 5.3. Task 3: Thermal Diffusivities/Thermal Conductivities A down-probe thermal conductivity sensor has been built, tested and calibrated in the laboratory. Preliminary calibration tests with a commercially available thermal conductivity device have yielded positive results. The sensor is ready to be deployed in the field, but a few additional laboratory tests are planned in order to refine and cross-check the initial calibrations. A field survey comprising 107 probe installations has been completed at the Desert Peak geothermal


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area. Steel probes have been left in the ground at all of these stations to facilitate thermal conductivity measurements once the device is ready for field work. Multiple temperature measurements over the course of a year will be used to separately estimate thermal diffusivities as a means of corroborating the field thermal conductivity measurements. Thus, this task is considered 90% complete, and could have been pushed to completion before year end 2010, but the time extension granted on this project will make it possible to conduct more systematic field testing of the thermal conductivity device. 5.4. Task 4: Soil Gas Measurements This task is largely complete with the purchase, installation, and laboratory testing of the Shimadzu GC-2014 gas chromatograph with flame photometric detector. Field testing of this equipment with soil gas samples from several known geothermal sites is not yet finished, but is scheduled for completion in the spring of 2011. 5.5. Task 5: Shallow Groundwater Sampling Task 5 has been suspended given the anticipated challenges with government permitting of groundwater sampling, and in consideration of alternative shallow groundwater sampling technologies recently developed by industry (see section 4.5 above). 6.0 Project Summary: Conclusions, Comments and Future Plans 6.1. Task 1: Drilling in Difficult Terrain The use of auger type drill steel has improved the ability to drill holes in rocky terrain. The auger drill steel is less prone to becoming stuck due to caving. Although drilling is somewhat slower than with conventional hexagonal drill steel, the lower incidence of getting drills stuck increases efficiency. Custom built 7 foot auger drill steel has made it possible to drill to the target 2 meter depth. The use of PVC pipe to case the upper 2 feet of holes in areas with sand or loose rubbly colluvium over lying bedrock was found to be effective to allow drilling in these areas. Attention to drilling techniques and experience has improved efficiency, and the use of a small air compressor to assist in clearing drill chips was not used for any of the locations at the study site. For 2011, additional testing of the soil auger and possible modification of the auger tip to reduce the potential for binding will be investigated. The masonry drill system will be tested for drilling in soft rock and clay, which tends to plug the airways of rock drill bits, and also for efficiency in drilling in hard rock as a substitute for using gas powered rock drill. Although the masonry drill system is not expected to be as effective as the heavier gas powered drill. However, because the same electric hammer can be used for drilling and driving the 2-meter probes, it can reduce the equipment needed where only minor bedrock, or caliche or sinter layers are expected to be encountered. 6.2. Task 2: Digital Corrections Task 2 work activities have successfully demonstrated the ability to make adjustments to the shallow temperature data to correct for sources of (non-geothermal) temperature noise such as produced by solar radiation and elevation variations. Future plans within the scope of this research project include positing relevant data and reports on the web site of the Great Basin Center for Geothermal Energy. A Geothermal Resources Council Transaction paper (Coolbaugh et al., 2010) describes many of the procedures used in this project for digitally correcting the 2-


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meter temperature data, but an additional paper may be presented at the 2011 Geothermal Resources Council annual meeting. It is anticipated that the techniques for calibrating 2-meter temperature data for the effects of solar radiation and elevation will be incorporated into other on-going research projects in which shallow temperature measurements are playing a role. The geothermal industry is keyed into the Geothermal Resources Council meetings where we have presented the results of this work, and we have been communicating these results to all industry representatives that have individually expressed an interest. 6.3. Task 3: Measurements of Thermal Diffusivity/Thermal Conductivity It has been possible to build a down-hole thermal conductivity device that can be inserted down the hollow steel 2-meter probes to measure the thermal conductivity of surrounding soils. This in turn will make it possible to approximately correct shallow temperature surveys for the effects of thermal diffusivity variations. This correction would be accomplished via a conversion of thermal conductivities to thermal diffusivities, which can be estimated where it can be assumed that moisture contents are primarily responsible for these variations. There are some limitations to the application of down-probe thermal conductivity measurements. In areas of bedrock, poor contact between the walls of the drill hole and the temperature probes will be inevitable, thus rendering the thermal conductivity measurements inaccurate. Thus, the effectiveness of down-hole measurements of this type are limited to areas of unconsolidated materials or very soft bedrock that can be directly penetrated by hammering without the need of a rock drill. The field survey conducted at the Desert Peak geothermal area has been designed to make it easy to come back during year 2011 to make down-hole thermal conductivity measurements, because all of the steel 2-meter probes have been left in the ground. Multiple measurements of 2-meter temperatures over the course of a year will make it possible to independently estimate thermal diffusivities, to help validate and optimize the ability of the down-hole sensors to be used to estimate this variable. 6.4. Task 4: Gas Chromatograph with Sulfur Detection It is anticipated that with the ability to detect sulfur gases, the Shimadzu GC-2014 gas chromatograph with flame photometric detector will significantly improve the ability to detect geothermal signatures in shallow soil gas samples. Now that the chromatograph is up and running in the NBMG laboratory, we are ready to run test orientation survey in the field at several known geothermal areas to assess the ability to detect anomalous concentrations of sulfur gases. This field work should be completed in the spring of 2011. 6.5. Task 5: Sampling of Shallow Groundwater Task 5 has been suspended given the anticipated challenges with government permitting of groundwater sampling, and in consideration of alternative shallow groundwater sampling technologies recently developed by industry (see section 4.5 above). It is proposed that the funds saved from not completing this task be used to purchase a new and slightly stronger ATV for transporting the portable temperature equipment and soil sampling equipment in the field. When portable rock drills and related support equipment are added to the existing ATV, the ATV


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becomes somewhat underpowered in hilly terrain. The use of an incrementally stronger ATV would help demonstrate the ability to conduct fully portable surveys in more challenging terrain. 7.0 Products Developed, Milestones, and Deliverables 7.1. Tasks 1: Drilling Equipment/Techniques The primary product developed in task 1 is an improved portable system of conducting shallow temperature surveys in the field. This equipment is fully functional and is being used on other geothermal research projects being conducted at the Great Basin Center for Geothermal Energy. This equipment is also made available for inspection by members of the geothermal exploration community, who are building similar equipment to conduct geothermal exploration. Additional testing in 2011 is planned to improve capabilities to drill in soft rock and clay. 7.2. Task 2: Temperature Data Processing The data processing techniques developed as a part of this research project are relatively easily implemented by geologists or engineers with training and education in the use of GIS software and statistics. Commercially available, off-the-shelf software such as those used in this project (ArcGIS for GIS data integration, ENVI for remote sensing processing, and Systat for statistical calculations) greatly facilitate the processing, but none of these software packages have a monopoly on the market, and multiple competing software products are available which can be used to perform the calculations. No proprietary code is involved and the approaches are described in sufficient detail in publication deliverables coupled with references. 7.3. Task 3: Thermal Conductivity Device The down-probe thermal conductivity device represents the principal physical “deliverable” for this task. Testing and calibration to improve the accuracy of this device will continue during year 2011. The procedures for constructing such a device, calibrating it, and using it in the field will be described in sufficient detail on the GBCGE web site and/or in one or more upcoming geothermal publications. 7.4. Task 4: Soil Gases The Shimadzu GC-2014 gas chromatograph with flame photometric detector is installed at the NBMG and constitutes the principal physical “deliverable” for this task. Testing of this equipment with soil gases taken from known geothermal systems will continue during the spring of year 2011. The ability of this equipment to detect sulfur gases associated with concealed geothermal activity will be assessed, and the results will be described on the GBCGE web site and/or in one or more upcoming geothermal publications. 7.5. Technology Transfer 7.5.1. Publications and Presentations: Two presentations/publications serve to document much of the research completed to date, and copies of those publications should be included with the submission of this report. One of these presentations is a poster presented at the 2nd annual meeting of the Nevada Renewable Energy Consortium at the University of Nevada, Las Vegas on Aug. 19th, 2010. The second presentation, which comes with an associated transactions paper (Coolbaugh et al., 2010), was given at the annual meeting of the Geothermal Resources Council in Sacramento, CA on Oct. 26th, 2010.


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Significant progress has been made since the Geothermal Resources Council meeting in regards to development of computer processing methodologies and development and calibration of the thermal conductivity sensor. New results from soil gas samples analyzed with the gas chromatograph are expected in the first part of year 2011. For this reason, we consider it appropriate to prepare at least one additional paper/presentation to document final results of this research project sometime during 2011. 7.5.2. Web Technology Transfer: Much of the work completed to date has not yet been documented on a local web site. However, the GBCGE has a large and comprehensive geothermal web site devoted solely towards the purpose of geothermal web technology transfer, with a staff actively working to improve and expand the site. It will be a relatively easy and smooth process to add all relevant data and reports to the GBCGE web site for availability to the greater public. This should be completed in the first half of 2011. 8.0 References Benoit, W.R., Hiner, J.E., and Forest, R.T., 1982, Discovery and geology of the Desert Peak

geothermal field: A case history: Nevada Bureau of Mines and Geology Bulletin 97, 82 p. Coolbaugh, M.F., Raines, G.L., and Zehner, R.E., 2007a, Assessment of exploration bias in data-

driven predictive models and the estimation of undiscovered resources: Natural Resources Research, v. 16, n. 2, p. 199-207.

Coolbaugh, M.F., Sladek, C., Faulds, J.E., Zehner, R.E., and Oppliger, G.L., 2007b, Use of rapid temperature measurements at a 2-meter depth to augment deeper temperature gradient drilling: Proceedings, 32nd Workshop on Geothermal Reservoir Engineering, Stanford University, Stanford, CA, Jan. 22-24, 2007, p. 109-116.

Coolbaugh, M.F., Kratt, C., Fallacaro, A., Calvin, W.M., and Taranik, J.V., 2007c, Detection of geothermal anomalies using Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) thermal infrared images at Brady’s Hot Springs, Nevada, USA: Remote Sensing of Environment, v. 106, n. 3, p. 350-359.

Coolbaugh, M.F., Sladek, C., and Kratt, C., 2010, Compensation for seasonal and surface affects of shallow (two-meter) temperature measurements: Geothermal Resources Council Transactions, v. 34, p. 851-856.

Drury, M.J., 1988, A simple needle-probe method for measuring thermal diffusivity of unconsolidated materials: Geothermics, v. 17, no. 5/6, p. 757-763.

Kratt, C., Coolbaugh, M., Peppin, B., and Sladek, C., 2009, Identification of a new, blind geothermal system with hyperspectral remote sensing and shallow temperature measurements at Columbus Salt Marsh, Esmeralda County, Nevada: Geothermal Resources Council Transactions, v. 33, p. 481-485.

Lange, A.L., Pilkington, H.D., and Deymonaz, J., 1982, Comparative studies of geothermal surveys in 3-meter and temperature-gradient holes: Geothermal Resources Council Transactions, v. 6, p. 133-136.

Lechler, P.J. and Coolbaugh, M., 2007, Gaseous emissions from Steamboat Springs, Brady’s Hot Springs, and Desert Peak geothermal systems, Nevada: Geothermal Resources Council Transactions, v. 31, p. 359-361.


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Lechler, P.J., Coolbaugh, M., and Sladek, C., 2009, Multi-gas geochemistry at the Desert Queen geothermal system, west-central Nevada: Geothermal Resources Council Transactions, v. 33, p. 491-494.

Sladek, C., Coolbaugh, M.F., and Zehner, R.E., 2007, Development of 2-meter soil temperature probes and results of temperature survey conducted at Desert Peak, Nevada, USA: Geothermal Resources Council Transactions, v. 31, p. 363-368.

Sladek, C., Coolbaugh, M., and Kratt, C., 2009, Improvements in shallow (two-meter) temperature measurements and data interpretation: Geothermal Resources Council Transactions, v. 33, p. 535-541.

Watson, K., 1973, Periodic heating of a layer over a semi-infinite solid: Journal of Geophysical Research, v. 78, p. 5904-5910.

Williams, C.F., Reed, M.J., DeAngelo, J., and Galanis, S.P. Jr., 2009, Quantifying the undiscovered geothermal resources of the United States: Geothermal Resources Council Transactions, v. 33, p. 995-1002.

Documents

Task 3.0 Geothermal Prospecting -- Subtask 3.2.1: SHALLOW ... · information on surface albedo, topographic slope orientation, and elevation to correct 2-meter temperatures for the