Ground Penetrating Radar in the Contextof Geothermal Reservoir Exploration

Jodutt BasrawiUniversity of California - Los Angeles, Department of Earth, Planetary, andSpace Sciences; Peking University, Department of Geophysicsjoduttbasrawi@jodutt.com — +1 (949) 332-0077

AbstractGround Penetrating Radar (GPR) has had nascent interactions with

geophysical exploration methods in the context of geothermal energy.With GPR’s ability to map the lateral and vertical extent of geologicbodies, it can be used as a means to prospecting geothermal reservoirs.The problems associated with GPR usage for geothermal explorationinclude GPR’s inability to map beyond the shallow subsurface (i.e., lessthan 50 meters), which is where most prospective geothermal reser-voirs are located. Another GPR problem lies in the need to balancefrequency (which is directly proportional to image clarity) and the ex-tent of depth (higher frequency entails lower extents of depth for craft-ing images) in GPR operation. This research aims to display GPR’scapabilities in scouting geothermal energy sources within any givenlandmass through the use of digital models, raw data associated withsubsurface bodies, and GPR relationships with a given area’s geochem-istry (e.g., sinters). What geoscientists know of GPR is its capability tocharacterize subsurface flow networks, map rock fractures, and detectvariances in moisture content, all of which contribute to understandinggeothermal exploration phases. Additionally, data from GPR, such aspermeability values, can be compared with MATLAB created modelsin order to relate electromagnetic waves with permeability values ofheterogeneous bodies. Ultimately, likelihoods exist for GPR to be anew and efficient geothermal exploration vehicle, notwithstanding itsinability to map deep subsurfaces.

IntroductionGround Penetrating Radar (GPR) uses radar waves to map theshallow subsurface (i.e., less than 50 meters). GPR devices dif-ferentiate subsurface bodies (including soil/rock strata, circularbodies, and plant material) by calculating the velocity of radarwaves reflected off of subsurface bodies alongside the amountof time passed since that same wave was released by the GPRdevice. GPR devices can also measure the dielectric constantthrough the following relationship:

ξr =(cv

)2(1)

where ξr is relative dielectric constant (unitless), c is the speedof light (≈ 3.0 ∗ 107 meters/second), and v is the velocity of anincoming radar wave (meters/second). Many subsurface materi-als each have their own dielectric constant or range of dielectricconstants as a result of their unique material properties.

Table 1: Table of material dielectic constants and the usual velocity(s) ofreflected radar waves

Geothermal energy is thermal energy that is stored and releasedfrom the earth. Enhanced Geothermal Systems (EGS) are flownetworks that are in contact with hot dry rock (HDR). Fluids thatflow through EGS settings can obtain geothermal energy andbring that energy to the surface for the purpose of generatingelectricity. Geothermal energy is an alternative energy sourcethat is available 24 hours a day, 7 days a week.

PurposeThe purpose of this project is to qualitatively and quantitativelyoutline shortcomings of GPR in geothermal contexts. Addition-ally, the project will show ways of circumventing these short-comings and thus render GPR into a potentially useful tool forthe geothermal industry.

MethodsGPR profiles were obtained from the Suiyuan Residential Dis-trict in Beijing.

Figure 1: Location from which raw data was collected relative to PekingUniversity

The raw data was then processed via MATGPR software. Theprocessing steps used for this raw data set are as follows:1. Set statics, polarity, and topographic corrections2. Remove low frequency components (i.e., de-wowing)3. Set a standard automatic gain control (standard AGC) to em-

phasize low amplitudes and de-emphasize high amplitudes ofthe radar waves (we care about the frequency of the waves andtheir velocities, not their amplitudes)

4. Remove global background noise (i.e., signals that remainconstant across a GPR profile of interest)

5. Trim the travel-time window to remove unclear signals fromdeep depths

6. Convert time to depth by means of a velocity model based onthe earth materials in a given GPR profile

7. Apply a centroid-frequency calculation algorithm across agiven GPR profile

2-D models (finite-difference time-domain and split-stepmodel methods provided by MATGPR) were constructed andmade to resemble the processed GPR data as much as possible.

Results: Raw and Processed GPR Profiles

Figure 2: A raw GPR profile showing normal displacement.

Figure 3: A processed GPR profile showing multiple anomalies (theparabolic lines) and a soil layer with a step on its right half.

Figure 4: Centroid frequency plot showing water contents

2-D Models

Figure 5: Model of figure 2, which includes three quartz sinters as anoma-lies and a clay aquifer through the middle of the profile. The model points tothe field area possibly possessing sinters, which are geochemical precursorsto geothermal reservoirs. This points to likelihoods of the Suiyuan Districtpossessing accessible geothermal reservoirs.

Figure 6: A model of a trench with two pipes as anomalies (the pipes couldpose as enhanced geothermal flow pathways).

Discussion & Conclusion

Figure 7: A satellite map of China showing sinter sites and documented hot-spots useful for geothermal energy extraction.

GPR’s ability to distinguish dielectric constants enables GPRdevices to detect sinters, which are geochemical products of hotalkali water flow that has been in contact with geothermal sys-tems. Additionally, flow networks and soil roughness can also bedetected, both of which indicate soil water content in prospectivegeothermal sites. With this information, prospective geother-mal sites can be mapped more easily than before the advent ofGPR’s application to geothermal exploration. Chinese reservoirlocations based on GPR data are oftentimes nearby documentedsinter sites, which points to a correlation between geochemicalmarkers detectable by GPR and geothermal reservoir locations.

References1. Clayton, Rob.“Notes on Ground Penetrating Radar. Rob Clayton, 2016.

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