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Application of Integrated Geophysical Techniques to Characterize the Edwards Aquifer, Texas
Integrated geophysical surveys can provide new insights into the geologic structures controlling groundwater flow in the Edwards Aquifer, one of the largest freshwater aquifers in the United States and the sole source of drinking water for the city of San Antonio, Texas. Here, we demonstrate the feasibility of integrated geophysical surveys at a site near two monitoring wells at the western edge of the Knippa Gap, where groundwater flows across the Balcones fault system and downdip into the Maverick Basin. In this area, normal faults controlling groundwater flow are partially obscured by Tertiary sediments, such as the Leona Gravel. Simultaneous inversion of transient electromagnetic (TEM) and direct current (DC) resistivity data picks the major stratigraphic boundaries, including the top of the Austin Chalk and the top of the Edwards Aquifer itself, at a depth of 237 m (780 ft). These results are in excellent agreement with gamma and lithology logs from the two wells located at the survey site. Large-loop (100-m-side; 324-ft-side) TEM profiling successfully delineates normal faults in the section, in excellent agreement with faults modeled from ground magnetic data. In total, four normal faults were identified by these surveys. Two of these faults were previously known from well log interpretation and two were previously unknown. In addition, the TEM profiles image these faults in the sedimentary section, to the top of the Edwards Aquifer. Thus, modeling TEM soundings, profiles, DC resistivity soundings, and magnetic data together provides a comprehensive view of the geologic structures influencing groundwater flow in this faulted, compartmentalized aquifer.
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