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Transient Electromagnetic Mapping and Monitoring of the Edwards Aquifer, 20-9212

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Principal Investigators
Charles B. Connor
Stewart K. Sandberg
Ronald T. Green
David A. Farrell

Inclusive Dates: 07/ 31/00 – 05/01/01

Background - Geophysical techniques are an important component of effective hydrogeological investigations. Geophysical surveys employing a variety of electrical and electromagnetic methods have been used successfully to map stratigraphy, geologic structure, and depth of the water table in major aquifer systems. The feasibility of using integrated geophysical surveys to characterize the Edwards Aquifer was evaluated at a site at the western edge of the Knippa Gap, where groundwater flows across the Balcones fault system and downdip into the Maverick Basin. This site was selected because of 1) the importance of accurately interpreting groundwater flow in the Knippa Gap to understand the regional flow regime in the Edwards Aquifer and 2) to evaluate the capability of surface geophysical techniques to interpret a geologically complex karst carbonate aquifer.

Approach - The utility of integrated surveys for characterization of the Edwards Aquifer was demonstrated using geophysical data gathered adjacent to two monitoring wells that penetrate the Edwards Aquifer at the western margin of the Maverick Basin, in Uvalde County, Texas. To the team's knowledge, this project represents the first application of integrated geophysical techniques to characterize this aquifer. Ground magnetics, transient electromagnetic (TEM), and direct current (DC) resistivity surveys were chosen as candidate techniques for the survey. Ground magnetic survey was chosen because of its ability to very rapidly map variations in the depth and thickness of the Cretaceous basalt found in this area. This method is particularly sensitive to lateral discontinuities in the basalt, produced by offsets across normal faults. TEM and DC resistivity were conducted to provide information about variation in electrical conductivity as a function of depth that can be correlated directly with the stratigraphic section. Data from TEM and DC soundings were jointly inverted to reduce uncertainty in the model results. TEM loop-profiling was also done to enable the team to build a section of apparent resistivity as a function of depth. Combined, these methods provided details of the geologic structure that have not been previously recognized.

Accomplishment: Integrated resistivity, electromagnetic, and magnetic surveys were effectively used to delineate geologic structures in a carbonate aquifer, on the scale of hundreds to thousands of feet. Simultaneous inversion of TEM and DC resistivity data was able to pick the major stratigraphic boundaries, including the top of the Austin Chalk and the top of the Edwards Aquifer itself, at a depth of 237 meters (780 feet). These results are in excellent agreement with gamma and lithology logs from the two wells located at the survey site. Large-loop (100-meter-side; 324-foot-side) TEM profiling successfully delineated normal faults in the section, in excellent agreement with faults modeled from ground magnetic data. Thus, modeling TEM soundings, TEM profiles, DC resistivity profiles, and magnetic data together provided a comprehensive view of the geologic structures influencing groundwater flow in this faulted, compartmentalized aquifer. This integrated surface geophysical approach proved to be an efficient and economical method to interpret a karst carbonate aquifer, particularly when compared to the cost and time required to acquire the same information using traditional drilling approaches.

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