San Antonio, Texas Area Audio-magnetotelluric Data

The San Antonio audio-magnetotelluric data sets are one part of a larger study of the Trinity and Edwards aquifers by the U.S. Geological Survey.  AMT surveys are typically collected along a profile across a structure of interest. The AMT line locations were sited to cover several different geologic structures. These include the fresh/saltwater interface southeast of the Balcones Escarpment, a near surface volcanic plug along the Frio River, and a water filled limestone cave system associated with a normal fault near Honey Creek.  All of the sounding sites in this survey used the Geometric EH-4 receiver augmented by a transmitter to fill in the middle frequencies (450-4,500 Hz) where the natural signal strength is sometimes below the detection limit of the instrument.

The San Antonio AMT profiles were constructed from station locations in 4 survey areas collected during the spring of 2001. The data from these surveys are of varying quality, due in large part to cultural noise.  The design (station spacing, line direction, and number of sites)  may vary between survey areas depending on the field conditions. Index maps and a data table give an overview of the survey sites and summarizes their locations.  The resulting electrical sections have associated data sets that can be downloaded.

These electrical sections are interim products. Considerable editing and one-dimensional modeling of the data was undertaken. Smoothing of the curves and resulting sections was necessary to reduce noise caused by cultural effects near the sites.  Resolution is better in the near surface and falls off with greater depth. Depth of exploration falls off in areas where the near surface rocks are conductive. One example where this happens is the Johnson Ranch near Uvalde, Texas electrical section. Another example where depth of exploration is impacted is the conductive salt water interface is near the surface beneath the southeast end of the Kyle electrical section.

Acknowledgements

This project was supported by the San Antonio Water System (SAWS), the U.S.G.S. Geologic Mapping Program, and the Water Resources San Antonio Sub-District Office of the USGS. Thanks to the following USGS colleagues: George Ozuna, Chuck Bloume, Rebecca Lambert, and Mark Warzecha for their assistance, Mike Nyman, Allen Clark, and Brian Petri, for data collection, and Jason Faith for digitizing analog induction well logs used for ground truth. Special thanks to Denise Renaghan of the San Antonio Water System for her help and savvy field guidance.

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