MAPPING OF HYDROSTRATIGRAPHIC UNITS AND STRUCTURE IN THE EDWARDS-TRINITY AQUIFER SYSTEM USING ELECTRICAL GEOPHYSICS; MEDINA AND BEXAR COUNTIES, TX

Two high resolution helicopter electromagnetic (HEM) resistivity surveys were completed over the Edwards aquifer lower confining units, recharge zone, and upper confining units in different depositional environments. These surveys yielded new information on the complex nature of the hydrostratigraphy and structure of one of the most productive and permeable carbonate aquifers in the United States. In many places alluvial deposits and Quaternary formations are less than a meter thick, therefore the very high frequency resistivity data (around 100 kHz) provide a surrogate map of the near surface bedrock geology and structure. Particular ranges of resistivity from the HEM survey can be associated with particular bedrock hydrostratigraphic units just as done from borehole resistivity (or electrical conductivity) logs. The lowest resistivities are associated with the Del Rio Clay, Eagle Ford Group (upper confining units, Medina County), and the upper-most part of the lower member of the Glen Rose Limestone in Bexar County. These are excellent “marker beds” for interpretation of stratigraphy. All of the hydrostratigraphic units of the Edwards Group have very high resistivities (hundreds of ohm meters). In Medina County, the upper and lower Devils River Formation can be divided based on lower overall resistivity of the upper unit observed from the HEM data. The upper Trinity aquifer (upper Glen Rose Limestone) has an overall lower resistivity when compared to the Edwards aquifer, which is to be expected based on lithology. Thin limestone beds with high resistivity in the Trinity aquifer can be mapped in detail by the airborne geophysics in the upper 20 meters. Hydrostratigraphic unit D in the upper Trinity aquifer is characterized by a very high resistivity that can be used as a “marker bed” in stratigraphic interpretation. Large reefal features in the lower Glen Rose Limestone have a high resistivity. These reefal units along Cibilo Creek may be important to recharge in the Lower Trinity aquifer. Current work utilizes the detailed airborne resistivity surveys to refine bedrock geologic maps and construct 3-dimensional geologic framework models. Limited drill hole information can be integrated with detailed interpreted 3-dimensional geophysical surfaces associated with marker beds.