A Multi-Tool Geophysical and Hydrogeological Investigation of a Karst Aquifer System, Cibolo Canyon Development Area, Bexar County, Texas

The U.S. Geological Survey, in cooperation with San Antonio Water System, conducted a geophysical and geological study of the Cibolo Canyon development area, northeast Bexar County, Texas, to characterize hydrostratigraphic features and to map surface and subsurface hydrogeologic features of the Edwards aquifer. Capacitively-coupled (CC) resistivity and frequency-domain electromagnetic (FDEM) profiling, direct current resistivity depth imaging (2D–DC) profiles, and timedomain electromagnetic (TDEM) soundings were used in conjunction with new geologic mapping, and borehole logging. This study shows that near surface geophysical methods were effective in defining electrical signatures of the various stratigraphic members of the Edwards aquifer. In general the upper hydrostratigraphic units of the Edwards aquifer (Kainer Formation) are less resistive than the lower units (Person Formation). The upper hydrostratigraphic units of the underlying Trinity aquifer have a lower resistivity than the overlying Edwards. The TDEM soundings were able to delineate changes in elevation of the base of the resistive units and define the amount of vertical displacement across the faults. Both CC resistivity and FDEM methods were used to map hydrostratigraphic contacts, fault zones, and possible karst features. The 2D–DC resistivity imaging techniques showed lateral changes in the subsurface lithology that correlate well with the CC and FDEM data. The TDEM and 2D–DC methods had limited application due to site accessibility and project time lines. Results from the study demonstrated that combining multiple geophysical methods and geological mapping significantly improved the hydrogeologic mapping the Cibolo Canyon development area.