Time series resistivity analysis to map hydrologic activity and predict karst conduits in unsaturated limestone above the recharge zone of the Edwards aquifer, central Texas

Time-series electrical resistivity is one method for locating preferential flow paths through the unsaturated zone across upland interstream areas overlying karst aquifers. Resistivity varies spatially with stratigraphy, karst features, and water content of the unsaturated zone. Temporal fluctuations of resistivity in the unsaturated zone, however, depend mainly on changes in water content. Temporal variance of water content is assumed to be greater in open fractures and karst conduits than in unfractured blocks of limestone matrix. Large-aperture fractures and conduits wet and drain with successive infiltration and dry periods because of the relation between water content, capillary pressure, and pore size. The volume of water in storage is transient because of a short-term imbalance between infiltration and drainage rates. Zones with greater resistivity variance, therefore, reflect greater hydrologic activity with more wetting and draining of pore space and mark preferential flow paths through which soil water moves downward toward the water table.

The Edwards karst aquifer in south-central Texas is recharged primarily from losing streams and secondarily from infiltration of water across upland interstream areas. The study area was in Edwards limestone overlying Headquarters Cave in an upland interstream setting in northwest San Antonio, Texas. Time-series resistivity involved repeated surveys over a 10 month period using a 48-electrode dipole-dipole array with 1-m electrode spacing. The two dimensional vertical pseudosection was discretized into specific sampling volumes for which resistivity was repeatedly measured. Temporal variance of resistivity was calculated for each sampling volume. Highest resistivity variance was in the uppermost 0.5-m depth, reflecting wetting and drying of surficial soil. Large variations in resistivity were also mappable along subvertical linear zones and in discrete horizons reflecting vertical and bedding-plane conduits within the epikarst. Additional work is needed to quantify water storage and flux in the unsaturated limestone for better calibration with electrical resistivity.