MULTIPLE GEOPHYSICAL METHODS FOR IDENTIFYING AND MAPPING CAVES IN THE RECHARGE ZONE OF THE EDWARDS AQUIFER, TEXAS

A hydrogeophysics class at the University of Texas at Austin evaluated five geophysical methods for identifying caves in the recharge zone of the Edwards Aquifer of Central Texas. Two sites were selected both of which occur in the Kainer Formation. Site 1 consists of a known cave while at site 2 a cave is inferred from surface observations. Electrical resistivity (ER) and electromagnetic (EM) methods were used to conduct line surveys along two subparallel transects crossing over the known cave at site 1, and nearly orthogonal North-South and East-West transects at site 2. Based on the electrical resistivity data, additional subsurface cavities were inferred. Two such locations at each site were chosen for collecting ground penetration radar (GPR) data in a gridded network. Gravity and seismic data were collected only at site 1. The results from the dipole-dipole ER surveys indicated areas of anomalously high resistivity, which were interpreted as air-filled caves. The resistivity signal at the known cave site was smaller, presumably due to highly conductive moist cave sediments (principle of suppression). In comparison, EM data collection was fast and efficient. In many cases, the EM-34 data confirmed the ER inferences about cave locations; however, EM31 and EM34 instruments were extremely sensitive to interference from power lines. Seismic data showed zones with very large attenuation indicating the presence of unconsolidated material or intense fracturing. Any inference about the known cave, however, remains unsubstantiated by current processing of the seismic data. The GPR data from site 1 showed a low velocity zone which correlate well with the ER data and hence provide confidence for locating an additional cave. The GPR data also showed influence from conductivity of near surface soils which resulted in reduced resolution of near surface features. The gravity data showed an anomaly consistent with the location of the known cave at site 1; however, the signal was on the order of the noise level, and it was determined that a gravimeter with accuracy in micro Gals would be needed to precisely locate the cave. Our conclusions are that field observations coupled with the EM method provide best reconnaissance at these sites. This combination should be followed by ER, GPR, and seismic methods, perhaps in this order.