ADVANCES IN ELECTRICAL RESISTIVITY IMAGING OF KARST VOIDS

Geophysical techniques applied to the problem of imaging near-surface void space in karst environments have a history of mixed results and qualified successes. Electrical, ground-penetrating radar, seismic, and micro-gravity methods have been utilized in attempts to locate and image karst voids of varying spatial dimension and morphology. No one method has proven itself to be the “ultimate solution” technique. However, experimentation with non-standard arrays in electrical resistivity imaging (ERI) has led to significantly improved delineation of void space in the near surface. An Advanced Geosciences Inc. (AGI) Supersting R8 DC resistivity system was the primary instrumentation utilized in multiple surveys of karst systems in the Edwards Plateau region of central Texas. Survey objectives were to locate and delineate karst voids as potential habitat for endangered karst invertebrate species. Non-standard arrays, typically referred to as mixed arrays, have demonstrated their utility in resolving both vertical and horizontal heterogeneity of void morphology at depths exceeding that commonly attained with the conventional dipole-dipole array. Where possible, surveys were conducted over known and mapped cave passage for validation of the techniques. Forward simulation modeling has been conducted to generate synthetic pseudosection profiles and to verify resistivity anomaly signatures of known void spaces. The problem of resistivity field sensitivity to off-line lateral anomalies is discussed with examples. Results have been highly encouraging and serve to reinforce the void imaging capabilities of electrical resistivity, especially when mixed array types are utilized.