DAVIS, DARREN and E. B. LEDGER*, Stephen F. Austin State University, Nacogdoches, TX

Abstract: Detection of Faults at the Earth's Surface by Hand-held Ground Conductivity Surveys, Eastern Texas

A portable Ground Conductivity Meter operated by one person has been used to detect normal faults in eastern Texas. These faults are usually difficult to locate on the ground because of soil and vegetation. The Eocene Claiborne Group is present in the study area. It contains alternating clay and quartzose sand units representing cyclic transgression (clay) and regression (sand).

Before attempting to detect a fault, baseline conductivities of each unit were measured at the outcrop in a grid pattern. Sands, even with admixed clay, had conductivities less than 20mS/m (millisiemens per meter). Clay units had conductivities greater the 50 mS/m. Multiple determinations in the field at the same grid show a variation of about 5 mS/m at each station. The small variation allows sand and clay units to be readily distinguished. The present antenna configuration measures the upper six m of the earth, so that soil and other superficial layers do not hinder measuring bedrock conductivity.

Four sites were chosen along the Mt. Enterprise Fault system. Data were taken along a line across the fault. Sand units were easily distinguished from clay units allowing faults to be located within 10 m on the ground. Faults in eastern Texas are not thought to produce large earthquakes, but they do disrupt ground water flow and serve as conduits for water flow to the surface. Knowing the exact location of these faults not only aids in detailed geologic mapping, but is important in hydrogeologic and environmental work. 

AAPG Search and Discovery Article #90924©1999 GCAGS Annual Meeting Lafayette, Louisiana