Mapping Subsurface In Karst Terrain Using 2-D Electrical Resistivity Tomography

Karst terrain is a unique and complex environment and has been a subject of increasing investigation for engineering, geotechnical, environmental, and archeological purposes. In this study, the electrical resistivity tomography (ERT) technique was used to image a test site in southwestern Missouri with the goal to map variable depth-to-bedrock and to characterize subsurface lithologic conditions. The ERT technique employed a multi-electrode resistivity system, powered by a 12-volt deep cycle marine battery, 5 feet electrode spacing, and a dipole-dipole array configuration with the intent to map the subsurface to a depth of at least 100 feet. The output, a two-dimensional (2-D) resistivity profile, was verified and constrained with active multichannel analysis of surface waves (MASW) data. The resistivity of the soils and pervasively fractured bedrock was observed to be mostly controlled by moisture content. Based on the moisture content, the soils and rocks were classified into moist soil (<50 ohm.m), dry soil (125 ohm.m), moist rock (≥125 ohm.m), and dry rock (≥1500 ohm.m). The bedrock surface topography in many instances appeared to depict the ground surface topographic expression.