ELECTRICAL RESISTIVITY FOR KARST GEOHAZARD IDENTIFICATION IN SOUTH CENTRAL KENTUCKY

Development of karst plain areas in the vicinity of Bowling Green, Kentucky and adjacent counties is increasingly being aided by pre-construction geophysical surveys in known karst geohazard areas. Such surveys conducted by us over the last few years have primarily been focused on the use of Electrical Resistivity (ER) but also some complementary Induced Potential (IP) and Refraction Microtremor (ReMi) seismic surveys. There has been an upswing in pre-construction site characterization due to hazards concerns and the need for changing local building codes or permits tied to rapid development. These actions have been spurred by significant media coverage of dramatic karst geohazards such as the 2002 collapse of Dishman Lane and certainly the 2014 collapse of the Corvette Museum. We present a series of survey profiles where karst conduits or caves can clearly be demarcated at multiple levels, some at construction sites and others in undeveloped areas such as the Green River Preserve and adjacent Mammoth Cave National Park.

Contrasting physical properties that typically are found to be useful for mapping soil and bedrock include electrical conductivity or resistivity, acoustic velocity and density. Of these, ER is commonly found to have the greatest range of contrast and is therefore the most applicable for detailed characterization of karst sites. Ground Penetrating Radar (GPR) has limited use in karst with the effects of wet, iron-rich clay soils atop the Mississippian Ste Genevieve or St. Louis limestone units limiting depth. We present several arrays to illustrate how subsurface visualization can vary and which arrays appear to be the most applicable for successful hazard identification and monitoring of such features such as using dipole-dipole and strong gradient combinations. Karst areas present challenges related to nonunique solutions due to the inherent variables in karst voids such as those containing air versus water versus clay or combinations. Nonetheless, ER is proving to be a time efficient and economical way to use geophysical tools to greatly aid in minimizing risks for development in karst areas.