IDENTIFYING SUBSURFACE VOIDS USING ELECTRICAL RESISTIVITY AND REMI

The world heritage site of Mammoth Cave reveals to us the beauty of karst. Karst can also cause nightmares as cars of the National Corvette museum and even people are consumed by sinkholes. The importance of knowing the location of subsurface voids is extremely important. The Laboratory for Applied Geophysics has collected data from multiple locations using Electrical Resistivity Tomography (ERT) to detect possible subsurface voids. An inherent problem with ERT is non-uniqueness where dry competent rock shows a high resistive signature similar to that of air filled voids. In an effort to produce accurate data, faster and cheaper, the laboratory has developed a system using the combination of ERT and the surface seismic method Refraction Microtremor (ReMi). The system uses ERT data processed in the field to determine possible void location targets. These targets are used to locate ReMi field surveys centered on the targets. Information from modeled ERT data is then used a priori to aid in constructing the ReMi model. The results of the ReMi surveys are superimposed on the ERT data to confirm voids or solid rock. Subsets of the ReMi data may be modeled to narrow down lateral target locations. Using the non-destructive tools of ERT and ReMi from the geophysics toolbox, karst and similar features can be rapidly and cost effectively assessed, identifying vulnerability and allowing for mitigation strategies to be emplaced. Results of studies conducted at multiple sites to develop this method will be presented.