3D resistivity survey over mapped caves in eogenetic karst terrane, west-central Florida, USA

Abstract

This study assesses the capability and practical applications of quasi-3 Dimensional (3D) Electrical Resistivity Tomography (ERT) for mapping air-filled conduits in eogenetic karst. A high-resolution quasi-3D ERT survey, consisting of multiple parallel 110m-long 2D profiles, was conducted over two mapped cave systems on the Brooksville Ridge, Florida. The irregularly shaped caves have diameters ranging up to 4 m and span depths from 3 m to 11 m below ground surface. Dipole-dipole array geometries with L2 (least squares) rather than L1 (robust) inversion produced the best fits of resistivity highs with the mapped cave locations. As expected, 3D inversions of sets of parallel lines produced better results than 2D inversions of individual transects. Better imaging was obtained of a cave over which cave-parallel profiles were run in addition to cross-cave profiles. However, even with the best acquisition and processing steps, there are significant misfits in the apparent size of the large cave sections, and narrower conduits are not imaged. Resolution decreased significantly with depth, as expected given the method limitations and the site constraints on profile lengths. 3D visualization techniques are explored, and found helpful in examining the data and comparing mapped caves and 3D resistivity datasets; however, when applied to eogenetic karst terrain, ERT has limited capacity to detect smaller cavities, which may require the additional use of other geophysical or subsurface investigative methods. With sinkholes continuing to be of concern to residential and urban development in west-central Florida, the results of this research present additional insight on the potential of quasi-3D ERT methods to map and characterize the potential hazards posed by karst terranes.