Publication Date: 2022/03/01
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.
SIMPLE LANGUAGE SUMMARY:
This research looked at using a method called quasi-3D Electrical Resistivity Tomography (ERT) to map out air-filled caves in a type of rock formation known as eogenetic karst. The researchers conducted a detailed ERT survey over two known cave systems in Brooksville Ridge, Florida. These caves are irregular in shape, up to 4 meters wide, and are located between 3 and 11 meters under the ground. The researchers used a specific method to analyze their data, which matched the known cave locations best. They also found that analyzing sets of parallel lines (3D) worked better than analyzing individual lines (2D). Additionally, they got a clearer picture of a cave when they ran their lines both along and across the cave. However, the method wasn't perfect. It was hard to get the exact size of the large sections of the cave and smaller channels couldn't be seen at all. Also, the further down they tried to look, the fuzzier the images became, due to limitations of the method and the site. They also looked at 3D visualization techniques to compare the mapped caves with their data. But, they found that ERT isn't great at detecting smaller cavities in this type of rock. So, other methods might also be needed to investigate beneath the ground. This research is useful because sinkholes are a big problem in west-central Florida. It shows that this quasi-3D ERT method might be helpful for mapping out and understanding the risks from the rocky terrain.