Quasi 3-Dimensional Electrical Resistivity Mapping of Air-filled Karst Conduits and Policy Implications

This study assesses the capability and practical applications of quasi 3-Dimensional (3D) electrical resistivity surveying (ER) for mapping air-filled karst conduits. Vadose zone caves within the Brooksville Ridge of West Central Florida are relatively similar in architecture, with N-S elongation, and do not consist of an interconnected network of conduits. A high resolution quasi-3D ER survey was performed over two mapped cave systems on the Brooksville Ridge. The resultant survey verified the general effectiveness of quasi-3D ER in locating the two known near-surface cave features. Several other locations in the survey show similar or stronger resistivity anomalies trending in a N-S direction; these are interpreted to represent previously unknown voids. The quality of inversion results were tested by comparing results against the known void locations and by computing cross-over errors from surveys conducted at the same point with different orientations. Our results show that 3D inversions of multiple adjacent parallel profiles produces higher quality results (lower cross-over errors, better fit to cave locations) than 2D inversions. The data indicate that no single value of resistivity can be used as a reliable indicator of the presence of a void, presumably due to variable void size and the complexity in resistivities in the host rock. With sinkholes continuing to be of concern to residential and urban development in West Central Florida, the results of this research show the necessity of applying geophysical techniques in order to reduce the potential hazards posed by karst terrain