SELECTION OF AN EFFECTIVE ELECTRODE ARRAY TO MAP SINKHOLES IN KARST TERRANES USING ELECTRICAL RESISTIVITY TOMOGRAPHY

When conducting environmental and engineering investigations in karst terranes, engineers and geologists often supplement exploratory borehole results with data gathered from surface geophysics to reduce the site-characterization cost and establish the most useful locations for borings or samples. When conducting resistivity investigations, a frequently occurring problem is the need to determine which of the many existing electrode configurations will respond best to the material changes in karst features. Each array has its advantages and disadvantages in terms of depth of investigation, sensitivity to horizontal or vertical variations, and signal strength. Numerical forward modeling was conducted of the Dipole-dipole, Schlumberger, and Wenner arrays, and they produced markedly different anomaly signatures for the same conceptual model of the development of a cover-collapse sinkhole. The resolution of the three above-mentioned arrays was further evaluated along a section of I-70 near Frederick, Maryland where a sinkhole had occurred in the median of the highway. The image from the dipole-dipole array appeared to be better than those from both Wenner and Schlumberger arrays in displaying the sinkhole collapse area. However, a mixed array, in which apparent resistivities from all the three arrays were combined, processed and modeled together appears to give a more detailed cross-section.