A Comparison of 2D and 3D Resistivity Soundings in Shallow Karst Terrane, Easton, PA

Abstract

At a site in Northampton, County, PA (Metzgar Field) of active sinkhole development (about one per month), I compared both 2D and 3D electrical resistivity for mapping the dimensions and orientations of voids within an terrain of highly variable bedrock depth using a multielectrode electrical resistivity system with a limited number of electrodes. My intention was to determine which method yields the best understanding of the subsurface and can be used to predict the potential development of sinkholes: numerous parallel 2D soundings or a single widely spaced 3D sounding. I targeted a shallow ( <30 ft deep) air-filled cavity within the carbonate bedrock of the Epler Fm. To further investigate the subsurface, I applied other methods, such as: electromagnetic induction, microgravity, drilling and the SearchCAM 2000. From the comparison, I conclude that, even though multiple 2D tests would take longer to collect data, this is the better approach to determining depth to bedrock and the possibility of cavities. This is not to say that the 2D approach is flawless. A drawback with the 2D resistivity approach is that 2D geology is assumed. This is the primary reason why the 3D approach was developed. A cavity can have an affect on a 2D test as current travels around it, even if it is offset several meters. This assumption clouds the accuracy of the 2D profile by producing anomalies in the results that may or may not be located there in the subsurface. 3D resistivity testing overcomes this assumption, but, it only enables a 7x8 electrode grid, with 56 electrodes, decreasing the electrode density along a given profile from 28 used in the 2D survey. Sounding to the depth of the cavity required an increased electrode separation from 2 m (2D test) to 10 m (3D test), resulting in an oversimplification of the subsurface, revealing the ineffectiveness of current 3D systems.