The purpose of a DC electrical survey is to determine the subsurface resistivity distribution of the ground, which can then be related to physical conditions of interest such as lithology, porosity, the degree of water saturation, and the presence or absence of voids in the rock. This information can be useful for locating geologic features such as ancient sinkholes, faults, environmental hazards such as contaminant plumes and landfill, as well as for general understanding of the distribution of various soil/rock types. Application of the surface resistivity method requires that an electrical current be injected into the ground by a pair of surface electrodes. The resulting potential field (voltage) is measured at the surface by a voltmeter between a second pair of electrodes. Electrical resistivity technology has recently evolved with the increased sophistication of computers and software. Formerly, two methods of subsurface imaging using electrical resistivity were available: vertical electrical sounding for investigating depth and horizontal electrical profiling for assessing lateral variations. Electrical soundings presented a one-dimensional vertical profile of limited lateral control, whereas horizontal electrical profiling generated a lateral profile but was limited to a constant depth and the bulk resistivity of only that depth interval. The newer automated electrical resistivity systems use instruments to acquire the data as a set of soundings comprising a two-dimensional cross-section or profile of the subsurface sometimes refered to as electrical resitivity imaging or electrical resistivity tomography.