Survey Types Overview: Soundings and Profiles

Thus far we have begun to see how geologically relevant structure can affect electrical current flow and measurements of voltage at the Earth's surface. We've described how depth variations in resistivity can be detected by increasing current electrode spacing by estimating apparent resistivities for various current electrode spacings. We have not, however, described the specific field procedures used in resistivity surveying.

Before describing these procedures, there is an important point to note about the geologic structures considered thus far. Notice that the resistivity method represents the first method that we have described which can detect depth variations in a geologically relevant parameter. For example, if we conducted gravity or magnetic surveys atop structures that varied in density or magnetic susceptibility only with depth, we would observe no spatial variation in the Earth's gravity or magnetic fields. Thus, these methods are insensitive to changes in density and magnetic susceptibility that occur solely with depth.