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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.

• Resistivity Soundings - As we've already shown, the resistivity method can detect variations in resistivity that occur solely with depth. In fact, this method is most commonly applied to look for variations in resistivity with depth. Surveys that are designed to determine resistivity variations with depth above some fixed surface location are referred to as resistivity soundings. In principle, the two-electrode experiments described previously are examples of soundings. In these experiments, electrode spacing is varied for each measurement. The center of the electrode array, where the electrical potential is measured, however, remains fixed. An example of a problem for which one might employ resistivity soundings is the determination of depth to the water table.

• Resistivity Profiles - Like the gravity and magnetic methods, resistivity surveys can also be employed to detect lateral variations in resistivity. Unlike soundings, profiles employ fixed electrode spacings, and the center of the electrode spread is moved for each reading. These experiments thus provide estimates of the spatial variation in resistivity at some fixed electrode spacing. Surveys that are designed to locate lateral variations in resistivity are referred to as resistivity profiles. An example of a problem for which one might employ resistivity profiles is the location of a vertical fault.