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 geological 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 in 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 in depth.
- Resistivity Soundings - As we've already shown,
the resistivity method can detect variations in resistivity
that occur solely in 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 the depth
to 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 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.
