DC Resistivity

DC Resistivity is a near-surface galvanic method for estimating the in-situ subsurface resistivity distribution.  Resistivity is a physical property controlled by various geological conditions, chiefly mineralogy, porosity, and water content.  Measurements are made on the ground surface by directly coupling or injecting low frequency, alternating current into the earth through two grounded electrodes (commonly labeled A and B, or C1 and C2), then measuring the potential difference, or voltage drop, between a pair of receiving electrodes (m and n, or P1 and P2).

The resistivity receiver computes impedance by calculating the ratio of the measured potential (V) to the transmitted current (I). This impedance is then multiplied by a geometric factor (k), determined by the electrode array geometry, to yield apparent resistivity (_Pa) _Pa = k(V/I).

This apparent resistivity value is not the true resistivity of the subsurface at the data point, because all the soil and/or rock between the surface electrodes and the data point contribute to the reading.  The relationship between apparent and true resistivity is complex; true resistivity must be estimated using an inversion modeling program.

Different electrode array geometries are employed to either obtain depth soundings or to profile the subsurface.  The diagram to the left shows different values of k for different array geometries.  In depth sounding arrays, the electrode spacings of the current and, usually, the potential electrodes are increased, usually in logarithmic increments, to measure apparent resistivities at increasing depths in a straight line down the center of the array.  This is sometimes referred to as electrical drilling, and yields a one-dimensional model of the subsurface, as shown below. 

In profiling, the electrode spacings are fixed, and the entire array is moved along a straight line for each reading.  The advantage of this method is that lateral, rather than vertical changes in the subsurface geology are detected, at the expense of detecting vertical changes.