Electrical Resistivity ER Methods

In ER methods, environmental engineers measure the resistivity of subsurface materials by injecting an electrical current into the ground through a pair of surface electrodes (current electrodes) and measuring the resulting potential field (voltage) from a second pair of electrodes (potential electrodes) as shown in Figure 9.14.2. Several types of electrode geometries can be used for resistivity measurements including the Wenner, Schlumberger, dipole, and others. The Wenner array is the simplest in terms of geometry and consists of four electrodes spaced equally in a line.

The ER measurements are a function of the soil or rock types, thickness of the soil and rock layers, moisture content, fluid conductivity, and depth to the water table. The ER of a geological formation is calculated based on the electrode separation, the geometry of the electrode array, the applied current, and the measured voltage.

As with the EM surveys, environmental engineers can use the ER surveys to obtain data by profiling or sounding. In profiling, engineers take measurements at a number of stations along a survey line to map lateral changes

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