The EM methods use a transmitter coil to generate an electromagnetic field that induces eddy currents in the ground below the instrument. A receiver coil measures secondary electromagnetic fields created by the eddy currents and produces an output voltage that can be related to variations in subsurface conductivity as shown in Figure 9.14.1. Variations in subsurface conductivity may be caused by changes in the basic soil or rock types, thickness of the soil and rock layers, moisture content, fluid conductivity, and depth to the water table.
Environmental engineers can use EM surveys to obtain data by profiling or sounding. In profiling, the engineer makes measurements at a number of stations along a survey line to map lateral changes in the subsurface electrical conductivity to a given depth. In sounding mode, the engineer places the instrument at one location and takes measurements at increasing depths, by changing coil orientation or coil spacing, to map vertical changes in electrical conductivity and, therefore, the soil and rock type at that location.
An advantage of the EM methods is that the surveys can be done quickly because direct contact of the instrument with the ground is not required. The disadvantage, however, is that the EM surveys are susceptible to the presence of metals and powerlines on the surface of the ground.
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