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Geophysical Support
Resistivity Method
The resistivity method
is used to measure the electrical resistivity of the geohydrologic section which includes
the soil, rock and ground water. Accordingly, the method may be used to assess the
lateral changes and vertical cross sections of the natural geohydrologic settings.
in addition, it can be used to evaluate contaminant plumes and locate buried waste
at hazardous waste sites.
Application of the method requires that an electrical current be injected into the ground by a pair of surface electrodes. The resulting potential field (voltage) is measured at the surface between a second pair of electrodes. The subsurface resistivity can be calculated by knowing the electrode separation and geometry ofthe electrode positions, applied current, and measured voltage.
In general, most soil and rock minerals are electrical insulators (highly resistive); hence the flow of current is conducted primarly through moisture-filled por spaces within the soil and rock. Therefore, the resistivity of soils and rocks is predominantly controlled by the porosity and permeability of the system, the amount of pore water, and the concentration of dissolved solids in the pore water.
The resitivity technique may ne used for profiling or sounding. Profiling provides a means of mapping lateral changes in subsuurface electrical properties. This field technique is well suited to the delineation of contaminant plumes and the detection and location of changes in natural geohydroloic conditions. Sounding provides a means of determining the vertical changes in subsurface electrical properties. Interpretation of sounding data provides the depth and thickness of subsurface layers having different resistivities Commonly up to 4 layers may be resolved with this technique.