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	Resistivity Methods
	The electrical resistivity method involves measuring the apparent resistivity of soils and rock as a function of depth or position. The resistivity of soils is a complicated function of porosity, permeability, ionic content of the pore fluids, and clay mineralization. The most common electrical methods used in hydrogeologic and environmental investigations are vertical electrical soundings (resistivity soundings) and resistivity profiling. During a resistivity survey, current is injected into the earth through a pair of current electrodes, and the potential difference is measured between a pair of potential electrodes. The current and potential electrodes are generally arranged in a linear array. The apparent resistivity is the bulk average resistivity of all soils and rock influencing the current. It is calculated by dividing the measured potential difference by the input current and multiplying by a geometric factor specific to the array used and electrode spacing. In a resistivity sounding, the distance between the current electrodes and the potential electrodes is systematically increased, thereby yielding information on subsurface resistivity from successively greater depths. The variation of resistivity with depth is modeled using forward and inverse modeling computer software. In resistivity profiling, the electrode spacing is fixed and measurements are taken at successive intervals along a profile. Data are generally presented as profiles or contour maps and interpreted qualitatively. When information on both the horizontal and vertical extent of a subsurface feature is desired, it is common to combine the sounding and profiling techniques. The use of rapid, automated data acquisition systems used to create 2-D "tomograms" has increased the flexibility of resistivity as a tool for environmental and geotechnical applications. Map faults Map aquitards Map paleo-channels Corrosion control design Delineate disposal areas Estimate landfill thickness Locate mine shafts and voids Determine depth of water table Investigate archaeological sites Determine overburden thickness Locate sinkholes and voids in karst Determine electrical characteristics of subsurface Map lateral extents of conductive contaminant plumes