Resistivity

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.

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.

Resistivity methods are typically employed in order to:

Map faults, aquitards, or 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