Electrical resistivity is an intrinsic property of all materials. The properties that affect the resistivity of soil or rock include: porosity, water content, composition (clay mineral and metal content), salinity of the pore water, and grain size distribution.  Therefore, the electrical resistivity method is ideally suited to provide information for ground water surveys and bedrock topography.  The electrical resistivity method is primarily deployed on land.  However, in addition to terrestrial surveys, marine electrical resistivity surveys can help delineate stratigraphy below a lake bottom.  See our Marine Geophysics page for more information on this method.

In an electrical resistivity exploration, electrical current is applied to the ground surface through two electrodes.  Two or three additional electrodes are placed in the ground to measure variations in the potential of the electrical field (voltage) that is set up within the earth by the current electrodes.  There are two basic field procedures which are commonly used in electrical resistivity exploration:

1) Electrical traversing in which the electrode separation remains constant during the survey; and

2) Electrical sounding, in which the center of the electrode spread is maintained at a fixed location and the electrode spacing is increased in increments.

  Electrical traversing is normally employed when a rapid survey of an area is desired.  It is particularly suited for prospecting for sand, gravel and ore deposits and for locating fault zones or contacts between steeply dipping layers of earth materials

Electrical sounding is designed to provide information on the variation in subsurface conditions with depth.  Sounding is typically used to help determine the depth to the water table, the thickness of sand, gravel and rock layers, and the actual value of electrical resistivity versus depth.  

Traversing and sounding can be combined using a multi-electrode system.  The multi-electrode system consists of over 28 electrodes and a resistivity meter that is capable of electronically switching the electrodes.  The data are analyzed with a 2-D or 3-D forward inverse modeling computer program.  In the program, a non-linear least-squares optimization technique is used to automatically determine the best fit to the data.  An example of a two dimensional electrical resistivity cross-section is shown in the Figure below.