How to Find Buried Objects.
Electrical Resistivity

(The material in this article has been provided by US EPA publications)

Electrical resistivity, also referred to as galvanic electrical methods, is occasionally useful at UST sites for determining shallow and deep geologic and hydrogeologic conditions. By measuring the electrical resistance to a direct current applied at the surface, this geophysical method can be used to locate groundwater/contaminant pathways, clay lenses and sand channels, perched water zones and depth to groundwater, and occasionally, large quantities of residual and floating product.

A variety of electrode configurations can be used depending on the application and the resolution desired. Typically, an electrical current is applied to the ground through a pair of electrodes. A second pair of electrodes is then used to measure the resulting voltage. The greater the distance between electrodes, the deeper the investigation. Because various subsurface materials have different resistivity values, measurements at the surface can be used to determine the vertical and lateral variation of underlying materials.

Electrical resistivity has a number of limitations:

Electrodes must be in direct contact with soil; if concrete or asphalt are present, holes must be drilled for inserting the electrodes and then refilled when the survey is complete.

The distance between outside electrodes must be 4 to 5 times the depth of investigation.

Measurements may be limited by both highly conductive or highly resistive surface soils. If shallow clays and extremely shallow groundwater are present, most of the current may concentrate at the surface. Although the condition is very rare, the presence of thick, dry, gravelly material (or massive dry material) at the surface may prevent the current from entering the ground.