Resistivity

Natural electrochemical reactions near the surface of the earth, where metallic sulfides may be subject to weathering, can be used in the simple self-potential (SP) method. The measuring instrument detects the electrical current developed during the weathering of the sulfide, as shown in figure 10.

A shortcoming of the SP method is the frequency and variety of spurious responses obtained. A more popular application of the electrical method is where controlled electrical energy is applied to the earth and the resulting electrical behavior of the ground is observed at closely spaced stations at regular intervals over the surface. An adaptation much used during the past decade is induced polarization (IF) where the conductivity of mineralized ground changes with variation of frequency of the applied current, while the conductivity of barren ground remains constant. As with the SP method, IP often produces misleading results and use of the method has declined recently.

Surface resistivity measures the electrical resistivity of the subsurface which includes soil and groundwater characteristics. Surface or ground resistivity (or earth resistivity) can be used to study lateral changes and vertical cross sections of the natural hydrogeologic setting. Electrical resistivity can also be used to study contamination of soil and groundwater.

Electrical Resistivity Applications:

soil or rock lithology
soil and groundwater contamination
saltwater/freshwater interface
mapping clay layers or sand deposits
mineral exploration
mapping water table
fracture location

Surface Resistivity Surveys:


ABEM Terrameter SAS 300 C Resistivity Meter	ABEM Terrameter SAS 300 C Resistivity Survey System

Surface Resistivity Method

Application of the surface resistivity 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 by a voltmeter between a second pair of electrodes.

The subsurface ground resistivity can be calculated by knowing the electrode spacing, geometry of the electrode positions, applied current, and measured voltage. Earth resistivity measurements are reported in the units of ohm-meters or ohm-feet.

The depth of measurement of ground resistivity is related to the spacings of the electrodes and may vary depending on the subsurface conditions. The electrical resistivity unit has a self-contained transmitter, capable of obtaining data to about 50 to 100 meters (160 to 300 feet), using self-contained, rechargeable batteries.