Electrical Methods

Electrical methods include electrical resistivity, induced polarization (IP), and spontaneous or self potential (SP). Electrical surveys are used for mapping the geological framework of aquifers, locating concentrated plumes of ground-water contamination, mapping the subsurface thickness of unconsolidated sediments and the depth to consolidated bedrock, and mineral exploration.

The Electrical Resistivity method measures the apparent bulk electrical resistivity by injecting current into the ground through current electrodes that are grounded at the earth's surface and measuring the difference of the electrical potential between the potential electrodes (see figure below). The amount of current (in amps) and the potential difference (in volts) is used to calculate the apparent resistivity (ohm-meters) at the midpoint of the array of the electrodes. Some common field arrays for the resistivity method are shown below.

The Induced Polarization method measures the slow decay of voltage when the injected current is stopped. The IP method therefore measures the bulk electrical chargeability of the rocks. IP data are also collected using the field arrays shown above.

The Spontaneous Potential or Self Potential method measures the natural variation of the ground voltage between two electrodes. The voltage variation is caused by electrochemical reactions at a conductive body (for example a sulfide body).

An example of the field equipment and crew needed to conduct an electrical survey

For more information on geophysical techniques, please refer to

Chapter 8 of the NJDEP's Field Sampling Procedures Manual, 2005.