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).
For more information on geophysical techniques, please refer to
Chapter 8 of the NJDEP's Field Sampling
Procedures Manual, 2005.