Electrical Methods Overview

Bridging our subdivision of geophysical techniques into passive and active methods are the electrical and electromagnetic methods. Taken as a whole, the electrical and electromagnetic methods represent the largest class of all geophysical methods, some passively monitor natural signals while others employ active sources.

In addition to their great variety, this group of geophysical techniques represents some of the oldest means of exploring the Earth's interior. For example, the SP method described below dates back to the 1830's when it was used in Cornwall, England by Robert Fox to find extensions of known copper deposits. Natural electrical currents in the Earth, referred to as telluric currents, were first identified by Peter Barlow (pictured) in 1847. The EM method was developed in the 1920's for the exploration of base-metal deposits.

Electrical methods employ a variety of measurements of the effects of electrical current flow within the Earth. The phenomena that can be measured include current flow, electrical potential (voltages), and electromagnetic fields. A summary of the more well-known electrical methods is given below. In this set of notes we will consider only one of these methods, the DC resistivity method.

• DC Resistivity - This is an active method that employs measurements of electrical potential associated with subsurface electrical current flow generated by a DC, or slowly varying AC, source. Factors that affect the measured potential, and thus can be mapped using this method, include the presence and quality of pore fluids and clays. Our discussions will focus solely on this method.

• Induced Polarization (IP) - This is an active method that is commonly done in conjunction with DC Resistivity. It employs measurements of the transient (short-term) variations in potential as the current is initially applied or removed from the ground. It has been observed that when a current is applied to the ground, the ground behaves much like a capacitor, storing some of the applied current as a charge that is dissipated upon removal of the current. In this process, both capacitive and electrochemical effects are responsible. IP is commonly used to detect concentrations of clay and electrically conductive metallic mineral grains.

• Self Potential (SP) - This is a passive method that employs measurements of naturally occurring electrical potentials commonly associated with the weathering of sulfide ore bodies. Measurable electrical potentials have also been observed in association with ground-water flow and certain biologic processes. The only equipment needed for conducting an SP survey is a high-impedence voltmeter and some means of making good electrical contact with the ground.

• Electromagnetic (EM) - This is an active method that employs measurements of a time-varying magnetic field generated by induction through current flow within the earth. In this technique, a time-varying magnetic field is generated at the surface of the earth that produces a time-varying electrical current in the earth through induction. A receiver is deployed that compares the magnetic field produced by the current-flow in the earth to that generated at the source. EM is used for locating conductive base-metal deposits, for locating buried pipes and cables, for the detection of unexploded ordnance, and for near-surface geophysical mapping.

• Magnetotelluric (MT) - This is a passive method that employs measurements of naturally occurring electrical currents, or telluric currents, generated by magnetic induction of electrical currents in the ionosphere. This method can be used to determine electrical properties of materials at relatively great depths (down to and including the mantle) inside the Earth. In this technique, a time variation in electrical potential is measured at a base station and at survey stations. Differences in the recorded signal are used to estimate subsurface distribution of electrical resistivity.