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 1830s 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 1920s 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 better-known electrical methods is given below. In this set of notes
we will consider only one of these methods, the DC resistivity method, in greater
detail.
- 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, or alternatively the variation in the response as
the AC frequency is changed. 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 capacitative 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 shallow
electrical conductors, such as sulfide ore bodies. Measurable electrical potentials
have also been observed in association with groundwater flow and certain biologic
processes. The only equipment needed for conducting an SP survey is a high-impedance
voltmeter and some means of making good electrical contact to 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, telluric currents, generated by
magnetic induction from 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.
