4.2
Electric and electromagnetic fields
The electrical and electromagnetic (EM) methods are used to map or
image the electrical conductivity () and dielectric () properties of the
ground.
They are active methods -- a controlled source is used to make current
(I) flow in the ground. The distribution of current in the ground depends on
the distribution of the conductivity. The current is measured indirectly either
by inserting pairs of electrodes in the ground to measure voltage drops or by
measuring the magnetic field produced by the current.
The current can be made to flow either by directly injecting it with
ground contacting electrodes or by inducing it to flow by creating a time
varying magnetic field. A time varying magnetic field in the conducting
earth produces an electromotive force (emf) via Faraday's Law of Induction
which in turn drives currents. A complete description of the fields created by
currents and the interaction of the resulting electric and magnetic fields in
the time varying case is provided by the laws of electromagnetic theory.
All the electrical and electromagnetic methods discussed in this course are
governed by these electromagnetic (EM) laws.
The DC Resistivity method is formally the low frequency limit of the
general electromagnetic (EM) method where the time rate of change of any
magnetic fields is so small that Faraday induction can be ignored. (DC or dc
is short for direct current, but the term is widely used to describe current and
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field behavior in the zero frequency limit. This occasionally leaves us with
redundant phrases like ‘dc current’).
At DC, or zero frequency, current can only be created by contacting
electrodes connected to a DC supply (i.e. a battery). At higher frequencies
the supply wires for the contacting electrodes also create time varying
magnetic fields and so the currents in the ground have a component created
by Faraday induction from the source. Purely inductive sources, such as
loops of wire carrying alternating current, need not contact the ground nor
will they induce any current at zero frequency (DC). Finally the time varying
currents in the ground have their associated magnetic fields which interact
with the source field modifying the whole induction process - this is the
subject matter of EM theory.
At very high frequencies (> 1.0 MHz), the concept of induction is
replaced by the concept of propagating waves (like radio, radar or light) in
which the electric and magnetic fields are tightly coupled. The response of
the ground is now described in terms of the propagation velocity of a wave
or wavelet, which depends on the dielectric properties; the conductivity
determines the attenuation or damping of the waves as they propagate. These
methods are usually grouped under the term Ground Penetrating Radar
(GPR).
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