Geoscience Reference
In-Depth Information
CHAPTER
5
Electrical and electromagnetic
methods
5.1
Introduction
There are a wide variety of geophysical methods based
on electrical and electromagnetic phenomena that
respond to the electrical properties of the subsurface
measurements of naturally occurring electrical or elec-
tromagnetic (EM) fields and use rather simple survey
equipment. Others are active methods that transmit a
signal into the subsurface and use sophisticated multi-
channel equipment to derive multiple parameters
related to the electrical properties of the subsurface.
EM and electrical surveys are routinely undertaken on
and below the ground surface, but electrical surveys
require contact with the ground so only EM measure-
ments are possible from the air.
Survey methods that involve the measurement of elec-
trical potentials (see
Sections 5.5
and
5.6
), associatedwith
the flow of subsurface current, by direct electrical con-
tact with the ground, are collectively known as
Appendix 5
)
with the results resembling the seismic
Electrical and EM methods both provide information
about electrical
, a measure of the ease with
which electrical currents flow within the subsurface. An
associated parameter is
conductivity
, which is the inverse
of conductivity and a measure of the difficulty with
which electrical currents flow (see
Section 5.6.2
)
. In par-
ticular, electrical methods depend upon the contrast in
electrical properties and respond well to regions where
current flow is inhibited, but these methods may be
unable to determine the absolute conductivity accur-
ately, especially of small targets. EM methods respond
primarily to the absolute conductivity of the ground
and the dimensions of that distribution, rather than just
the conductivity contrast, and are most sensitive to
regions where current flow is least inhibited, i.e. con-
ductive targets.
The rock-forming minerals that form the basis of the
geological classification of rocks exert little influence
over the electrical properties of rocks. The main control
is porosity and the contents of the pore space, so rock
electrical properties are extremely variable and hard to
predict geologically. Anomalous electrical properties
are observed from many types of ore minerals and their
occurrences, but responses are widely variable
depending upon the geological environment. As with
most types of geophysical data, a direct correspondence
resistivity
electrical
methods
. These include the
self-potential
(SP),
resistivity
,
induced polarisation
(AP)
methods. With the exception of the SP method which
measures natural potential, all the others depend on the
electrical transmission of the current into the ground.
Electromagnetic
(IP) and
applied potential
(EM) methods use the phenomenon of
electromagnetic induction (see
Section 5.7
) to create the
subsurface current flow and measure the magnetic fields
associated with it. A variant of the EM method uses high
(radio and radar) frequency electromagnetic waves (see
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