Geology Reference
In-Depth Information
8
ELECTROMAGNETIC METHODS
Electromagnetic (EM) induction, which is a source of noise in resistivity
and IP surveys (Chapters 6 and 7), is the basis of a number of geophysical
methods. These were originally used mainly in the search for conductive
sulphide ores but are now being increasingly used for depth-sounding and
area geological mapping. Because a small conductive mass within a poorly
conductive environment has a greater effect on induction than on 'DC'
resistivity, and also because the responses to conductors of similar size and
shape are proportional to their conductivities, discussions of EM methods
tend to focus on conductivity (
σ
), the reciprocal of resistivity, rather than on
resistivity itself.
There are two limiting situations. In the one, eddy currents are induced in
small conductive bodies embedded in insulators, producing discrete anoma-
lies that provide information on body location and conductivity. In the other,
the effects produced at the surface by horizontal currents induced in hori-
zontally layered media can be interpreted in terms of apparent conductivity.
Most real situations involve combinations of layered and discrete conduc-
tors, making greater demands on interpreters, and sometimes also on field
crews.
Wave effects are important only at frequencies above about 10 kHz, and
the methods can otherwise be most easily understood in terms of varying
current flow in conductors and varying magnetic fields in space. Where the
change in the inducing primary magnetic field is produced by the flow of
sinusoidal alternating current in a wire or coil, the method is described as
continuous wave (CWEM). In transient electromagnetic (TEM) methods,
induction arises from the abrupt termination of current flow.
8.1 Two-Coil CW Systems
A current-carrying wire is surrounded by circular, concentric lines of mag-
netic field. Bent into a small loop, the wire produces a magnetic dipole
field (Figure 1.5), which can be varied by alternating the current. This vary-
ing magnetic field causes currents to flow in nearby conductors (see Sec-
tion 5.2.1).
