Geoscience Reference
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the alternating current is flowing in the transmitter. As a
consequence, induction is continually taking place in the
coil so the induced emf is continually changing in ampli-
tude and polarity at the frequency of the alternating cur-
rent in the transmitter coil, but 90° out-of-phase to it.
Continuous induction is the basis of frequency domain
EM measurements (see Section 5.7.1 ) .
Electromagnetic induction also occurs when the coil is
replaced by a body of conductive material ( Fig. 5.9c ). The
time-varying magnetic field intersecting the conductor
induces an emf into the conductor causing current to
circulate in it. This is known as an eddy current, and its
strength and direction of flow at the instant of induction
are also governed by Faraday
Electromagnetic disturbances occur over a wide range of
frequency and can be from natural or arti cial sources.
They include such well known phenomena as radio waves,
microwaves, radar, visible light, ultraviolet light, gamma-
rays (see Section 4.2 ) and X-rays. The various ranges of
frequencies used in geophysical prospecting are shown in
Fig. 5.1 .
As noted in Section 5.2.2 , the magnetic field of the
electromagnetic disturbance is caused by the current asso-
ciated with mobile charge carriers and the displacement
current, but the latter is only significant in geophysical
measurements at the higher radio and radar frequencies.
In the electrical environment of the subsurface, the behav-
iour of electromagnetic fields at frequencies below about
1000 Hz is controlled by diffusive processes; i.e. the fields
diffuse into their surrounds. At higher frequencies, like
those used in radio and radar techniques, and provided
the environment is not highly conductive, the electromag-
netic disturbance moves, or propagates, and behaves as a
wave ( Fig. 5.10 ) and wave phenomena such as attenuation,
re ection and diffraction dominate (cf. the behaviour of
seismic waves described in Chapter 6 ) . In these cases the
dielectric properties of the subsurface control the geophys-
ical response. Waves are re ected at interfaces where there
is a contrast in dielectric constant; attenuation is con-
trolled primarily by electrical conductivity and velocity
(v), which depends on the dielectric constant (
s Law ( Eq. (5.7) ). Note how
the magnetic field associated with the eddy currents, rep-
resented by that of a loop of wire as shown in Figs. 5.7c and
5.9d , mimics the primary
'
field in the vicinity of the con-
ductor. We describe further characteristics of eddy cur-
rents in terms of geophysical surveying in Section 5.7.1.4 .
5.2.3 Electromagnetic waves
Electromagnetic fields comprise both electric and magnetic
fields which are inextricably associated. A varying current
is surrounded by a varying magnetic field and, simultan-
eously, the varying magnetic field induces a varying electric
field which, in turn, gives rise to a varying magnetic field,
and so on. Oscillating electric and magnetic fields regener-
ate each other, effectively riding on each other
κ
) and the
relative magnetic permeability (
μ r ,see Section 3.2.3.3 ) and
given by:
s back. At
suitably high frequencies they move as waves travelling
away from their source, a radio wave being the common
example. The two fields oscillate in planes perpendicular to
each other and perpendicular to their direction of motion,
forming a transverse electromagnetic wave ( Fig. 5.10 ).
'
c
μ r κ
ν ¼
ð
5
:
8
Þ
p
For most rocks
1 (see Zhdanov and Keller, 1994 ).
Velocity in the geological environment is typically about
20
μ r
60% of the speed of light (c, the propagation speed of
EM waves in free space, 3
-
10 8 m/s).
E lec tri c
fie ld
5.2.3.1 Attenuation of electromagnetic fields
An important characteristic of time-varying electromag-
netic fields is their attenuation with distance through a
conductive medium. Attenuation is a consequence of
energy lost by the circulating eddy currents and their
magnetic fields. The phenomenon is known as the skin
effect and it depends on the conductivity of the material, its
dielectric properties and the rate of change (the frequency)
of the field. It determines the penetration of electromag-
netic fields into a medium and strongly in uences the
depth of investigation of EM measurements in geophysics.
M a g n e t ic
fi e l d
Direction
of EM wave
Figure 5.10 Schematic illustration of an electromagnetic wave
illustrating its component electric and magnetic
fields which
fluctuate normal to each other and in the plane normal to the
direction of propagation.
 
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