Geoscience Reference
In-Depth Information
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
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
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
'
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 (
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
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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