Geoscience Reference
In-Depth Information
5.3
Electrical properties of the
natural environment
For a sinusoidally varying
field penetrating an in
nite
uniform conductive medium, and where the effects of the
dielectric properties are very small compared with the
effects of conductivity, skin effect is quanti
ed in terms
of a parameter known as skin depth (
The most important electrical property in
uencing geo-
electrical measurements is electrical conductivity, and its
reciprocal resistivity, since it controls the responses meas-
ured in the resistivity, electromagnetic and self-potential
methods. Also signi
cant are electrical polarisation, a
phenomenon related to capacitance and measured by the
induced polarisation method, and other dielectric proper-
ties which control the responses measured in high- (radar-)
frequency EM methods.
δ
). It is the distance
'
over which the electromagnetic
eld
is amplitude is attenu-
ated by 1/e (i.e. 37%) of its surface value and is given by:
1
πμσ
δ ¼
p
ð
5
:
9
Þ
f
μ
where
is the magnetic permeability (see
Section 3.2.3.3
)
of a homogenous medium (henry/m),
is conductivity in
S/m, f is frequency in hertz and the skin depth is in metres.
The magnetic permeability of most rocks is nearly the
same as that in a vacuum (
σ
5.3.1
Conductivity/resistivity
μ ≈ μ
0
¼
4
π
Making representative measurements of electrical conduct-
ivity/resistivity on samples of natural materials is particu-
larly dif
cult. In porous rocks, electrical properties are
often predominantly controlled by the
fluids occupying
the pore space, which may not be retained during sampling
or properly reproduced during the measuring process.
Also, electrical properties may be highly heterogeneous,
introducing a scale-dependence and possibly unassailable
problems of representative sampling. For example, frac-
tures or mineralised veins in the rock may provide the
main path for electrical current
ow, so measurements
on vein/fracture-free specimens are unlikely to be repre-
sentative of the whole rock. When multiple measurements
are available, the largest conductivity is most likely to be
representative of the overall conductivity.
The electrical conductivity of natural substances varies
greatly, more than any other physical property relevant to
geophysical surveying. The conductivity and resistivity
ranges of various rocks and selected minerals are shown
less than 22 orders of magnitude. Even individual minerals
show variations in conductivities/resistivities that span
several orders of magnitude, often exceeding those
exhibited by the various rock types (
Fig. 5.13
). In fact,
there is little agreement amongst published sources as to
the limits of the ranges because of factors such as: different
methods of measurement; whether individual crystals or
aggregates were tested; the controlling effects of cracks and
even small quantities of impurities and lattice defects in
individual crystals; and the variation in electrical properties
related to the direction of current
flow through individual
crystals or the sample as a whole, i.e. electrical anisotropy
(see
Section 5.3.1.4
). From a practical perspective,
503
8
σ
:
δ ¼
p
ð
5
:
10
Þ
f
Note that this is not the limiting distance/depth of the
s penetration, but a convenient measure of the attenu-
ation in terms of distance or depth. The equations show
that EM
eld
'
fields attenuate faster in materials of high con-
ductivity and that attenuation is greater for
fields of higher
frequency.
Figure 5.11
shows skin depths for frequency
and conductivity ranges relevant to geophysical measure-
ments, and where the material acts chie
y like a conductor
and not a dielectric. At radio and radar frequencies the
dielectric properties signi
cantly in
uence attenuation (see
online
Appendix 5
)
.
Conductivity (S/m)
10
-4
10
-3
10
-2
10
-1
10
0
10
1
10
-1
10
0
10
1
10
2
10
3
Approximate conductivity
range of near-surface materials
Conductive
regolith
10
4
Permafrost
Figure 5.11
Skin depth for the range of frequencies used in EM
surveying and for the range of electrical conductivities found in the
geological environment.
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