Geology Reference
In-Depth Information
5.1.3 Apparent resistivity
A single electrical measurement tells us very little. The most that can be
extracted from it is the resistivity of a completely homogeneous ground (also
described as a
homogeneous half space
) that would produce the same result
under the same survey conditions. This quantity is known as the
apparent
resistivity
(
ρ
α
). Variations in apparent resistivity or its reciprocal,
apparent
conductivity,
provide the raw material for interpretation in most electrical
surveys.
Where electromagnetic methods are being used to detect very good con-
ductors such as sulphide ores or steel drums, locating the target is more
important than determining its precise electrical properties. Since it is
difficult to separate the effects of target size from target conductivity for
small targets, results are sometimes presented in terms of a
conductivity-
thickness product
.
5.1.4 Overburden effects
Build-ups of salts in the soil produce high conductivities in near-surface
layers in many arid tropical areas. Conductive overburdens will effectively
short-circuit any currents produced by sources situated at or above the ground
surface and therefore pose problems for all electrical methods. Continuous-
wave electromagnetic methods are the most severely affected.
Highly resistive surface layers are obstacles in surveys using electrodes
but may actually be advantageous in EM surveys, because attenuation is
reduced and the depth of investigation is increased. Capacitive coupling can
also be used, provided that the very resistive layer is less than about a metre
thick.
5.1.5 Anisotropy
Most elementary analysis of electrical data (and certainly most analyses
performed in the field) assume that resistivity is the same in all directions.
This is usually true where current is carried by ions in pore waters, but not
necessarily in other cases. For example, a graphitic shale usually conducts
electricity much more readily along the bedding planes than across them.
Currents and the electric fields that drive them are vectors but in anisotropic
media are not necessarily in the same direction. The resistivity or conduc-
tivity that describes their relationship is therefore a tensor.
Currents may also be driven by alternating magnetic fields, as described
in Section 5.2, and there is therefore a magnetic resistivity/conductivity
tensor as well as an electric one. The use of full tensor information is still
uncommon, but where it is required by the interpreters, field crews will find
their lives made considerably more complicated, because more sensors will
