Geology Reference
In-Depth Information
8
Electrical surveying
8.1 Introduction
There are many methods of electrical surveying. Some
make use of fields within the Earth while others require
the introduction of artificially-generated currents into
the ground.The
resistivity
method is used in the study of
horizontal and vertical discontinuities in the electrical
properties of the ground, and also in the detection
of three-dimensional bodies of anomalous electrical
conductivity. It is routinely used in engineering and
hydrogeological investigations to investigate the shallow
subsurface geology. The
induced polarization
method
makes use of the capacitive action of the subsurface to lo-
cate zones where conductive minerals are dissemin-ated
within their host rocks. The
self-potential
method makes
use of natural currents flowing in the ground that are
generated by electrochemical processes to locate shallow
bodies of anomalous conductivity.
Electrical methods utilize direct currents or low-
frequency alternating currents to investigate the electri-
cal properties of the subsurface, in contrast to the elec-
tromagnetic methods discussed in the next chapter that
use alternating electromagnetic fields of higher frequen-
cy to this end.
8.2.2 Resistivities of rocks and minerals
The
resistivity
of a material is defined as the resistance
in ohms between the opposite faces of a unit cube of
the material. For a conducting cylinder of resistance
d
R
, length
d
L
and cross-sectional area
d
A
(Fig. 8.1) the
resistivity
r
is given by
RA
L
dd
d
=
r
(8.1)
The SI unit of resistivity is the ohm-metre (ohm m) and
the reciprocal of resistivity is termed
conductivity
(units:
siemens (S) per metre; 1 S m
-1
= 1 ohm
-1
m
-1
; the term
'mho'for inverse ohm is sometimes encountered).
Resistivity is one of the most variable of physical
properties. Certain minerals such as native metals and
graphite conduct electricity via the passage of electrons.
Most rock-forming minerals are, however, insulators,
and electrical current is carried through a rock mainly
by the passage of ions in pore waters. Thus, most rocks
conduct electricity by electrolytic rather than electronic
processes. It follows that porosity is the major control of
8.2 Resistivity method
δ
L
I
8.2.1 Introduction
In the resistivity method, artificially-generated electric
currents are introduced into the ground and the resulting
potential differences are measured at the surface. Devia-
tions from the pattern of potential differences expected
from homogeneous ground provide information on
the form and electrical properties of subsurface
inhomogeneities.
δ
A
δ
R
δ
V
Fig. 8.1
The parameters used in defining resistivity.
