Geology Reference
In-Depth Information
sulphide ores, as well as in environmental investigations where metallic
objects are often the targets. Graphite is a good 'electronic' conductor and,
since it is not itself a useful mineral, is a source of noise in mineral explo-
ration. Conduction in most other types of rock is by ions in the pore waters,
and electrical methods are important in water resource investigations.
5.1.1 Ohm's Law and resistivity
The current that flows in a conductor is in most cases proportional to the
voltage across it, i.e.:
V
= IR
This is Ohm's Law . The constant of proportionality, R , is known as the
resistance and is measured in ohms when current ( I ) is in amps and voltage
( V ) is in volts. The reciprocal, conductance, is measured in Siemens, also
known as mhos.
The resistance of a metre cube of a material to current flowing between
opposite faces defines the resistivity (
) of that material and is measured in
ohm-metres ( m). The reciprocal, conductivity ( σ ), is expressed in Siemens
per metre (S m 1 ) or mhos per metre. The resistance of a rectangular block
to current flow between opposite faces is proportional to the resistivity
and to the distance x between the faces, and inversely proportional to their
cross-sectional area, A , i.e.:
ρ
R = ρ ( x / A )
Isotropic materials have the same resistivity in all directions. Most rocks
are reasonably isotropic but strongly laminated slates and shales are more
resistive across the laminations than parallel to them.
Geophysicists working with galvanic methods generally talk about resis-
tivity , while those working with induction methods talk about conductivity .
Both quantities are, in fact, complex (using the word in its strict mathematical
sense), involving both amplitude and phase (see Section 5.2.3). Amplitudes
reflect the bulk resistance of the ground, while phase is determined by the
ground's ability to store electric charge, i.e. the chargeability .
5.1.2 Electrical resistivities of rocks and minerals
The resistivity ranges for some common rocks and minerals are listed in
the fourth column of Table 1.2. Conductivity ranges can, of course, be
calculated from these values but, for convenience, are listed in the fifth
column. Table 1.2 emphasises the possible ambiguities in interpretation,
implying, for example, that the resistivity contrast between a wet sand and
underlying competent limestone bedrock may be the reverse of that between
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