Geology Reference
In-Depth Information
6
RESISTIVITY METHODS
Nomenclature in electrical geophysics can be confusing. The flow of current
in the so-called
direct current
(DC) surveys is usually reversed at intervals
of one or two seconds, and CCR surveys, in which alternating currents are
introduced into the ground by capacitive coupling, have more in common
with DC than EM methods, despite the use of kHz frequencies. These two
methods are discussed in this chapter.
6.1 DC Survey Fundamentals
Surface resistivity methods are based on the principle that the electric poten-
tials (voltages) measured around a current-carrying electrode are affected
by the electrical resistivities of the underlying materials.
6.1.1 Apparent resistivity
The 'obvious' method of measuring ground resistivity, by simultaneously
passing current through and measuring voltage between a single pair of
grounded electrodes, does not work, because of contact resistances, which
depend on such things as ground moisture and contact area and may amount
to thousands of ohms. The problem can be avoided if, as illustrated in
Figure 6.1, voltages are measured between a second pair of electrodes using
a high-impedance voltmeter that draws virtually no current. The voltage
drop through the voltage electrodes is then negligible, and although the
resistances at the current electrodes limit current flow, they do not affect
resistivity calculations. Usually the current electrodes are in line with the
voltage electrodes but they can be located anywhere. A geometric factor is
needed to convert the readings obtained with these four-electrode
arrays
to
resistivity.
The result of any single measurement with any array could be interpreted
as due to a homogeneous ground with constant resistivity. The geometric
factors used to calculate this
apparent resistivity
,
ρ
α
, can be derived by
applying, to each current-electrode/voltage-electrode pair, the formula
V
=
ρ
I
/
2
π
a
