Geology Reference
In-Depth Information
(1990) have also described how the resistivity method can
be used to study acid mine drainage and, in a similar envi-
ronmental context, Carpenter et al . (1991) have reported
the use of repeated resistivity measurements to monitor
the integrity of the cover of a landfill site in Chicago.
two manifestations of the capacitance property of the
ground provide two different survey methods for the
investigations of the effect.
The measurement of a decaying voltage over a certain
time interval is known as time-domain IP surveying. Mea-
surement of apparent resistivity at two or more low AC
frequencies is known as frequency-domain IP surveying.
8.3 Induced polarization (IP) method
8.3.2 Mechanisms of induced polarization
8.3.1 Principles
When using a standard four-electrode resistivity spread
in a DC mode, if the current is abruptly switched off, the
voltage between the potential electrodes does not drop
to zero immediately. After a large initial decrease the
voltage suffers a gradual decay and can take many sec-
onds to reach a zero value (Fig. 8.28).A similar phenom-
enon is observed as the current is switched on. After an
initial sudden voltage increase, the voltage increases
gradually over a discrete time interval to a steady-state
value.The ground thus acts as a capacitor and stores elec-
trical charge, that is, becomes electrically polarized.
If, instead of using a DC source for the measurement
of resistivity, a variable low-frequency AC source is used,
it is found that the measured apparent resistivity of the
subsurface decreases as the frequency is increased.This is
because the capacitance of the ground inhibits the pas-
sage of direct currents but transmits alternating currents
with increasing efficiency as the frequency rises.
The capacitive property of the ground causes both the
transient decay of a residual voltage and the variation of
apparent resistivity as a function of frequency. The two
effects are representations of the same phenomenon
in the time and frequency domains, and are linked
by Fourier transformation (see Chapter 2). These
Laboratory experiments indicate that electrical energy is
stored in rocks mainly by electrochemical processes.This
is achieved in two ways.
The passage of current through a rock as a result of an
externally imposed voltage is accomplished mainly by
electrolytic flow in the pore fluid. Most of the rock-
forming minerals have a net negative charge on their
outer surfaces in contact with the pore fluid and attract
positive ions onto this surface (Fig. 8.29(a)). The con-
centration of positive ions extends about 100 m m into the
pore fluid, and if this distance is of the same order as the
diameter of the pore throats, the movement of ions
in the fluid resulting from the impressed voltage is inhib-
ited. Negative and positive ions thus build up on either
side of the blockage and, on removal of the impressed
voltage, return to their original locations over a finite
period of time causing a gradually decaying voltage.
This effect is known as membrane polarization or elec-
trolytic polarization . It is most pronounced in the presence
of clay minerals where the pores are particularly small.
The effect decreases with increasing salinity of the pore
fluid.
When metallic minerals are present in a rock, an
alternative, electronic path is available for current flow.
Figure 8.29(b) shows a rock in which a metallic mineral
Δ
V c
Fig. 8.28 The phenomenon of induced
polarization. At time t 0 the current is
switched off and the measured potential
difference, after an initial large drop from
the steady-state value D V c , decays gradually
to zero. A similar sequence occurs when the
current is switched on at time t 3 . A
represents the area under the decay curve
for the time increment t 1 - t 2 .
Time
A
t 0
t 1
t 2
t 3
Δ
V c
 
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