Geology Reference
In-Depth Information
most observed IP effects are due to membrane polarisation. Changes in ion
concentration (e.g. salinity levels) affect both types of polarisation. Non-
ionic fluids (e.g. oil contamination) can also change polarisation behaviour.
7.2.3 The square wave in chargeable ground
When a steady current flowing in the ground is suddenly terminated, the
voltage
V
o
between any two grounded electrodes drops abruptly to a small
polarisation voltage V
p
and then declines asymptotically to zero. Similarly,
when current is applied to the ground, the measured voltage first rises rapidly
and then approaches
V
o
asymptotically (Figure 7.3). Although in theory
V
o
is never reached, in practice the difference is not detectable after about a
second.
Chargeability
is formally defined as the polarisation voltage developed
across a unit cube energised by a unit current, and is thus in some ways analo-
gous to magnetic susceptibility. The
apparent chargeability
of an entire rock
mass is defined, in terms of the square wave shown in Figure 7.3, as the ratio
of
V
p
to
V
o
. This is a pure number but, in order to avoid very small values,
it is generally multiplied by a thousand and quoted in millivolts per volt.
The
V
p
/
V
o
ratio cannot be measured directly because electromagnetic
transients are dominant in the first tenth of a second after the original current
has ceased to flow, and the practical definition of time-domain chargeability,
in terms of the decay voltage at some specified delay time, is only tenuously
linked to the theoretical definition. Not only do different instruments use
different delays, but it was originally essential and is still quite common
to measure an area under the decay curve using integrating circuitry, rather
than an instantaneous voltage. The results then depend on the length of the
integration period as well as on the delay and are quoted in milliseconds.
7.2.4 Frequency effects
Figure 7.3 also shows that if a current were to be terminated almost im-
mediately after being introduced, a lower apparent resistivity, ρ
hf
, equal to
2
I
multiplied by the array geometrical factor, would be calcu-
lated. The IP frequency effect is defined as the difference between the 'high
frequency' and 'DC' resistivities, divided by the high-frequency value. This
is multiplied by 100 to give an easily handled whole number, the
Percent
Frequency Effect
(
PFE
). The theoretical relationship between the PFE and
the chargeability is given by:
π
(
V
o
−
V
p
)
/
M
=
[PFE]
/
(100
+
[PFE])
Figure 7.3 illustrates the derivation of this relationship.
Because of electromagnetic transients, the theoretical PFE cannot be
measured and the practical value depends on the frequencies used. To cancel
