Geology Reference
In-Depth Information
(a)
(b)
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Rock
Rock
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Pore
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Pore
+
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ε
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Fig. 8.29
Mechanisms of induced
polarization: (a) membrane polarization
and (b) electrode polarization.
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Mineral
grain
grain blocks a pore, When a voltage is applied to either
side of the pore space, positive and negative charges are
imposed on opposite sides of the grain. Negative and
positive ions then accumulate on either side of the grain
which are attempting either to release electrons to the
grain or to accept electrons conducted through the
grain. The rate at which the electrons are conducted is
slower than the rate of electron exchange with the ions.
Consequently, ions accumulate on either side of the
grain and cause a build-up of charge. When the im-
pressed voltage is removed the ions slowly diffuse back to
their original locations and cause a transitory decaying
voltage.
This effect is known as
electrode polarization
or
overvolt-
age
.All minerals which are good conductors (e.g. metallic
sulphides and oxides, graphite) contribute to this effect.
The magnitude of the electrode polarization effect de-
pends upon both the magnitude of the impressed voltage
and the mineral concentration. It is most pronounced
when the mineral is disseminated throughout the host
rock as the surface area available for ionic-electronic
interchange is then at a maximum. The effect decreases
with increasing porosity as more alternative paths be-
come available for the more efficient ionic conduction.
In prospecting for metallic ores, interest is obviously
in the electrode polarization (overvoltage) effect.
Membrane polarization, however, is indistinguishable
from this effect during IP measurements. Membrane
polarization consequently reduces the effectiveness of
IP surveys and causes geological 'noise' which may be
equivalent in magnitude to the overvoltage effect of a
rock with up to 2% metallic minerals.
Resistive region
1
ρ
a
2
Warberg region
Electromagnetic
induction
3
10
-1
10
1
10
2
10
3
1
Log current frequency
Fig. 8.30
The relationship between apparent resistivity and log
measuring current frequency.
A
VV
vt
1
t
Ú
2
M
=
=
( )
d
t
(8.22)
D
D
t
1
c
c
Chargeability is measured over a specific time interval
shortly after the polarizing current is cut off (Fig. 8.28)
The area
A
is determined within the measuring appara-
tus by analogue integration. Different minerals are dis-
tinguished by characteristic chargeabilities, for example
pyrite has
M
= 13.4 ms over an interval of 1 s, and mag-
netite 2.2 ms over the same interval. Figure 8.28 also
shows that current polarity is reversed between succes-
sive measurements in order to destroy any remanent
polarization.
Frequency-domain techniques involve the measure-
ment of apparent resistivity at two or more AC fre-
quencies. Figure 8.30 shows the relationship between
apparent resistivity and log current frequency.Three dis-
tinct regions are apparent: region 1 is in low frequencies
where resistivity is independent of frequency; region 2 is
the Warberg region where resistivity is a linear function
of log frequency; region 3 is the region of electromag-
netic induction (Chapter 9) where current flow is by
induction rather than simple conduction. Since the rela-
tionship illustrated in Fig. 8.30 varies with rock type
8.3.3 Induced polarization measurements
Time-domain IP measurements involve the monitoring
of the decaying voltage after the current is switched off.
The most commonly measured parameter is the
charge-
ability M
, defined as the area
A
beneath the decay curve
over a certain time interval (
t
1
-
t
2
) normalized by the
steady-state potential difference
D
V
c
(Fig. 8.28)
