Geology Reference
In-Depth Information
clearly show the presence of the mineralization, allow its
limits to be determined and provide estimates of the
depth to its upper surface.
X
c
c
p
p
Double-dipole
c
8.4 Self-potential (SP) method
X
c
p
p
Pole - dipole
8.4.1 Introduction
The self-potential (or spontaneous polarization)
method is based on the surface measurement of natural
potential differences resulting from electrochemical re-
actions in the subsurface.Typical SP anomalies may have
an amplitude of several hundred millivolts with respect
to barren ground. They invariably exhibit a central
negative anomaly and are stable over long periods of
time.They are usually associated with deposits of metal-
lic sulphides (Corry 1985), magnetite or graphite.
X
c
p
p
c
Schlumberger
c
p
X
= current electrode
= potential electrode
= reading plotted
Fig. 8.33
Electrode configurations used in induced polarization
measurements.
2000
20
8.4.2 Mechanism of self-potential
0
100 m
Field studies indicate that for a self-potential anomaly
to occur its causative body must lie partially in a zone
of oxidation. A widely-accepted mechanism of self-
potential (Sato & Mooney 1960; for a more recent analy-
sis see Kilty 1984) requires the causative body to straddle
the water table (Fig. 8.36). Below the water table elec-
trolytes in the pore fluids undergo oxidation and release
electrons which are conducted upwards through the ore
body. At the top of the body the released electrons cause
reduction of the electrolytes. A circuit thus exists in
which current is carried electrolytically in the pore fluids
and electronically in the body so that the top of the body
acts as a negative terminal.This explains the negative SP
anomalies that are invariably observed and, also, their
stability as the ore body itself undergoes no chemical
reactions and merely serves to transport electrons from
depth. As a result of the subsurface currents, potential
differences are produced at the surface.
M
10
1000
ρ
a
0
0
Fault
Devonian (Old
Red Sandstone)
Carboniferous
dolomitic limestone
1-2 0.5 -1 0.25 - 0.5
% Cu
Fig. 8.34
Time-domain IP profile using a pole-dipole array over
the Gortdrum copper-silver body, Ireland. (After Seigel 1967.)
8.4.3 Self-potential equipment
and survey procedure
corresponding apparent resistivity profile reflects the
large resistivity contrast between the Old Red Sandstone
and dolomitic limestone but gives no indication of the
presence of the mineralization.
A further example of an IP survey is illustrated in Fig.
8.35 which shows a traverse over a copper porphyry
body in British Columbia, Canada. IP and resistivity
traverses were made at three different electrode spacings
of a pole-dipole array.The CST results exhibit little vari-
ation over the body, but the IP (chargeability) profiles
Field equipment consists simply of a pair of electrodes
connected via a high-impedance millivoltmeter. The
electrodes must be non-polarizing as simple metal spikes
would generate their own SP effects. Non-polarizing
electrodes consist of a metal immersed in a saturated so-
lution of its own salt, such as copper in copper sulphate.
The salt is contained in a porous pot which allows slow
leakage of the solution into the ground.
