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and also depend on the electrical properties of the pore
fluid and the country rocks. Potentials of tens to hundreds
of millivolts are typical, but their amplitude may be
affected by rainfall, albeit with suf
cient time lag to allow
this to translate into changes in groundwater
ow (Ernst-
son and Scherer,
1986
). Potential anomalies occur where
groundwater
flows across the boundaries between mater-
ials with different electrical properties. Anomalies may be
positive or negative, with variations in the potentials often
correlating with topography, higher ground being almost
always more negative than lower ground. Several authors
have reported measuring negative potentials of around
2000 mV over high ground.
Differences in the concentration and mobility of ions in
groundwater are another source of electrical potentials.
Potential differences occur when electrolytes having differ-
ent concentrations of ions come into contact. This is par-
ticularly relevant
a)
+
0
-
SP
b)
O
2
O
2
Surface
--
-
-
-
-
HFeO
2
Fe
+++
O
2
+
Fe
++
OH
-
+
Cathode
H
2
O
2
OH
-
-
Water table
-
+
Fe(OH)
2
+
+
Fe(OH)
3
H
+
+
Anode
H
+
Fe
++
Fe(OH)
3
+
-
FeS
2
+
Mineralisation
Figure 5.27
Schematic illustration of an electrochemical mechanism
for the self-potential produced by an iron sulphide body. Based on a
diagram in Sato and Mooney (
1960
).
uid
encounters natural groundwaters. The diffusion rates of
ions in solution are related to their mobilities; so in elec-
trolytes composed of different types of ions, their different
mobilities produce a heterogenous mixture of cations and
anions causing liquid-junction potentials. These are usually
tens of millivolts. Nernst or shale potentials occur when the
two
fluids are separated by semipermeable formations. The
negative charges on clay minerals in the formations restrict
the diffusion of negative ions in the solution, but not the
positive ions. This leads to an uneven distribution of ions,
and the resulting potential differences are usually a few
tens of millivolts.
The adsorption of ions onto the charged surfaces of min-
eral grains produces positive anomalies of up to about
100 mV. Anomalies of this type occur in association with
quartz veins, pegmatites and concentrations of clay minerals.
Biogenic causes of SP anomalies are also known, possibly
caused by ion selectivity and water pumping by plant roots.
These cause decreases in potential of up to about 100 mV.
for SP logging when drilling
The most comprehensive explanation of mineral
potentials is that of Sato and Mooney (
1960
) and involves
electrochemical interaction between the mineralisation and
the Sato and Mooney model, the mineralisation straddles
regions with differing oxidation potential, most likely
regions above and below the water table, and this is
assumed in the following description. Potential differences
are formed by the different, but complementary, electro-
chemical reactions occurring above and below the water
table. Below the water table there is an anodic half-cell
reaction with ions in the solution in contact with the
mineralisation being oxidised to release electrons. Above
the water table, ions in solution are reduced by a cathodic
half-cell reaction, so a supply of electrons is required. This
causes the shallower outer-part of the mineralisation to
have a negative potential relative to the deeper outer-part.
The free electrons created by the reactions below the water
table are electronically conducted upwards through the
(electrically conductive) mineralisation to take part in the
reduction reactions above the water table. The mineral-
isation has a passive role acting as a conduit from anode to
cathode for the electrons involved in electronic conduc-
tion. In the surrounding electrolytes, current is passed by
ionic means. Weathering of the near-surface part of the
mineralised body removes it from the self-polarisation
process.
5.5.1.2
Mineralised sources
Potential anomalies ranging from hundreds to more than a
thousand millivolts occur in association with bodies of
disseminated and massive sulphide, graphite, magnetite,
anthracite coal and manganese mineralisation. These are
known as mineral or sulphide potentials, and they form the
signal in mineral SP surveys. The causes of the potential
variations associated with coal are poorly understood, and
even those due to metallic mineralisation are not fully
explained.
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