Page 117 - Geochemical Remote Sensing of The Sub-Surface

94 S.M. Hamilton

ground and positive charge to discharge into the electrode by the conversion of Cu2+(aq)
to Cu(s) at the surface of the copper, and a corresponding migration of cations up through
the membrane and anions down through the membrane. There is, therefore, an electrical
potential difference between the two areas which is measurable with the millivolt meter
and which would result in current in the wire if a direct connection were made.
An important interference often encountered during the measurement of SP using
Cu-CuSO4 electrodes occurs due to variable moisture conditions. These can cause false
anomalies which can seriously complicate the interpretation of survey data, especially in
areas of thick overburden. Wet areas have often been observed to cause high positive
readings relative to adjacent areas (e.g., Parasnis, 1979; Burr, 1982). This is perhaps
counterintuitive because it appears to imply that wet soils are more oxidising than
unsaturated soils. Burr (1982) attributed higher SP readings in swamps and wet soil to
pH effects. The Eh, and therefore SP, have a general pH dependency because many
natural redox reactions involve hydrolysis. For example, for the half-reactions that
involve either the oxidation or reduction of water, a decrease of 1 pH unit will result in
an increase in the Eh of the reaction of approximately 60 mV. Therefore, the higher Eh
of peat and moist humus layers is consistent with the fact that these materials are usually
more acidic than are mineral soils. Lower electrode-ground resistance has also been
noted in moist areas and in humus soil layers relative to underlying mineral soils, and
has been suggested as a possible contributor to moisture-related anomalies (R. Chaplain,
pers. comm., 1998). Streaming potentials as a result of groundwater discharge in low-
lying areas has also been suggested as a possible source of SP (Dobrin and Savit, 1988)
and therefore could be a source of false anomalies in low-lying areas. Streaming
potentials are generated by the movement of water through a porous medium that is
capable of ion exchange, such as clay or oxides on sand.
Other factors that can affect SP surveys tend to be less significant than the problems
due to moisture. Magnetic storms (Burr, 1982), radar and other electromagnetic radiation
can cause induction in the long SP wire, particularly when it is fully extended. Telluric
currents, which are global-scale electrical currents in the Earth induced by the Earth's
magnetic field, could conceivably affect SP surveys but typically result in a SP
difference of only a few millivolts per kilometre.
The use of SP surveys as an exploration tool has waned since the 1950s with the
increasing sophistication of other electrical geophysical techniques such as induced
polarisation (IP) and ground resistivity. Part of the reason is that the interpretation of
these non-passive techniques is easier because electrical theory and electronics theory
can be applied. Since the causes of natural SP above mineralisation are still widely
misunderstood (Hamilton, 1998), the interpretation of the results of SP surveys is
difficult.