Geology Reference
In-Depth Information
Figure 7.1
Sources of self-potential (SP) effects. The sulphide mass strad-
dling the water table concentrates the flow of oxidation-reduction currents,
producing a negative anomaly at the surface. The downslope flow of ground-
water after rain produces a temporary SP, in this case inversely correlated
with topography.
Small potentials, seldom exceeding 100 mV and usually very much less,
may accompany ground-water flow and water seepage. Polarity depends on
rock composition and on the mobilities and chemical properties of the ions
in the pore waters, but usually the region towards which ground water is
flowing becomes more electropositive than the source area. These
streaming
potentials
are sometimes useful in hydrogeology and in leak detection but
can make mineral exploration SP surveys inadvisable for up to a week after
heavy rain.
Movements of steam or hot water can explain most of the SPs associated
with geothermal systems, but small (
<
10 mV) voltages, which may be pos-
itive or negative, are produced directly by temperature differences. Geother-
mal SP anomalies tend to be broad (perhaps several kilometres across) and
have amplitudes of less than 100 mV, so very high accuracies are needed.
