SP AND IP - Field Geophysics - page 139

7.1.3 Errors and precautions
If two estimates of a base/sub-base difference disagree by more than 3 mV,
work should be stopped until the reason has been determined. Usually it will
be found that copper sulphate solution has either leaked away or become
undersaturated. Electrodes should be checked every two to three hours by
placing them on the ground a few centimetres apart. The voltage difference
should not exceed 1 or 2 mV.
Accumulation of errors in large surveys can be minimised by working in
closed and interconnecting loops around each of which the voltages should
sum to zero (see Section 1.6.3).
7.2 Polarisation Fundamentals
Induced polarisation (IP) methods are perhaps the most popular of all geo-
physical techniques in mineral exploration, being the only ones responsive
to low-grade disseminated mineralisation. There are two main and incom-
pletely understood mechanisms of rock polarisation and three main ways in
which polarisation effects can be measured. In theory the results obtained
by the different techniques are equivalent but there are practical differences.
7.2.1 Membrane polarisation
The surfaces of clays and some other platey or fibrous minerals are negatively
charged and cause membrane polarisation in rocks with small pore spaces.
Positive ions in the formation waters in such rocks congregate near the pore
walls, forming electrical double layers . If an electric field is applied, the
positive ion clouds are distorted and negative ions move into them and are
trapped, producing concentration gradients that impede current flow. When
the applied field is removed, a reverse current flows to restore the original
equilibrium.
7.2.2 Electrode polarisation
The static contact potentials that exist between metallic conductors and
electrolytes were discussed in Section 6.2.2. Additional over-voltages are
produced whenever currents flow. This electrode polarisation occurs not
merely at artificial electrodes but wherever grains of electronically conduct-
ing minerals are in contact with the ground water. The degree of polarisation
is determined by the surface area, rather than the volume, of the conduc-
tor present, and polarisation methods are thus exceptionally well suited to
exploration for sulphides in disseminated porphyry ores. Strong anomalies
are also usually associated with massive sulphide mineralisation, because
of surrounding disseminated haloes.
Although, for equivalent areas of active surface, electrode polarisation is
the stronger mechanism, clays are much more abundant than sulphides and