Page 136 - Geochemical Remote Sensing of The Sub-Surface

Spontaneous potentials and electrochemical cells 113

that these cells should occur wherever the presence of conductive mineralisation results
in redox anomalies on the bedrock surface.
Part of the reason overburden cells such as these have not been recognised in the past
might be that they would be extremely difficult to detect in the vadose zone. Much
higher contrasts are expected between redox conditions inside and outside the column
below the water table than would occur above the water table where oxidising agents are
abundant and far more mobile. As such, the cells would develop best in areas of high
water table and in these areas SP surveys have traditionally not been carried out because
of false anomalies that often occur due to variable moisture content and pH. This is not
to suggest that the cells cannot exist in the unsaturated zone above the water table, just
that they are likely to be subtler. The presence of SP lows on surface above
mineralisation in areas of a thick unsaturated zone (Parasnis, 1979; Burr, 1982; Govett et
al., 1984) suggests that oxidising agents are consumed and that cells, as defined above,
can also exist in the unsaturated zone.
There is abundant field evidence for the existence of SP cells over petroleum
reservoirs. Indeed this evidence itself was used to find petroleum reservoirs for almost
100 years before the cells were recognised. It includes gaseous and liquid hydrocarbon
seeps, paraffin deposits, halo-type metal anomalies, iron and manganese deposition,
carbonate cementation, hard-drilling areas, magnetic anomalies (associated with
magnetite mineralisation) and elevated uranium concentrations (Tomkins, 1990).
Chapters 5-7 of this volume and references provided therein document many of these
features.

GEOCHEMICAL RESPONSE TO SPONTANEOUS POTENTIAL CELLS

Ion mobility

One of the problems with the understanding of surface selective leach anomalies is
their development in thick, young deposits. Glacial materials in Canada only 8000 years
old have well-developed surface anomalies spatially associated with underlying
mineralisation. As argued above, diffusion, gaseous carriers and advective groundwater
transport can be ruled out as the primary transport mechanisms in these environments
because they are too slow to account for most anomalies. The rate of movement of
charged species in an electrical field, however, can far exceed the rate of other dispersal
mechanisms. Ionic migration rates in an electric field are described by the equation
(Webber, 1975; Glasstone and Lewis, 1960):

s = ()~+/F)-(V/d)