Page 124 - Geochemical Remote Sensing of The Sub-Surface

Spontaneous potentials and electrochemical cells 101

immersed in an electrolytic conductor. In a voltaic cell, electronic current is spontaneous
and therefore to induce current through the wire joining the electrodes there must be
physical separation of oxidising and reducing agents in the solution. The redox field in
the Earth represents an electrolyte within which there is vertical separation of oxidising
and reducing agents. For the purpose of the following discussion, an SP cell is defined
here as a natural system that induces the spontaneous, long-term flow of electrical
current in a focused area within the Earth, i.e., the current flux has to be anomalous with
respect to background current.
Although there must be constant ionic current in the Earth's redox field, cells as just
defined can only develop due to major inhomogeneities in either the electrical
conductivity of Earth materials or the SP gradient in a particular area. Indeed the
development of Earth cells of this type should be predictable by Ohm's law. If an area of
increased electrical conductivity occurs across an otherwise uniform voltage gradient, an
increase in current density in that zone must also occur. As discussed below, a vertically-
oriented geological conductor crossing horizontal redox equipotential lines in the Earth
is one example of such a system. Likewise if an area of increased voltage gradient occurs
in a medium of uniform electrical conductivity, an increase in current density must also
occur. Examples of this type of cell are also discussed below.
The term conductor, when used in a geological context, usually refers to
electronically-conductive or semi-conductive materials in bedrock, such as graphite or
metallic sulphide mineralisation. These substances conduct electricity far better than
low-porosity silicate or carbonate bedrock. However, they are typically poorer
conductors than most groundwater-saturated overburden. Part of the reason why this fact
is not widely recognised among geologists is that the electrically-conductive properties
of groundwater and bedrock have typically been expressed in inverse ways. The
electrical conductivity of groundwater is most commonly reported in conductivity units
whereas the conductive properties of bedrock are reported in resistivity units. Table 3-11
shows the electrical conductivities of some typical groundwaters and those of a number
of common bedrock materials converted from their usual resistivity units. It is evident
that very few common bedrock conductors begin to approach the electrical conductivity
of most groundwaters. Overburden is usually more electrically conductive than the
groundwater within it because of the additional conductivity imparted by clay minerals
and oxide surfaces (Keller and Frischknecht, 1966). This demonstrates that the electrical
conductivity of overburden far exceeds that of low-porosity bedrock and usually also
exceeds that of most bedrock conductors.

Cells associated with electronic conductors in bedrock

Geobatteries centred on electronically conductive, steeply-dipping mineralisation
have been described for many years and most geologists are aware of their existence.
However, several widely-referenced geochemical models developed to account for SP