Spontaneous potential in Earth materials

The existence of spontaneous potentials in the Earth has been known for at least 150 years and their measurement has been used systematically in the search for buried ore deposits since the 1920s (Parasnis, 1979). Spontaneous or self potentials (SP) are natural voltage differences between two points in the Earth which result in electrical 

In practice, ORP probes have inherent limitations that often render this approach to SP measurement unworkable. Because platinum is effectively inert, the gain or loss of electrons on the electrode surface necessitates the attenuation of ions from the sample by their conversion to neutral species or to charged species of a higher or lower oxidation state. For the readings to be reproducible, one or more reversible redox-couples, such as Fe / Fe , HS / S04 or Cu / Cu, must be present in solution near the electrode. 

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. 

---