These two methods are mainly used to prospect for conductive ores. Self-potential (or spontaneous potential), SP, depends on small potentials or voltages being naturally produced mainly by some massive ores. Induced polarisation, IP, in contrast, depends upon a small amount of electric charge being stored in an ore when a current is passed through it, to be released and measured when the current is switched off. IP is significant only for disseminated ores but can often be used to locate massive ores as these are commonly surrounded by disseminated ore. For both methods, potentials can also arise in other, usually unwanted, ways.
Both methods require electrodes and wires to make contact with the ground.
Induced polarisation, IP
What induced polarisation is
Induced potential is a potential difference that sometimes exists briefly after the current in a resistivity array has been switched off. It arises from the presence of small particles of conductor in rocks, so it is used to detect disseminated ores, which are composed of discrete particles of conducting minerals, in a nonconducting matrix.
In rocks other than ores, current is conducted by positive and negative ions (Section 12.2.1) moving through the groundwater, often in tiny channels formed of interconnecting pores (Fig. 13.1). If a channel is blocked by a grain that is insulating no current can flow through it, but if the grain is conducting electrons can pass through, though ions cannot.
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