Offshore Application of Self-potential Prospecting
An offshore self-potential prospecting system, consisting of a portable chart recorder and a pair of towed electrodes, is shown to be capable of locating both onshore and submerged offshore deposits of conductive minerals. The silver - silver chloride electrodes, specially designed for this work, are shown to have a small and predictable response to changes in environmental parameters.
The background noise level is generally less than a few tenths of a millivolt, allowing visual recognition of offshore self-potential anomalies with a gradient in excess of one millivolt over the electrode separation. However, some man-made structures, or areas of geothermal activity, may generate fields large enough to interfere with self-potential profiling.
Laboratory and field experiments show that a conductive body extending from a reducing sea floor into oxidizing sea water, generates a potential field consistent in magnitude and polarity with those seen over sulfide ore bodies on land. As an oxidation - reduction mechanism appears to offer the best explanation for the self-potential phenomenon on land, conductive mineral deposits in the sea floor also should be capable of generating self-potential fields.
The self-potential field generated by a submerged ore body may be simulated by a point current sink or dipole buried in the sea floor, and the potential field at the water surface studied as a function of depth of burial, sea-floor resistivity, water depth, and dipole angle and separation. It is seen in most cases that an ore body of economic size will generate an easily detectable potential field at the water surface.
Field work was conducted in areas of known sulfide mineralization (southeastern Alaska and Penobscot Bay, Maine) and onshore and off-shore geothermal activity (California, Nevada, and Ensenada, Mexico). Increased self-potential background noise levels were seen in the geothermal areas, along with some large anomalies. In Maine, large offshore self-potential fields were found to be generated by onshore sulfide and graphite deposits, and submerged mineral deposits are thought to be responsible for other observed offshore anomalies.