Self potential exploration of the continental shelf and mid-ocean ridges

(G.S. Heinson, A. White, N. Fathianpour, B.D. Perkins and R. Walker)

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Self Potential (SP) anomalies are measurable voltages arising naturally from the Earth, and are, in principle, simple to measure, requiring only a set of electrodes and a voltmeter. The SP method commonly used on land may be extended offshore. Seawater has a reduction-oxidation (redox) potential (Eh) of +200 to +400 mV, while marine sediments have Eh values of -100 to -200 mV. The sea floor thus is a strong redox boundary. A conducting medium, such as a mineral vein, will act as a short circuit and the return current in the seawater and sediments produces an SP anomaly. As seawater is highly conducting SP anomalies are reduced to those on land by the ratio of the bulk conductivity of the sediments to the conductivity of the seawater. However, as sea floor sediments have high porosity this ratio is generally close to 1.

We are developing a new SP instrument for shallow and deep water geophysical investigation, incorporating state-of-the-art technologies. Two types of marine SP anomalies are being investigated. Firstly, we are attempting to map and interpret mineralisation on the continental shelf around Australia and thus develop a new geophysical exploration technique. Marine SP exploration will be somewhat analogous to marine magnetic surveys, but is sensitive to different mineralisations, principally graphite and sulphides, which have economic importance. Secondly, the work is leading to an investigation of hydrothermal sulphide mineralisation at mid-ocean ridges (MOR), which are of great current interest. Such hydrothermal deposits are a key indicator of the processes active at MORs and are modern analogues of many ancient ophiolite-hosted deposits, such as those exposed in Cyprus. Sulphides have SP anomalies of the order of 400 mV on land and should be easily detectable in the marine environment.

In 1997 we conducted a number of tests from the Flinders University R/V Hero in the Gulf of St Vincent and in the Spencer Gulf, successfully locating mineralisation offshore in Wallarroo Bay. A two-dimensional finite-element code was developed to help interpret the results.

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