Marine self potential exploration^*

Recent marine self potential (SP) measurements south of Eyre Peninsula, South Australia and Rose Canyon, San Diego, California have been made. In both cases, a series of horizontal electrode dipoles were towed close to the seafloor in water depths of up to 100 m to measure the electric potential gradients generated by mineralisation beneath the seafloor. A proton-precession magnetometer was also towed at the surface in the Eyre Peninsula experiment. Marine SP measurements show significantly lower noise levels than land measurements, due to the uniform marine environment and low contact resistance of the electrodes, so that anomalies of less than a few tens of microvolts per metre can be detected. The major source of noise is from ocean swell and waves, which may be minimised by coherent stacking of signals, and by bandpass filtering. South of Eyre Peninsula SP electric field anomalies of 100 μV/m and width 2 km were observed in a number of traverses perpendicular to the trend of an onshore mylonite zone. Little correlation exists between the SP and magnetic data, suggesting that the SP sources are probably due to non-ferrous minerals such as graphite, and/or from the electrokinetic effect of groundwater flow through the fracture zone.