Abstract

The recent discoveries of polymetallic sulfide deposits on the deep sea floor have created an interest in geophysical techniques for mapping them. A magnetometric resistivity method (MINI-MOSES) has been developed for detecting the distribution of sub-sea-floor electrical resistivity. A vertical, long-wire bipole feeds electric current into the ocean. Some of the current enters the sea floor, and its magnetic field is measured at various distances by a low-frequency induction coil magnetometer. The method is sensitive to conductive and resistive zones at depth, and information about three-dimensional resistivity structures can be obtained.

The small load resistance of an electric transmitter grounded in the ocean allows high current to be driven by a battery-powered instrument. The induction coil receiver is more sensitive and less noisy than conventional flux-gate magnetometers over the selected frequency band. This receiver is therefore well-adapted to the almost noise-free deep sea-floor environment. Both the transmitter and the receiver are completely self-contained, recording averaged transmitted and received signals in random access memory.

The technique was tested near a sulfide deposit at 47° 57′N, 129°06′W on the axis of the Juan de Fuca Ridge. The deep submersible ALVIN positioned and moved the instruments about on the sea floor. Just enough data were obtained to demonstrate the viability of the technique for this kind of geologic mapping and mineral exploration. The electrical resistivity of 14 Ω · m obtained for a uniform half-space model is consistent with Deep Sea Drilling Project (DSDP) results for the seawater-saturated basalt.

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