First results of the MOSES experiment: Sea sediment conductivity and thickness determination, Bute Inlet, British Columbia, by magnetometric offshore electrical sounding

Abstract

A static electrical method has been developed to determine the electrical resistivity of crustal rock beneath the sea. The transmitter is a vertical, long‐wire bipole, extending from the sea surface to the sea floor. A commutated current, generated on the ship, is fed to two large electrodes: one near the sea surface, the other at the end of a long insulated wire. The return path for the current is through the sea and the subjacent crust. The receiver is a self‐contained, remote, microprocessor‐controlled magnetometer which is deployed from the ship to the sea floor and subsequently recovered. The data are measurements of the azimuthal component of the magnetic field as a function of transmitter‐receiver horizontal separation. The acronym MOSES has been coined for the method. The choice of the name MOSES is appropriate because the system geometry is carefully arranged to remove many of the adverse effects of the relatively conductive sea water. In particular, accurate estimates of sea floor resistivity are possible because the data are proportional to the transmitted current from the source into the crustal material. A sea test of the method in a water depth of 640 m was conducted in the “V” shaped Bute Inlet, British Columbia. Transmitted power was 1.25 kW; averaging time at each transmitter location was 1 hour. Transmitter‐receiver separations ranged from 150 to 2 000 m. The resistivity and thickness of a sedimentary section beneath the sea were determined as 1.9⋅Ω m and 560 m, respectively. The interpretation was accomplished both by matching the data converted to apparent resistivity to corresponding model type curves and by generalized linear inverse theory. Errors in the final parameters were estimated at about 9.2 percent using a parameter eigenvector analysis. The interpreted resistivity is in accord with direct measurement on core samples of sediment porosity. The interpreted thickness is less than an upper limit determined by extrapolating local inlet topography beneath the sea.