On measuring the electrical conductivity of the oceanic crust by a modified magnetometric resistivity method

Magnetotelluric sounding reveals a conductive zone beneath the Pacific Ocean at depths in excess of 60 km but does not resolve the resistivity of the lithosphere above this zone. Further resolution can be obtained by controlled source electrical methods. The simplest of these are the galvanic techniques. Dipole-dipole resistivity sounding is not suitable because dipole separations of thousands of kilometers would be required to obtain values of the resistivity. A viable alternative is to measure on the ocean floor the magnetic field of a vertical bipolar source extending from the sea surface to the seafloor. Magnetometer transmitter separations of only a few kilometers are sufficient to determine the resistivity of a half space beneath the ocean. Sounding curves similar to those of the resistivity method may be constructed to resolve the resistivity of a layered lithosphere. The curves constructed are valid at alternating frequencies which are small compared with a skin frequency not in the ocean but in the lithosphere. The depth of penetration is of the order of half the transmitter-receiver separation. Magnetic field amplitudes are in the range of picotesla for reasonable lithospheric resistivities and separations up to 10 times the length of the bipole. Modern instrumentation, modified for the ocean floor, can detect such signals at a range of 20 km at a frequency of about 0.02 Hz averaged over several hours.