Controlled-source electromagnetic sounding of the oceanic lithosphere

Abstract

The attenuation of ionospheric signals in the frequency range 0.06–24 Hz by sea water effectively precludes using the magnetotel-luric method to study the electrical structure of the upper oceanic lithosphere. We have carried out a dipole–dipole electromagnetic sounding in the North Pacific by injecting electromagnetic signals into the ocean and sea bed. The crust at the site is 25 Myr old and has a thin sediment cover. The technique, similar to that used in earlier work^1,2, involves dragging a horizontal dipole antenna along the sea floor. The electric fields that propagated through the resistive basement were detected by seafloor receivers at ranges of 10–65 km. As the ambient electric field is very small (varying from 10⁻¹⁸ V² m⁻² Hz⁻¹ at 0.1 Hz to 10⁻²⁴ V² m² Hz⁻¹ above 1 Hz; ref. 3), the controlled-source signals could be easily monitored. Our data are consistent with a simple one-dimensional Earth model consisting of a 3–7-km-thick crustal layer of moderate conductivity (∼0.001 S m⁻¹) underlain by a thicker region of very low conductivity (<2 × 10⁻⁵ S m⁻¹). The results suggest an upper mantle water content of at most 0.1% by volume.