Electromagnetic fields induced at the seafloor by Rayleigh‐Stoneley waves

Abstract

We model oceanic acoustic and seismic disturbances as Rayleigh‐Stoneley waves in the layered medium consisting of ocean, sediment and rock. The waves induce electromagnetic fields because of motion of conducting materials through the geomagnetic field. Low‐frequency disturbances in deep water are efficient electromagnetic generators because the motions are coherent over large volumes of highly conducting seawater. For wave frequencies below 0.1 Hz in deep water, the geomagnetic field is nearly frozen to the moving water. Consequently, the electric field measured by a voltmeter attached to the seabed is approximately Δu × F, where Δu is the difference between the velocity of the water and of the voltmeter itself as it is moved by the motion of the seafloor, and F is the geomagnetic field. This result applied to Love waves suggests that the electric field will result largely from the movement of the detector but detailed calculations have not been made. Although the fields are weak they should be detectable because the main interference is from ionospheric sources and the fields from these sources are greatly attenuated at the seabed by the overlying oceanic shield. The effectiveness of the shield diminishes sharply at frequencies below 0.03 Hz. At high frequencies the principal limitation to detectability is in the inherent noise level of detectors whether electric or magnetic. At present, electric detectors are more effective than magnetic. They appear to be competitive with accelerometers for seismic detection in the deep ocean in a restricted frequency band near 0.05 Hz. A layer of unconsolidated sediment underlying the ocean profoundly affects the dispersion relation of Rayleigh‐Stonely waves and thereby affects the electromagnetic induction process. The shunting effect of a normal deep ocean sediment layer is not large on the electric field, but the low shear modulus of the sediments influences sea bottom motions and therefore greatly changes the fields sensed by bottom mounted instruments especially at higher frequencies.