The seismo-acoustic response to the nonlinear interaction of ocean gravity waves.

Abstract

In the absence of shipping, high sea/wind states can produce measurable noise in the sea, various noise mechanisms dominating in different parts of the spectrum. In the band 1–3 to 50 Hz, the noise source is believed to be breaking waves. Between 0.1 to 1–3 Hz, commonly called the microseism band, the high noise levels observed are believed to arise from nonlinear interactions between surface gravity wave trains. At still lower frequencies (0.02–0.1 Hz), noise levels are generally very low and are thought to be controlled by currents and turbulence in the seafloor boundary layer. In the final band from dc to 0.02 Hz the noise again increases. It is felt that the noise in this band originates from very long wavelength swell. This paper examines the generation of ocean and seafloor noise in the microseism band, discusses the question of the source level associated with the wave-interaction process, and examines the dependence of the observed seismo-acoustic noise spectrum on wind speed, on seastate (PM and JONSWAP seas), on water depth, and on the geoacoustical environment. Experimental data from onshore seismic sensors will demonstrate the possibility of using the ULF noise field to measure seastate remotely and to provide insight into the processes controlling the growth and decay of wind-driven seas.