Acoustic source levels associated with the nonlinear interactions of ocean waves

Abstract

Spectra of wave-induced microseismic noise are calculated as a function of the distance from the center of an active wave region of finite size, to examine the influence of the inhomogeneous component of the pressure field arising from nonlinear wave interactions in a shallow water environment. The contribution of this component is shown to be very dependent on the properties of the bottom structure, especially the rigidity of the top sedimentary layer and the shear-wave velocities of the sublayers. Calculations for a typical six-layered bottom with an overlying unconsolidated sediment in a shallow water environment (100-m water layer) confirms that the inclusion of the inhomogeneous component of the pressure field can raise the total seismic spectral level up to 30–40 dB, as indicated recently by Schmidt and Kuperman [J. Acoust. Soc. Am. 84, 2153–2162 (1988)] in an analysis of ‘‘bottom magnification.’’ When the bottom is composed of a solid half-space, however, this contribution is found to drop to only a few dB. It is also demonstrated that for the multilayered model being discussed the contribution from the inhomogeneous component drops quickly and by as much as 30 dB, when the observation point is moved outside the active region. These results confirm the interpretation of the ULF noise-source levels, reported earlier by Kibblewhite and Ewans [J. Acoust. Soc. Am. 78, 981–994 (1985)] on the basis of onshore microseismic spectra, and show why they are reasonably representative of the wind-dependent ULF noise spectra observed in deep water.