Effect of a rough seabed on the spectral composition of deep ocean infrasonic ambient noise

Abstract

In deep water, the vertical separation of surface noise sources from the elastic bottom is too large for direct excitation of the experimentally observed seismic interface waves in the frequency regime 1–10 Hz. It is here proposed that these strong seismic components are generated by scattering at rough elastic interfaces in the bottom, thereby coupling the noise field produced by the primary surface sources into evanescent waves in the bottom. This hypothesis is investigated by combining a previously developed perturbation theory of rough interface scattering with a model for surface generated noise in a stratified ocean environment. The resulting theory predicts a spectral composition of the noise field near the seabed that is consistent with the spectral structure of relevant seismic noise data recently collected in the Pacific. The present analysis, in combination with earlier analysis of a data set gathered in a shallow-water region, makes it quite clear that waveguide propagation mechanisms have an extreme coloring effect on the spectral shape of ocean ambient noise; hence, these mechanisms must be subtracted from measured noise data before inferring the spectral level of surface noise sources.