Estimation of seafloor properties in shallow oceans using minimally processed vertical directivity of surface-generated ambient noise

Abstract

Typically, data from an array with vertical aperture will be beam-formed to provide the level and variation of spectrally averaged ocean ambient noise as a function of vertical angle. This paper considers the potential for such measurements to provide information about the acoustical properties and layering of a seafloor in a shallow ocean. First, with the assumption of dipole surface sources, it is feasible to describe the ambient noise in the near-horizontal in terms of the Weston α parameter (Weston, J. Sound Vib. 18, 271–287, 1971). Furthermore, measurements of ambient noise in the positive and negative vertical directions may be used to provide a spectrally averaged determination of bottom loss. With the presence of a surficial seafloor layer, each of the near-horizontal and near-vertical noise intensities will exhibit spectral variation. Techniques are described by which such relative variations in noise directivity, over a frequency span of some octaves, enable a reasonable estimation of, first, the thickness of a seafloor layer, and second, the acoustical properties of the layer and the seafloor basement.