Estimation of geoacoustic models of the seabed from bottom loss measurements

Abstract

Measurements of bottom loss obtained in deep water sites in the S.W. Pacific have been used to infer geoacoustic models of the ocean bottom. The measurements were grouped into two types according to the acoustic reflectivity at frequencies below 100 Hz. The first type was characteristic of deep ocean basins and the second was typical of shallower regions of the continental margin. This classification was supported by archival seismic and nonacoustic information that indicated that the geology and the geophysical properties of the sediment were significantly different for each type. The former was composed of a layer of silty‐clay material overlying the ocean crust, whereas the latter consisted mainly of sandy turbidite layers. A ray model, which was developed to simulate bottom loss in thin sediment layers, was used to estimate values of the geoacoustic properties for each bottom type by comparing model results with the measurements. The data at low frequencies were adequately described by simple geoacoustic models consisting of a layer of sediment with a constant sound‐speed gradient overlying a solid substrate. For the first bottom type the substrate material was basalt, whereas for the second type the substrate was consolidated sediment. Results are presented for an example of each bottom type.