Low frequency acoustic propagation over calcarenite seabeds with thin, hard caps.

Abstract

Much of Australia's continental shelf consists of a relatively soft limestone called calcarenite, which is variable in geoacoustic properties and covered by a thin veneer of unconsolidated sediment vanishing in some areas. Low frequency underwater acoustic propagation in such environments is strongly influenced by the geoacoustic properties of the calcarenite, which typically has a shear speed slightly lower than the sound speed in water. This often results in strong frequency dependence of the acoustic transmission loss with some frequency bands having a much lower transmission loss than the nearby frequencies. In some cases the upper part of the calcarenite consists of a thin (~1m) layer of hard, well-cemented calcarenite overlaying softer, semi-cemented layers. This paper considers the effect that this hard cap has on the acoustic reflectivity of the seabed and on the resulting acoustic propagation at frequencies sufficiently low that the upper, well-cemented layer is thinner than its shear and compressional wavelengths.