A finite element model of propagation on the Southern and Western Australian continental shelf

Much of the littoral region of Southern and Western Australia is composed of a soft limestone bed covered by a layer of unconsolidated sand. The limestone bed, composed of calcarenite, has a high shear wave speed and there is often efficient coupling between the water born wave and the shear mode. Although studies of the effect of the elastic mode in the calcarenite on transmission loss have been undertaken, the effects of the thin sand layer and interface roughness must be quantified in order to determine a robust inversion scheme. It is found that a 1.0 m sand layer decreases the transmission loss by more than 5 dB while a 2.5 m layer can decrease the loss by as much as 20 dB. Interface roughness affects higher frequencies by increasing transmission loss and a rough interface waveguide with a sand layer can have a similar level of transmission loss as a waveguide with a bare calcarenite bottom. However, the frequency dependence and model interference patterns of the two waveguides are different. An inversion scheme based on a bare calcarenite model which would lead incorrect results.