investigation of the acoustic field in wedge-shaped scale models of continental shelf regions

The acoustic field in the ocean is difficult to model theoretically, due to the complexity of the environment. This is particularly true if the water depth is range dependent, such as in the coastal region, where a fully three dimensional description is required. Propagation effects result in horizontal refraction, shadow zones, and the existence of regions with strong interference patterns. As a result, all of the existing theoretical models are based on significant simplifying assumptions. One such assumption is to model a region of the ocean as a water column overlying a planar sloping bottom. To test the accuracy of these theories model scale measurements of the acoustic field under highly controlled conditions have been undertaken in this study. Two experiments were performed on models with a sloping bottom. The first model consisted of a fast fluid bottom, and the second model consisted of a thin epoxy layer, to model a sediment, overlying a concrete layer, which modelled the substrate rock. The measurements performed included pulse, CW traverse, and depth profile measurements in both the across slope and down slope directions, in order to demonstrate the three dimensional features of the field. The features of the results are discussed and where possible are compared with existing theories. The results indicated that the three dimensional propagation effects in a fluid bottom wedge are described accurately by a theoretical model which uses an effective depth correction. No three dimensional theory was available for the shear wave supporting bottom case but the fluid bottom theory was found to provide accurate predictions. Down slope propagation over a shear wave supporting bottom was also shown to be accurately predicted using a two dimensional finite element parabolic equation code.