A coupled normal-mode approach to three-dimensional sound propagation including elastic effects from ocean bottom sediments

Sound propagation in the ocean has been extensively studied both experimentally and theoretically. Most theoretical models have been based on the two-dimensional environments, hence they do not consider azimuthal coupling and variation. The most recent developments of three-dimensional (3-D) ocean acoustic modeling are based on ray trace or PE methods that do not include shear waves in elastic layers of bottom sediments.;A new coupled normal-mode approach to 3-D sound propagation including elastic effects from ocean sediments has been devised. This method is based on a normal mode method which is able to tackle elastic layers in the ocean bottom and the coupled mode theory. The 3-D varying environment is divided into a mesh grid in horizontal, and layers in depth to meet the requirements of oceanography modeling and the accuracy of computation. The attenuation coefficients in seawater are neglected, but are three-dimensionally varied in the ocean bottom for both compressional and shear waves. In the case of a rigid boundary, the eigenvalue and eigenfunction problem is solved by a finite difference algorithm; while for the elastic sea floor, the local modes can be obtained by using propagating and matching matrix algorithm employed in normal mode theory. The complete acoustic field is then solved by integrating coupled equations for the envelopes of local mode. This new approach is the first method in the 3-D sound propagation problem that includes elastic effects from ocean bottom sediments.;Several satisfactory results of prediction are obtained from interpretations of both laboratory and field experiments. The laboratory experiment is a simulation of wave propagation in typical shallow water with elastic sediments, while the field experiments are conducted in deep ocean over a large range with complicated ocean bottom structure. The acoustic fields in 3-D environment are calculated by using realistic 3-D oceanography data of the north-east Pacific Ocean, and assuming sound speed variations in the elastic ocean bottom. The occurrence of significant azimuthal coupling is demonstrated. It follows that the 3-D ocean acoustic problem cannot be adequately solved without the use of a 3-D propagation model as it was employed here.