Narrow Results

The accuracy of the seismo-acoustic parabolic equation is tested for problems involving sloping solid–solid interfaces and variable topography. The approach involves approximating the medium in terms of a series of range-independent regions, using a parabolic wave equation to propagate the field through each region, and applying a single-scattering approximation to obtain transmitted fields across the vertical interfaces between regions. The accuracy of the parabolic equation method for range-dependent problems in seismo-acoustics was previously tested in the small slope limit. It is tested here for problems involving larger slopes using a finite-element model to generate reference solutions.

The accuracy of the seismo-acoustic parabolic equation is tested for problems involving sloping fluid–solid interfaces. The fluid may correspond to the ocean or a sediment layer that only supports compressional waves. The solid may correspond to ice cover or a sediment layer that supports compressional and shear waves. The approach involves approximating the medium in terms of a series of range-independent regions, using a parabolic wave equation to propagate the field through each region, and applying single-scattering approximations to obtain transmitted fields across the vertical interfaces between regions. The accuracy of the parabolic equation method for range-dependent problems in seismo-acoustics was previously tested in the small slope limit. It is tested here for problems involving larger slopes using a finite-element model to generate reference solutions.

Single-scattering operators are used to extend the seismo-acoustic parabolic equation to problems involving transitions between areas with and without ice cover, which are common in the marginal ice zone. Gradual transitions are handled with single-scattering operators for sloping fluid–solid interfaces. Sudden transitions, which may occur when the ice fractures and drifts, are handled with a single-scattering operator that conserves normal displacement and tangential stress across the vertical interfaces between the range-independent regions that are used to approximate a range-dependent environment. The approach is tested by making comparisons with a finite-element model for problems involving range-dependent features in the ice cover and in a sediment that supports shear waves.

This paper reviews some of the highlights of selected topics in ocean acoustics during the thirty years that have passed since the founding of the Journal of Theoretical and Computational Acoustics. Advances in computational methods and computers helped to make computational ocean acoustics a vibrant area of research during that period. The parabolic equation method provides an unrivaled combination of accuracy and efficiency for propagation problems in which the bathymetry, sound speed, and other environmental parameters vary in the horizontal directions. The extension of this approach to cases involving layers that support shear waves has been an active area of research throughout the thirty year period. Interest in basin-scale and global-scale propagation was stimulated by the Heard Island Feasibility Test for monitoring climate change in terms of changes in travel time that occur as the temperature of the ocean rises. Diminishing ice cover in the Arctic, which is one of the consequences of climate change, has stimulated renewed interest in Arctic acoustics during the past decade. Reverberation is a challenging problem that was the topic of a major research program during the beginning of the thirty year period. An innovative approach for making it feasible to solve such problems was applied to data for reverberation from the seafloor and from schools of fish, and some of the findings were featured in Science and Nature. Source localization is one of the core problems in ocean acoustics. When applied on a 2-D array of receivers, an approach based on the eigenvectors of the covariance matrix is capable of separating the signals from different sources from each other, determining when this partitioning step is successful, and tracking sources that cross each other in bearing; one of the advantages of this approach is that it does not require environmental information or solutions of the wave equation. Geoacoustic inversion for estimating the layer structure, wave speeds, density, and other parameters of ocean bottoms has also been a topic of interest throughout the thirty year period.