Journal of Computational Acoustics

The effect of uncertainty in the parameters that define the experimental geometry is investigated to determine the performance of matched-field inversion for geoacoustic properties. The investigation is carried out using simulated data for a vertical line array in a North Pacific environment. The results indicate that there can be significant degradation of inversion performance for small errors (~1/3 wavelength) in source depth, depth and tilt of the array, ocean depth and source/array range. The relatively high sensitivity to mismatch in experimental geometry suggests that uncertainties of this type can be tolerated in the inversion by including the geometrical parameters in the global search algorithm. The simulations show that performance is improved by inverting for both the ocean bottom properties and the geometrical parameters simultaneously.

A set of experiments were performed in the offshore area off the coasts of Taiwan and three-dimensional (3-D) measurements recorded. The 3-D effect on underwater propagation due to azimuthal variation of bottom topography is studied for the offshore regions southwest of Taiwan, where submarine canyons exist. A 3-D acoustic propagation model, FOR3D, is used to detect the 3-D effect. Computational results show that the 3-D effect is more prominent along the axis of the canyon than across it. Calculations show a very good agreement with field data, which indicate that the 3-D effect exists in this realistic ocean environment.

A method is developed for increasing the efficiency of broadband parabolic equation modeling. For broadband adiabatic mode calculations, the smoothness of the wavenumbers and mode functions in frequency permits interpolation across frequency. In order to permit frequency interpolation, the pressure field computed by the PE is transformed to mode space and the complex mode amplitudes are interpolated across frequency. For mildly range dependent environments a speed increase of a factor of ten can be achieved. A convergence test of propagation through internal waves indicates that a frequency interpolation across four frequencies is possible.

Nonstandard finite difference schemes offer the potential for either constructing exact discrete models of differential equations or obtaining discrete models that do not have the elementary numerical instabilities. While the general laws for constructing such schemes are not precisely known at the present time, a number of important rules have been discovered. This paper provides an introduction to the nonstandard finite difference rules, explains their significance, and applies them to several model ordinary and partial differential equations. Several major unresolved issues and problems are briefly discussed.

The theory of antisymmetric localized elastic modes propagating along edges of immersed wedge-like structures is developed using the geometrical-acoustics approach to the description of flexural waves in elastic plates of variable thickness. The velocities of these modes, often called wedge acoustic waves, are calculated using solutions of the dispersion equation of the Bohr-Sommerfeld type following from the geometrical-acoustics description of localized wedge modes. In a subsonic regime of wave propagation, i.e. for wedge modes slower than sound in liquid, the influence of liquid loading results in significant decrease of wedge wave velocities in comparison with their values in vacuum. This decrease is a nonlinear function of a wedge apex angle θ and is more pronounced for small values of θ. In a supersonic regime of wedge wave propagation, a smaller decrease in velocities takes place and the waves travel with the attenuation due to radiation of sound into the surrounding liquid. The comparison is given with the recent experimental investigations of wedge waves carried out by independent researchers.