Narrow Results

Today's minimum requirements for ocean acoustic models are to be able to simulate broadband signal transmissions in 2D varying environments with an acceptable computational effort. Standard approaches comprise ray, normal mode and parabolic equation techniques. In this paper we compare the performance of four broadband models on a set of shallow-water test environments with propagation out to 10 km and a maximum signal bandwidth of 10–1000 Hz. It is shown that a computationally efficient modal approach as implemented in the model is much faster than standard, less optimized models such as and . However, the handling of range dependency in the adiabatic approximation is not always sufficiently accurate, and it is suggested that a mode coupling approach be adopted in . Moreover, the interpolation of modal properties in range could lead to a further significant speed-up of mode calculations in range-dependent environments. It is concluded that coupled modes with wavenumber interpolation in both frequency and range remain the most promising wave modeling approach for broadband signal simulations in range-dependent shallow water environments. At higher frequencies (> 1 kHz) there is currently no alternative to rays as a practical signal simulation tool.

Parabolic equation models in 3D usually apply the "staircase" approximation to general range–varying interfaces between adjacent layers. This is the simplest technique available: it consists in neglecting range and azimuthal derivatives in the associated interface conditions. Our aim in this paper is to analyze the influence of the stair-step approximation technique, common to most 3D PE models, on a one-way sound wave propagation problem. We present a new finite-element 3D narrow-angle PE model which accurately treats the variable interface conditions. This is accomplished by using (i) an appropriate parabolized condition of the same aperture as the parabolic equation used, and (ii) a new change-of-variable technique which does not require any homotheticity condition of the layers as in previous works. Numerical simulations for the 3D wedge problem are presented. The convergence of the numerical solutions with respect to the azimuth is investigated. Unlike other 3D PE models working in cylindrical coordinates, the convergence tests have been carried out using a range-dependent number of points in azimuth. Numerical solutions obtained with the newly developed model are compared with a reference solution based on the image source and with a solution obtained with a 3D PE model that uses a stair-step technique.