Coupled-mode prediction of backscatter

The application of coupled-mode theory to ocean acoustic propagation and scattering requires that ideal boundary conditions be applied at the surface and within the seabed. The depth of the lower boundary imposes limits on the ability of coupled-mode models to treat propagation and scattering at high grazing angles. Selecting this depth to predict the contributions of the continuous spectrum in a range-independent two-layered waveguide is not practical, and other methods must be introduced to apply coupled-mode theory in the near field of a source. An example of up-slope propagation in a range-dependent waveguide in which backscatter is governed by ray steepening and reversal is also treated. With a careful choice of the depth at which the lower boundary condition is applied, an estimate of the backscattered field is obtained.