Improving a practical broadband adiabatic normal mode model by including untrapped modes

An adiabatic normal mode model that neglects untrapped mode contributions will give inaccurate transmission loss predictions for environments with bottom slopes when modes transition between being trapped and untrapped. For example, initially untrapped, but strongly excited, modes that subsequently become trapped away from the source may contribute significantly to water column fields. Results of Tindle and Zhang [Oceans ’93 Proceedings, I‐81] show that including untrapped modes in an adiabatic description significantly improves transmission loss predictions for the ASA benchmark wedge problem. A practical implementation of an adiabatic broadband propagation model for both trapped and untrapped modes will be described. Difficulties associated with the transition region between trapped and untrapped modes near the critical angle will be discussed, and the results of inserting a gradient in the lower half‐space to solve the problem will be presented. [Work supported by ONR.]