Abstract:
A normal mode description is often useful for interpreting long-range propagation in deep water. For modal travel time measurements to be effective in applications such as acoustic thermometry, an understanding of the effects of interactions with internal waves is necessary. In this work, individual realizations of an ocean with a GM internal wave spectrum are generated. A wide-angle, finite-element PE code is used to provide the full-wave acoustic solution, which is then projected onto a set of mode shapes to obtain the complex mode coefficients. Particular attention is paid to the phase accuracy of the coefficients, a potential problem area in propagating the PE to very long ranges. The sensitivity of the coefficients to the choice of mode basis set (local modes versus a background mode set) is demonstrated. The main area of focus is understanding the resulting time spread of the individual modal arrivals. While most of the spread is due to mode coupling, a portion of it results from ``adiabatic dispersion,'' where a single mode sees different parts of the water column as frequency varies over the source spectrum. The general frequency dependence of the modal propagation and coupling is investigated as well. [Work supported by ONR.]