Pulse propagation at long ranges in an underwater surface duct

Abstract

In a previous communication, we have presented a hybrid-ray-mode theory for coping with convergence difficulties occurring near the source depth in the ray acoustic model of long range propagation in a surface duct. The hybrid formulation replaces the problematic acoustic ray fields with a uniquely defined group of surface ducted modes, while retaining the legitimate ray fields intact. Detailed numerical calculations for a high frequency Gaussian pulse propagating in a model environment with exponential velocity profile in depth have now revealed the inherent features of the hybrid scheme. With a solution obtained by modal summation as a reference, the hybrid form is found to be accurate and numerically efficient. It is also capable of explaining in cogent physical terms the features of the received signal by blending automatically and self-consistently the ray arrivals dominant at early times with mode arrivals dominant at later times. The interpretations follow from separate examination of ray fields and mode fields, with clear indications of ray field failure in the transition region noted above. [Work supported by ONR Underwater Acoustics.]