Analysis of pulse propagation in a bottom-limited sound channel with a surface duct

Abstract

Measurements of acoustic transmission in a bottom-limited sound channel with a surface duct, are compared with model predictions using broadband ray, normal mode, fast-field, and parabolic approximation methods. At a range of 42 km, six surface-ducted arrivals are evident in the data set and are predicted by all models except simple ray theory. Surface-ducted propagation is compared to propagation without a surface duct. The surface duct is found to generate a series of modes which display a resonance between the highly dispersive surface reflected bottom reflected (SRBR) propagation and less dispersive refracted bottom reflected (RBR) propagation. The six arrivals contain energy from three types of propagation paths. Faster precursors are purely diffracted energy, while slower precursors have RBR and SRBR contributions. Ray theory predicts the SRBR contribution, but not the diffracted energy. The precursors are generated at discrete ranges where RBR and SRBR phase fronts exchange energy with the RSR phase front in the duct. Energy leaking from the duct also reenforces the slower precursors. Since range-independent models were used, mode coupling cannot be the cause of energy exchange.