Acoustic Propagation in a Wedge-Shaped Ocean with Perfectly Reflecting Boundaries
Interim rept. Dec 1982-Sep 1983
NAVAL RESEARCH LAB WASHINGTON DC
Pagination or Media Count:
A new solution for the acoustic field produced by a point source in a wedge-shaped ocean channel with pressure-release boundaries is presented. The solution is in the form of a sum of normal modes, which reduces in the immediate vicinity of the source point to the free-field solution for a point source. The radiation field associated with each mode forms a well-defined beam which diverges as the energy propagates out towards deep water. Outside the beam, shadow zones are formed, where there is essentially no energy in the mode. The modal beams are nested together, with the inner and outer beams having the highest and lowest mode numbers, respectively. Thus, the spacial extent of the field in the direction parallel to the shore line is determined by the lowest order mode. The modal beams are interpreted in terms of rays, by invoking the concept of raymode duality. The criterion for a ray to correspond to a mode is given. Each ray path undergoes curvatures in the horizontal direction, or horizontal refraction, due to the multiple acoustic interactions with the inclined boundaries of the wedge, and the rays corresponding to a given mode are found to be constrained to fall precisely within the modal beam. From simple geometric arguments, these modal rays cannot extend beyond the beam, and shadow zones are formed.