Three-dimensional propagation effects near the mid-Atlantic Bight shelf break (L)

Significant three-dimensional~3-D! environmental variability exists in the vicinity of the shelf break along the mid-Atlantic Bight. This study examines the influence of azimuthal coupling due to this variability on acoustic propagation in this region. Numerical studies employing a 3-D ray code, a hybrid ray-mode code, and a 3-D parabolic equation model are used to study the significance of azimuthal coupling on various propagation paths. These paths include up-slope, slant-slope, and cross-slope propagation. The numerical analysis suggests that, for the propagation ranges less than 60 km examined, the influence of azimuthal coupling is negligible compared to the inherent Continental slopes represent some of the steepest large- scale bathymetric features in the world's oceans, averaging 2-3 degrees and extending for several miles out from the continental shelf. The boundary between these regions and the shallow, littoral zones on the shelf is the shelf break, typically an area of significant oceanographic variability. Such three-dimensional environmental variability exists in the vicinity of the shelf break along the mid-Atlantic Bight and is characterized by relatively strong bathymetric slopes, cold shelf water interacting with warm Gulf Stream waters, and diurnal generation of internal solitary waves onto the shelf. The influence of such a highly variable environment on acoustic propagation is of great interest as is the impact on inversion techniques such as tomography and localization, and sonar system performance estimations. In the summer of 1996, the Office of Naval Research sponsored an acoustic field study, called the Shelf Break Primer, off the coast of New Jersey near the mid-Atlantic Bight. Details of the experimental design and data collection are provided by Lynch et al. ~1997!. This letter provides an overview of several different nu- merical propagation studies performed for this region. Spe- cifically, each study examined the predicted influence of azi- muthal coupling due to the three-dimensional character of the environment on acoustic propagation. A 3-D ray code was used to estimate horizontal refraction of propagation paths and differences in arrival times when contrasted with simple 2-D ray calculations. A hybrid ray-mode code was also used to investigate the influence of the refracted paths on mode amplitude and phase. The modes were assumed to be adiabatic in this treatment. Finally, a fully coupled 3-D parabolic equation model was used to investigate travel time differences and to determine the dominant mechanism for 3-D effects. II. ENVIRONMENTAL'EXPERIMENTAL DESCRIPTION