Acoustic beam broadening by solitary bores during ASIAEx 2001 (South China Sea)

Abstract

Previous theoretical and modeling studies have predicted that acoustic interaction with anisotropic internal wave fields will introduce mode-order-dependent horizontal refraction, thereby limiting the resolution of horizontal receiving arrays. An experiment to observe this phenomenon was performed on the continental shelf southwest of Taiwan. A nonlinear, bottom-laid horizontal array received signals over an 18-day period from two sound sources (300 and 500 Hz) located 18 km to the northeast. Water depth along the propagation path was in the range 110–180 m. The prevailing downward-refracting sound speed profile was modulated by internal waves. At semidiurnal intervals strong internal bores originating in the Luzon Strait entered the propagation path at the source position, introducing strongly anisotropic sound speed perturbations. The wavecrests of the internal bores’ surface expression were oriented at typical angles 20–30 deg with respect to the acoustic propagation path. Aperture-limited signal beam widths (FWHM), nominally 1.5 deg, were broadened while the semidiurnal bores were in the acoustic propagation path. Details of the acoustic and oceanographic measurements and model calculations of beam broadening by the sound speed perturbations will be presented. [Work supported by ONR.]