Surface and deep ducting of sound parallel to strong nonlinear internal wave fronts.

Abstract

Acoustic ducting effects observed in a nonlinear internal wave field on the shelf of the South China Sea northeast of Tung-Sha Island (water depth of ∼120 m) are investigated. Environmental measurements from stationary instruments in the area show that the nonlinear internal waves have large amplitudes (over 40 m) and strong curvature. A principal component interpolation method is used to reconstruct the time-dependent three-dimensional internal wave field. A three-dimensional parabolic equation model is then employed to simulate monochromatic 400-Hz acoustic propagation parallel to the internal wave fronts for comparison to field data. The passing internal waves alternately elevate and depress the thermocline to form strong three-dimensional near-bottom acoustic ducts between the wave troughs and strong surface ducts of mixed layer water in the wave troughs. These alternating, depth-dependent acoustic ducts create asynchronous, vertically-bifurcated sweeping beams of ducted acoustic energy. The three-dimensional acoustic propagation modeling results are shown to mimic the field observations to a strong degree and elucidate the propagation physics in this highly complex environment.