Island diffraction and other three-dimensional effects in global acoustics

Abstract

The adiabatic mode parabolic equation (PE) is an efficient approach for solving three-dimensional ocean acoustics problems [M. D. Collins, J. Acoust. Soc. Am. 94, 2269–2278 (1993)]. This method is used to solve the outgoing wave equations for the mode coefficients, which depend on range and azimuth. The range- and azimuth-dependent wave numbers appearing in these equations correspond to the eigenvalues. The adiabatic mode PE is applied to solve global-scale problems involving diffraction around islands and other three-dimensional effects. The significance of azimuthal coupling is illustrated by comparing solutions generated both with and without azimuthal coupling. The coupling azimuth solutions predict that the shadow zone behind an island is wider than uncoupled azimuth solutions predict. Since the modal phase velocities increase as ocean depth decreases, this result is consistent with the expected result based on ray theory. Adiabatic mode PE solutions are in qualitative agreement with observations from the Perth–Bermuda experiment [Heaney etal., J. Acoust. Soc. Am. 90, 2586–2594 (1991)] and the Heard Island Feasibility Test [Baggeroer and Munk, Phys. Today 45(9), 22–30 (1992)]. a)Permanent address: Planning Systems, Inc., McLean, VA 22102.