Three-dimensional acoustic effects due to ocean currents

Abstract

In the presence of currents, vertically stratified fluids become acoustically anisotropic in the horizontal plane and the group and phase velocities of the acoustic normal mode are generally not parallel. In this paper, the resulting 3-D effects and their implications for acoustic monitoring of ocean currents are discussed in terms of the normal modes. The sensitivity of various acoustic quantities to the vertical and in-plane and out-of-plane horizontal components of the flow velocity is estimated. Despite anisotropy, there is no azimuth coupling at long-range sound propagation in a stratified moving medium. When a waveguide varies in the horizontal plane, the 3-D effects due to current-induced anisotropy and horizontal refraction are superimposed. The relative importance of these two phenomena in shallow- and deep-water scenarios is analyzed by considering horizontal (modal) rays in the presence of currents. For the purposes of numerical simulations, a parabolic approximation is considered for sound in a moving fluid. A new parabolic equation is proposed for efficient numerical modeling of the 3-D acoustic effects in the ocean with currents. [Work supported by NSERC.]