The effect of internal waves on long range acoustic propagation in the ocean

Abstract

The acoustic-internal wave interaction in an acoustic waveguide is investigated using wave techniques. Refractive index fluctuations due to the vertical displacements of the internal waves create an inhomogeneous waveguide. The analysis uses weak scattering theory based upon the Rytov perturbation technique. It is found that the internal wave field acts as a diffraction grating in permtting only certain scattered acoustic waves to propagate through the waveguide. Since the internal waves are continuously distributed in wavenumer space, the acoustic fluctuations become a statistical average with a bias toward particular spatial internal wavelengths. The multimode nature of acoustic propagation precludes the linear relationship of internal wave statistics to acoustic amplitude and phase fluctuations. Assuming statistical independence between amplitude and phase fluctuations within a mode and between different modes, it is shown that the total phase-rate fluctuation is a weighted sum of the phase-rate fluctuations in the individual modes. Using a statistical internal wave model [C. Garrett and W. Munk, Geophys. Fluid Dynam., 2, 225-264 (1972)] predictions of acoustic fluctuations are made. Over much of the internal wave frequency band the slope of the phase rate frequency spectrum is between -0.5 and -1.0. The depth dependence for the mean-square phase rate fluctuation has been found. Largest fluctuations occur for shallow and deep receivers. The predicted fluctuations compare favorably with experimental data.