Long-range acoustic propagation through internal wave fields.

Abstract

Sound can travel long distances through the deep ocean due to a naturally formed waveguide. Internal waves, similar to ocean surface waves but occurring throughout the ocean depths, are believed to be the main cause of perturbation to propagating acoustic waves. A Munk profile, to model the deep-ocean waveguide, and a set of propagating internal-wave modes are used to construct a three-dimensional, time-varying ocean sound speed model. The purpose of this work is to study the effects of internal waves on acoustic propagation. Three-dimensional ray tracing techniques are developed to simulate long-range sound propagation of low-frequency broadband acoustic signals. These methods combine the computational efficiency and extendibility to full 3D ocean models of ray tracing and accurate modeling of low-frequency broadband acoustic propagation previously only contained in other more computationally intensive solutions of the wave equation. The internal wave induced sound speed variations are modeled deterministically and the model acoustic receptions are analyzed deterministically. A single internal-wave mode that is spatially synchronized to an arrival can coherently focus and defocus the acoustic energy. Comparisons of two-dimensional and three-dimensional computational ocean model results show significant differences in the internal wave induced acoustic amplitude fluctuations, suggesting three-dimensional computational ocean models may be required for certain analysis of the acoustic reception. These internal waves can cause an arrival's amplitude fluctuation to mimic Rayleigh Fading; however, the time-domain phase is stable, in contradiction to the classical Rayleigh Fading environment where the received phase is uniformly distributed. For example, the received power attributed to an early arrival propagated over a 750 km range can fluctuate over 40 dB while the time-domain phase remains within a quarter of a 75-Hz cycle. The characteristics of the time-domain phase are important for establishing coherent integration times at the receiver. The effects of acoustic diffraction are analyzed for long-range, deep-ocean propagation and a ray tracing based method is developed for modeling diffracted wavefronts. Range limitations for deep ocean acoustic propagation through internal wave fields are established based on the overall strength of the internal wave field. It is shown that the internal wave propagation direction of a spatially synchronized internal wave may be determined from the received acoustic amplitude along a 2D array of hydrophones for simulated results.