Effects of Internal Waves on Low Frequency, Long Range Acoustic Propogation in the Deep Ocean

Jinshan Xu, Ph.D., 2007
Tim Duda, John Colosi, Advisors

This thesis covers a comprehensive analysis of long-range, deep-ocean, low-frequency, sound propagation experimental results from the North Pacific Ocean. The statistics of acoustic fields after propagation through internal-wave-induced sound-speed fluctuations are explored experimentally and theoretically. The thesis investigated the environmental data by exploring the space-time scales of ocean sound speed variability and the contributions from different frequency bands. The results impose hard bounds on the characteristic scales of sound speed fluctuations in this region for both internal-wave and mesoscale band fluctuations. The thesis then analyzed the low frequency, broadband sound arrivals obtained in the North Pacific Ocean. The observed acoustic variability is compared with predictions based on the weak fluctuation theory, and direct parabolic equation (PE) Monte Carlo simulations. The comparisons show that a resonance condition exists between the local acoustic ray and the internal wave field such that only the internal-waves whose crests are parallel to the local ray path will contribute to acoustic scattering. This is the first observational evidence for the acoustic ray and internal wave resonance. Finally, the thesis examined the evolution with distance, of the acoustic arrival pattern of the off-axis sound source transmissions in the Long-range Ocean Acoustic EXperiment. The observations of mean intensity time-fronts are compared to the deterministic ray, PE and normal mode calculations. It is found the diffraction effect is dominant in the shorter range. In the longer range, the scattering effect smears the energy in both the spatial and temporal scales, and thus has a dominant role in the finale region.