Internal Wave Induced Fluctuations of Low-Frequency Long-Range Acoustic Propagation

Abstract

Some of the fluctuation of acoustic signals at long ranges is due to the passage of acoustic radiation through the internal gravity wave field. A ray theory is presented that predicts the amplitude and phase of the acoustic field perturbed by the internal wave field. A postulated internal wave model [C. Garrett and W. Munk, Geophys. Fluid Dynam. 2, 225–264 (1972)] is used to estimate the perturbation of the sound-velocity field. The acoustic-gravity interaction is assumed concentrated near the top of the main thermocline. It is found that ray cycle lengths comparable to, or a multiple of, an internal wavelength induce large phase fluctuations at internal wave frequencies and high-frequency random amplitude variations. Based on the proposed internal wave spectrum, predictions of phase variations as a function of range, frequency, source, and receiver locations are possible. Comparison between measured and predicted acoustic fluctuations is discussed.