A numerical study of the validity regimes of weak fluctuation theory for ocean acoustic propagation through random internal wave sound speed fields
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Results of the Acoustic Thermometry of Ocean Climate (ATOC) project's experiments have shown that at 75 Hz, the Rytov acoustic scattering theory using the Garrett-Munk (GM) internal wave spectrum may be used for predicting the variations of log-amplitude and phase. Using Monte Carlo methods, this paper is forcused on establishing the regimes of validity of the Rytov theory within the 75-400 Hz acoustic frequency and for propagation ranges up to 200 km. Ray paths corresponding to grazing angles of 0 degrees, 5 degrees, 10 degrees and 14 degrees are considered, thus spanning the range of possible ray geometry from surface reflection to axial propagation. Investigations show that the Rytov and simulation spectra are in very good agreement in the frequency range from the buoyancy frequency up to a grazing-angle-dependent transition frequency between 1 and 0.2 cph. For frequencies less than the transition, the Rytov spectra are in fairly good agreement with the simulations for all ranges and grazing angles between 0 degrees anf 10 degrees. For the 14 degree beam, the Rytov theory dramatically underpredicts the spectral energy at frequencies less than 1 cph. When there is a strong variability in phase and log-amplitude, analysis shows significant spectral energy can exist at frequencies greater than the buoyancy frequency. This energy is not predicted by the Rytov model and represents the effect of strong interference and scattering not treated in the weak fluctuation approach of the Rytov theory. This study increases the relevance of the weak fluctuation theory (WFT) as an acoustic prediction tool.
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