A Numerical Study of the Regimes of Validity of Weak Fluctuation Theory for Ocean Acoustic Propagation through Random Internal Wave Sound Speed Fields

Results of the ATOC project’s AET experiment have shown that at 75 Hz Rytov theory may be used for predicting the phase variations. This paper is focused on establishing the regimes of validity for Rytov theory at 75-400 Hz acoustic frequency range and up to 200 km distance. Ray paths correspond to grazing angles of 0°,5°,10° and 14° are considered, thus spanning the range of possible ray geometry from surface reflection to axial propagation. We find 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 on the transition frequency between 1 and 0.2 cph. For frequencies less than the transition frequency the Rytov spectra are in fairly good agreement with the simulations for all ranges and grazing angles between 0º and 10º. For the 14º beam the Rytov theory dramatically under predicts the spectral energy at frequencies less than 1 cph. When there is significant variability in phase and log-amplitude, we also find that 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 will increase the interest in the weak fluctuation theory (WFT) as an acoustic prediction tool.