Progress in our understanding of forward acoustic scattering

Recent solutions of the parabolic fourth moment equations, pioneered by Shishov have led to good predictions of the power spectral density of temporal intensty fluctuations as meaured in the Cobb71 and MATE experiments for a wide range of scattering parameters. Comparison of Monte‐Carlo solutions of the parabolic wave equation with the above predictions for the vertical spatial fluctuations of intensity produces excellent agreement between simulations and theory. The discrepancies between experiment and theory which remain appear to be due to our lack of understanding of the index of refraction field itself. The intensity moments m_q, q > 2 of a propagating wavefield in a random medium exhibit log‐normal behavior at short ranges when observed as a function of m₂; subsequently they rise to a maximum in the second moment and then drop back at long ranges to the exponential moments. The shape of the moment curves and their point of departure from log‐normal are dependent on the specific correlation function of the index of refraction. Prediction of the higher moments using a model which spans the regime between log‐normal and exponential, and is based on the m₂ and m₃ moments as parameters, has proved quite successful at explaining the Cobb71, MATE, AFAR, and SW Bermuda results. Thus, if a theoretical prediction of m₃ can be obtained, the probability distribution and the spectral decomposition of the lower moments of intensity fluctuations due to forward scattering should be predictable in the parabolic approximation. The development of the theory‐experiment comparisons will be presented and so will the authors views on what else needs to be measured for a complete test of the current theories.