Mean, variance, and temporal coherence of the 3D acoustic field forward propagated through random inhomogeneities in continental-shelf and deep ocean waveguides

摘要

When an acoustic field propagates through a multimodal waveguide, the effect of variations in medium properties induced by 3D random inhomogeneities accumulates by multiple forward scattering over range. This causes significant random fluctuations in the received field and greatly affects underwater acoustic sensing and communication systems, such as Ocean Acoustic Waveguide Remote Sensing(OAWRS). In order to characterize this effect, analytical expressions are derived for the mean, variance and temporal covariance of the acoustic field forward propagated through an ocean waveguide containing internal waves, fish shoals, wind-generated bubble clouds and krill. These expressions account for the accumulated effects of multiple forward scattering through temporally and spatially varying scatter function densities of the 3D inhomogeneities. In order to quantify the statistics of the scatter function densities, physical models and statistical descriptions of these inhomogeneities are developed.Acoustic field transmission through internal waves in both continental shelf and deep ocean waveguides is investigated. Stratified ocean models are used to describe physical and statistical internal waves properties. Simulations for a typical continental-shelf environment show that when the standard deviation of the internal wave displacement exceeds the acoustic wavelength, the acoustic forward field becomes so randomized that the expected total intensity is dominated by the variance field and lacks a the coherent interference structure beyond moderate ranges. This leads to an effectively saturated field that decays monotonically. It is found that 3D scattering effects become pronounced when the acoustic Fresnel width exceeds the cross-range coherence length of the internal waves. This leads to frequency and range-dependent power losses in the forward field that explains some of the attenuation observed in acoustic transmission through typical continental shelf and deep ocean waveguides.