Fluctuation Statistics of Sea-Bed Acoustic Backscatter

A theoretical model is developed that applies to statistics of sea-bed backscatter, including non Rayleigh envelope distribution, finite reverberation envelope auto-correlation at 1ag >1 pulse width, and pulse-pulse envelope correlation. The model is based on a two-phase patch surface assumption, generated by a Markov process, and defined through parameters ρ = high scatter area fraction, γ = high/low backscatter strength ratio, and λ = effective resolution cell 1 patch size ratio. Experiments at 1.8, 4.1 and 8.1 kHz with beamwidth 3.6° to 17° at 5 sites and 3 range bands to minimum grazing angle 2.5° in 70 to 160 m water depth are analysed, indicating non-Rayleigh envelope distributions, power envelope autocorrelations of 0.1 to 0.4 at >1 pulse lag, as expected. Close experimental-model fitting requires ρ = 10-4 to 2.10-2, γ = 3.5 to 80, and a patch dimension of 100 to 400 m. Factorial analysis on model fitting parameters, within experimental sensitivity, indicated beam-width dependence of statistics as predicted; site, grazing angle and site-frequency dependence were all significant at <1% level; simple frequency dependence was not significant.