Abstract
At high sea states and low grazing angles, it has been hypothesized that surface reverberation is dominated by backscatter from the bubble layer beneath the sea surface. Assuming the hypothesis is true, a simple stochastic bubble-layer backscatter model has been used to analyze surface reverberation data from four different deep-ocean areas. In every area, the inferred wave-number spectrum for the horizontal structure of the bubble layer is an inverse power law of the form P(K)=AK−β, where A is a constant, K is the horizontal wave number of the structure, and 3≲β≲4. The existence of power-law wave-number spectra indicates that the horizontal structure of the bubble layer is fractal. Using D=4−β/2 for the fractal dimension, 2≲D≲2.5 is obtained for measurements analyzed so far. Fractal structure in the bubble layer implies that horizontal inhomogeneities exist on a wide range of scales and possess scale invariance. Hence, on the average, the medium looks the same at all scales between some ‘‘inner scale’’ and ‘‘outer scale.’’ (Presently, the inner and outer scales are not known.) In this paper some physical processes are considered that could generate fractal structure in the bubble layer and some experimental means are suggested for identifying the processes. [Work supported by ONR.]
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