The variability of high-frequency acoustic backscatter from the region near the sea surface

The temporal variability of acoustic backscattering from the region near the sea surface is examined for frequencies in the 30–70 kHz range. A variance spectrum of the scattering strength exhibits effects associated with three different processes. Below about 0.1 Hz, the spectrum contains a large contribution associated with temporal variations in the advection of bubble clouds through the measurement volume by large-scale processes. At high frequencies, the spectrum asymptotes to a level characteristic of a Gaussian backscattered pressure field from randomly moving bubbles within the scattering volume. The overall variability is treated as a slow modulation of this Gaussian process by larger-scale processes and a probability density function is derived for the scattering strength using Bayes’ theorem. Finally, in some cases, the spectrum exhibits a peak at the frequency of the dominant surface waves. Attempts to compute coherence functions between the backscattered acoustic power and surface wave orbital...

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