THE VARIABILITY OF HIGH-FREQUENCY ACOUSTIC BACKSCATTER FROM THE REGION NEAR THE SEA-SURFACE

The temporal variability of acoustic backscattering from the region ne ar the sea surface is examined for frequencies in the 30-70 kHz range. A variance spectrum of the scattering strength exhibits effects assoc iated 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 le vel characteristic of a Gaussian backscattered pressure field from ran domly moving bubbles within the scattering volume. The overall variabi lity is treated as a slow modulation of this Gaussian process by large r-scale processes and a probability density function is derived for th e scattering strength using Bayes' theorem. Finally, in some cases, th e spectrum exhibits a peak at the frequency of the dominant surface wa ves. Attempts to compute coherence functions between the backscattered acoustic power and surface wave orbital velocities, measured by a mic rowave system observing the same spot as the acoustic system, resulted in very low values. This leads to the belief that the wave-induced pe ak in the acoustic backscatter variance spectrum is caused by highly n onlinear processes. A time series of acoustic backscatter from a verti cally pointing system confirms the existence of this modulation at the dominant wave frequency and also suggests its nonlinear character. (C ) 1997 Acoustical Society of America.