Vertical Incidence Backscatter and Surface Forward Scattering from Near-SurfaceBubbles

摘要

This report presents results of acoustic measurements of vertical incidencebackscattering and surface forward scattering from near-surface bubbles. Applications of this work are directed toward torpedo guidance control, and countermeasure technologies, such as detection and signal-processing constraints imposed by the near-surface environment. Acoustic measurements of the concentration and vertical extent of near-surface bubbles generated by breaking waves were made in the frequency rane 15-50 kHz. The vertical incidence measurements, which were interspersed with the surface forward scattering measurements, produced a vertical profile of volume scattering cross section due to bubbles that decreased exponentially with depth. By integrating this profile in depth, a direct estimate of integrated volume scattering strength sigma l was made, which is a measure of total bubble concentration. The surface forward scattering measurements show a loss for a single interaction. This loss, attributable to bubbles, is referred to as the surface bubble loss (SBL), and an infrared estimate of sigma l was obtained from the SBL through a transfer function that incorporates the total extinction cross section from bubbles. The vertical incidence backscattering data and the forward scattering data, interpreted in terms of scattering and absorption by resonant bubbles, did not produce comparable trends in frequency and wind speed functional dependence, and differed significantly in overall level. For example, the forward scattering data indicate bubble concentrations that are as much as an order of magnitude greater than those indicated by the vertical incidence measurements. A likely explanation for the observed differences between the two data sets discussed.