Motion tracking of fish and bubble clouds in synthetic aperture sonar data.

Data captured by a Synthetic Aperture Sonar (SAS) near Mobile Bay during the 2021 Undersea Remote Sensing experiment funded by the Office of Naval Research reveals near surface bubble clouds from wave breaking events and a large aggregation of fish. Tools developed for using SAS data to image hydrodynamic features in the water column were applied to observations of the bubble clouds and fish aggregation. Combining imagery and height data captured by the sonar array with a detection and tracking algorithm enables the trajectories, velocities, and behavior of fish in the aggregation to be observed. Fitting the velocity and height data of the tracked objects to a Gaussian mixture model and performing cluster analysis enables an estimate of the near-surface ambient velocity via observation of the movement of the bubble traces and the general direction of motion of the fish aggregation. We find that the velocity traces associated with bubbles are consistent with ambient currents as opposed to the direction of propagating wave crests while velocities of fish indicate relatively large, pelagic species.

Backscattering by a tilted intermediate thickness cylindrical metal empty shell in water.

Backscattering by metal shells in water was investigated by Morse, Marston, and Kaduchak [J. Acoust Soc. Am. 103(2), 785-794 (1998)]. The evolution of the backscattering as a function of frequency and tilt angle, the "acoustic color," depended on the shell's thickness. The present study concerns scattering by a shell with an intermediate thickness-to-radius ratio (0.106). The results include: (a) meridional rays associated with a supersonic antisymmetric guided wave cause a prominent tilt-angle dependent enhancement at frequencies above the coincidence frequency; (b) a subsonic guided wave is also prominent over a range of frequencies and tilt angles; and (c) meridional ray contributions are evident in the time-frequency-angle domain.