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.

Three-dimensional observations of tidal plume fronts in estuaries using a synthetic aperture sonar array.

Synthetic aperture sonar (sas) systems are designed to observe stationary scatterers located near the sediment interface. Less commonly, a sas system may be used to observe scattering features located above the sonar in the water column. The Undersea Remote Sensing (USRS) project, sponsored by the Office of Naval Research, was a collaborative Directed Research Initiative (DRI) focused on studying dynamic estuarine water column features. During the USRS DRI, researchers from multiple institutions gathered to observe tidal features at various estuaries along the coast of the United States using both in situ and remote sensing techniques, including sas. The first studied estuary was the mouth of the Connecticut River (CTR). Data captured by a sas system deployed during a tidal event were post-processed to create three-dimensional observations of the structure of the leading edge of the CTR's ebb plume front. From these observations, lobed structures similar in scale to previously reported instabilities are revealed, with the present observations providing additional insight regarding the structure of the bubble distribution behind the front. Velocity estimates of plume features were also determined from sas data and shown to compare favorably with concurrent marine radar estimates.

Omega-K beamforming: Practical ramifications of alternative formulations.

The Omega-K algorithm is widely used for creating imagery from synthetic aperture sonar and radar data. In early literature related to Omega-K beamforming, two alternative forms of the algorithm exist: one in which a critical matched-filtering stage is applied before Stolt-mapping and one in which it is applied after. The former was adopted as the standard approach in both fields. In the present study, it is demonstrated that applying the matched-filter prior to Stolt-mapping has the potential to alias acoustic energy associated with wide aperture angles, causing artifacts in imagery, whereas the alternative formulation avoids these artifacts.

High frequency imaging and elastic effects for a solid cylinder with axis oblique relative to a nearby horizontal surface.

The calibrated acoustic backscattering spectrum versus aspect angle, also called the "acoustic color" or "acoustic template," of solid cylinders located near a flat interface was previously studied for the case where the cylinder axis was vertically oblique relative to the interface and was insonified by a beam at a non-zero grazing angle. The presence of the interface allows for multiple paths by which sound is backscattered. These multipaths are highly dependent on the relative orientations of the target, the interface, and the source/receiver. In this work, the effects of vertical obliquity on the reconstructed synthetic aperture acoustic images is presented. Several robust orientation dependent features are considered and the physical mechanisms responsible identified through geometric arguments. Information about a target's three-dimensional orientation and size may be gleaned from these images, aiding the interpretation of features within the target's acoustic template. Specific features observed in the reconstructed image are associated with features in the acoustic color. The coupling conditions for surface elastic waves are also considered.

Circular synthetic aperture sonar imaging of simple objects illuminated by an evanescent wavefield.

This paper is motivated by the case where an underwater object located within the sediment is illuminated by a grazing acoustic beam below the critical angle. The included experimental work uses a liquid-liquid interface and vertically inverted geometry as a stand-in for the water-sediment boundary. In the super-critical regime sound in the water column refracts into the sediment before scattering. However, for sub-critical illumination a rapidly decaying evanescent wavefield is generated in the sediment near the water-sediment interface. For compact objects located in the sediment near the interface this can result in strong backscattering signals suitable for acoustic image reconstruction using synthetic aperture sonar techniques. Certain properties of the evanescent wavefield such as the vertical phase-locking behavior, the rapid amplitude decay with distance from the interface, and the low-pass filter effect have understandable ramifications for the image formation process and for characteristics of the reconstructed image. In particular, circular imaging techniques require correct placement of the imaging plane to properly focus an object; however, for backscattering (monostatic) evanescent image formation the imaging plane may be placed at the interface and the target will remain in focus regardless of burial depth. A laboratory experiment using simple scatterers is presented.

High frequency backscattering by a solid cylinder with axis tilted relative to a nearby horizontal surface.

The backscattering spectrum versus azimuthal angle, also called the "acoustic color" or "acoustic template," of solid cylinders located in the free water column have been previously studied. For cylinders lying proud on horizontal sand sediment, there has been progress in understanding the backscattering spectrum as a function of grazing angle and the viewing angle relative to the cylinder's axis. Significant changes in the proud backscattering spectrum versus the freefield case are associated with the interference of several multipaths involving the target and the surface. If the cylinder's axis has a vertical tilt such that one end is partially buried in the sand, the multipath structure is changed, thus modifying the resulting spectrum. Some of the changes in the template can be approximately modeled using a combination of geometrical and physical acoustics. The resulting analysis gives a simple approximation relating certain changes in the template with the vertical tilt of the cylinder. This includes a splitting in the azimuthal angle at which broadside multipath features are observed. A similar approximation also applies to a metallic cylinder adjacent to a flat free surface and was confirmed in tank experiments.

Fast nearfield to farfield conversion algorithm for circular synthetic aperture sonar.

Monostatic circular synthetic aperture sonar (CSAS) images are formed by processing azimuthal angle dependent backscattering from a target at a fixed distance from a collocated source/receiver. Typical CSAS imaging algorithms [Ferguson and Wyber, J. Acoust. Soc. Am. 117, 2915-2928 (2005)] assume scattering data are taken in the farfield. Experimental constraints may make farfield measurements impractical and thus require objects to be scanned in the nearfield. Left uncorrected this results in distortions of the target image and in the angular dependence of features. A fast approximate Hankel function based algorithm is presented to convert nearfield data to the farfield. Images and spectrograms of an extended target are compared for both cases.

Acoustic scattering from a water-filled cylindrical shell: measurements, modeling, and interpretation.

Understanding the physics governing the interaction of sound with targets in an underwater environment is essential to improving existing target detection and classification algorithms. To illustrate techniques for identifying the key physics, an examination is made of the acoustic scattering from a water-filled cylindrical shell. Experiments were conducted that measured the acoustic scattering from a water-filled cylindrical shell in the free field, as well as proud on a sand-water interface. Two modeling techniques are employed to examine these acoustic scattering measurements. The first is a hybrid 2-D/3-D finite element (FE) model, whereby the scattering in close proximity to the target is handled via a 2-D axisymmetric FE model, and the subsequent 3-D propagation to the far field is determined via a Helmholtz integral. This model is characterized by the decomposition of the fluid pressure and its derivative in a series of azimuthal Fourier modes. The second is an analytical solution for an infinitely long cylindrical shell, coupled with a simple approximation that converts the results to an analogous finite length form function. Examining these model results on a mode-by-mode basis offers easy visualization of the mode dynamics and helps distinguish the different physics driving the target response.