Sea Technology

JUN 2017

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12 st / June 2017 PCD and APL-UW have placed specif- ic emphasis on making volume CSAS work reliably without the use of any external navigation aids. The result is that these scans are simple and efficient to perform, and in only three field tri- als several dozen multipass scans have already been collected, in a variety of environments and around targets as di- verse as UXO, plastic and steel barrels, lobster pots, an airplaneā€”even a table lamp. Other sonars, such as multibeam systems, are capable of generating volumetric imagery; however, to attain similar resolution would require oper- ating at very high frequencies and very close ranges. CSAS volume images can be generated from remarkably long standoff distances by virtue of the large synthetic apertures and relatively low frequencies used by these systems. Fur- thermore, in contrast to high-frequency systems, the use of comparatively low frequencies in SAS systems often en- ables target penetration, frequently re- vealing internal structure and possibly opening up some avenues for remote underwater nondestructive testing and evaluation (NDTE). As an example, a scan of a wooden lobster pot was re- cently conducted, and from cutaways of the volume image several key inter- nal features could be resolved, such as the bait spike, netting and inlet rings. As in medical tomography, the interior and exterior features of these objects can be visualized using programs that render the data cubes with transpar- ency or visualized as slices through the image. Multipass volume imaging also solves another problem faced by lin- ear SAS imaging, namely, the geomet- ric distortion caused by projecting all scatterers onto a common slant plane. This can also cause problems for inter- ferometry, which makes certain sim- plistic assumptions about the distribu- tion of scatterers within a given range cell. In contrast, large synthetically generated multidimensional arrays are able to generate dimensionally correct volume images and resolve complex scattering distributions in both the ver- tical and horizontal dimensions. 3D CSAS volumetric imaging could also be used to enhance tactical aware- ness by providing ground truth for the distribution and configuration of tar- gets of interest. Field trials have shown that information such as target orien- tation and burial depth can be readily determined from these high-resolution volume images. This kind of informa- tion can be used to plan mission-criti- cal tasks such as remediation efforts or threat mapping for follow-on neutral- ization, or used for obstacle avoidance during marine navigation. In addition, the volume images can also be used for generating 3D-printed models, which can provide intuitive informa- tion about targets or a maritime scene of interest. Lastly, there are many other data products that can be obtained from the large multipass arrays used to gener- ate volume images. For example, us- ing these arrays, it is possible to make high-resolution maps of the temporal changes in the seabed. The seabed can evolve rapidly with time due to biological activity and other physi- cal processes. Volumetric imaging, by virtue of the relatively long amount of time required to image scenes (e.g., anywhere from 15 minutes to more than an hour) can provide the ability to monitor the temporal evolution of the seabed, tracking such things as benthic organism activity, while also provid- ing data to support acoustic seafloor modeling efforts and understanding of sediment scattering mechanisms and characterization. Development and Field Testing In order to provide insight on the practicality and feasibility of long- range 3D volumetric sonar imaging using an AUV platform, the Office of Naval Research (ONR) and U.S. De- partment of Defense Strategic Envi- ronmental Research and Development Program (SERDP) leveraged sponsor- ship of a multiyear project investigating this technology. The project utilized a "crawl, walk, run" technical approach which, in the initial funding year, used data collected in a controlled environ- ment by means of the NSWC PCD's circular rail system to develop co-reg- istration and three-dimensional beam- forming algorithms. This rail system, which is immersed in a large artificial test pond, uses a sonar tower mounted on a circular rail trolley cart to inson- ify the enclosed sediment and targets. The tower has a six-element (vertically spaced) 1-m receiver array, which is used to form a vertical aperture. Addi- tionally, interchangeable staves can be used to raise and lower the array, fa- cilitating insonification over disparate

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