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ArticleMacroscopic observations of diel fish movements around a shallow water artificial reef using a mid-frequency horizontal-looking sonar(Acoustical Society of America, 2018-09-18) Lee, Wu-Jung ; Tang, Dajun ; Stanton, Timothy K. ; Thorsos, Eric I.The twilight feeding migration of fish around a shallow water artificial reef (a shipwreck) was observed by a horizontal-looking, mid-frequency sonar. The sonar operated at frequencies between 1.8 and 3.6 kHz and consisted of a co-located source and horizontal line array deployed at 4 km from the reef. The experiment was conducted in a well-mixed shallow water waveguide which is conducive to characterizing fish aggregations at these distances. Large aggregations of fish were repeatedly seen to emerge rapidly from the shipwreck at dusk, disperse into the surrounding area during the night, and quickly converge back to the shipwreck at dawn. This is a rare, macroscopic observation of an ecologically-important reef fish behavior, delivered at the level of aggregations, instead of individual fish tracks that have been documented previously. The significance of this observation on sonar performance associated with target detection in the presence of fish clutter is discussed based on analyses of echo intensity and statistics. Building on previous studies of long-range fish echoes, this study further substantiates the unique utility of such sonar systems as an ecosystem monitoring tool, and illustrates the importance of considering the impact of the presence of fish on sonar applications.
ArticleShallow Water ’06 : a joint acoustic propagation/nonlinear internal wave physics experiment(Oceanography Society, 2007-12) Tang, Dajun ; Moum, James N. ; Lynch, James F. ; Abbot, Philip A. ; Chapman, Ross ; Dahl, Peter H. ; Duda, Timothy F. ; Gawarkiewicz, Glen G. ; Glenn, Scott M. ; Goff, John A. ; Graber, Hans C. ; Kemp, John N. ; Maffei, Andrew R. ; Nash, Jonathan D. ; Newhall, Arthur E.Since the end of the Cold War, the US Navy has had an increasing interest in continental shelves and slopes as operational areas. To work in such areas requires a good understanding of ocean acoustics, coastal physical oceanography, and, in the modern era, autonomous underwater vehicle (AUV) operations.
ArticleHigh-frequency bistatic scattering by sub-bottom gas bubbles(Acoustical Society of America, 1997-08) Chu, Dezhang ; Williams, Kevin L. ; Tang, Dajun ; Jackson, Darrell R.A previous study of high-frequency acoustic backscattering data collected at Eckernfoerde Bay, Germany revealed that scattering is mainly due to methane gas bubbles buried about a meter beneath the seafloor [Tang et al., J. Acoust. Soc. Am. 96, 2930–2936 (1994)]. A backscattering model was developed [Tang, Geo-Marine Lett. 16, 161–169 (1996)] where the gas bubbles were approximated by oblate spheroids. In this paper, a bistatic scattering model is proposed as an extension of the previously developed backscattering model. In this model, gas bubbles are again assumed to be oblate spheroids with varying aspect ratios and a single-scattering approximation is used. The model is compared to bistatic data acquired in Eckernfoerde Bay, Germany. In particular, the azimuthal dependence of the bistatic scattering strength predicted by the model is tested against experimental data and it is found that both the model and the bistatic scattering strength data exhibit a mild azimuthal dependence. Best agreement between model and data requires a 35% reduction in areal bubble density relative to that used in the backscattering model/data comparison. Possible reasons for this are discussed including multiple scattering effects.