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Xu G, Harcourt RR, Tang D, Hefner BT, Thorsos EI, Mickett JB. Subsurface acoustic ducts in the Northern California current system. J Acoust Soc Am 2024; 155:1881-1894. [PMID: 38451134 DOI: 10.1121/10.0024146] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/16/2023] [Accepted: 12/08/2023] [Indexed: 03/08/2024]
Abstract
This study investigates the subsurface sound channel or acoustic duct that appears seasonally along the U.S. Pacific Northwest coast below the surface mixed layer. The duct has a significant impact on sound propagation at mid-frequencies by trapping sound energy and reducing transmission loss within the channel. A survey of the sound-speed profiles obtained from archived mooring and glider observations reveals that the duct is more prevalent in summer to fall than in winter to spring and offshore of the shelf break than over the shelf. The occurrence of the subsurface duct is typically associated with the presence of a strong halocline and a reduced thermocline or temperature inversion. Furthermore, the duct observed over the shelf slope corresponds to a vertically sheared along-slope velocity profile, characterized by equatorward near-surface flow overlaying poleward subsurface flow. Two potential duct formation mechanisms are examined in this study, which are seasonal surface heat exchange and baroclinic advection of distinct water masses. The former mechanism regulates the formation of a downward-refracting sound-speed gradient that caps the duct near the sea surface, while the latter contributes to the formation of an upward-refracting sound-speed gradient that defines the duct's lower boundary.
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Affiliation(s)
- Guangyu Xu
- Applied Physics Laboratory, University of Washington, Seattle, Washington 98105, USA
| | - Ramsey R Harcourt
- Applied Physics Laboratory, University of Washington, Seattle, Washington 98105, USA
| | - Dajun Tang
- Applied Physics Laboratory, University of Washington, Seattle, Washington 98105, USA
| | - Brian T Hefner
- Applied Physics Laboratory, University of Washington, Seattle, Washington 98105, USA
| | - Eric I Thorsos
- Applied Physics Laboratory, University of Washington, Seattle, Washington 98105, USA
| | - John B Mickett
- Applied Physics Laboratory, University of Washington, Seattle, Washington 98105, USA
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Yang J, Nystuen JA, Riser SC, Thorsos EI. Open ocean ambient noise data in the frequency band of 100 Hz-50 kHz from the Pacific Ocean. JASA Express Lett 2023; 3:036001. [PMID: 37003715 DOI: 10.1121/10.0017349] [Citation(s) in RCA: 1] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/19/2023]
Abstract
Bubbles from wind generated breaking surface waves are the dominant ambient noise source [Dean and Stokes, Nature 418, 839-844 (2002)]. With ambient noise data collected in the open ocean between 100 Hz and 50 kHz from 1999 to 2022, the ambient noise level is observed to sharply decrease as wind speed increases beyond 15 m/s for frequencies higher than 4 kHz. Data-model comparisons show a mismatch, as existing models including the Wenz curves [Wenz, J. Acoust. Soc. Am. 34, 1936-1956 (1962)] are monotonic in nature. The decrease at high wind speeds and frequencies is likely due to attenuation when ambient sound propagates through the deeper and denser bubble layer for high sea conditions [Farmer and Lemon, J. Phys. Oceanogr. 14, 1761-1777 (1984)].
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Affiliation(s)
- Jie Yang
- Applied Physics Laboratory, University of Washington, Seattle, Washington 98105, USA
| | - Jeffery A Nystuen
- Applied Physics Laboratory, University of Washington, Seattle, Washington 98105, USA
| | - Stephen C Riser
- School of Oceanography, University of Washington, Seattle, Washington 98105, USA , ,
| | - Eric I Thorsos
- Applied Physics Laboratory, University of Washington, Seattle, Washington 98105, USA
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Lee WJ, Tang D, Stanton TK, Thorsos EI. Macroscopic observations of diel fish movements around a shallow water artificial reef using a mid-frequency horizontal-looking sonar. J Acoust Soc Am 2018; 144:1424. [PMID: 30424649 DOI: 10.1121/1.5054013] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/05/2018] [Accepted: 08/14/2018] [Indexed: 06/09/2023]
Abstract
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.
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Affiliation(s)
- Wu-Jung Lee
- Applied Physics Laboratory, University of Washington, 1013 Northeast 40th Street, Seattle, Washington 98105, USA
| | - Dajun Tang
- Applied Physics Laboratory, University of Washington, 1013 Northeast 40th Street, Seattle, Washington 98105, USA
| | - Timothy K Stanton
- Department of Applied Ocean Physics and Engineering, Woods Hole Oceanographic Institution, 86 Water Street, Woods Hole, Massachusetts 02543, USA
| | - Eric I Thorsos
- Applied Physics Laboratory, University of Washington, 1013 Northeast 40th Street, Seattle, Washington 98105, USA
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Williams KL, Kargl SG, Thorsos EI, Burnett DS, Lopes JL, Zampolli M, Marston PL. Acoustic scattering from a solid aluminum cylinder in contact with a sand sediment: measurements, modeling, and interpretation. J Acoust Soc Am 2010; 127:3356-3371. [PMID: 20550236 DOI: 10.1121/1.3419926] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/29/2023]
Abstract
Understanding acoustic scattering from objects placed on the interface between two media requires incorporation of scattering off the interface. Here, this class of problems is studied in the particular context of a 61 cm long, 30.5 cm diameter solid aluminum cylinder placed on a flattened sand interface. Experimental results are presented for the monostatic scattering from this cylinder for azimuthal scattering angles from 0 degrees to 90 degrees and frequencies from 1 to 30 kHz. In addition, synthetic aperture sonar (SAS) processing is carried out. Next, details seen within these experimental results are explained using insight derived from physical acoustics. Subsequently, target strength results are compared to finite-element (FE) calculations. The simplest calculation assumes that the source and receiver are at infinity and uses the FE result for the cylinder in free space along with image cylinders for approximating the target/interface interaction. Then the effect of finite geometries and inclusion of a more complete Green's function for the target/interface interaction is examined. These first two calculations use the axial symmetry of the cylinder in carrying out the analysis. Finally, the results from a three dimensional FE analysis are presented and compared to both the experiment and the axially symmetric calculations.
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Affiliation(s)
- Kevin L Williams
- Applied Physics Laboratory, College of Ocean and Fishery Sciences, University of Washington, Seattle, Washington 98105, USA
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Lim R, Williams KL, Thorsos EI. Acoustic scattering by a three-dimensional elastic object near a rough surface. J Acoust Soc Am 2000; 107:1246-1262. [PMID: 10738781 DOI: 10.1121/1.428414] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/23/2023]
Abstract
The ensemble-averaged field scattered by a smooth, bounded, elastic object near a penetrable surface with small-scale random roughness is formulated. The formulation consists of combining a perturbative solution for modeling propagation through the rough surface with a transition (T-) matrix solution for scattering by the object near a planar surface. All media bounding the rough surface are assumed to be fluids. By applying the results to a spherical steel shell buried within a rough sediment bottom, it is demonstrated that the ensemble-averaged "incoherent" intensity backscattered by buried objects illuminated with shallow-grazing-angle acoustic sources can be well enhanced at high frequencies over field predictions based on scattering models where all environmental surfaces are planar. However, this intensity must compete with the incoherent intensity scattered back from the interface itself, which can defeat detection attempts. The averaged "coherent" component of the field maintains the strong evanescent spectral decay exhibited by flat interface predictions of shallow-angle measurements but with small deviations. Nevertheless, bistatic calculations of the coherent field suggest useful strategies for improving long-range detection and identification of buried objects.
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Affiliation(s)
- R Lim
- Coastal Systems Station/Dahlgren Division, Naval Surface Warfare Center, Panama City, Florida 32407-7001, USA
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Thorsos EI, Jackson DR, Williams KL. Modeling of subcritical penetration into sediments due to interface roughness. J Acoust Soc Am 2000; 107:263-277. [PMID: 10641636 DOI: 10.1121/1.428303] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/23/2023]
Abstract
Recent experimental results reveal acoustic penetration into sandy sediments at grazing angles below the critical angle. A mechanism for this subcritical penetration is described based on scattering at a rough water-sediment interface. Using perturbation theory, a numerically tractable three-dimensional model is used for simulating experiments. The rough interface scattering has been treated using formally averaged methods as well as with single rough surface realizations. Data-model comparisons show that scattering by interface roughness is a viable hypothesis for the observed subcritical penetration.
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Affiliation(s)
- EI Thorsos
- Applied Physics Laboratory, University of Washington, Seattle 98105, USA
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Thorsos EI, Winebrenner DP. An examination of the “full-wave” method for rough surface scattering in the case of small roughness. ACTA ACUST UNITED AC 1991. [DOI: 10.1029/91jc00417] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
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