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Observations of Broadband Acoustic Backscattering From Nonlinear Internal Waves: Assessing the Contribution From Microstructure

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  • In this paper, measurements of high-frequency broadband (160–590 kHz) acoustic backscattering from surface trapped nonlinear internal waves of depression are presented. These waves are ideal for assessing the contribution from oceanic microstructure to scattering as they are intensely turbulent. Almost coincident direct microstructure measurements were performed and zooplankton community structure was characterized using depth-resolved net sampling techniques. The contribution to scattering from microstructure can be difficult to distinguish from the contribution to scattering from zooplankton using a single narrowband frequency as microstructure and zooplankton are often colocated and can have similar scattering levels over a range of frequencies. Yet their spectra are distinct over a sufficiently broad frequency range, allowing broadband backscattering measurements to reduce the ambiguities typically associated with the interpretation of narrowband measurements. In addition, pulse compression signal processing techniques result in very high-resolution images, allowing physical processes that are otherwise hard to resolve to be imaged, such as Kelvin–Helmholtz shear instabilities. In this study, high-resolution acoustic observations of multiple nonlinear internal waves are presented and regions with distinct scattering spectra are identified. Spectra that decrease in level across the available frequency band were highly correlated to regions of intense turbulence and high stratification, and to Kevin–Helmholtz shear instabilities in particular. Spectra that increase in level across the available frequency band were consistent with scattering dominated by small zooplankton. Simple inversions for relevant microstructure parameters are presented. Limitations of, and improvements to, the broadband system and techniques utilized in this study are discussed.
  • Keywords: Turbulent oceanic microstructure, Zooplankton, Broadband, Acoustic scattering, Nonlinear internal waves
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  • Lavery, A. C., Chu, D., & Moum, J. N. (2010, October). Observations of Broadband Acoustic Backscattering From Nonlinear Internal Waves: Assessing the Contribution From Microstructure. IEEE journal of oceanic engineering, 35(4), 695-709. doi:10.1109/JOE.2010.2047814
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  • 35
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  • This work was supported in part by the Woods Hole Oceanographic Institution and the U.S. Office of Naval Research under Grant N000140210359.
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