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Characterization of range‐dependent shallow water waveguides

The Journal of the Acoustical Society of America 2005年第S1期80卷 S52-S52页

作者： George V. Frisk Ferdinand J. Diemer Peter H. Dahl Department of Ocean Engineering Woods Hole Oceanographic Institution Woods Hole MA 02543 MIT/WHOI Joint Program in Oceanography/Oceanographic Engineering Woods Hole MA 02543

A technique for characterizing range‐dependent shallow water waveguides is described. The method consists of determining the beamformed output of a horizontal array over short apertures for signals due to a cw point source. By modeling the acoustic field locally as a sum of damped normal modes and using Prony's method to perform the beamforming, the local modal structure of the waveguide can be resolved. As a result, the modal composition of the waveguide as a function of range can be determined and interpreted in terms of range‐dependent mode theories (e.g., adiabatic mode theory). In addition to identifying important propagation characteristics such as mode cutoff, the method can be used to determine range‐dependent acoustic properties of the bottom. Examples of the application of the technique to the case of propagation in a wedge‐shaped ocean are presented. [Work supported by ONR.]

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An integral equation approach to range‐dependent boundary value problems in ocean acoustics

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The Journal of the Acoustical Society of America 2005年第S1期83卷 S117-S117页

作者： Peter H. Dahl George V. Frisk MIT/WHOI Joint Program in Oceanography/Oceanographic Engineering Woods Hole Oceanographic Institution Woods Hole MA 02543 Department of Ocean Engineering Woods Hole Oceanographic Institution Woods Hole MA 02543

In ocean acoustics, the introduction of range discontinuities, for example, the water‐to‐ice canopy surface, creates a mixed boundary value problem. In this paper, an exact solution of certain mixed boundary value problems is discussed using the Wiener‐Hopf method. A key attribute of this approach is that it is not fundamentally numerical in nature and allows additional insight into the mathematical and physical structure of the acoustic field due to range discontinuities. The problem discussed here is a canonical one: A plane wave is incident upon a planar surface where the boundary condition changes from Dirichlet (free surface) to Neumann (hard). The boundary conditions addressed in this problem are highly idealized renditions of what happens, when, say, a plane wave is incident upon a water‐to‐ice canopy surface; nevertheless, important features of the diffraction process are produced here, and the solution gives considerable insight into the process. The solution of the diffracted potential is partitioned into two components: a field consisting of cylindrical waves weighted by a polar gain function, resulting from the artificial source created by the discontinuity in the boundary; and a field containing a residue contribution which restores field continuity along the line corresponding to specular reflection. Contour plots of equal pressure amplitude show how the component fields superimpose such that the boundary conditions are maintained, and how energy is redistributed across the angular spectrum in the diffraction process. The latter is related to mode coupling due to boundary changes in waveguide problems. [Work supported by ONR.]

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Lagrangian surface signatures reveal upper-ocean vertical displacement conduits near oceanic density fronts

arXiv

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arXiv 2022年

作者： Aravind, H.M. Verma, Vicky Sarkar, Sutanu Freilich, Mara A. Mahadevan, Amala Haley, Patrick J. Lermusiaux, Pierre F.J. Allshouse, Michael R. Mechanical and Industrial Engineering Northeastern University BostonMA02115 United States Mechanical and Aerospace Engineering University of California San Diego La Jolla CA92093 United States MIT–WHOI Joint Program in Oceanography CambridgeMA02139 United States Woods Hole Oceanographic Institution Woods HoleMA02543 United States Department of Mechanical Engineering Massachusetts Institute of Technology CambridgeMA02139 United States

Vertical transport in the ocean plays a critical role in the exchange of freshwater, heat, nutrients, and other biogeochemical tracers. While there are situations where vertical fluxes are important, studying the vertical transport and displacement of material requires analysis over a finite interval of time. One such example is the subduction of fluid from the mixed layer into the pycnocline, which is known to occur near density fronts. Divergence has been used to estimate vertical velocities indicating that surface measurements, where observational data is most widely available, can be used to locate these vertical transport conduits. We evaluate the correlation between surface signatures derived from Eulerian (horizontal divergence, density gradient, and vertical velocity) and Lagrangian (dilation rate and finite time Lyapunov exponent) metrics and vertical displacement conduits. Two submesoscale resolving models of density fronts and a data-assimilative model of the western Mediterranean were analyzed. The Lagrangian surface signatures locate significantly more of the strongest displacement features and the difference in the expected displacements relative to Eulerian ones increases with the length of the time interval considered. Ensemble analysis of forecasts from the Mediterranean model demonstrates that the Lagrangian surface signatures can be used to identify regions of strongest downward vertical displacement even without knowledge of the true ocean state. © 2022, CC BY.

关键词： Surface measurement

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Tracking Sargassum in the Caribbean: The Deployment and Validation of a Low-Cost Surface Drifter

SSRN

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SSRN 2023年

作者： Pixa, Chase Manganini, Kevin Myers, Alyson Michel, Anna P.M. Department of Applied Ocean Physics and Engineering Woods Hole Oceanographic Institution 266 Woods Hole Road Woods HoleMA02543 United States Department of Mechanical Engineering Massachusetts Institute of Technology 77 Massachusetts Ave CambridgeMA02139 United States MIT-WHOI Joint Program in Oceanography/Applied Ocean Science & Engineering Cambridge and Woods Hole MA United States Fearless Fund 3029 Woodland Drive NW WashingtonDC20008 United States

Sargassum blooms have been causing significant ecological and economic damage to coastal regions in the Northern Equatorial Atlantic since 2011. To better understand the movement and effects of this macroalgae, there is a need to track its transport. In this study, a low-cost drifter was deployed, designed to entangle with Sargassum and aid in its tracking. The design was based on the results of twenty-seven drifter designs and five days of field trials. The tracking data was validated using gridded wind and current products, as well as high resolution satellite imagery. The successful entanglement and tracking with the Sargassum demonstrated in this study can provide in situ movement data to ground-truth models and supplement gaps in satellite imaging. The results can guide future studies and further our understanding of its movement in the great Atlantic Sargassum belt (GASB). © 2023, The Authors. All rights reserved.

关键词： Costs

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Low‐frequency diffraction from a free surface coupled to a semi‐infinite elastic surface as modeled by sea ice properties

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The Journal of the Acoustical Society of America 2005年第S1期85卷 S50-S50页

作者： Peter H. Dahl George V. Frisk MIT/WHOI Joint Program in Oceanography/Oceanographic Engineering Department of Ocean Engineering Massachusetts Institute of Technology Cambridge MA 02139 Department of Ocean Engineering Woods Hole Oceanographic Institution Woods Hole MA 02543

This paper discusses the solution of a low‐frequency plane wave incident upon a semi‐infinite elastic plate, such as an Arctic ice lend or free edge, using the Wiener‐Hopf method. By low‐frequency it is meant that the elastic properties of the plate are adequately described by the thin plate equation. For example, in a floating ice sheet, this translates into frequency‐ice thickness products that are ≲ 150. A key issue here is the fluid loading pertaining to sea ice and low‐frequency acoustics, which cannot be characterized by simplifying heavy or light fluid loading limits. An approximation to the exact kernel of the Wiener‐Hopf functional equation is used here, which is valid in this midrange fluid loading regime. The farfield diffracted pressure is found, which includes a fluid‐loaded, sub‐sonic (relative to the water) flexural wave in the ice plate. Comparisons are also made with the locally reacting approximation to the input impedance of an ice plate. The combined effects of the ice lead diffraction process represent loss mechanisms that contribute to the transmission loss in long‐range Arctic acoustic propagation.

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A self‐consistent theory of wave propagation and scattering through weakly inhomogeneous media

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The Journal of the Acoustical Society of America 2005年第S1期85卷 S70-S70页

作者： Dajun Tang George V. Frisk MIT/WHOI Joint Program in Oceanography/Oceanographic Engineering Department of Ocean Engineering Massachusetts Institute of Technology Cambridge MA 02139 Department of Ocean Engineering Woods Hole Oceanographic Institution Woods Hole MA 02543

A unified perturbation approach to wave propagation and scattering in weakly inhomogeneous media is presented. By separating the total wave field into an averaged coherent propagation part and a random scattering part, a set of coupled wave equations is found. The problem of a wave propagating in a random medium is changed to a problem of a pair of coupled waves propagating in a deterministic medium with randomly distributed sources. This method not only improves upon the conventional Born approximation for the scattered field, but also gives the attenuation behavior of the coherent propagating field related to the statistics of the random media. Examples of underwater acoustic waves in an inhomogeneous water column and an inhomogeneous bottom are given.

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Numerical modeling of the scattered acoustic field from elastic ice

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The Journal of the Acoustical Society of America 2005年第S1期83卷 S37-S37页

作者： J. Robert Fricke Ralph A. Stephen Arthur B. Baggeroer MIT/WHOI Joint Program MIT Cambridge MA 02139 Woods Hole Oceanographic Institution Woods Hole MA 02543 Department of Ocean MIT Cambridge MA 02139 Department of Electrical Engineering MIT Cambridge MA 02139

A two‐dimensional elastic finite difference modeling program written for marine geophysical applications [M. E. Dougherty and R. A. Stephen, J. Acoust. Soc. Am. 82, 238–256 (1987)] has been modified to study the ice scattering problem. The modified code was checked by comparing modeled plane‐wave reflection coefficients at various angles of incidence with the analytical solutions provided by Zoeppritz equations. These tests were calibrated for a flat interface with homogeneous half‐spaces of water and ice and resulted in favorable comparisons. Subsequently, various ice models were run to observe the scattered acoustic field. These models include flat ice, flat ice with a single keel, and ice with randomly rough surfaces (top and bottom) derived from different process models (e.g., Gaussian and Poisson). With this program, controlled numerical experiments can be performed for single realizations of ice roughness including full elastic behavior of the ice which is usually neglected in scattering theory. Such a capability can be used as an analysis tool for evaluating scattering approximations (say, the Kirchhoff approximation). In addition, this modeling capability can be used as the synthesis step of a scattering inversion program where ice roughness parameters are to be estimated from a measurement of the scattered field. [Work supported by ONR Arctic Acoustics program.]

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Reconstruction of a complex acoustic field from its real or imaginary part

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The Journal of the Acoustical Society of America 2005年第S1期78卷 S30-S30页

作者： Michael S. Wengrovitz George V. Frisk Alan V. Oppenheim MIT/WHOI Joint Program in Oceanography/Oceanographic Engineering Woods Hole MA 02543 Woods Hole Oceanographic Institution Woods Hole MA 02543 Research Laboratory of Electronics Department of EECS Massachusetts Institute of Technology Cambridge MA 02139

In underwater acoustics, the sound pressure field in a horizontally stratified, range‐independent medium due to a continuous‐wave point source can be described by a Hankel transform of the depth‐dependent Green's function. Although the total field consists of both outgoing and incoming components, a reasonable assumption is that the incoming components can be neglected. This assumption is the basis for a number of synthetic field generation techniques such as the Fast Field program (FFP) [F. R. DiNapoli and R. L. Deavenport, J. Acoust. Soc. Am. 67, 92–105 (1980)]. It is shown that the condition that the field consists only of outgoing components implies a relationship between the real and imaginary parts of the field. An implication is that the real (imaginary) part of the pressure field can be reconstructed from the imaginary (real) part. An algorithm for performing this reconstruction is presented and examples of its application to synthetic and experimental acoustic fields is discussed.

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Corrigendum to “Lipid composition, caloric content, and novel oxidation products from microbial communities within seasonal pack ice cores” [Geochimica et Cosmochimica Acta 368 (2024) 12–23]

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Geochimica et Cosmochimica Acta 2024年 382卷 161-161页

作者： Henry C. Holm Helen F. Fredricks Shavonna M. Bent Daniel P. Lowenstein Kharis R. Schrage Benjamin A.S. Van Mooy Marine Chemistry and Geochemistry Department Woods Hole Oceanographic Institution Woods Hole MA 02543 USA MIT-WHOI Joint Program in Oceanography/Applied Ocean Science & Engineering Cambridge MA 02139 USA

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A linear inverse scheme for the extraction of ocean bottom geoacoustic parameters from modal eigenvalues

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The Journal of the Acoustical Society of America 2005年第S1期76卷 S94-S95页

作者： S. D. Rajan M. S. Wengrovitz J. F. Lynch G. V. Frisk MIT/Woods Hole Oceanographic Institution Joint Program in Oceanography/Oceanographic Engineering Woods Hole MA 02543 Woods Hole Oceanographic Institution Woods Hole MA 02543

Shallow water theoretical and experimental results recently obtained by Frisk, Lynch, Wengrovitz, and Rajan [G. V. Frisk and J. F. Lynch, J. Acoust. Soc. Am. 76, 205–216 (1984)] indicate that one can perform a relatively simple cw surveying experiment which would, after processing of the data via the Hankel transform, yield both the discrete and continuous modal spectra. Extraction of bottom geoacoustic information can then be accomplished by applying linear inverse theory (e.g., the Backus‐Gilbert approach) to the eigenvalues thus obtained. Specifically, one deals with a mathematical form derived from first‐order perturbation theory, di = ∫0ZmaxGi(z)m(z)dz (i = 1,2,3,…,N), where di represents data (the modal eigenvalues), Gi(z) is the kernel for some initial guess model (currently taken as a Pekeris waveguide, but not restricted to it), and m(z) stands for a profile of some quantity (such as velocity or density) in our desired earth model minus the profile for our initial model. Results of preliminary inversions for typical bottom velocity profiles will be presented, as well as discussion of other topics of interest such as attenuation, density profiles, and resolution criteria. [Work supported by ONR.]

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