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

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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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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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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Visually augmented navigation in an unstructured environment using a delayed state history

Visually augmented navigation in an unstructured environment...

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IEEE International Conference on Robotics and Automation (ICRA)

作者： R. Eustice O. Pizarro H. Singh MIT/WHOI Joint Program in Applied Ocean Science and Engineering Massachusetts Institute of Technology Cambridge MA USA Department of Applied Ocean Physics and Engineering Woods Hole Oceanographic Institution Woods Hole MA USA

This work describes a framework for sensor fusion of navigation data with camera-based 5 DOF relative pose measurements for 6 DOF vehicle motion in an unstructured 3D underwater environment. The fundamental goal of this work is to concurrently estimate online current vehicle position and its past trajectory. This goal is framed within the context of improving mobile robot navigation to support sub-sea science and exploration. Vehicle trajectory is represented by a history of poses in an augmented state Kalman filter. Camera spatial constraints from overlapping imagery provide partial observation of these poses and are used to enforce consistency and provide a mechanism for loop-closure. The multi-sensor camera + navigation framework is shown to have compelling advantages over a camera-only based approach by: 1) improving the robustness of pairwise image registration, 2) setting the free gauge scale, and 3) allowing for a unconnected camera graph topology. Results are shown for a real world data set collected by an autonomous underwater vehicle in an unstructured undersea environment.

关键词： Navigation Delay History Cameras Underwater vehicles Robot vision systems Sensor fusion Motion measurement Mobile robots Robustness

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Micro-bathymetric mapping using acoustic range images

Micro-bathymetric mapping using acoustic range images

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OCEANS

作者： C. Roman H. Singh MIT/WHOI Joint Program in Applied Ocean Science and Engineering Massachusetts Institute of Technology Cambridge MA USA Department of Applied Ocean Physics and Engineering Woods Hole Oceanographic Institution Woods Hole MA USA

This work focuses on the creation of high resolution micro-bathymetric maps using a high frequency pencil beam sonar. These maps typically cover areas of 10's to 100's of square meters. Data is collected using a sonar mounted to an underwater vehicle that can be positioned at discrete locations on the sea floor or flown in a survey pattern above the bottom. Specifically, we are focused on improving the accuracy of these terrain maps by merging sonar pings taken from multiple vantage points over the same location. This requires the adaption of data registration techniques to handle errors related to the sonar sensor. The registration can also be modified to incorporate the navigation data available from an instrumented underwater vehicle. A two axis scanning sonar was used to gather data in the form of densely sampled range images. The data registration works on the 3D projection of these range images in a scan-matching manner. Through this process regions occluded in a single scan are filled with data available from other scans and individual pings which show a poor return are removed. Refined estimates of the scan vantage points are also generated during the registration. The initial estimates of these locations are provided by navigation data. The practical application of this work complements photographic mapping when visibility is limited. It also provides additional measurements which can be used to improve vehicle navigation.

关键词： Sonar navigation Sonar measurements Remotely operated vehicles Underwater vehicles Sea floor Underwater acoustics Layout Oceans Acoustical engineering Acoustic beams

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Development of a six-degree of freedom simulation model for the REMUS autonomous underwater vehicle

Development of a six-degree of freedom simulation model for ...

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作者： T. Prestero MIT/WHOI Joint Program in Oceanographic Engineering USA

Describes the development and verification of a six degree of freedom, non-linear simulation model for the REMUS AUV, the first such model for this platform. In this model, the external forces and moments resulting from hydrostatics, hydrodynamic lift and drag, added mass, and the control inputs of the vehicle propeller and fins are all defined in terms of vehicle coefficients. The paper briefly describes the derivation of these coefficients. The equations determining the coefficients, as well as those describing the vehicle rigid-body dynamics, are left in non-linear form to better simulate the inherently non-linear behavior of the vehicle. Simulation of the vehicle motion is achieved through numeric integration of the equations of motion. The simulator output is then verified against vehicle dynamics data collected in experiments performed at sea. The simulator is shown to accurately model the motion of the vehicle. The paper concludes with recommendations for future model validation experiments.

关键词： Underwater vehicles Vehicle dynamics Remotely operated vehicles Propellers Nonlinear equations Marine vehicles Hydrodynamics Automotive engineering Life testing Costs

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Scattering of acoustic tomographic signals in shallow water by internal waves

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The Journal of the Acoustical Society of America 1995年第5_SUPPLEMENT期97卷 3264-3264页

作者： James F. Lynch Guoliang Jin Peter Traykovski Ching‐sang Chiu James H. Miller Woods Hole Oceanogr. Inst. Woods Hole MA 02543 Academia Sinica People’s Republic of China MIT/WHOI Joint Program in Oceanography and Oceanographic Engineering Woods Hole MA 02543 Naval Postgraduate School Monterey CA 93943

This talk examines how internal waves in shallow water cause travel time fluctuations in acoustic tomographic transmissions. Ray and mode theory expressions are developed and calculations are compared to travel time fluctuation data taken from the 1992 Barents Sea Polar Front experiment. While there is basic agreement, there is still much room for improvement of the basic theory and experiments. Directions of further research in both theory and experiment are discussed. [Work supported by ONR.]

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