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 ...
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.]
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 p...
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.]
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 vert...
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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 ...
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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...
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.
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 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.
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...
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.]
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'...
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.
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 relati...
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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