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Dec 1986

Volume 80, Issue S1, pp. S1-S128

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back to top Session BBB. Underwater Acoustics X: Scattering from the Seafloor and Ice Canopy
Contributed Papers
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An all‐frequency solution of sound scatter by hard angular surfaces (A)

I. Tolstoy

J. Acoust. Soc. Am. Volume 80, Issue S1, pp. S114-S114 (1986); (1 page)

Online Publication Date: 13 Aug 2005

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In estimating plane sound‐wave scatter by hard bodies of simple prismatic cross section or by corrugated surfaces with angular (i.e., sawtooth or facetted) profiles it is possible to replace the actual bodies by suitable systems of single (infinite) wedges and corners plus line sources (at the vertices) plus images thereof. This procedure differs fundamentally from approximate superpositions of rigorous wedge/corner solutions used by other writers in that all orders of multiple scatter interaction are taken into account via the standard self‐consistent algorithm. An infinite series representation of the field was obtained; the coefficients of which are determined by an infinite system of simultaneous linear equations, which appear to be soluble in a number of cases. The resulting solution for the total field is convergent everywhere and satisfies all the boundary conditions. In the case of an angular depression in a plane, or in the troughs of a corrugated surface with a sawtooth profile, a simple, highly accurate solution is obtained for all frequencies. [Work supported by ONR.]
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A study of bathymetric scattering using simulation techniques (A)

R. N. Baer, J. S. Perkins, and D. H. Berman

J. Acoust. Soc. Am. Volume 80, Issue S1, pp. S114-S114 (1986); (1 page)

Online Publication Date: 13 Aug 2005

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Seafloor scattering can significantly affect the acoustic field. The statistics of ocean floor scattering were simulated by using the split‐step parabolic equation with an ensemble of possible bathymetries in a manner analogous to that of Tappert and Nghiem‐Phu [J. Acoust. Soc. Am. Suppl. 1 77, S102 (1985)]. The bathymetry was taken to have a Gaussian spectrum. Situations with various horizontal correlation lengths and rms roughness were examined. Different acoustic sources were also considered. An ensemble of 100 to 200 parabolic equation runs was found that in general was sufficient to give predictions of the mean and standard deviations of the acoustic field. The ensemble‐mean distribution differs significantly from both an individual ensemble member and field with the average (flat) bathymetry. In the convergence zone, the variance of the acoustic field correlates with the mean transmission loss and it decreases with decreasing rms roughness or increasing horizontal correlation length. Outside the convergence zone, the variance is high even for relatively smooth cases. [Work supported by NRL and ONR.]
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Backscatter at a model of the Arctic ice canopy (A)

Kevin R. Johnson, Patrick L. Denny, Ken J. Reitzel, and Herman Medwin

J. Acoust. Soc. Am. Volume 80, Issue S1, pp. S114-S114 (1986); (1 page)

Online Publication Date: 13 Aug 2005

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The mechanisms which may cause low‐frequency sound to backscatter from the Arctic ice canopy include rough surface backscatter, diffraction at leads, and reradiation of flexural waves. These phenomena have been studied by observing underwater sound pulses backscattered from an acrylic model of the Arctic ice floating on water. The flexural wave velocity and the acoustic roughness are accurately scaled and the ρc impedance contrast is approximately modeled by the selection of the acrylic material and the dimensions of the plate and roughness elements. The 3‐mm‐thick plate backscattering at 50 kHz then represents an Arctic ice cover 1.5 m thick and about a kilometer in extent, backscattering 100‐Hz sound. The physical contributors of backscatter from the Arctic surface are studied for models of plane ice, an Arctic ice ridge, edges of leads, and a rubble field of ice. [Work supported by ONR.]
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Forward scatter at a model of the Arctic ice canopy (A)

Patrick L. Denny, Kevin R. Johnson, Ken J. Reitzel, and Herman Medwin

J. Acoust. Soc. Am. Volume 80, Issue S1, pp. S114-S115 (1986); (2 pages)

Online Publication Date: 13 Aug 2005

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When low‐frequency underwater sound “reflects” from the Arctic ice cover, not only will it be reflected from the plane, and specularly scattered from roughness elements, but it will also be diffracted at leads and reradiated from flexural waves in the ice. In the case of near‐grazing forward scatter from closely spaced roughness elements there may also be a coherent forward scattered boundary wave detectable near the under‐ice surface. These phenomena have been studied in an anechoic tank by pulse transmission from an underwater point source to a large floating acrylic model of the Arctic ice. Scaling of the flexural wave velocity B(ωh)1/2 at frequency ω and ice thickness h is accurately achieved because B, for the acrylic model, is well within the range of typical values in arctic ice. Scaling of the sound scattered from roughness elements is also accomplished in terms of frequency and the length dimension. Consequently, the laboratory model at frequency 50 kHz represents 100‐Hz sound encountering a typical Arctic canopy which is 1½ m thick and about a kilometer in extent. The physical contributors to the gross “reflection coefficient” are studied for models of a plane ice layer, an Arctic ice ridge, edges of leads, and a rubble field of ice. [Work supported by ONR.]
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Laboratory‐scale simulation of under‐ice reflectivity at low frequencies (A)

T. McLanahan, O. I. Diachok, and S. C. Wales

J. Acoust. Soc. Am. Volume 80, Issue S1, pp. S115-S115 (1986); (1 page)

Online Publication Date: 13 Aug 2005

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Under‐ice reflectivity is dominated by sea ice ridges, long, randomly spaced, randomly oriented rubble piles of broken ice. The effective low‐frequency shear and compressional velocities of these formuations have been estimated to be between 1100–1700 and 2700–3400 m/s, respectively (Diachok, Proceedings International Congress on Acoustics, 1986). To be consistent with these estimates, Lucite, a readily available and easily machined material, for which Vs = 1260 m/s and Vp = 2600 m/s was used to construct laboratory scale analogs of sea ice ridges situated on a thin plate. Both parallel and randomly oriented ridge models were construtted, and the effects of ridge elasticity and orientation on under‐ice reflectivity (amplitude and phase) were examined for 0.67 < ka < 6 (k = wavenumber and a = mean depth). Initial reflection coefficient data at the lowest frequencies are approximately consistent with Twersky's predictions for reflectivity from both hard and soft half‐cylinders on a soft boundary. Higher frequency results, including phase versus grazing angle, will also be reported, and implications for under‐ice propagation modeling will be discussed.
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Arctic low‐frequency propagation loss from shallow cracks in ice plates (A)

Jacques R. Chamuel and Gary H. Brooke

J. Acoust. Soc. Am. Volume 80, Issue S1, pp. S115-S115 (1986); (1 page)

Online Publication Date: 13 Aug 2005

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At the 111th Meeting of the Acoustical Society of America, the authors reported on the effects of shallow cracks in floating ice plates on lowering the flexural wave dispersion curve creating discrepancies in plate thickness determination. In this paper, the effects of shallow cracks in sea‐ice plates on low‐frequency propagation loss in the water waveguide underneath the ice are demonstrated using laboratory ultrasonic models. Air‐filled shallow cracks extending near vertically in the ice plate present sharp impedance discontinuities to the low‐frequency water acoustic waves coupled to the ice plates. Upward refraction in arctic water increases the interaction of the acoustic waves with the cracks causing distributed backscatter along the water waveguide. The importance of the cracks increases as the effective water waveguide depth is decreased. Experimental laboratory results are presented for shallow water waveguide conditions. The new findings may explain the very high backscatter levels and high propagation losses of low frequencies observed in the Arctic. [Work supported by DREP.]
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Geoacoustic scattering from seafloor features in the ROSE area (A)

Martin E. Dougherty

J. Acoust. Soc. Am. Volume 80, Issue S1, pp. S115-S115 (1986); (1 page)

Online Publication Date: 13 Aug 2005

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A strong “refraction branch diffraction” has been observed on ocean bottom hydrophone data from the Rivera Ocean Seismic Experiment (ROSE). Modeling of the interaction between seismic/acoustic energy and the ocean bottom has shown that this arrival could be caused by scattering from common seafloor features. Seismic/acoustic wave propagation was computed for models of marine seismic refraction lines using a 2D heterogeneous formulation of the elastic wave equation. These models demonstrate that a significant amount of energy is diffracted from seafloor structures such as hills and valleys when both the direct water wave and the refracted compressional wave (traveling through the upper crust) are incident upon a structure. Since much of the diffracted energy travels through the crust, the diffracted arrivals appear on the refraction branch of the seismograms at large ranges. Energy partitioning at the seafloor of the two incident wave types produce both compressional (P) and shear (S) diffracted phases in the upper oceanic crust. The large models used also clearly demonstrate the existence of phases which are theoretically possible but rarely identified in marine seismic data such as the pseudo‐Rayleigh wave and the P and S interference head waves.
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An nth order solution of the Neumann series in scattering from rough interfaces (A)

M. F. Werby and Richard Keiffer

J. Acoust. Soc. Am. Volume 80, Issue S1, pp. S115-S115 (1986); (1 page)

Online Publication Date: 13 Aug 2005

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Scattering from rough interfaces has been treated by the integral representation of the Helmholtz equation. This equation involves surface integrals and is well suited for rough interfaces which are piecewise continuous. The well‐known Kirchoff approximation involves replacing the unknown surface pressure field and its gradient by the incident field as a first‐order approximation to the integral equation. This leads to an expression which involves direct integration and will be noted to represent the first term in the Neumann solution of an integral equation of the second kind. Physically, it corresponds to one encounter of the field with the interface. It can be shown that the nth solution corresponds to n encounters with the interface. We derive expressions for the nth solutions of the Neumann series. The expressions are exact for each order for the three‐dimensional case and include an out‐of‐plane scatter.
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Single backscatter reverberation (A)

Johanan L. Codona and Richard S. Patton

J. Acoust. Soc. Am. Volume 80, Issue S1, pp. S115-S115 (1986); (1 page)

Online Publication Date: 13 Aug 2005

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By making use of Green's function techniques, a self‐consistent, nonperturbative, single backscatter model for acoustic reverberation in the ocean was constructed. The model accounts for multiple forward scattering on both the outgoing and return paths. The model can be made to use any one‐way wave equation: e.g., the standard parabolic equation or the Thomson‐Chapman high‐angle equation. When the standard parabolic equation is used, the technique lends itself to a numerical implementation that is much faster than competing schemes. While the model is particularly useful at low frequencies, its validity does not depend on frequency. The model's numerical speed and versatility suggest its use for the study of low‐frequency reverberation from rough ocean surfaces, deterministic and random variations in the ocean volume, and range dependence in the bottom (both bathymetric and compositional).
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Sound propagation through random currents using parabolic approximations (A)

R. I. Brent, M. J. Jacobson, and W. L. Siegmann

J. Acoust. Soc. Am. Volume 80, Issue S1, pp. S115-S115 (1986); (1 page)

Online Publication Date: 13 Aug 2005

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The influence of random, horizontal, depth‐dependent ocean currents in the parabolic approximation is studied. Emphasis is placed on received acoustic intensity. Parabolic equations including currents, derived recently by our group, are examined using perturbation and asymptotic methods. Expressions are derived for the mean and standard deviation of intensity. The development assumes that the random current is wide‐sense stationary. For convenience, an exponential‐cosine autocorrelation function with depth is taken, although any such function can be used. Formulas obtained are sufficiently general to permit many types of depth‐dependent sound speeds and bottom interface models. We illustrate current fluctuation effects on intensity moments for the specific case of an isospeed channel with a perfectly hard bottom. The acoustic consequences of changes in physical parameters directly related to current fluctuations, such as its standard deviation and correlation length, are discussed. Also exhibited are changes in random current effects on intensity which result from changes in noncurrent related parameters, such as acoustic frequency and channel depth. [Work supported by ONR.]
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Re‐examination of convergence of rough‐surface perturbation theory (A)

Darrell R. Jackson

J. Acoust. Soc. Am. Volume 80, Issue S1, pp. S116-S116 (1986); (1 page)

Online Publication Date: 13 Aug 2005

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Several numerical studies have been reported which suggest that the rough‐surface perturbation series can give accurate results even when the Rayleigh hypothesis is untrue. The question of convergence or nonconvergence is important for applications of perturbation theory and other theories based upon it (composite roughness, smoothing, phase perturbation). Numerical convergence will be demonstrated for large‐amplitude sinusoidal surfaces of sufficiently small slope. Partial theoretical arguments will be given in support of the convergence criterion suggested by the numerical results. [Work supported by ONR.]
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Studies of the validity of the Kirchhoff approximation for rough‐surface scattering (A)

Eric I. Thorsos

J. Acoust. Soc. Am. Volume 80, Issue S1, pp. S116-S116 (1986); (1 page)

Online Publication Date: 13 Aug 2005

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The accuracy of the Kirchhoff approximation for randomly rough, pressure‐release surfaces has been examined through comparison with exact numerical results based on solving an integral equation. A Gaussian roughness spectrum was chosen for the surfaces, and a Monte Carlo procedure was used to obtain the average bistatic scattering cross section. Results will be presented that illustrate the breakdown of the Kirchhoff approximation as the incident grazing angle is reduced and as the average radius of curvature is reduced. The accuracy of shadowing corrections to the Kirchhoff approximation will also be discussed. [Work supported by ONR.]
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Shallow water reverberation: A heuristic ray‐mode model (A)

Dale D. Ellis

J. Acoust. Soc. Am. Volume 80, Issue S1, pp. S116-S116 (1986); (1 page)

Online Publication Date: 13 Aug 2005

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A shallow water reverberation model has been developed based on normal mode theory together with various ray‐mode analogies. The model seems to be different from other shallow water reverberation models [K. V. Mackenzie, J. Acoust. Soc. Am. 34, 62 (1962); E. C. Shang, 12th ICA Symposium on Underwater Acoustics, Halifax, N.S., Canada (1986)] in that the reverberation at time t is due to contributions from a number of different ranges. Acoustic energy propagates from the source to the scattering element via mode n, and returns via mode m. The scattering is assumed to take place at the water‐surface or water‐bottom interface in accordance to the scattering function M(θn, θm). The angles are determined from the mode wavenumbers in accordance with the usual ray‐mode analogy. The travel times for the different paths are obtained from the modal group velocities. The final result is a double sum over the modes, but is computationally quite efficient since the mode functions and scattering angles are independent of range.
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Resonance reflections from a stratified ocean bottom (A)

P. D. Jackins, G. C. Gaunaurd, and J. Arvelo

J. Acoust. Soc. Am. Volume 80, Issue S1, pp. S116-S116 (1986); (1 page)

Online Publication Date: 13 Aug 2005

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The echoes reflected by a stratified ocean bottom that is insonified by acoustic plane waves emerging from a distant source are studied. The various strata in the bottom are modeled as elastic layers bonded together and allowing longitudinal (acoustic) and transverse (shear) waves to penetrate and be propagated through, and reflected from, the overall configuration. There can be as many layers as one wishes, all of arbitrary compositions. The bottom layer is a half‐space of infinite thickness upon which all other layers rest. The top layer is the fluid layer that simulates the ocean and contains the source. The reflections versus frequency at fixed angles of incidence, or versus angle of incidence at fixed frequencies are studied. Numerous resonances are present in these returns and emerge in the analysis. This investigation has the eventual, ultimate, goal of determining details about the bottom composition and stratification, from the remotely sensed reflections. Of particular interest is a five‐layer ease in which the consolidation and rigidity of the sediments in the various layers increases as one penetrates deeper into the bottom.
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