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Apr 1982

Volume 71, Issue S1, pp. S1-S113

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back to top Session E. Underwater Acoustics I: Shallow Water Propagation
Invited Papers
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Shallow‐water acoustic research at the Naval Research Laboratory (A)

F. Ingenito and S. N. Wolf

J. Acoust. Soc. Am. Volume 71, Issue S1, pp. S10-S10 (1982); (1 page)

Online Publication Date: 12 Aug 2005

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This paper reviews shallow‐water acoustic research performed by the Naval Research Laboratory during the past ten years. The earlier work concentrated on acoustic propagation modeling and measurements. Major achievements were the development of techniques to measure the excitation, depth distribution, and attenuation of individual normal modes and successful comparison of the measurements with theoretical predictions. This work culminated in a tested propagation loss model capable of representing the complex conditions characteristic of the shallow‐water environment. More recently, a model of wind‐generated noise based on the transmission loss model has been developed. This model predicts relative level and spatial coherence of the noise field. The experimental program has been expanded to include measurements of signal coherence, ambient noise level and coherence, and array gain. Examples of these measurements and comparisons with predictions will be presented.
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Broadband sound propagation in shallow water (A)

T. Akal and F. B. Jensen

J. Acoust. Soc. Am. Volume 71, Issue S1, pp. S10-S10 (1982); (1 page)

Online Publication Date: 12 Aug 2005

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A review of ten years of research in shallow water acoustics at SACLANTCEN is presented. Both experimental and theoretical results are given, with general features of shallow water propagation being emphasized. The experimental basis for this study is an extensive set of shallow water propagation data collected in different coastal areas of the Mediterranean Sea and the eastern North Atlantic. These data, in turn, have been subjected to a comprehensive modeling effort using wave‐theory models (normal mode and parabolic equation) in order to identify the fundamental mechanisms governing sound propagation in shallow water. As a result of this model/data comparison we have obtained a better understanding of the complicated frequency‐dependent changes in propagation conditions with changing environment, i.e., sound‐speed profile, water depth, bottom type, etc. Propagation levels in shallow water are generally found to be highly dependent on bottom type. In particular the coupling of sound into shear waves in the bottom is found to be an important low‐frequency loss mechanism. It is also shown that the optimum frequency of propagation in shallow water is strongly dependent on water depth but less dependent on bottom type. This phenomenon will be dealt with in some detail.
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Geoacoustic models for propagation modeling in shallow water (A)

D. M. F. Chapman and Dale D. Ellis

J. Acoust. Soc. Am. Volume 71, Issue S1, pp. S10-S10 (1982); (1 page)

Online Publication Date: 12 Aug 2005

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Acoustic propagation in shallow water is viewed as a guided‐wave phenomenon, with the sea surface and seabed forming the boundaries. At subkilohertz frequencies, the acoustic properties of the seabed to a depth of several wavelengths can have a strong effect on propagation. The computer modeling of propagation requires estimates of such parameters as sound speed, density, attenuation, and layer thicknesses, which collectively are called the geoacoustic model of the seabed. Direct measurement of these quantities is difficult, and methods must be devised to infer these values from other experiments, often employing acoustic techniques. At DREA, we have adopted the approach of independently determining as many geoacoustic parameters as possible, and adjusting less precisely known parameters within reasonable limits to effect an agreement between theory and experiment. To this end, we have used sub‐bottom reflection profiles to determine sediment types and layer thicknesses, large and small scale seismic refraction experiments to estimate sound speeds, and processing of sub‐bottom vertical reflection data to estimate volume attenuation. Techniques used by other researchers will be reviewed. Examples of geoacoustic models and comparisons with experiment will be presented for shallow water sites on the Scotian Shelf and the southwestern approaches to the English Channel.
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Coupling of acoustic normal modes in shallow water (A)

Suzanne T. McDaniel

J. Acoust. Soc. Am. Volume 71, Issue S1, pp. S11-S11 (1982); (1 page)

Online Publication Date: 12 Aug 2005

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In shallow water, coupling between acoustic normal modes may occur due to interaction with irregular boundaries. Two treatments of mode coupling are presented and discussed: a stochastic theory applicable to scattering due to ocean surface and bottom roughness, and a deterministic theory applicable to large‐scale range dependent features. The advantages and limitations of each of these methods is discussed and results are compared with those obtained from alternative methods of treating the same phenomena. It is concluded that both the stochastic and deterministic mode‐coupling models provide a powerful tool for increasing our understanding of the effect of boundary irregularities on shallow water acoustic propagation. [Work supported by NAVSEASYSCOM, NSEA‐63R‐1.]
Contributed Papers
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Shallow‐water propagation: The role of the branch‐line integral in the Pedersen‐Gordon model (A)

Marshall Hall

J. Acoust. Soc. Am. Volume 71, Issue S1, pp. S11-S11 (1982); (1 page)

Online Publication Date: 12 Aug 2005

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Solutions to the wave equation for sound‐speed profiles which are terminated by an isospeed half‐space can be expressed as a sum of discrete modes plus a branch‐line integral (BLI). The BLI is often insignificant, but can be of great significance in duct propagation at frequencies near the cutoff freqency of the duct. In one example [D. C. Stickler, J. Acoust. Soc. Am. 57, 856–‐861 (1975)], the BLI dominated the mode contribution for ranges out to 30 km. The Pedersen‐Gordon normal‐mode model, on the other hand, terminates the profile with a “furry” half‐space in which the sound speed approaches zero as z−1/2 as the depth (z) approaches infinity. The Pedersen‐Gordon model is applied to Stickler's shallow‐water duct, and the limit of the results for the sound field (based on only a mode sum), as the sound‐speed gradient in the half‐space (evaluated at its interface with the duct) approaches zero, is determined. This limit is found to be equal to Stickler's result for the sum of both the BLI and the mode contributions. The theoretical reasons for this equality are discussed.
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Comparison between perturbative and exact treatment of bottom attenuation for shallow‐water, low‐frequency conditions (A)

DeWayne White and Stanley A. Chin‐Bing

J. Acoust. Soc. Am. Volume 71, Issue S1, pp. S11-S11 (1982); (1 page)

Online Publication Date: 12 Aug 2005

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A comparison between treating bottom attenuation perturbatively and exactly in a normal‐mode expansion has been made for a representative shallow‐water, low‐frequency problem. The exact treatment introduces bottom attenuation through complex sound speed and thus complex depth functions, whereas the perturbative approach uses real valued depth functions and introduces mode attenuation only in the range function. Cases examined indicate transmission loss versus range calculations resulting from the two approaches disagree significantly near cutoff. Comparison with FFP calculations indicates the exact (complex depth function) solution is correct. When more than one mode is present, the other modes dominate the mode nearest cutoff; and although this mode is in error, it does not seriously affect the results. For those cases where the perturbative and exact solutions significantly differ, the error in the perturbative approach is mostly due to an incorrect normalizing factor; mode attenuation differences also account for part of the error. Results are discussed and a correction to the mode normalization factor derived from the perturbative solution is suggested.
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Investigation of forward sound field amplitude and phase fluctuations by boundary perturbations in a normal‐mode duct (A)

Michael R. Layton

J. Acoust. Soc. Am. Volume 71, Issue S1, pp. S11-S11 (1982); (1 page)

Online Publication Date: 12 Aug 2005

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A physical model of a shallow‐water duct has been constructed with a 5‐cm (4λ) isovelocity water layer overlying a 10‐cm homogeneous sand bottom. Using a projector operating cw at 100 kHz, a propagating normal‐mode pressure distribution consisting primarily of the lowest‐order mode was established in the duct. A pair of small hydrophones were then positioned in the water layer at identical range and depth, with a horizontal separation between them on the order of 1.5 cm (one wavelength). Prior to investigating the effect of a small perturbation either on the water surface or at the water‐sand boundary, the difference between the received signals from each hydrophone was nulled using electronic means. The differential signal behavior was then examined for a variety of experimental conditions, which included surface roughness, bottom roughness, receiver depth, receiver range (from projector), and receiver separation. [Work supported by ONR and DARPA.]
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