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Nov 1982

Volume 72, Issue S1, pp. S1-S108

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back to top Session II. Underwater Acoustics V: Propagation (Précis‐Poster Session)
Contributed Papers
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A study of secondary sound channels due to temperature inversions in the Northeast Pacific Ocean (A)

R. K. Chow and D. G. Browning

J. Acoust. Soc. Am. Volume 72, Issue S1, pp. S57-S57 (1982); (1 page)

Online Publication Date: 12 Aug 2005

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Above 45 °N latitude the circulation of the Northeast Pacific Ocean is controlled by the counterclockwise Alaskan Gyre of subarctic water. Due to a greater freshwater influx than evaporation, the water column is characterized by the three distinct layers: 0–100 m depth, cold, low salinity water; 100–200 m, zone of strong salinity gradient (halocline); 200 m and below, gradual changes in temperature and salinity. Roden [J. Geophys. Res. 69, 2899–2914 (1964)] has shown that the middle halocline layer is the site of a complex temperature structure with numerous temperature inversions. These temperature inversions result in the formation of secondary sound channels which our analysis shows are widely distributed. Using a PE prediction model the cutoff frequency of a typical secondary sound channel was determined to be approximately 80 Hz. Since depth excess exists at most locations, energy not trapped in the secondary duct remains in the deep sound channel. Relative loss in the two channels is presented as a function of frequency (25–200 Hz) and range (0–100 km) for various source and receiver depths. Under optimum conditions the existence of a secondary sound channel can decrease the average propagation loss by 20 dB.
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Low‐frequency attenuation in the North Pacific subarctic‐subtropical transition zone (A)

D. G. Browning and R. K. Chow

J. Acoust. Soc. Am. Volume 72, Issue S1, pp. S57-S58 (1982); (2 pages)

Online Publication Date: 12 Aug 2005

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The water mass regions in the North Pacific are relatively well defined. Kibblewhite et al. [J. Acoust. Soc. Am. 61, 1169–1177 (1977)] have shown that each water mass has a characteristic low frequency (below 200 Hz) attenuation. They find the highest attenuation to be in the Subarctic‐Subtropical Transition Zone and these values are the highest yet reported in any ocean for this frequency range. It has recently been reported by Focke et al. [J. Acoust. Soc. Am. 71, 1438–1444 (1982)] that such values of attenuation can be modeled using a frequency independent mechanism, exponentially decreasing from the surface, the origin of which is unspecified. We compare these results with attenuation coefficients we have obtained in this zone. The oceanographic and biological properties of this region are analyzed to determine possible scattering mechanisms and their distribution. The effect of the two frontal areas bounding this region on the determination of attenuation coefficients is also examined.
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More on replacing transitional acoustic ray fields by a bundle of modes (A)

E. Niver, A. Kamel, and L. B. Felsen

J. Acoust. Soc. Am. Volume 72, Issue S1, pp. S58-S58 (1982); (1 page)

Online Publication Date: 12 Aug 2005

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The failure of ray acoustics in transition regions near single and multiple caustics, bottom glancing and critically incident rays in a vertically stratified ocean can be repaired by replacing these ray fields with a bundle of guided modes plus truncation remainders. The theory for this equivalence is exact, and has been verified by direct numerical implementation of the various constituents [E. Niver, A. Kamel, and L. B. Felsen, J. Acoust. Soc. Am. Suppl. 1 71 S66 (1982)]. The study has now been extended by treating the remainder not only as a modified (“collective”) ray field as presented previously [L. B. Felsen, J. Acoust. Soc. Am. 69, 352 (1981)] but alternatively as a modified (collective) modal field. Moreover, asymptotic considerations have been employed to simplify the computer program. Detailed numerical comparisons for a model profile reveal the accuracy and numerical efficiency of various mode and remainder options as applied to the transitional ray fields noted above. It is shown that modal replacement of rays near single or multiple caustics also accounts for non‐negligible evanescant ray fields. Moreover, it is preferable to treat the remainders as a modified modal field instead of a modified ray field since the width of the equivalent mode bundle can be reduced thereby. [Work supported by ONR Ocean Acoustics Branch.]
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A comparison of acoustic propagation predictions for surface duct environments using ray‐theoretic and parabolic‐equation computer codes (A)

A. Tolstoy, E. R. Franchi, and K. R. Nicolas

J. Acoust. Soc. Am. Volume 72, Issue S1, pp. S58-S58 (1982); (1 page)

Online Publication Date: 12 Aug 2005

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An important type of under water acoustic environment involves a deep water SOFAR channel with a superimposed (winter) surface duct capable of trapping acoustic energy. In this paper we examine the effects predicted by a computer model of three ducted sound‐speed profiles on acoustic propagation at 300 Hz. The purpose of this study was to better understand the capabilities and limitations (both theoretical and numerical) of a ray theoretic model for predicting mid‐ and long‐range signal transmission loss (TL) in the winter North Atlantic. Several source and receiver configurations (combinations with one or both in and out of the duct) have been studied. To test validity, the ray theoretic calculations are compared to previously accepted results predicted by a propagation model based upon the parabolic equation. It is found that both model predictions agree qualitatively (TL measurements and acoustic field patterns are comparable), while any quantitative discrepancies are explained by the theoretical or numerical limitations of the different approaches. We conclude that when both the source and receiver are at least several wavelengths from the sea surface, ray theory can be quite accurate in its prediction for these ducted environments. [Work supported by Naval Electronic Systems Command, Code 612.]
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Multipath propagation in a deep ocean including modified rays near grazing incidence at the bottom (A)

Ronald F. Pannatoni

J. Acoust. Soc. Am. Volume 72, Issue S1, pp. S58-S58 (1982); (1 page)

Online Publication Date: 12 Aug 2005

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A ray description of acoustic propagation between communicators submerged in a deep ocean may include rays that almost graze the interface between water and sea‐floor sediment. For example, this can occur in range‐independent environments where the squared refractive indices in water and in sediment decrease linearly with depth. Four pairs of such rays connect communicators near the surface and separated in range by at most 4(H/γ)1/2, where H is the ocean depth and γ is the gradient of square refractive index in water. One ray in each pair is entirely waterborne; the other intercepts the sediment and grazes a caustic. We present a uniform asymptotic analysis of these ray pairs that incorporates diffractive finite frequency effects as in the modified ray theory of Murphy and Davis. These effects include the appearance of a frequency‐dependent caustic and a modification of the usual π/2 phase shift experienced by the classical ray that intercepts the sediment. Comparison of these results with normal mode calculations is excellent. Vector diagrams (after Bartberger) also identify mode groups corresponding to the modified rays.
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A formula for the ray density over a caustic surface for variable index media (A)

Donald G. Burkhard

J. Acoust. Soc. Am. Volume 72, Issue S1, pp. S58-S58 (1982); (1 page)

Online Publication Date: 12 Aug 2005

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The acoustic ray density becomes infinite over caustic surfaces and therefore cannot be used as a measure of the concentration of rays at or near the caustic. A finite substitute measure, the density of rays tangent of the caustic, may be obtained by dividing an element of incident flux by the area of the caustic formed by the associated rays. This provides a measure of the energy density over different regions of the caustic. As an example the ray density over the caustic is evaluated for a linear depth dependent index of refraction.
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Application of two‐variable Taylor series to the ray theory of propagation in an unbounded medium (A)

Marshall Hall

J. Acoust. Soc. Am. Volume 72, Issue S1, pp. S58-S58 (1982); (1 page)

Online Publication Date: 12 Aug 2005

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The ensonified acoustic field near the initial caustic in the first convergence zone is considered. This field is formed by a pair of rays that are heading downward at the source and upward at the receiver. The ray parameter (namely the phase velocity) of each ray is expressed as a two‐variable Taylor series in horizontal range and depth relative to a fixed expansion point. The terms of the series are calculated for derivatives up to fifth order. The method is applied to a typical oceanic example in which the caustic of the rays passes the expansion point at a vertical distance of 15 m and at a horizontal distance of 200 m. The Taylor‐series phase velocities are inserted into the usual ray theory expressions for the relative energy and travel time. It is found that satisfactory accuracy is obtained over a region of the ocean that is sufficiently large for several sonar applications. The advantage of this new approach is that, to calculate the sound field at a grid of points within a region, it is sufficient to determine (by iteration) the ray parameters of only the two rays that pass through the expansion point.
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Sound propagation in a surface duct: Can the deep water profile be neglected? (A)

Marshall Hall

J. Acoust. Soc. Am. Volume 72, Issue S1, pp. S58-S58 (1982); (1 page)

Online Publication Date: 12 Aug 2005

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In calculating normal‐mode acoustic propagation in an ocean surface duct, the sound‐speed profile is often represented by the simple bilinear profile. The question arises as to whether the positive gradient in the sound‐speed profile that occurs beneath the SOFAR axis has a significant effect on surface duct propagation. Calculations have been made for typical oceanic profiles and it has been found that the bilinear profile and the complete profile yield the same result as a function of range (even though the individual normal modes are different) until the convergence zone effect becomes significant. The threshold range at which this effect becomes significant decreases as frequency is decreased below the surface duct's cutoff frequency (since the energy propagated within the duct decreases while the width of the convergence zone increases). For a duct that has a cutoff frequency of 200 Hz for example, the threshold range decreases from 30 to 15 km as the frequency decreases from 200 to 100 Hz.
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Acoustic sensitivity to sound‐speed profile selection in the deep ocean (A)

P. Bilazarian, W. L. Siegmann, and M. J. Jacobson

J. Acoust. Soc. Am. Volume 72, Issue S1, pp. S58-S59 (1982); (2 pages)

Online Publication Date: 12 Aug 2005

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The effect of the selection of a deep‐ocean sound‐speed profile on the sensitivity of acoustic receptions is examined. In our initial study [J. Acoust. Soc. Am. Suppl. 69, S34 (1981)], the fixed sound source and receiver were positioned on the ocean boundaries. In this work, a variety of locations of submerged source and receiver, with separations of less than 50 km, is considered so that different types of ray propagation must be examined. Given a particular profile, procedures are prescribed for constructing a simpler profile for which all significant ray geometric and acoustic quantities, such as ray types, numbers, phase, and amplitudes, are either identical or virtually so. Also, the sensitivity of performance measures for horizontal linear receiving arrays to profile selection is studied. Conditions on array length and source‐receiver range and orientation are developed for which a simpler profile may replace a given profile, and still maintain essentially equivalent array performance. [Work supported by ONR.]
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Analysis of short‐range sound transmission through fronts in shallow water (A)

T. H. Rousseau, M. J. Jacobson, and W. L. Siegmann

J. Acoust. Soc. Am. Volume 72, Issue S1, pp. S59-S59 (1982); (1 page)

Online Publication Date: 12 Aug 2005

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Effects of sound‐speed variations, produced by fronts in shallow‐ocean regions, on short‐range ray transmissions between fixed source and receiver are investigated. A parametric model is used in which the front is represented by a sound‐speed jump in an otherwise constant‐speed channel. The propagation and frontal models are sufficiently general to permit determination of acoustical effects for fronts of varying strengths and for arbitrary orientations and positions relative to the propagation range. Frontal influences on travel time and geometrical spreading loss are determined, and expressions for per‐ray and total field quantities are developed for various source and receiver depths. Using our results, it is shown how the determination of the location of a source is influenced by the presence of a front. Further, it is demonstrated how predictions of the location, strength, and orientation of fronts may be obtained from acoustic receptions at a receiver using the known location of a source. [Work supported by ONR.]
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Model experiments on the spatial altering of modes (A)

Yunyu Wang, Ji‐xun Zhou, Hong‐hai Li, Zhen‐zhong Zhang, Shen‐li Jiang, Er‐chang Shang, Qing‐hua Bao, Long‐jiang Zu, and Guo‐guang Yan

J. Acoust. Soc. Am. Volume 72, Issue S1, pp. S59-S59 (1982); (1 page)

Online Publication Date: 12 Aug 2005

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The spatial filtering of modes is very important both for theoretical and practical purposes. Spatial filtering of modes can verify the normal‐mode theory and is also a way to match the waveguide fields. It will have an important application in the investigation of the interaction between the sound wave and environmental factors such as the acoustic characteristics of ocean bottom, internal wave,…etc., because it is easier to study separate modes. In this paper, the model experiments include the following: (1) The spatial filtering of modes in a long water tank without absorbers on the side wall. (2) The suppression of the near interference point source by means of selected mode. (3) The effects of surface waves on each individual mode and the sum of all modes at a point receiver.
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Normal mode filtering in shallow water (A)

En‐cen Lo, Ji‐xun Zhou, and Er‐Chang Shang

J. Acoust. Soc. Am. Volume 72, Issue S1, pp. S59-S59 (1982); (1 page)

Online Publication Date: 12 Aug 2005

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The filtering of mode 1 and mode 2 by using a vertical array of nine hydrophones has been realized in the frequency range of 250–800 Hz in the shallow water of the Yellow Sea with a near‐perfect isovelocity condition. The eigenfunctions of the mode were calculated by using two parameters (P, Q) to describe the characteristics of the bottom reflections approximately at small grazing angles. The results of the mode filtering were quite good: (1) The group delay measurements of mode 1 and mode 2 agree well with the theoretical value calculated by using the bottom reflection phase shift parameter P; (2) The parameter Q of the bottom reflection loss can be extracted from the amplitude ratio of mode 1 and mode 2. The extracted values of Q were quite near to the values of Q obtained by other approaches.
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Some theoretical problems on mode filtering in shallow water (A)

Er‐Chang Shang and Tain‐Fu Gao

J. Acoust. Soc. Am. Volume 72, Issue S1, pp. S59-S59 (1982); (1 page)

Online Publication Date: 12 Aug 2005

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Since the mode filtering is a certain sort of “space filtering,” the matter of the utmost concern is the “side lobe” produced by some simplified processing in practice instead of the ideal orthogonal operation. In this paper the following problems were discussed theoretically: (1) “side lobe” produced by using an integration in water column instead of half‐space; (2) “side lobe” produced by using a finite discrete sampling; (3) “side lobe” produced by using a hard‐clipped eigenfunction; (4) “side lobe” produced by the inclination of the vertical array. Some analytic results and numerical examples were shown for a special case in which the sound‐speed profile is isovelocity in water column. It was found that the effect of array inclination was serious and a phase compensation approach was proposed.
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A coupled mode solution for acoustic propagation in a waveguide with stepwise depth variations (A)

Richard B. Evans

J. Acoust. Soc. Am. Volume 72, Issue S1, pp. S59-S59 (1982); (1 page)

Online Publication Date: 12 Aug 2005

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A coupled mode solution is formulated for the problem of acoustic propagation in a cylindrically symmetric ocean divided, in range, into a finite number of adjoining Pekeris waveguides of differing water depths. The problem is discretized by assigning a pressure release boundary condition at some large finite depth and including sufficient attenuation to exclude any significant energy returning to the water. This formulation includes backscatter from the depth variations of the waveguide and full cross coupling between a finite number of propagating and nonpropagating modes determined by the range and strength of the depth variations. Numerical results based on an implementation of this solution are presented. [Work supported by NORDA.]
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An improved numerical ordinary‐differential‐equation solution to the parabolic wave equation (A)

Ding Lee and Kenneth Jackson

J. Acoust. Soc. Am. Volume 72, Issue S1, pp. S59-S59 (1982); (1 page)

Online Publication Date: 12 Aug 2005

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The solution of the parabolic wave equation can be approximated by the solution of a stiff Initial Value Problem (IVP) associated with a system of Ordinary Differential Equations. The Generalized Adams Methods (GAM) based upon a stable rational approximation to the matrix exponential has been shown to be an effective method for solving this problem. However, previous implementations of the GAM suffered from severe storage limitations because of the use of an explicit rational approximation to the matrix exponential. In this presentation, we describe a new implementation of the GAM that is based upon an implicit use of a “Restricted‐Pade” approximation to the matrix exponential. This implementation is particularly effective for problems for which the associated matrix is banded, as is the case for the IVP approximating the parabolic wave equation. A problem of propagation in a wedge shaped region is presented to illustrate the strong limitation of the previous implementation and how this limitation is removed in the new implementation. [Work is supported by ONR.]
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A comparison of two range‐dependent parabolic equation models: IFD and split‐step (A)

Ding Lee, Martin H. Schultz, and Kenneth E. Gilbert

J. Acoust. Soc. Am. Volume 72, Issue S1, pp. S59-S60 (1982); (2 pages)

Online Publication Date: 12 Aug 2005

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The introduction of the IFD (implicit finite‐difference) model for the prediction of propagation loss offers users an additional tool for solving the parabolic wave equation. Numerical results comparing the IFD and split‐step models for a set of selected problems using the VAX 11/780 computer are presented. In particular, we compare these two models with respect to generality, accuracy, speed, user effort, flexibility, reliability, and other features. [Work supported jointly by NUSC and NORDA.]
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Attenuation of low‐ and high‐frequency waves in a random medium (A)

Alan R. Wenzel

J. Acoust. Soc. Am. Volume 72, Issue S1, pp. S60-S60 (1982); (1 page)

Online Publication Date: 12 Aug 2005

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A theoretical analysis of the wavefield radiated by a point source in a one‐dimensional random medium has been carried out. Approximate expressions for the mean intensity and mean energy flux as a function of propagation range, based on general results obtained previously, have been derived for the limiting cases of low and high frequencies. These expressions show that, in both cases, the mean intensity and mean energy flux decrease more rapidly with range than would be the case in the absence or randomness. This more rapid rate of decrease with range can be interpreted as an excess attenuation. These results are similar, in general, to those obtained previously for the intermediate‐frequency case. The low‐frequency results are found to be in qualitative agreement with observations of excess attenuation of sound propagating through turbulence. At high frequencies, the results indicate that the excess attenuation is always much smaller than the attenuation due to absorption. This is also in agreement with observations, which generally show no significant excess attenuation at high frequencies. [Work supported by NORDA.]
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Signal statistics near a caustic in the presence of fluctuations (A)

Thomas L. Clarke

J. Acoust. Soc. Am. Volume 72, Issue S1, pp. S60-S60 (1982); (1 page)

Online Publication Date: 12 Aug 2005

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Signal fluctuations in the field near a caustic produced by random inhomogeneities in sound speed are examined. Statistics are calculated by Monte‐Carlo simulation based on a phase‐screen approximation. The diffraction parameter for the phase‐screen approximation serves to generalize the Λ parameter. The statistics are found to depend on the strength of the fluctuations in a complicated way when the generalized diffraction parameter is near unity. In particular, the scintillation index is large at intensity minima even for weak fluctuations.
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Short‐range, high‐frequency acoustic propagation fluctuations (A)

Robert J. Vent and G. Thomas Kaye

J. Acoust. Soc. Am. Volume 72, Issue S1, pp. S60-S60 (1982); (1 page)

Online Publication Date: 12 Aug 2005

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A series of one‐way high frequency acoustic propagation experiments were conducted off southern California in an area east of San Clemente Island. Source and receiver were located at depths of 137 and 107 m, respectively, below the diurnal thermocline. The source frequency was 30 kHz; the pulse duration was 0.5 ms and the pulse repetition rate was 0.5 s−1. A series of fixed ranges were used extending from 180 to 920 m. In addition, temperature fluctuations were observed over the experiment depth range with a thermistor array. For a representative data set, energy density spectra of the acoustic and temperature fluctuations reveal similar forms. At a range of 385 m with a record length of 1 h (1800 transmissions), the standard deviation of the received acoustic intensity was 0.5 dB with a coefficient of variation of 12%. The sound‐speed fluctuation patch size was calculated to be 1 m. [Work supported by NAVSEA.]
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Propagation in a random medium: An experiment using heating and cooling in a large water tank (A)

Joe W. Posey and Coleman Levenson

J. Acoust. Soc. Am. Volume 72, Issue S1, pp. S60-S60 (1982); (1 page)

Online Publication Date: 12 Aug 2005

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One millisecond tone bursts in the frequency range 0.8 to 2.0 MHz were propagated to a range of 10 m through a water tank having a 3.7×3.7 m cross section. Sound speed distribution along the propagation path was randomized by either heating or cooling. The heating technique has been used by several previous investigators. This method has been criticized because degassification bubbles rise through the propagation path and may act as resonant scatterers. The cooling approach does not produce bubbles. A three‐dimensional, fast response thermistor array was used to measure temporal and spatial statistics of the temperature fluctuations. Variations in the complex amplitude of received acoustic signals are compared with Wenzel's theory [AIAA paper 75–546 (1975)]. He defines the coherent wave as having the mean complex amplitude of the total wave and derives a conservation relation for the sum of coherent and incoherent wave energy. He predicts that coherent energy will decay exponentially with characteristic length and mean‐square amplitude of sound speed fluctuation as well as with range and signal frequency squared.
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