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May 1984

Volume 75, Issue S1, pp. S1-S93

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back to top Session O. Underwater Acoustics II: High Frequency I
Invited Papers
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Surface and volume backscattering of broadband acoustic pulses normally incident on the sea floor: Observations and models (A)

D. J. Dodds

J. Acoust. Soc. Am. Volume 75, Issue S1, pp. S29-S29 (1984); (1 page)

Online Publication Date: 12 Aug 2005

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When a broadband (1–10 kHz) impulsive (0.2‐ms duration) acoustic source is directed vertically at the sea floor, the resulting echo contains a significant proportion of energy which does not remain coherent during horizontal translations of the acoustic system. This indicates that scattering is taking place. The scattering effects are a function of frequency, and the interaction of the source beam function with the bottom produces a time dependence as the expanding wave front encounters the sea floor at an increasing distance off the axis of the source. These effects can be displayed in a contour diagram of signal power as a function of time and frequency, called a sonogram. By using models of surface and volume scattering, a synthetic sonogram can be calculated from parameters of volume scattering and surface roughness. Such a synthetic sonogram can be fitted to an actual sonogram, yielding estimates of the parameters. These parameters promise to be useful in characterizing sediments, their acoustic properties, and their surfaces.
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Acoustic volume scattering: 100 kHz to 10 MHz (A)

D. V. Holliday and R. E. Pieper

J. Acoust. Soc. Am. Volume 75, Issue S1, pp. S29-S29 (1984); (1 page)

Online Publication Date: 12 Aug 2005

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High resolution profiles of acoustic volume scattering strength were made to a depth of 100 m at 18 frequencies spaced approximately logarithmically in a band between 100 kHz and 10 MHz. Stations were located in a complex marine environment extending offshore from Los Angeles, CA. High resolution temperature, conductivity, and chlorophyll fluorescence data were also collected and comparisons are made with the acoustic profiles. Interpretations of the data are made in relation to size and abundance of zooplankton collected at selected depths during the same cruise. Comparisons are also made to data collected in different seasons in the same geographic area. [Work supported by ONR and NSF.]
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Bubbles near the surface of the ocean and their influence on wind‐generated ambient noise (A)

David M. Farmer and David D. Lemon

J. Acoust. Soc. Am. Volume 75, Issue S1, pp. S29-S29 (1984); (1 page)

Online Publication Date: 12 Aug 2005

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Bubbles of radius 40–400 μm are formed by breaking wind waves and are known to influence significantly air‐sea gas exchange. Do they also modify wind generated ambient noise? For frequencies greater than 500 Hz, wind‐generated noise typically has a constant spectral slope, but a layer of bubbles will scatter and absorb the sound, changing its spectral and directional properties at high wind speed. Observations in Queen Charlotte Sound, B. C., in bands centered at 4.3, 8.0, 14.5, and 25.0 kHz illustrate these effects; at sufficiently high wind speeds noise at 14.5 and 25.0 kHz actually decreases with increasing speed. The changes in spectral slope as a function of wind speed and frequency allow bubble populations and size distributions to be inferred. These were found consistent with previous photographic and bubble trap measurements, but the range of wind speeds encountered permits determination of a more complete relationship. Scattering and absorption by the bubble layer has implications for the use of ambient noise in passive remote sensing of wind speed and precipitation.
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Hybrid ray‐mode methods for underwater acoustic propagation (A)

L. B. Felsen

J. Acoust. Soc. Am. Volume 75, Issue S1, pp. S29-S30 (1984); (2 pages)

Online Publication Date: 12 Aug 2005

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Ray fields and guided mode fields form alternative and complementary building blocks for modeling high‐frequency acoustic propagation in the ocean environment. Since the ocean waveguide is large compared to the local wavelength, a mode formulation requires many modes. In the presence of surface ducts and submerged ducts, a ray formulation at long ranges requires many rays. Since either formulation is therefore inconvenient, a hybrid combination in terms of rays and modes can introduce efficiencies and greater physical clarity. The foundation for the hybrid scheme is the ray‐mode equivalent that permits a finite spectral interval of continuously distributed local plane‐wave fields to be filled either with rays or with modes plus a truncation remainder. The equivalent can then be used to eliminate troublesome ray or mode spectral intervals and replace these by troublefree modes or rays, respectively. Exact for range‐independent environments, including those with elastic layers where P‐SV coupling may occur and ray species proliferate in consequence, the equivalent can also be formulated approximately for weak range dependence through use of adiabatic invariants. These concepts are developed, quantified, and illustrated on examples involving modal substitution for ray clusters in caustic forming surface ducts, for bottom glancing and refracting ray transitions, for collective treatment of ray fields in elastic layers, and for ocean channels with sloping penetrable bottom. Alternatively, rays are efficient for tracking interference maxima of groups of modes. Implications of these concepts for time‐dependent propagation are also discussed. [Work supported by ONR, Ocean Acoustics Branch.]
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Scale‐model studies of normal mode field (A)

E. C. Shang, C. S. Clay, and Y. Y. Yang

J. Acoust. Soc. Am. Volume 75, Issue S1, pp. S30-S30 (1984); (1 page) | Cited 1 time

Online Publication Date: 12 Aug 2005

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In recent years scale‐model studies of normal mode fields in waveguides have been conducted by the authors both in the Institute of Acoustics, Academia Sinica, the Institute of Oceanography in China, and in the Department of Geology and Geophysics, University of Wisconsin‐Madison in the United States. Some results concerning the implementation of mode filtering and some new techniques of signal processing in waveguides are summarized. The results of the study of source location‐source ranging, source depth estimation, and source bearing in waveguides are presented.
Contributed Papers
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High‐frequency propagation modeling using HYPER (A)

Fred Tappert, Ding Lee, and Henry Weinberg

J. Acoust. Soc. Am. Volume 75, Issue S1, pp. S30-S30 (1984); (1 page)

Online Publication Date: 12 Aug 2005

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The hybrid parabolic equation‐ray (HYPER) model previously described [J. Acoust. Soc. Am. Suppl. 1 74, S96 (1983)] has been further developed both theoretically and numerically. The small‐angle Newtonian ray equation that is consistent with the parabolic approximation has been integrated numerically through strongly range‐dependent ocean environments and compared to the exact rays with satisfactory agreement. The modified parabolic equation that correctly describes amplitude and phase within a ray bundle has been integrated numerically at kHz frequencies through strongly range‐dependent ocean environments and the caustic structure at convergence zones has been examined. Comparisons to available caustic theories show that the HYPER model provides fully diffractive results that are uniformly valid.
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Acoustic backscattering at low grazing angles from the ocean bottom. I. Bottom backscattering strength (A)

H. Boehme, N. P. Chotiros, L. D. Rolleigh, S. P. Pitt, A. L. Garcia, T. G. Goldsberry, and R. A. Lamb

J. Acoust. Soc. Am. Volume 75, Issue S1, pp. S30-S30 (1984); (1 page)

Online Publication Date: 12 Aug 2005

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Acoustic backscattering measurements on a sand bottom were made at grazing angles in the range of about 2°–10° in water depth of approximately 15.5 m near San Diego, CA [T. G. Goldsberry, S. P. Pitt, and R. A. Lamb, J. Acoust. Soc. Am. Suppl. 1 72, S74 (1982)]. Data from these measurements have been analyzed to determine the mean value and standard deviation of the bottom backscattering strength per m2 as a function of grazing angle, insonified area, transmit signal type, and frequency. A curved ray path propagation model and measured sound speed profiles were used to determine grazing angle versus time. The mean value followed Lambert's law for the range of grazing angles measured and for all frequencies used. No significant differences in mean value were observed when the insonified area and transmit signal type were varied. The observed frequency dependence of the bottom backscattering strength per m2 falls in the range from f1.5 to f1.8 for this relatively flat, sandy bottom. [Work supported by NAVSEA 63R and NORDA Code 113.]
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Acoustic backscatter at low grazing angle from the ocean bottom. II: Statistical characteristics of bottom backscatter at a shallow water site (A)

N. P. Chotiros, H. Boehme, T. G. Goldsberry, S. P. Pitt, R. A. Lamb, A. L. Garcia, and R. A. Altenburg

J. Acoust. Soc. Am. Volume 75, Issue S1, pp. S30-S30 (1984); (1 page)

Online Publication Date: 12 Aug 2005

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Analyses of the statistical characteristics of bottom backscatter, measured in shallow water off San Diego, CA [T. G. Goldsberry, S. P. Pitt, and R. A. Lamb, J. Acoust. Soc. Am. Suppl. 1 72, S74 (1982)] are presented. An experimental sonar, operating at 30 kHz, mounted on the sea bottom was used to gather data over a wide sector of the bottom within its operating range. The bottom was comprised of areas of coarse and fine sand. The distribution function and probability of false alarm function of the detected envelope of a “widebeam” and a “narrowbeam” signal were measured. Some spatial and temporal correlation functions of the signal amplitudes were measured. A limited attempt was made to compare the results with existing theoretical models. [Work supported by NAVSEA 63R and NORDA Code 113.]
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A semi‐empirical model for high‐frequency bottom backscattering (A)

Darrell R. Jackson

J. Acoust. Soc. Am. Volume 75, Issue S1, pp. S30-S30 (1984); (1 page)

Online Publication Date: 12 Aug 2005

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Expressions have been developed for bottom scattering strength as a function of frequency and four bottom parameters. These parameters are rms roughness. sound speed, mass density, and a volume scattering parameter. Procedures have been developed for assigning best values to these parameters for poorly characterized sites, e.g., for sites at which mean sediment grain size or porosity is the only known quantity. This model is based upon the composite roughness and Kirchhoff approximations, but empirical adjustments have been made to fit data that lie outside the region of applicability of these approximations. [Work supported by NAVSEA.]
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Angle spreads of broadband high‐frequency signals reflecting from a random surface (A)

Michael H. Brill and Xavier Zabal

J. Acoust. Soc. Am. Volume 75, Issue S1, pp. S30-S31 (1984); (2 pages)

Online Publication Date: 12 Aug 2005

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A simple geometric‐acoustic model (appropriate for high frequencies) has recently been used to compute time and frequency spreading of a broadband acoustic signal reflecting from an ocean bottom [M. H. Brill, X. Zabal, and S. L. Adams, J. Acoust. Soc. Am. Suppl. 1 73, S11 (1983); also 74, S76 (1983)]. In that model, the expected power received from a bottom facet dxdy at (x,y,0) is computed as a function of the x and y coordinates of the reflecting point on the bottom. A ray arriving from any reflecting point has a well‐defined travel time, relative frequency shift (for a given source and receiver motion), polar arrival angle, and azimuthal arrival angle. In the present work, the received power from a grid of facets is histogrammed in the two arrival angles. The resulting histogram, called the angle‐spread function, is a measure of the spatial coherence of the reflected signal. Computed angle‐spread functions will be presented for various source/receiver geometries, and implications for sonar signal processing will be discussed.
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High resolution bottom backscatter measurements (A)

W. I. Roderick, R. K. Dullea, and J. M. Syck

J. Acoust. Soc. Am. Volume 75, Issue S1, pp. S31-S31 (1984); (1 page)

Online Publication Date: 12 Aug 2005

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Acoustic bottom backscattering measurements and the corresponding geoacoustic properties of the ocean bottom are presented for an experiment conducted in the shallow waters of the North Atlantic. The bottom scattering strength data, which were obtained with a high resolution (narrow beamwidth) parametric sonar, were measured as a function of frequency (5–20 kHz), grazing angle (4°–10°), azimuthal angle (± 55°), and pulse length (0.4–10 ms). The supporting environmental measurements included box cores for determining the acoustic properties of the sediment and stereo photography for calculating the two‐dimensional roughness spectrum of the sea floor. [Work supported by NAVSEA 63R.]
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High‐frequency acoustic backscatter sea surface (A)

W. I. Roderick, R. K. Dullea, and J. B. Chester

J. Acoust. Soc. Am. Volume 75, Issue S1, pp. S31-S31 (1984); (1 page)

Online Publication Date: 12 Aug 2005

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A high resolution acoustic surface scattering experiment was conducted in the shallow waters of the North Atlantic. A narrow beam parametric array, which was rotatable in both azimuth and elevation, was utilized as a broadband high‐frequency acoustic projector. Acoustic surface scattering data were obtained at normal incidence and low grazing angles (less than 10°) as a function of acoustic transmit frequency and sea state conditions. Meteorologic and oceanographic data were obtained in concert with the acoustic measurements and included wind speed and direction, ocean surface wave spectra and currents, and ocean sound speed. Surface back‐scattering strength, Doppler spectra (shift and spread), and envelope statistics were some of the measured parameters. It will be shown that the Doppler spectra are approximately Gaussian and the spectral shift could be predicted from Bragg diffraction theory modified by the induced Doppler due to surface currents. At normal incidence, the surface loss varied 20 dB as the frequency changed from 5 to 80 kHz, under nearly constant sea surface conditions. [Work supported by NAVSEA 63R.]
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