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Nov 1989

Volume 86, Issue S1, pp. S1-S125

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back to top Session M. Underwater Acoustics II: High‐Frequency Acoustic Imaging
Invited Papers
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The STRESS sonar experiment (A)

Darrell R. Jackson and J. George Dworski

J. Acoust. Soc. Am. Volume 86, Issue S1, pp. S32-S32 (1989); (1 page)

Online Publication Date: 13 Aug 2005

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A high‐frequency scanning sonar experiment has been conducted as part of the STRESS Program (sediment transport events: slopes and shelves). An autonomous platform was used to scan a circular region 100 m in radius with 5° angular resolution. The acoustic frequency was 40 kHz and the transmitted signal was an FM pulse of 2‐kHz bandwidth. The objective was to quantify changes in bottom scattering due to storm‐induced sediment transport events. The acoustic experiment was conducted in water of depth 90 m at a silty site off the California coast. Other experiments measured environmental parameters such as sediment properties, bottom currents, and suspended sediment concentration. Scans were made at a rate of ten per day over a period of 13 weeks during the winter of 1988–1989. The acoustic data have been processed to determine scattering strength and the correlation between scans as a function of time separation. These results are presented in the form of images showing the evolution of the scattering process during quiet and stormy periods. Very little change is seen in the scattering strength, but the correlation magnitude exhibits a monotonic decrease that accelerated during a storm. The phase of the complex correlation provides a very sensitive indicator of small changes. Two applications of these phase data will be presented. [Work supported by ONR.]
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High‐frequency acoustic imaging of the seafloor (A)

C. de Moustier

J. Acoust. Soc. Am. Volume 86, Issue S1, pp. S32-S33 (1989); (2 pages)

Online Publication Date: 13 Aug 2005

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For decades, sidescan sonars have been the primary tool to obtain acoustic images of the seafloor. Such images provide qualitative information on the seafloor surveyed based on amplitude variations of the backscattered acoustic signals received. In the 1980s, bathymetric sidescan sonar systems, capable of simultaneously producing acoustic images and measuring depth at numerous points across the swath, added a quantitative description of the seafloor in the form of a depth contour map. Similar claims can be made with multibeam echo sounders well known for their high‐resolution swath bathymetry capabilities. Taking advantage of this high bathymetric resolution, the beamformed acoustic backscatter data can also be displayed as a geometrically correct acoustic image of the seafloor and provide textural information not available in the contoured bathymetry of the same area. Likewise, knowledge of the bathymetry, particularly bottom slopes, is needed to correct for the angular dependence of seafloor acoustic backscatter and construct a map of acoustic backscattering strength over the area. Such a map will give clues to regional variations in lithelogies.
Contributed Papers
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Evaluation and verification of bottom acoustic reverberation statistics predicted by the point scattering model (A)

D. Alexandrou, G. Haralabus, and C. de Moustier

J. Acoust. Soc. Am. Volume 86, Issue S1, pp. S33-S33 (1989); (1 page)

Online Publication Date: 13 Aug 2005

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The point scattering model offers a parametrization of the reverberation envelope probability density function (pdf) in terms of the average number of scatterers contributing to the return and the presence of a coherent component in the received process. Computer simulations were used to verify model predictions and to evaluate their usefulness in the context of sea floor classification. Central to the verification strategy was the successful solution of the inverse problem based on synthetic reverberation data. The average number of scatterers was determined from estimates of the kurtosis of the instantaneous reverberation pdf. The magnitude of a coherent component embedded in the scattered return was recovered from envelope histograms with the assistance of the Kolmogorov goodness‐of‐fit test. Following the verification study, the model was perturbed by introducing clustered and ordered distributions of scatterers in addition to the standard Poisson. The initial results indicate that the reverberation envelope pdf differs significantly for the three scatterer distributions. The clustered distribution led to a rapid increase in kurtosis, while the ordered distribution displayed evidence of intermittent coherent scattering. The usefulness of this parametrization was further tested with real reverberation data representing several distinct seafloor regimes. [Research funded by ONR.]
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Estimation of sediment volume scattering strength and absorption coefficient (A)

Robert A. Stewart and Nicholas P. Chotiros

J. Acoust. Soc. Am. Volume 86, Issue S1, pp. S33-S33 (1989); (1 page)

Online Publication Date: 13 Aug 2005

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A simple theoretical model for estimating sediment volume scattering strength and absorption coefficient from the reverberation tail of an acoustic pulse observed by a point receiver within the ocean sediment is described. This model assumes an infinite plane wave entering the sediment at normal incidence and scattering isotropically from a uniform field of random inhomogeneities. The model predicts the magnitude and decay rate of the volume scattering at a point receiver in terms of the volume scattering strength and the absorption coefficient of the medium. By fitting the predictions of the model to experimentally observed volume scattering data, the volume scattering strengths and absorption coefficients of real sediments were inferred. Expeimental data were obtained from insediment acoustic probes deployed at sites off Kings Bay, GA, and off Panama City, FL. The absorption coefficient results were compared with corresponding measurements from core samples. [Work supported by ONT with NORDA management.]
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Plane‐wave analysis of acoustic signals in a sandy sediment (A)

Robert A. Altenburg, Carl Faulkner, and Nick Chotiros

J. Acoust. Soc. Am. Volume 86, Issue S1, pp. S33-S33 (1989); (1 page)

Online Publication Date: 13 Aug 2005

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Acoustic pusles entering a sandy sediment through the water‐sediment interface were observed. An acoustic projector located in the water column was used to insonify an array of sensors embedded in the sediment. Carrier frequencies of from 5 to 80 kHz were projected toward the sediment interface at grazing angles below, near, and above the critical value. Sound‐pressure levels and time of arrival differences associated with the passage of the acoustic pulse were determined from the sensor data. A weighted least‐squares algorithm was used to fit a plane to the arrival time differences yielding wave‐front direction and speed estimates. Estimates of acoustic attenuation and wave‐front coherence were also obtained. [Work supported by ONT under NORDA management.]
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Remote sensing of particle motion by cross correlation of acoustic backscatter (A)

Nicholas P. Chotiros

J. Acoust. Soc. Am. Volume 86, Issue S1, pp. S33-S33 (1989); (1 page)

Online Publication Date: 13 Aug 2005

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In a previous design study, it was determined that clouds of passive riders in the upper ocean, such as bubbles and suspended matter, could produce detectable and unique acoustic backscatter signatures. It is postulated that the acoustic signatures may be processed to yield the movement of the passive riders, hence image the motion of the water itself. Methods based on cross correlation processing were developed. Computer simulations and experiments were carried out to test the methods. [Work supported by ONR.]
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Angular dependence of seafloor acoustic backscatter at 12 kHz (A)

C. de Moustier and D. Alexandrou

J. Acoust. Soc. Am. Volume 86, Issue S1, pp. S33-S33 (1989); (1 page)

Online Publication Date: 13 Aug 2005

Full Text: | Download PDF

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The angular dependence of seafloor acoustic backscatter measured with a 12‐kHz multi‐narrow‐beam echo sounder over a variety of seafloors, ranging in depth from 500 to 5000 m, has been determined for incidence angles from 0 to 25 deg. The acoustic data consist of quadrature samples of the beamformed echoes received on each of the sixteen 2.66‐deg beams of a Sea Bearn echo sounder. These data are subjected to adaptive noise canceling for sidelobe interference rejection and to beam‐to‐beam “decorrelation” to ensure that the returns are separated in angle with minimal cross talk between adjacent beams. The measurements are then corrected for geometric effects, taking into account the ensonified area for each beam, which varies as a function of the ship's roll and bottom slopes. It was found that apparent bottom slopes of less than 2 deg in the athwartships direction are noticeable in the data and must be compensated for. For lack of system calibration, the data are presented as relative mean energy levels in 1‐deg bins from 0‐ to about 25‐deg incidence. These results are compared with the results era REVGEN computer simulation which faithfully reproduced the acoustic geometry of the Sea Beam system and generated scattered returns devoid of a specular component. Results are discussed based on geometric and physical arguments, and the ability to differentiate seafloor types using such angular dependence functions as assessed. [Research funded by ONR.]
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