Direct measurement of acoustic scattering from the seabed at shallow grazing angles and low kilohertz frequencies presents a considerable challenge in littoral waters. Specifically, returns from the air-water interface typically contaminate the signals of interest. To address this issue, DRDC Atlantic has developed a sea-going research system for measuring acoustic scatter from the seabed in shallow-water environs. The system, known as the wideband sonar (WBS), consists of a parametric array transmitter and a superdirective receiver. In this paper, backscatter measurements obtained with the WBS at two sandy, shallow-water sites off North America's Atlantic coast are presented. Data were collected at 4 and 8 kHz at grazing angles from 3 degrees-15 degrees. The backscattering strength is similar at both sites and, below about 8 degrees, it appears to be independent of frequency within the statistical accuracy of the data. The measurements show reasonable agreement with model estimates of backscatter from sandy sediments. A small data set was collected at one of the sites to examine the feasibility of using the WBS to measure the azimuthal variability of acoustic scatter. The data set--although limited--indicates that the parametric array's narrow beamwidth makes the system well-suited to this task.
It has been established that the first and second normal incidence acoustic returned from the seabed can be used as a classification tool to discriminate between different sediments. Typically this is accomplished using monostatic echosounder systems that operate in the range of several 10 s of kHz so that bottom roughness at the seabed interface is the dominant scattering mechanism. Model results (Hines and Heald, Proceedings of the Institute of Acoustics Conference on Acoustical Oceanography, Southampton, UK, April 2001) indicate that at lower frequencies, normal incidence returns from the sub-bottom should provide an additional discrimination tool without significantly corrupting the information contained in the interface scatter returns. This paper presents an experimental methodology for making these measurements in the 1 to 10 kHz frequency band using a vertical line array of receivers and a downward-looking superdirective projector array. Results from experiments at the ONR Strataform area and Scotian Shelf will be compared with independent geo-acoustic information.
Direct measurement of acoustic scattering from the seabed at shallow grazing angles presents a considerable challenge in littoral waters. Specifically, returns from the air–water interface typically contaminate the signal of interest. To address this issue DREA has developed a pair of sea going research systems for measuring acoustic scatter from the seabed in shallow water environs. The first system is a bottom mounted parametric transmitter with a 6-channel superdirective line array receiver. The second system is a vertical line array of 8 hydrophones and 2 transmitters. Operating the systems in tandem allows measurements of backscatter and forward scatter from the same scattering patch at the same time. In this paper the acoustic attributes of both systems will be outlined, the experimental geometry will be described, and measurements of acoustic backscatter and forward scatter collected during one such experiment will be presented.
The acoustic backscattering strength of the seabed has been demonstrated to be one of the key inputs required in sonar performance prediction models. Yet, direct measurement of acoustic scattering from the seabed at shallow grazing angles presents a considerable challenge in littoral waters. The DRDC Atlantic (formerly DREA) Wide Band Sonar system, which consists of a parametric transmitter and a superdirective receiver, is ideally suited to make this measurement. The system was used to measure backscatter as a function of grazing angle and azimuth on the Scotian Shelf off the coast of Nova Scotia and on the Strataform site off the coast of New Jersey. Interpretation of the data set is enhanced with swath bathymetry measurements made at one of the experimental sites. In this paper the experimental geometry is described and the backscatter measurements are presented and discussed in light of the swath bathymetry results.
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