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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.]

A parametric sonar has been used to measure volume scattering strength at a number of ocean locations. The narrow beamwidth and absence of sidelobes of the parametric sonar make it an ideal tool for measuring volume scattering strength as a function of depth. The resolution of this device is such that in some circumstances individual scatterers can be identified. An example of such a situation is presented. The parametric sonar system is described. Additional sets of examples from nearshore and off-shore areas are presented for both day and night conditions.

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Parabolic equation calculations are compared with stepwise coupled mode COUPLE calculations for a set of runs for which the two models have overlapping capability. The PE calculations were then extended to consider the additional case of refracting sediment. Normal mode start-up fields were used in the calculations. PE models have capabilities differing in the areas of wide angle capability, treatment of the bottom, and numerical methods. The intent of this paper is to discuss which of these differences make a difference. The environment chosen was that presented in the previous paper. Calculations were performed at 50 and 300 Hz. Water depths varied somewhat, but averaged about 150 m, corresponding to 3 wavelengths and 15 wavelengths at 50 and 300 Hz, respectively. The dependence of depth with range took two forms. The first consisted of smoothly varying linear segments, the second was the same as the first with a number of spikelike irregularities added in. Differences are discussed in terms of numerical issues and the physics incorporated in the models.