The ocean bottom scattering function depends, in general, on the grazing angles and the azimuthal angles of the incident and scattered energy. However, most measurements are for backscatter only. The few general measurements that are available indicate strong forward scattering near the angle of the specularly reflected ray and weaker, azimuthally isotropic, diffuse scattering away from the specular angle. By combining Lambert’s law scattering with a surface scattering function based on the Kirchhoff approximation, Ellis and Haller [J. Acoust. Soc. Am. Suppl. 1 82, S124 (1987)] proposed a function that incorporated these features. The function is quite simple, and depends on three parameters that can be fitted to backscatter measurements. The functional form thus allows a reasonable extension from backscatter to the general three-dimensional scattering function, which can then be used in bistatic reverberation calculations. It is an improvement over two commonly used methods (which do not include azimuthal dependence) for extrapolating backscattering to general scattering: the separable approximation, and the half-angle approximation. This paper discusses the three-dimensional function in more detail, and presents some comparisons between model predictions and measured bistatic reverberation.
Acoustic backscatter data were collected from the ocean bottom at four sites on the Sohm Abyssal Plain. The bottom sediment at the four sites varied from mud/clay to silt/gravel. Using a simple array consisting of a free-flooding-ring projector which was omnidirectional in azimuth, and an omnidirectional hydrophone, data were collected over the frequency range of 800–2400 Hz [Hines and Barry, J. Acoust. Soc. Am. 92, 315–323 (1992)]. To obtain sufficient signal-to-noise ratio without impairing spatial resolution, 200-Hz-wide linear FM pulses were employed. By deploying the array at a height of 500 m above the seabed, bottom backscatter data were obtained at grazing angles down to 4 degrees before the onset of the first surface return. For data arriving after the first-bottom–first-surface interaction, the geometry approximates an in-plane bistatic experiment. Estimates of the in-plane bistatic scattering strength were obtained for pairs of incident and scattered grazing angles (φi,φs)<(50 degrees,88 degrees), at frequencies of 900, 1200, and 1600 Hz. At one site the grazing angle dependence matched Lambert’s rule; at all other sites the data exhibited a steeper slope than that of Lambert’s rule. There was insufficient data to extract the frequency dependence. The fathometer returns were used to estimate the normal-incidence bottom loss at the four sites. The measured bottom loss ranged from a low of 2 dB to a high of 16 dB. The in-plane bistatic data and the monostatic data were compared to examine the validity of the separable and half-angle approximations [Ellis and Crowe, J. Acoust. Soc. Am. 89, 2207–2214 (1991)]. The separable approximation provided a reasonable fit to the data at three of the four sites; the half-angle approximation did not match the data well at any of the sites.
Acoustic backscatter data were collected from the ocean bottom at four sites on the Sohm Abyssal Plain. Using a simple array consisting of a free-flooding ring projector, omnidirectional in azimuth, and an omnidirectional hydrophone, data were collected over the frequency range of 800 to 1700 Hz. To obtain sufficient signal-to-noise ratio without impairing spatial resolution, 200-Hz-wide linear FM pulses were employed. The array was deployed 500 m above the seabed and bottom backscatter data were obtained at grazing angles down to 4° before the onset of the first surface return. For data arriving after the first-bottom first-surface interaction, the geometry approximates an in-plane bistatic experiment. Therefore, the in-plane bistatic data and the monostatic data can be compared to examine the validity of using the separable and half-angle approximations used in some bistatic scattering models [J. Acoust. Soc. Am. 89 (1991)]. In this paper the bistatic geometry is explained and the data are interpreted in light of the bistatic arrivals. Then, an algorithm is presented for extracting the in-plane bistatic scattering strength from the data. Finally, bistatic and monostatic data are compared using the separable and half-angle approximations.
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