Extracting in-plane bistatic scattering information from a monostatic experiment

Abstract

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.

Constrained comparison of ocean waveguide reverberation theory and observations

“…This is an important point for purposes of this paper, because in practice it is very difficult to directly measure the dependence of seabed scattering upon vertically bistatic angles. While measuring the vertically bistatic scattering strength is possible at relatively high angles in deep 21 and shallow water, 5 it is very difficult to measure the vertical bistatic angle dependence at low angles because of hybrid multipaths ͑see Figs. 2,3 in Ref.…”

Seabed Classification Using Normal Incidence Backscatter Measurements in the 1-10 Kh Frequency Band

“…This is depicted (in cross-section) by the ellipse in the figure and shows paths EOE| 3EO and 130846580 which have the same time of ight. For further discussion of this see [18]. Of course, resolving layers from roughness features in this way requires the transducer to be well removed from the sea surface since the ellipse collapses to a circle as the transducer approaches the air-water interface.…”