MEASUREMENTS OF ACOUSTIC BACKSCATTER FROM THE SOHM ABYSSAL-PLAIN

Acoustic backscatter data were collected from the ocean bottom at four essentially smooth 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 in 200-Hz bandwidths. By deploying the array to a depth approaching 5000 m, bottom backscatter data were obtained at grazing angles down to 4-degrees before the onset of the first surface return. To obtain sufficient signal-to-noise ratio without impairing spatial resolution, linear FM pulses were employed. In this paper, the experiment and the bottom topography of the four sites are described, and the backscatter data are presented.