IMAGING OF SUBMARINE LANDSLIDES WITH WIDE-SWATH SONAR William B. F. Ryan Lamont-Doherty Geological Observatory of Columbia University Palisades, New York INTRODUCTION Avalanche scars, landslide scree and debris flow tongues have been imaged in plan view on the contin- ental slope of the western margin of the North Atlantic. Their detection and mapping has been accomplished with a tethered deep-ocean side-scan sonar having a swath width of 5 km. The sonar was constructed to search for and locate the wreck of the S. S. TITANIC (Wilford, 1980). We have subsequently used it in cooperation with the U.S. Geological Survey to explore the continental slope of New England and the mid-Atlantic margin of the United States (Robb et al., 1981, a, b; McGreggor et al., 1981). DESCRIPTION OF THE SONAR SYSTEM The sonar system is contained in a neutrally buoyant vehicle that is tethered 100 m behind a 1,000 kg depressor (Figure 1). The depressor is guided with- in 200-400 m of the seafloor behind an oceanographic survey vessel with a 7,500 m armored co-axial cable. The sonar vehicle is towed at speeds of 1-2.5 knots, in sea states of up to Force 6 and winds up to 40 knots, although in such conditions the survey vessel needs a bow thruster or active stern rudders for maneuverability. The sonar vehicle is equipped with left and right looking transducers that operate with a horizontal 175 S. Saxov et al. (eds.), Marine Slides and Other Mass Movements © Plenum Press, New York 1982