High-resolution seafloor mapping using the DSL-120 sonar system: Quantitative assessment of sidescan and phase-bathymetry data from the Lucky Strike segment of the Mid-Atlantic Ridge

High-resolution, side-looking sonar data collected near the seafloor (simil ar to 100 m altitude) provide important structural and topographic informat ion for defining the geological history and current tectonic framework of s eafloor terrains. DSL-120 kHz sonar data collected in the rift valley of th e Lucky Strike segment of the Mid-Atlantic Ridge near 37 degrees N provide the ability to quantitatively assess the effective resolution limits of bot h the sidescan imagery and the computed phase-bathymetry of this sonar syst em. While the theoretical, vertical and horizontal pixel resolutions of the DSL-120 system are < 1 m, statistical analysis of DSL-120 sonar data colle cted from the Lucky Strike segment indicates that the effective spatial res olution of features is 1-2 m for sidescan imagery and 4 m for phase-bathyme try in the seafloor terrain of the Mid-Atlantic Ridge rift valley. Comparis on of multibeam bathymetry data collected at the sea-surface with deep-tow DSL-120 bathymetry indicates that depth differences are on the order of the resolution of the multibeam system (10-30 m). Much of this residual can be accounted for by navigational mismatches and the higher resolving ability of the DSL-120 data, which has a bathymetric footprint on the seafloor that is similar to 20 times smaller than that of hull-mounted multibeam at thes e seafloor depths (similar to 2000 m). Comparison of DSL-120 bathymetry wit h itself on crossing lines indicates that residual depth values are +/- 20 m, with much of that variation being accounted for by navigational errors. A DSL-120 survey conducted in 1998 on the Juan de Fuca Ridge with better na vigation and less complex seafloor terrain had residual depth values half t hose of the Lucky Strike survey. The quality of the bathymetry data varies as a function of position within the swath, with poorer data directly benea th the tow vehicle and also towards the swath edges. Variations in sidescan amplitude observed across the rift valley and on Luc ky Strike Seamount correlate well with changes in seafloor roughness caused by transitions from sedimented seafloor to bare rock outcrops. Distinct ch anges in sonar backscatter amplitude were also observed between areas cover ed with hydrothermal pavement that grade into lava flows and the collapsed surface of the lava lake in the summit depression of Lucky Strike Seamount. Small features on the seafloor, including volcanic constructional features (e.g., small cones, haystacks, fissures and collapse features) and hydroth ermal vent chimneys or mounds taller than similar to 2 m and greater than s imilar to 9 m(2) in surface area, can easily be resolved and mapped using t his system. These features at Lucky Strike have been confirmed visually usi ng the submersible Alvin, the remotely operated vehicle Jason, and the towe d optical/acoustic mapping system Argo II.