8 . Seamarc Ii Sonar Imagery a N D Bathymetry of the Nazca Plate a N D Peru Forearc , Odp Leg

In March 1985, SeaMARC II side-scanning sonar and swathbathymetry surveys were conducted on the submarine Peru forearc as part of the studies to select drilling sites for Leg 112 of the Ocean Drilling Program. Most of these surveys were concentrated in two areas: (1) between 8°45'S and 9°35'S, which is north of where the Mendana Fracture Zone (MFZ) enters the Peru-Chile Trench, and (2) between 11°00'S and 11°50'S, on the southern side of the MFZ/trench intersection (Fig. 1). Both surveys extended from the Nazca Plate across the trench axis to the upper continental slope. We refer to the northern survey area (which includes Sites 683 and 685) as the Yaquina Basin survey and to the southern survey area (including Sites 682 and 688) as the Lima Basin survey. SeaMARC II is a 12-kHz, towed sonar array that produces a long-range, side-scanning image, while simultaneously measuring bathymetric depths within the ensonified swath. The system typically is towed at speeds of about 7 kt at a depth of about 100 m and can map in any ocean depth. A more complete description can be found in Blackinton et al. (1983) and Hussong and Fryer (1983). For the site-selection surveys, the SeaMARC II was set to produce an acoustic back-scatter (side-scanning) image that was a constant 10 km wide and that measured bathymetry in a swath at least three times as wide as the water depth. The acoustic-image data are composed of 1024 pixels (picture elements) per side per ping. Each pixel in the resulting image represents 5 m of seafloor across the track. These backscatter magnitude data are displayed with stronger returns in darker shades of gray. Thus, low back-scatter regions such as sediments are very light, while strong returns from reflectors such as outcropping rock are represented by dark areas. As the wavelength of the 12-kHz signal is approximately 12 cm in water, the character of the side-scanning image is particularly sensitive to reflectivity changes caused by back scatter from seafloor relief on the order of 10 cm. However, recognition of seafloor patterns is a function of the shape of the features causing the echoes. Typically, linear features (such as faults and debrisflow channels) are recognizable, even with dimensions of only a few to several tens of meters. Three-dimensional features, such as small mounds or sediment ponds, must have dimensions on the order of 100 m or more before a shape can be recognized. SeaMARC II bathymetry is determined by measuring the phase angle of echoes returning to arrays of sonar receivers in the towed body. These data produce hundreds of independently determined soundings for each ping. The ping repetition rate is 8 to 11 s during these surveys, which are subsequently smoothed to produce swath bathymetry having 100% coverage. The accuracy of the bathymetry data during these surveys was about 50