Hydrosweep Measurements During the Expedition ARK XX-2 to Lena Trough and Western Gakkel Ridge

The region of Lena Trough and Western Gakkel Ridge in the Arctic Ocean was the object of an expedition in the summer of 2004. This region is of particular geoscientific interest because of its extremely slow spreading rates and the variety of morphologic forms that are produced in this tectonic environment. Therefore, the multibeam measurement system was of particular importance to the scientific goals of the cruise. The main characteristic of the Hydrosweep DS-2 deep-water sounding system aboard RV Polarstern is the 90° or 120° coverage angle in which the seafloor is depicted with 59 specific values for water depths perpendicular to the ship's long axis. The accuracy of the measurement is approx. 1% of water depth, the frequency of the acoustic signal is 15.5 kHz. The refraction of the sonar beams was corrected by automatic crossfan calibration. By regular transmission and measurement of a sweep profile in the ship's longitudinal direction and comparison of the slant beams with the vertical beam, the mean sound velocity over the vertical water column is determined and is used for the depth computation. The data collected include depth, sidescan (2048 values per scan), and backscatter information on each of the 59 beams. During this cruise, the Lena Trough was surveyed systematically for the first time by a multibeam sonar system. The recorded area has an expanse of approx. 100000 km² and connects previously mapped areas of the Eurasian - North-American plate boundary between Fram Strait and Gakkel Ridge. The region of Western Gakkel Ridge, mapped in 2001 (AMOR-Expedition) by RV Polarstern and USCGC Healy (USA), was extended by two more profiles (each 220 km long) along the ridge. In order to produce working maps for the expedition, the multibeam sonar data were gridded with a spacing of 50 m, producing plots with various contour line intervals. For further morphological interpretation of Lena Trough and Gakkel Ridge slope magnitude maps, slope direction maps and two-dimensional slope histograms were produced. They show clearly the predominant slope direction and are helpful in distinction between sediment basins, volcanos and ridge flanks.