Iceberg A74: Seafloor circumnavigation images collected during COSMUS expedition PS124

During the RV POLARSTERN expedition PS124 to the Weddell Sea during Jan - April 2021 the Ocean Floor Observation and Bathymetry System (OFOBS) was used to collect still and video images, as well as sidescan data, from various locations surveyed during the research cruise. During the expedition, the giant iceberg A74 calved off from the Antarctic mainland. In this data set we present images taken from within the water column and the seafloor during part of the iceberg circumnavigation conducted by FS POLARSTERN on the 14th of March, 2021. The OFOBS system consisted of a towed underwater camera system equipped with both a high-resolution photo-camera (iSiTEC, CANON EOS 5D Mark III) and a high-definition video-camera (iSiTEC, Sony FCB-H11) as well as an integrated sidescan sonar system. The cameras were mounted on a steel frame (140L x 92W x 135H cm), together with two strobe lights (iSiTEC UW-Blitz 250, TTL driven), three laser pointers spaced with a distance of 50 cm used to estimate the size of seafloor structures, four LED lights, and a USBL positioning system (Posidonia) to track the position of the OFOBS during deployments, with additional positioning information provided by the integrated INS and DVL systems. In automatic mode, a seabed photo, depicting an area of approximately 4-10 m**2, with variations depending on the actual height above ground, was taken every ~15 seconds to obtain series of TIMER stills distributed at regular distances along each of the survey profiles. Profile lengths varied in length depending on duration of the cast. At a ship speed of 0.5 kn, the average distance between seabed images was approximately 5 m, with this spacing being 15 m at 1.5 kt speed. Additional HOTKEY photos were taken from interesting objects (organisms, seabed features, etc) when they appeared in the live video feed. One minute POSIDONIA position fixes were used in this current data set, so there is some offset of a few 10s of meters for some of the collected images.