Multi-beam sea-ice draft from remotely operated vehicle (ROV) surveys during the MOSAiC expedition 2019/20, version 2.0

The three-dimensional under-ice topography has been mapped regularly during the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition in 2019/20 (Nicolaus et al., 2022) using a multibeam echosounder system (DT101, Imagenex Technology Corp., Port Coquitlam, BC, Canada). The system was attached to a remotely operated vehicle (ROV, M500, Ocean Modules, Åtvidaberg, Sweden, Katlein et al., 2017). The ROV was operated directly from the sea ice during most dive days on all legs of the MOSAiC expedition. Standard multibeam surveys were performed at a dive depth of 20 m while high-resolution surveys at 10 m depth, both in a grid/mattress pattern. The area covered resulted mostly from the dive time available on the respective day. The system consists of a multibeam sonar, an integrated sound velocity probe, and a motion reference unit. The operating frequency was 240 kHz. The sonar was used with 480 beams emitted at the same time with a swath width (nominal beam geometry) of 120° (across track) x 3° (along track) and an effective beam width of 0.75°. The angular resolution resulting from the chosen number of beams and the sector size was 0.25°. Horizontal distances between lines were between 20 m and 25 m to achieve some 30% overlap of the area scanned by the outer beams of neighbouring lines. The ping rate was automatically set and was based on the range setting, numbers of beams selected and specification of the computer system used for operation. Those settings resulted in a mean ping rate of 9.37 Hz for all surveys. A constant sound speed of 1436 m/s was applied as the surveys were performed within the mixed layer with no significant distortions due to density expected. Returns from three points were averaged to derive the distance from the multibeam to the ice underside. Subtracting the distance to the ice from the vehicle depth yielded the sea-ice draft. The position is in a floe-fixed, relative coordinate system (X, Y) with the origin (X=0 m, Y=0 m) at the ROV hole. Data were processed using the CARIS HIPS and SIPS software (Teledyne CARIS, Fredericton, NB, Canada, version 10 and 11) applying swath and subset editors for data cleaning. The horizontal resolution for standard surveys resulted in 0.5 m. The abbreviation '_HR' in the file names indicates higher lateral resolution processing with resolutions of 0.1 m instead of 0.5 m. An offset of 0.05 m between the ROV depth calibration reference (bumper bars on top of the ROV) and the multibeam is accounted for in the draft data. The depth used for the multibeam data processing is a cross-calibration product between the depth derived from the ROV pressure sensor and depth obtained by a Glider-Payload CTD sensor also attached to the ROV. More technical details of the system are available here: https://imagenex.com/products/dt101xi. The software to operate the MBES and record data is available from the Imagenex website (https://imagenex.com/interior-page/software-dowload).