DOI: 10.5061/dryad.fn2z34v3k
title: Grounding line remote operated vehicle (GROV) exploration of the ice shelf cavity of Petermann Glacier, Greenland

description: The melting of ice by ocean waters along the periphery of ice sheets is a
 major physical process driving their evolution in a warming climate. Using
 the fiber-optic-tethered Grounding line Remote Operated Vehicle (GROV), we
 explored the ice shelf cavity of Petermann Glacier, in Northwestern
 Greenland, in May 2023, using a novel interferometric multibeam sonar
 operating at 117 KHz with 360° viewing capability. The seafloor depth is
 uniform at 820 m and 200 m deeper than anticipated. At the ice shelf base,
 we find a succession of terraces interrupted by 20-40 m ice cliffs that
 have no signature at the surface, but are consistent with double-diffusive
 convection. The central melt channel deviates by ± 80 m from flotation, is
 smoother than indicated by the surface, and reveals asymmetric melt. The
 results demonstrate the fundamental importance of surveying the geometry
 of ice shelf cavities to document ice-ocean interaction.
description: Petermann Glacier is a major outlet glacier in North Greenland
 (60°W, 81°N) that forms a 20-km wide and 45-km long floating ice shelf
 (Fig. 1). Petermann drains 4% of the Greenland Ice Sheet in area. The ice
 shelf undergoes massive ice melt in contact with the ocean waters that
 removes more than half of its ice thickness in the first 10 km. In 2010
 and 2012, the ice shelf underwent two major calving events, which
 truncated its length by 25 km. In 2018-2022, the glacier grounding line
 retreated for the first time in this century, by 7 kilometers at the
 center, 4 km on the sides, and the glacier sped up by 100 - 150
 m/yr. <strong>GROV:</strong> The``Grounding
 line Remote Operate Vehicle” (GROV) uses the Graaltech, torpedo shaped,
 X-300 Autonomous Underwater Vehicle (AUV), which is a portable, modular
 AUV, with an open control system, fully holometric five degrees of freedom
 (no fin), 15.5-cm diameter, 29 kg in weight, and a 14-hour endurance
 (untethered). We built a sonar module based on the Bathyswath-3 multibeam,
 phase-differencing, interferometric sonar from ITER-Systems Ltd. operating
 at a frequency of 117 KHz (12 mm in water). The interferometric capability
 achieves a wider swath in shallow water (&lt; 200-400 m depth)
 compared to standard MBES systems, typically 10 times the water depth
 instead of 3-4 times, which is ideal for cavity exploration. We molded 12
 transducer antennas on a cylindric module, with 4 transmitting/receiving
 and 8 receiving-only elements to enable 360° imaging with a 400-m nominal
 range, 600-m maximum. The sonar module is 22 cm in diameter. The Inertial
 Navigation Unit is a Spatial-FOG dual INS from Advanced Navigation which
 measures roll and pitch within 0.01°, heading within 0.01°, and with a
 bias instability of 0.05° per hour without external aiding (e.g. GNSS or
 magnetometers) . Navigation is assisted by Doppler Velocity Logger (DVL)
 DVL-A125 from Waterlink, with 5 cm-125 m range, 600 m depth rating, and
 maximum velocity of 9 cm/s, which locks on the ice bottom. Initial
 position and heading is determined using a the dual GNSS receiver build-in
 the INS. During the dive, DVL-aided navigation, East and North uncertainty
 is expected to be within 0.1% of the distance travelled.
 <strong>CTD:</strong> We use a Conductivity
 Temperature Depth (CTD) sensor Star-Oddi DST online, with a temperature
 accuracy of 0.1°C and a precision of 0.032°C; a salinity accuracy of 0.1
 psu and a precision of 0.02 psu. The fiber optic tether is lightweight
 (4.2 mm diameter, 95 kg with 5 km cable and motored winch) and high
 resistance (200 kg breaking strength). The GROV is depth-rated to 600 m,
 capable of a 1 km swath, weighs 56 kg in air, and is 2.2 m in
 length.  <strong>Access Hole:</strong> We
 drilled a 25-cm hole in newly-formed ice with an ice Auger into 10 cm
 thick ice. The three-person science team plus a two-person film crew
 established a camp above the bore hole. We lowered a Minos XL CTD down the
 hole to record temperature, salinity, and depth up to 700 m depth, which
 was above the sea floor. The GPS location was noted every day before each
 dive. We lowered the GROV through the access hole and performed a series
 of eleven dives, with the longest dive corresponding to a
 tethered-traveling distance of 4 km under the ice shelf over a duration of
 4 hours. The GROV was brought back to the surface regularly to re-charge
 its batteries in about 5 hours.   <strong>Data
 Processing: </strong>The MBES data were processed using ITER System
 freeware software and post-processed with self-developed python-based
 interferogramm method and filtered using Cloud Compare freeware. The
 seafloor was not detected in the first couple of dives performed 50 m
 below the ice bottom. In subsequent dives, we unlocked the DVL from the
 ice base and surveyed at 400-500 m depth, at which point we detected both
 top and bottom of the cavity. Navigation was controlled by maintaining a
 constant heading and keeping track of distance with the fiber optic
 markings. Prior testing had indicated an exceptional stability for the IMU
 when the DVL had a good lock on the ground, i.e. a deviation of less than
 2 meter in 1.5 km of travelled distance. As we collected data both on the
 way in and on the way out, we also overlapped tracks for consistency. The
 data are processed in x, y, z points available as a point cloud data set
 and gridded on a 25 m x 25 m horizontal using data averaging. The point
 cloud data include recordings from the CTD probe at each x,y,z
 location.  <strong>Ancillary data:
 </strong>We employed a World View Digital Elevation Model (DEM)
 referenced to mean sea level and ortho-rectified imagery at 2 m spacing
 dated May 10, 2023 to conduct the data analysis. We used WV imagery from
 April 2023 to forecast the field operation. The data spacing is 8 m with a
 vertical precision of 1 m.
description: # Grounding line Remote Operated Vehicle (GROV) exploration of the ice
 shelf cavity of Petermann Glacier, Greenland
 [https://doi.org/10.5061/dryad.fn2z34v3k](https://doi.org/10.5061/dryad.fn2z34v3k) ## Description of the data and file structure The data are provided in UTM format Zone 20N. The multibeam echo sounding data are provided as x, y, and z files. 1. Merged bed.xyz : ASCII point cloud, bathymetry surveyed by GROV. Format is X, Y, Z (in meters), Raw_Amplitude (uncalibrated), PingNumber, Coord._Z (meters), Original_cloud_index. 2. Merge ice.xyz: ASCII point cloud, ice bottom elevation (in meters) of the Petermann ice shelf from GROV mixing data from 11  missions.  Format is X, Y, Z (in meters), Raw_Amplitude (uncalibrated), PingNumber, Coord._Z (meters), Original_cloud_index. 3. CTD rift Petermann 230512.txt: Ascii file (with header) of the conductivity, temperature, depth collected in the bore hole of Petermann Ice Shelf on May 12, 2023 including temperature (degree Celcius), salinity (psu), depth (meters) and dissolved oxygen (ppm). 4. SETSM_s2s041_WV03_20230510_10400100852E8500_1040010085BB2F00_2m_lsf_seg1_dem_msl.tif: World View DEM acquired on May 10, 2023 at 2 m spacing, UTM zone 20N and referenced to the geoid using the geoid in BedMachine Greenland Version 5 available at the National Snow and Ice Data Center, Boulder, CO.  ## Code/Software You can visualize the Merged*.xyz data file using the free software Cloud Compare. We use the qGIS free software to analyze all of our products.