Oceans Melting Greenland

Data from the OMG mission:

Bathymetry The bathymetry survey was primarily conducted using multi-swath Multibeam Echo Sounder System (MBES) equipment and procedures. MBES uses a sonar that produces an angular swath of beams to the port and starboard side in a single ping to effectively map a corridor with a width approximately 3-4 times the water depth. In addition to the multibeam echosounder, several more instruments are required on the vessel to georeference the raw multibeam data into corrected soundings. A GPS installed on the vessel is used to accurately position the location of the multibeam. Data from a pitch, roll, heave and heading sensor is used to correct the raw sounding based on the vessels attitude at the time the sonar ping. The sound velocity of the water column is required to convert the soundings from raw time-of-flight measurements to a corrected range as well as correct the sounding for refraction as it passes through the water column. Additional bathymetry data was collected by a smaller vessel using a Singlebeam Echo Sounder System (SBES). The SBES data provides only a single measurement of water depth, directly beneath the ship, but this data was collected in a few regions where the MBES survey was not able to reach.

CTD Temperature and salinity data are collected primarily using air-deployed sensors called Airborne eXpendable Conductivity Temperature Depth (AXCTD) instruments. These expendable instruments are launched from an aircraft, fall under a small parachute and float on the surface after impact. The floating portion then release a probe, which sinks to a depth of up to 1000 meters. The probe is connected to the float by a thin wire which unspools at the probe sinks, measuring temperature and conductivity as a function of time. This information is sent by radio to the aircraft, where it is used to compute temperature and salinity as a function of depth. In addition, during the ship survey more traditional lowered CTD data was collected. These instruments measure temperature and conductivity as a function of pressure and the data is downloaded and stored when the instrument is taken out of the water.

Elevation The surveys of Greenland’s ice sheet were conducted with the GLacier and Ice Surface Topography INterferometer (GLISTIN-A), which aims to produce high spatial resolution (25 m), high-precision (< 50 cm) height maps of Greenland’s coastal glaciers, at 10 to12-km wide swaths using Ka-Band (8.4 mm wavelength) single-pass interferometry. By measuring ice surface elevation changes over several years, volume changes of marine terminating glaciers can be inferred. The GLISTIN-A radar is mounted in a pod under a Gulfstream III airplane. Operating at Ka-Band enhances interferometric accuracy, reduces penetration into the top layers of snow and firn and limits signal attenuation in the atmosphere.

The swaths generally cover the lower parts of the glaciers. The near edges of most swaths are set as close as possible to, and just downstream from, glacier fronts. The remainder of the swaths extend up-glacier from the fronts. Most swaths are flown across glacier flow, capturing as many glacier fronts as possible in each single swath. In the cases of a few large glaciers, swaths are flown along glacier flow, again extending from the front upstream towards the interior of the ice sheet.

Gravity he airborne gravity survey was carried out by Sander Geophysics Limited using a fixed-wing aircraft flying at speeds of about 120 nautical miles per hour at a line spacing of 2 to 4 kilometers. The data, collected using the Airborne Inertially Referenced Gravimeter (AIRGrav) instrument, include both free air gravity in milligal (mGal) and its first vertical derivative. This is provided in the form of Level 1 gridded files (two file types, .xyz and .gxf) as well as data along the flight lines. As part of OMG, this data will be combined with other bathymetry data to produce higher-level maps of the sea floor depth. Since these data will be used to improve maps of sea floor shape and depth, the word "bathymetry" is used to describe several of the products. The underlying measurement, however, was free-air gravity.