The topographic survey of the underside of the sea ice will be carried out using a
Remote-Operated vehicle (ROV), in this case a wire-guided submersible. Our expedition will be using a Super Achille ROV equipped with electric motors, navigation/positioning systems and hydrographic sensors. It will be lowered into the water via a hole cut in the ice. We will be operating it at a depth of 20-30 metres to avoid the deepest ice keels, or sails, under pressure ridges.
The motors
The motors enable the ROV to move in any direction, to turn on the spot and to remain at a constant depth while it takes measurements, but they use a lot of energy.
The positioning system
This is based on acoustic signals. At all times during its operation, the ROV emits an acoustic signal that is picked up by a sensor/receiver at the bottom of a mast that is lowered 5 metres below the ice. This sensor has a trihedral shape and each of its four peaks are equipped with a hydrophone (or underwater microphone) to pick up the ROV signals. The receiver uses triangulation to calculate the exact position of the ROV relative to the mast. So we know the exact position of the ROV in relation to the receiver, which acts as a geographic reference point. It is essential that this receiver does not move relative to the ice, so the mast it is attached to is solidly moored to the ice surface above. It is also important that nothing comes between the ROV and the receiver during measurement (an ice keel, for instance). To avoid the inaccuracy that this would induce, the mast will be positioned at various positions within the zone being surveyed, so that it can pick up a signal from a spot that was previously masked.
The single-beam depth-sounder
The ROV is equipped with a single-beam depth sounder or upward-looking sonar (ULS). This acts rather like an altimeter, telling the ROV at any moment the vertical distance from the ROV to the underside of the ice pack. Coordinates (x,y,z) are then assigned to each point relative to the geographical reference (the receiver). New points are plotted as the ROV continues its to-and-fro grid pattern under the ice, yielding a 3D topographical “picture” of underside of the ice.
The bathy-sounder or CTD device
The accuracy with which the moving ROVs position can be determined depends on the speed at which its signal travels, and that speed depends in turn on temperature and salinity of the water. So the ROV is equipped with a bathy-sounder or CTD (conductivity, temperature, depth) device that measures in real time both the temperature and the salinity so that they can be fed into positioning data. These parameters are particularly important because there is high variability in both temperature and salinity between the water immediately under the ice (colder and more salty) and the water at lower levels.
In addition, the CTD device measures water pressure, which gives the ROV's depth below sea-level. The difference between this figure and the distance to the underside of the ice pack (given by the ULS) gives the thickness of the submerged part of the sea ice.
Pilot and navigator
ROV guidance and data acquisition (ULS, CTD, position) are controlled from a heated tent set up on the sea ice. Here, a bank of computers, monitors and recording devices allow the pilot and navigator to control the ROV and manage the acquired data. The pilot does the actual “driving”, while the navigator determines the route to be followed, depending on the grid being surveyed, and handles data acquisition.
Exploiting the data Once the data is processed, it will be used to map the topography of the underside of the ice relative to sea level.
By superimposing this point-by-point grid on the surface topography of the same zone, a 3D model will be built and used to calibrate the EM-Bird.
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