September 3, Acquisition of multibeam data north of Greenland during the LOMROG expedition 2007
Received from Dennis Anthony, The Royal Danish Administration of Navigation and Hydrography (Oceanographic Department), Denmark .
Edited by Jane Holst and Torsten Hoelstad, GEUS
We have now reached the Morris Jesup Rise – the submarine extension of the northernmost point of Greenland. This is an area of great importance to the Continental Shelf Project. We have now taken measurements of the outer part of the continental shelf.
The ice-breaker Oden has an extremely powerful multibeam echo sounder. It is built into the hull of the ship and is designed to withstand the same ice-pressure as the ship. The multibeam is a specially designed for deep-sea measurements, sending out powerful sound pulses at 12 kHz. It is capable of covering a broad track of the seabed with up to 191 separate and regular ‘shots’ athwart the ship’s course. When conditions are good the width of the track covered by the multibeam can be up to three times the water depth.
The multibeam echo sounder is relatively sensitive to noise, so it is not without problems to acquire bathymetric data in the Arctic Ocean. The echo sounder must be able to detect the exact signal it sent to a precise point in the seabed, calculating its return time, and in this way establish the precise depth of this point. The detection is disturbed by the vicious noise coming from the actual breaking of the ice and from ice and air bubbles inevitably drawn under the ship in front of the multibeam. The working conditions for the echo sounder during heavy ice-breaking is like trying to have a deep conversation during a rock concert – it can be done, it takes a long time and you have to repeat yourself a lot not to be misunderstood!
Bathymetric data are acquired along the entire LOMROG expedition route. The quality of the acquired data varies with the changing ice conditions. In the areas where the depth conditions are especially important, we have to acquire the depth data at a much reduced speed. The various types of ice conditions also play a major part in our acquisition technique. If there are leads in the ice we prefer to follow them, because the data we acquire here are practically undisturbed. Since the multibeam covers such a broad track on the seabed, it is of little consequence that we often move in a zigzag course. We still get a 3-D model in a wide zone.
In the area around the Morris Jesup Rise, a slightly alternative method of measurement has proved to be very suitable. Sporadic open waters, forming small lakes, have allowed us to let the Oden do a very slow ‘pirouette’. This results in a data coverage that looks like a series of overlapping rosette-like patterns, but with a much higher data quality than if we had tried to use data from ice-breaking alone.
The data from the echo sounder are continuously being processed, i.e. adjusted for data errors and calibrated with the relevant sound-velocity profiles calculated from the regular CTD measurements that the oceanographers make. From the depth data, 3-D models are designed which give us a fantastic overview of the seabed relief. The 3-D models are also very suitable to adjust the on-going investigation programmes, if the seabed turns out to be different from expected. This often happens in a place such as the Arctic Ocean, since in many areas the data are scarce.
From the existing depth data, which are primarily based on Russian maps, we knew more or less where the foot of the continental slope on the Morris Jesup Rise would be and we started the hunt for it on 31 August in the morning – slightly farther away from land than the map indicated to be sure to cover the entire area.
Due to the uncertainty of the existing depth maps and the difficult ice conditions the multibeam echo sounder kept showing a depth of approx. 4000 m as the day wore on. The day turned into evening and into night and our world still seemed to be as flat as a pancake – that is at a depth of four km. We knew it had to become shallower at some point, but doubt crept in, did we measure in the right place? A vivid debate took place; among other topics were the rather narrow form of the Morris Jesup Rise.
Close to six o’clock the next morning at long last something happened. Without warning a wall rose from the seabed. We were slowly closing in on the continental slope and the multibeam was made to perform at its best, the ice still giving considerable problems with noise in the data. For the next three kilometres the seabed rose approx. two km, and after a further couple of kilometres’ of measurements we had reached the top of the plateau at a depth of approx. one km. We had now found the location of the continental slope. The form of the slope, however, turned out to be much steeper and better defined than we had expected and than the previous information indicated.
It was a couple of intense days and nights working with the multibeam, but now we had established a well-defined location of the northernmost tip of the Morris Jesup Rise. The existing depth maps of the Arctic Ocean in many instances being rather inaccurate makes the measuring challenge the greater and further emphasises the usefulness of the efforts.