Cruise Report for Seismic Reflection trials from the CCGS Louis St Laurent.

The objectives of the reflection seismic work onboard CCGS Louis S. St-Laurent
were to test the newly designed airgun tow sled in Arctic sea ice, and to collect some
initial seismic data in Canada Basin that can be used for the planning of future UNCLOS experiments in the Canadian High Arctic. The transit from Halifax to Kugluktuk was used to install the seismic gear on the ship and to do initial deployment tests of the tow sled. The deployment with the two cranes on the quarter deck is difficult and time consuming, but the better-suited A-frame was not ordered intime to have it ready for this cruise. The aging compressor had some downtime, which resulted in the premature end of one seismic line. However, a new compressor will be available for future work.

The airgun tow sled behaved very well in the ice as long as the centre shaft of the
shift was not used. When the centre shaft is used, the extra water pressure pushes the sled away from the stem, which increases the likelihood that the tow wires and umbilical cords of the sled get caught by ice, which can pull the entire sied out of the water. The use of the centre shaft is necessary in multiyear ice and in ice ridges, where the ship needs the extra power to break the thick ice. If a camera was mounted on the quarter deck so that the bridge could monitor the sled position, the risk to the gun array could probably be reduced.

Thick multiyear ice impeded CCGS Louis S. St-Laurent when the airgun sled was
m the water, because the ship can neither operate at full speed nor can it move
backwards. Both techniques are required to go through the 5-m-thick ice that was
encountered in the survey area. Hence, the major recommendation for future seismic data acquisitions in the Arctic is to use a second icebreaker that makes a lead for the shooting vessel. The benefits of a two-ship experiment are that the gun array will be at lower risk, the noise levels will be reduced, the speed of the data acquisition can probably be kept close to 4 knots even in multiyear ice, and the ship can stay on the desired track lines.

A total of 405 km of seismic data were acquired during the cruise. In western
Canada Basin, sediments were detected down to a depth of 3.5 seconds below seafloor, which implies that a Canadian claim for an extended continental shelf can go out to 350 km from the foot of the slope if UNCLOS' sediment thickness rule is applied. However, there was no clear indication of basement in the record sections obtained :from minimal onboard processing. When the ship sailed through ice, the signal-to-noise ratio was significantly reduced (up to a factor of ten). Two lines were initially shot with three guns and later with one or two guns. The data quality did not change significantly when only one airgun was used. This suggests that the low seismic penetration is more a function of the ambient noise and the streamer than of the airgun source. Bence, it is recomrnended to use a different streamer and run a short one-da y cruise to test the streamer. Sorne of the streamer options are: 1) a single-channel streamer with better hydrophones (noise cancellation); 2) a longer streamer so that the hydrophones are farther away from the main noise source (the ice-breaking ship) - this would require the development of an appropriate deployment method; 3) a multi-channel streamer. In addition, the shot rate should be reduced to 20 seconds or less (60 seconds in this survey), which should not be a problem with the new compresser. If the guns are operated at a pressure of 3000 psi, the signal will also be significantly stronger than during this experiment (1750 psi).

The lengthening of the leading cab le of the streamer :from 100 to 306 fi has slightly
improved the signal but is still short of the 600 fi used during the USGS seismic surveys in the Arctic. Background noise from the ship ( engine, propeller, and icebreaking) was generally below 25 Hz, peak noise levels were below 12 Hz. This is in the same frequency range as the deeper reflections. This suggests that the streamer should be towed as far behind the ship as possible, which is basically determined by a safe deployment technique in the ice.

The test of a sonobuoy was not satisfactory as the signais from the radio transmitter
were below the squelch level at a distance of ~8 km, which is too close to observe
refractions from deeper sedimentary layers in the water depths of >3500 m. During the test, the antenna on the ship was mounted at a height of 7 to 12 m above sea level.
Mounting the antenna higher up on the ship could increase the sonobuoy signal range. The direct wave and the water bottom reflection recorded by the buoy did not have a high amplitude (at least not with the processing carried out on board). This could be due to several reasons: 1) ambient noise from ice, even though the buoy was deployed in an area that was relatively ice-free; 2) the settings on the buoy were not optimal (e.g. hydrophone depth); 3) the energy of the guns is too weak.

Engine problems on the CCGS Louis S. St-Laurent caused the ship to drift for 4.5
days. In 2005, engine problems resulted in a downtime of 6 days. These incidents
indicate that the ship is prone to have failures due to its aging technical equipment. If it is decided to use a second icebreaker for future UNCLOS projects in the Arctic, downtime could become a serious financial burden. In addition, the tight deadlines of the UNCLOS program are at risk. Hence, every effort should be made to reduce downtimes at sea and this should be seriously discussed with the Canadian Coast Guard.

This is also the reason that we suggest the use of two tow sleds for the seismic
work. This would allow the continuation of data acquisition while one array
wasrepaired. Given the high costs for a possible two-ship operation, the investment in a second sled would soon be recovered, as damage to the array cannot be fully prevented in ice-infested waters.

Minor damage to the tow sled occurred at the electric connectors of the air guns and
at the fittings of the air hoses. This is caused by the airguns hitting the frame of the sled. Additional protection for the connectors is planned for future experiments (plastic block). Another problem was that the regulator of the anti-freeze system that froze at temperatures around the freezing point. This problem can be fixed by moving the system into the compressor container.