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"8th Field Report "
Print date: Sunday, August 18 2019 - 3:23
Page last modified: December 5, 2009
© Kontinentalsokkelprojektet lng_uk/main.html
Seismic data are important for the Continental Shelf Project since one of the provisions in Article 76 of the United Nations Convention on the Law of the Sea (UNCLOS) deals with the thickness of sediments:
In principle the seismic method makes use of a strong sound signal, usually produced by an airgun, which is transmitted through the water column and into the subsurface below the seabed. Boundaries between different geological layers in the subsurface reflect some of the seismic energy back toward the sea surface. The reflections are then registered by a long cable (the "streamer"), which is equipped with pressure sensitive sensors called hydrophones. The streamer is pulled behind the ship in water depths of 6 to 8 meters. The arrival time of the different reflections depends on the depth to the different geological layers, while the strength of the signal to some degree characterizes the boundaries between the layers. Data are stored on a computer and on tape and then must be processed before a useful image of the subsurface is obtained. Processing is not completely automated and typically requires inspection of records to select the correct parameters needed to create the seismic images.
Acquisition of seismic data in open water is technically and logistically demanding - especially doing 3D surveys with up to 16 or more streamers for the oil industry. Seismic data acquisition in the ice filled waters of the Arctic Ocean, however, presents unique challenges never encountered by these conventional seismic surveys.
In order to survive under these harsh conditions, the seismic equipment has to be modified considerably. These modifications have been made in cooperation with the Department of Earth Sciences at the Århus University based on previous experience with data acquisition in ice filled waters:
Oden's normal mode of operation under heavy ice conditions, such as found in the area north of Greenland, is to break ice at as high a speed as possible. If the ship gets stuck in the ice, it would normally back and ram as many times as necessary to pass the obstacle. However neither high speed nor backing and ramming are possible with seismic gear deployed behind the ship:
In easier ice conditions, where Oden can break ice continuously at 3 to 4 knots approximately along a pre-planned heading, seismic data of reasonable data quality can be acquired. However, long continuous profiles are often not possible since ice conditions change rapidly and evaluation of ice conditions from the helicopter is not always easy or accurate. This is a particular challenge for this project since UNCLOS requires data to be collected at a certain density. The ice conditions often prevent Oden from being able to acquire data where needed.
A second option is to have Oden break a 25 nautical mile long lead or track along a pre-planned line, going back along the same lead to make it wider, and finally to acquire the seismic data while passing through the lead a third time. This option, which was suggested by the captain of Oden, Erik, and the first mate, Thomas, has some obvious advantages. Data can most likely be acquired along pre-planned lines since ice conditions can be evaluated during the first pass and changing ice conditions can be evaluated during the second pass. Data quality is better since Oden does not need full engine power on the third pass and can keep a more steady speed. In addition, the risk of loosing or damaging the seismic gear is reduced considerably. However, data acquisition using this option is more time consuming.
A third option is to use two icebreakers for collection of seismic data in ice filled waters. A lead icebreaker - as powerful as possible - breaks a lead along a pre-planned line, possibly several times in order to prepare as wide a lead as possible. Oden trails behind acquiring seismic data. Using two icebreakers will of course increase the cost for the operations considerably. This is however partly balanced out by a faster and better data acquisition as well as providing a way to collect seismic lines longer than 25 nautical miles. A Russian nuclear icebreaker - 50 let Pobedy - was used for this purpose during the first LOMROG cruise in 2007. Under very severe ice conditions with sea ice under compression, this option also has limitations.
Unfortunately, none of the above mentioned options can guarantee that the necessary seismic data can be acquired. Ice conditions vary considerably from year to year, from month to month, and from week to week. The ice conditions for any particular area are nearly impossible to predict prior to a cruise. Thus we are completely dependent on trying to reach a target area and evaluating the conditions on the spot. If the conditions allow deployment of seismic gear, we can proceed. Otherwise, we must attempt to reach other target areas and hope for better ice conditions.
During the LOMROG II cruise approximately 380 km of seismic data have been acquired mostly by having Oden to break a track (option 2 as described above). This year, none of the seismic gear was lost in the ice as happened during the first LOMROG cruise, and is a common occurrence in arctic seismic experiments. Nevertheless, sometimes it was very close (see photo of streamer on the ice). In fact, only one section of the streamer was damaged by the ice. In general, the data quality is better than that obtained during LOMROG I in 2007. Last but not least the LOMROG II cruise has considerably increased our knowledge on how to operate in ice filled waters. The "learning by doing" principle is especially appropriate when working in the Arctic.