Acquisition of seismic data in Arctic sea ice
Written by Christian Marcussen, Thomas Funck, John Hopper, Per Trinhammer, Holger Lykke Andersen, Lars Rödel, Anja Gunvald, Esben Villumsen Jørgensen
Web-edition Torsten Hoelstad, GEUS
September 5, 2009
On board the Swedish icebreaker Oden
Position: 82° 40’N, 13° 36’E (in transit to Longyearbyen on Svalbard)
Weather: -2.5° C, northerly wind, overcast
One of the main goals of the LOMROG II cruise is to acquire seismic data in the Amundsen and Makarov Basins on both sides of the Lomonosov Ridge (for a map see field report no. 5
). Seismic methods can be used to map sediment thickness, as is commonly done by the oil industry prospecting for oil and gas.
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:
- a line delineated in accordance with paragraph 7 by reference to the outermost fixed points at each of which the thickness of sedimentary rocks is at least 1 per cent of the shortest distance from such point to the foot of the continental slope; ( or
- (ii) a line delineated in accordance with paragraph 7 by reference to fixed points not more than 60 nautical miles from the foot of the continental slope.)”
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:
- The streamer is considerably shorter than in open water. For the LOMROG II cruise we use a 250 meter long streamer. There are many advantages to using a short streamer in the Arctic. Seismic streamers are designed to maintain a constant depth in the water only while the ship is in motion, if the ship’s speed falls below 2 knots, the streamer will sink. Below 300 meter, the electronics in the streamer will be crushed by the water pressure. With a 250 meter streamer, we are able to deploy and recover the streamer with the ship at a standstill without risk of damage.
- The seismic source is considerably smaller and therefore also more compact than for open water surveys. This simplifies deployment and recovery in the event that equipment must be brought on board quickly, for example when the ship becomes stuck in ice and must backup to free itself.
- Both the streamer and guns are towed at a depth of approximately 20 meters, which is more than twice as deep as normal. This is below the wash from the ship’s propellers, which can be a source of considerable noise, especially when extra power is needed to break ice and keep the ship moving forward. In addition, a deep towing depth helps to prevent the equipment from coming in contact with ice, which can cause damage. In addition, the streamer can become pinched in the ice and pulled off.
- Both the airguns and streamer are connected with only one cable to the ship (the “umbilical”). This minimizes the risk of damage by ice and serves to simplify deployment and recovery of gear so that it can be done quickly when necessary.
Because we record only the time it takes for energy to return to the surface, it is necessary to know the seismic velocity of the subsurface to compute the exact thickness of the sediments. This can be derived from processing of data acquired with a long streamer. When a short streamer is used, as in ice filled waters, sonobuoys are deployed instead. These can detect the seismic signals up to 25 km away from the ship. The buoys send the signals back to Oden with a radio transmitter, where the data are recorded. The only challenge is to deploy them in the wake of Oden without having them destroyed by ice.
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:
- High speed would create an unacceptable noise level behind the ship and the seismic gear is not designed to withstand a high speed.
- As the ship travels faster, the towed gear gets pulled toward the surface, risking damage by ice.
- Oden can not back due to the risk of getting the seismic gear tangled in the propellers.
To meet the above limitations there are different options:
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.
A – Seismic equipment on Oden‘s aftdeck: winch container with streamer (blue) and umbilical (orange). B – Anja and Esben holding the streamer. Parts of the airguns can be seen to the left. C – Thomas ready to launch a sonobuoy. D – Holger, Per and John in the registration container monitor the data acquisition.
A – Oden collecting data along a pre-broken track in the sea ice. B – Open lead behind Oden indicates fairly light ice conditions. C – The umbilical (orange) is the only cable connecting the seismic gear with the ship. D – One of the excitements doing of seismic work in dense sea ice: the streamer on top of the ice. Just before the picture was taken the airguns received the same treatment. However the seismic gear survived!