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Results of the LOMROG cruise

Lomonos ov Ridge off Greenland (LOMROG) 2007: Danish Continental Shelf Project

Christian Marcussen1 (principal investigator)

Dennis Anthony2, Trine Dahl-Jensen1, Rene Forsberg3, Thomas Funck1, Timothy Janzen4, Holger Lykke-Andersen5, Lars Rödel1, Morten Sølvsten2, Per Trinhammer5 & Thomas Vangkilde-Pedersen1

1Geological Survey of Denmark and Greenland (GEUS), Copenhagen, Denmark
2Royal Danish Administration of Navigation and Hydrography (RDANH), Copenhagen, Denmark
3Danish National Space Center (DNSC), Technical University of Denmark, Copenhagen, Denmark
4Canadian Hydrographic Services (CHS), Burlington, Ontario, Canada
5Department of Earth Science, University of Aarhus, Århus, Denmark


Dansk sammendrag

Den danske del af LOMROG togtet er et led i det danske Kontinentalsokkelprojekt i området nord for Grønland, der har til opgave at tilvejebringe det tekniske grundlag for en eventuel udvidelse af kontinentalsoklen ud over 200 sømil i henhold til FN’s Havretskonvention.

Isforholdene i undersøgelsesområdet hører til de vanskeligste i det Arktiske Ocean og er således en stor udfordring for dataindsamlingen. Formålet med LOMROG var således at teste dataindsamlingskonceptet under disse vanskelige isforhold og her specielt det nyudviklede seismiske udstyr.

Der er under LOMROG togtet indsamlet følgende data:

  • Bathymetriske data i Amundsen Bassinet og op på Lomonosov Ryggen, en bathymetrisk kortlægning af yderspidsen af Morris Jesup Rise samt en kortlægning af det område, hvor Østgrønlands Ryggen møder den østgrønlandske shelf. Indsamling af multibeam data og subbottom profiler data er sket i tæt samarbejde med de svenske forskningsgrupper.
  • Seismiske data langs et ca. 130 km langt profil i Amundsen Bassinet, et 15 km langt profil på sydspidsen af Lomonosov Ryggen samt langs et 165 km langt profil ved Østgrønlands Ryggen.
  • Under hele togtet blev der endvidere kontinuert indsamlet gravimetriske data samt indsamlet gravimetriske punktdata ved hjælp af Oden's helikopter.

LOMROG togtet har vist, at indsamling af bathymetriske og seismiske data til brug for det danske Kontinentalsokkelprojekt er muligt under de vanskelige isforhold, der findes i området nord for Grønland. Dataindsamlingen ville dog ikke have været muligt uden meget kompetent hjælp fra den russiske atomisbryder 50 let Pobedy.


Introduction and background

The area north of Greenland is one of three potential areas off Greenland for extension of the continental shelf beyond 200 nautical miles according to the United Nations Convention on the Law of the Sea (UNCLOS), article 76 (Marcussen et al. 2004). The technical data needed for a submission to the Commission on the Limits of the Continental Shelf (CLCS) include geodetic, bathymetric, geophysical and geological data. Due to the ice conditions in the area north of Greenland acquisition of the necessary data faces large logistical problems.

As the Danish part of the LOMROG expedition was the first Danish ship borne activity in the Arctic Ocean ever, one of the main goals of the expedition was to test the data acquisition concepts and especially the new reflection seismic equipment on board Oden under severe ice conditions (10/10 of several meters thick multi-year sea ice under compression). It was furthermore planned to acquire as much bathymetric data as possible to support the delineation of the foot of the continental slope, to map the sediment thickness in the Amundsen Basin and to investigate the bathymetric through between the Lomonosov Ridge and the Canada/Greenland shelf. The Canadian Continental Shelf project participated with a small bathymetric programme planned in Canadian waters. Finally acquisition of gravity data was planned to support the mapping of the sediment thickness.

The Danish Continental Shelf Project (www.a76.dk) funded half of the costs for Oden and the full costs for the Russian nuclear icebreaker 50 let Pobedy.


Seismic data acquisition

The seismic reflection equipment used during the LOMROG cruise has been developed in cooperation with the Department for Earth Science, University of Aarhus and is based on the experience gained by various people who have acquired reflection seismic data in the Arctic Ocean from icebreakers since 1991 (especially Yngve Kristoffersen, Art Grantz and Wilfried Jokat)

The crucial issue of seismic data acquisition in ice covered waters is to protect the seismic equipment from being damaged by sea ice and still getting useful data in this very noisy environment. It was therefore decided to tow both airgun and streamer very deep (approximately 20 meters below water surface) also in order to stay clear from the propeller wash behind the ship. However, for such a large source depth the surface ghost reflections will cause undesirable notches in the source spectrum at 30 Hz and 60 Hz.

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Sercel 605 cu.in. linear airgun cluster

Geometrics GeoEel digital streamer

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Winch container

Inside view of the recording container

Photos 1. Seismic equipment used during the LOMROG cruise (Photos by Thomas Vangkilde-Pedersen).


The seismic equipment is fully containerized and consists of a winch container with three winches, a compressor container, a recording container and a storage container. A heavy duty umbilical both feeds the airguns with compressed air and connects with the seismic streamer. For smooth launch and retrieval of the equipment the winches and Oden’s A-frame were used. Acquisition parameters are summarized in table 1.

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Photo 2. Launching of the seismic equipment: Airgun cluster lower left and umbilical head above the two persons (Photo by Trine Dahl-Jensen)

Table 1 Summary of acquisition parameters

Source

2 Sercel G and 1 Sercel GI gun

Chamber volume

605 cu.inch (250 + 250 + 105)

Pressure

200 bar (3000 psi)

Mechanical delay

16 ms

Nominal tow depth

20 m



Streamer

Geometrics GeoEel

Length of tow cable

43 m

Length of stretch section

50 m

No. of active sections

6 / 5 / 4 / 3

Length of active sections

300 / 250 / 200 / 150 m

No. of groups in each section

8

Total no. of groups

48 / 40 / 32 / 24

Group interval

6.25 m

No. of hydrophones in each group

8

Depth sensor

In each section

Nominal tow depth

20 m



Acquisition system

Geometrics GeoEel controller

Sample rate

1 ms

Low-cut filter

Out

High-cut filter

Anti-alias (=500 Hz)

Gain setting

6 dB

No. of recording channels

48 / 40 / 32 / 24

No. of auxiliary channels

4

Shot spacing

25 m

Record length

Variable between 8.5 and 11 s


map
Figure 1. Trackmap for LOMROG 2007
Due to the short length of the seismic streamer velocity information from the sedimentary units is very limited. However, sediment velocities are important for the documentation of the sediment thickness of the extended continental shelf, if the 1-%-sediment-thickness formula (Gardiner line) has to be applied. Therefore, sonobuoys were deployed along the seismic lines to record the seismic signals at larger offsets.

On-board processing (using ProMax software) concentrated on the evaluation of data quality, noise reduction and acquisition parameters. It showed that the noise level after processing is acceptable considering the conditions under which the data were acquired. Analysis of subsets of the seismic data indicated that a streamer with four sections instead of six sections would provide record sections with only slightly reduced data quality.

Seismic data acquisition started in the Amundsen Basin, where an approximately 130 km long profile was acquired over the course of two days (see figure 1 for location). A brute stack section from onboard data processing is shown in Figure 2. At the southern part of the Lomonosov Ridge area only 15 km of seismic data were acquired due to severe ice conditions. The most vulnerable part of the seismic equipment is the streamer and due to the ice conditions parts of the streamer were lost on two occasions. Finally a 165 km long seismic line supplemented by seven sonobuoys was acquired in the area where the East Greenland Ridge abuts on the North East Greenland shelf.

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Figure 2. Part of seismic line acquired in the Amundsen Basin (water depth approximately 4000 metres).

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Photo 3. 50 let Pobedy sailing along Oden to break the ice and 50 let Pobedy sailing backwards to “pick up Oden”. These operations occurred often during seismic data acquisition due to the severe ice conditions in the area of investigation (Photos by Thomas Vangkilde-Pedersen).


Bathymetric data acquisition

During LOMROG, bathymetric data were recorded continuously along the ship’s track. The areas of specific bathymetric interest to the Danish UNCLOS project were

  1. the Amundsen Basin,
  2. the slope of the Lomonosov Ridge, where the exact location of the foot of the continental slope was identified,
  3. the crest of the Lomonosov Ridge, where the general outline of the Lomonosov Ridge was mapped and verified,
  4. the zone where the Greenland shelf and the Lomonosov Ridge connects where very few bathymetric data exists,
  5. the Morris Jesup Rise, where the precise location of the foot of the slope was mapped,
  6. and finally, the East Greenland Ridge, where a high-resolution full-coverage multibeam survey was carried out in order to reveal the morphological features of the shelf-ridge complex.

Data quality was very dependent on the ice conditions. At the Lomonosov Ridge, where the ice thickness was up to 4 m with abundant pressure ridges, it was impossible to acquire data during ice-breaking and hence, alternative hydrographical surveying methods had to be employed (e.g. the "pirouette method", see Field report no. 5 ). These methods are, however, more time consuming.

The East Greenland Ridge survey area was partly in open water and partly covered by polar drift ice. The data coverage was almost 100 % in all areas including the outer shelf, the slope, the abyssal plain and the central part of the East Greenland Ridge. Five lines extending from the upper shelf break and across the ridge were surveyed with the multibeam echo sounder. Figure 3 shows a bathymetric map that was compiled from portions of two of the surveyed multibeam lines. The multibeam data from the East Greenland Ridge reveal important details of high value for the interpretation of the area and for planning of additional work.

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Figure 3 (top). Part of the bathymetric grid in the East Greenland Ridge area based on two of the surveyed multibeam lines, (bottom) Bathymetric profile - units in meters.


Gravimetric data acquisition

Gravity measurements were carried out with a marine gravimeter of type Ultrasys Lacoste and Romberg (Serial no. S-38). The gravimeter was mounted in the engine room close to the centre of mass of Oden (Photo 4). The system records data every 10 sec, which after processing and reference measurements in the harbours of Tromsø and Svalbard yield gravity values at an accuracy of approximately 1 mgal and 2-500 m resolution, depending on the speed of Oden and ice conditions.

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Photo 4. Marine gravimeter mounted in the engine room on board Oden (Photo by Rene Forsberg).


As a complement to the marine gravity data, the ship’s helicopter was used to make measurements with land gravimeters along profiles across the flanks of the Lomonosov Ridge and the Morris Jesup Rise (Figure 4).

Gravimeter data were collected on Oden at 10 s interval, and processed using the DNSC marine software “eotvos”, that incorporates Eötvös corrections from the ship GPS navigation (logged at 5 s intervals), outlier and spike detection, and a zero-phase filtering scheme. Some minor data gaps occurred for a short period of time due to some serial communications problems. However, most gaps were sufficiently short (a few minutes) to allow interpolation.

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Figure 4. Location of the off-ship helicopter gravity profiles. Greenland at lower left.


Oden provided an excellent platform for marine gravity measurements, and measurements in the ice were superior to data from many other icebreakers or even submarines, in spite of the irregular navigation with frequent course and speed changes. The used Utrasys Lacoste Romberg gravimeter proved stable and reliable with only a small drift. The collected gravimetric data will be useful both in connection with the Danish UNCLOS project, as well as an important new data contribution to the Arctic Gravity Project.

The helicopter measurements were swift and efficient, but limited by the range of the helicopter and by the weather (frequent white-out conditions). However, with a functioning echo sounder and an increased operation range, this could be an efficient way to collect relevant additional geophysical data on future cruises.


Other activities

A research project with the aim to investigate and quantify the importance of sea ice in transporting carbon dioxide from the atmosphere to the ocean in areas with different types of sea ice has been associated with the Danish part of the LOMROG cruise. Furthermore a Danish media crew (Suvi Helminen and Kenneth Sorento) from STV Nature and Science was on board Oden during the LOMROG cruise.


Conclusion

The LOMROG cruise demonstrated that acquisition of bathymetric and seismic data of sufficient quality for substantiating a claim according to article 76 of UNCLOS by a two ship operation is possible under the severe ice conditions that prevail in the area north of Greenland, and data acquisition would not have been feasible without the competent support from the Russian nuclear icebreaker 50 let Pobedy.


References

Marcussen, C., Christiansen, F.G., Dahl-Jensen, T., Heinesen, M., Lomholt, S., Møller, J.J. and Sørensen, K. 2004. Exploring for extended continental shelf claims off Greenland and the Faroe Islands – geological perspectives. Geological Survey of Denmark and Greenland Bulletin 4, 61–64
www.geus.dk/publications/bull/nr4/nr4_p61-64-uk.htm

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