About 100 kilometres of seismic data were acquired within a small area at 87°30’N on the Lomonosov Ridge in support of ODP proposal 533 for scientific drilling. Target oriented seismic surveys represent a greater gamble than the reconnaissance studies hitherto conducted in leads of opportunity. As a result of relatively difficult ice conditions in the target area, data collection was only realised on three of the five days available for the survey and 400 m hydrophone cable were also damaged.

Aim of the project

Geophysical surveys by Polarstern in 1991 and 1998, Oden in 1996 and SCICEX in 1999 have provided the database for a proposal for scientific drilling on Lomonosov Ridge. The proposal, backed by a proponent grouped of 10 scientists from seven countries (NAD Regional Working Group on Lomonosov Ridge), is presently the highest ranked undrilled proposal in the Ocean Drilling Program. The objective is to collect a unique climate record, spanning the last 50 million years, from a drape of pelagic mud that rests unconformably over dipping sediments along the ridge top. Further drilling through the basal unconformity would sample the Mesozoic passive margin and provide constraints on the tectonic development of the Arctic’s Amerasian Basin.

All the proposed sites are based on single seismic lines. During several decades of scientific drilling in the world’s oceans, targets were identified and drilled based on single seismic lines. Considering the difficult accessibility of the Lomonosov Ridge our initial expectations were that, although an additional site survey was desirable, the requirements would be relaxed. At the Site Survey Panel meeting in February 2000 the consensus was:

• Significant data in support of this drilling proposal have been deposited at the Data Bank. A significant amount of critical data is also known to exist and should be deposited in the near future. However, seismic reflection cross lines for the proposed drill sites have not yet been collected. We encourage the proponents to explore means to collect these important data.

Work onboard

On approach to the Lomonosov Ridge we started deploying two G-guns (8.5 litre) and the 300 m streamer (8 channels, 200 m active length) at 87°40’N, 132°E for a short line segment in search for a site where bathymetric contours gave some hope for conditions where tectonic objectives could be met. The ice was considerably tighter than during the survey in 1996. Leads were short, narrow and disconnected. Moving pieces of ice in the wake of the vessel created immediate problems for the streamer and 200 m of the active sections were damaged in the first attempt to deploy. New sections were put in, but the line had to be aborted after 5 hours.

We then moved to 87°40’N, 136° to start on a line across the ridge between the two lines acquired in 1991. The vessel advanced only 6 n.m. during 8 hours. We therefore aborted the line and steamed towards a planned grid around the proposed sites LORI-1 and -3.

Helicopter reconnaissance showed a limited system of parallel NNW trending leads in the area of the proposed sites. We were able to complete 3 parallel NNW–SSE lines and an E–W segment in a 10×7 n.m. grid around sites LORI-1 and -3. In total we completed 39 n.m. at an average speed of 2 knots. A new line crossed the old AWI-90091 line at each of the proposed sites. At this time about 2.5 days of survey time had been spent, and the opportunity to sleep while the vessel departed on a 48 hour excursion into the Makarov Basin was highly appreciated.

Upon return from the Makarov Basin, we were able to complete a line along the Makarov Basin side of the Lomonosov Ridge crest connecting lines –091 and –090 acquired in 1991. Ice conditions dictated that only NNW–SSE trending tracks were possible.

The vessel then moved north towards a saddle in the central part of the Lomonosov Ridge. The goals were to:

1.  Search for a site suitable for tectonic objectives
2. To investigate the significance of the saddle as a pathway for water mass exchange across the ridge.

Unfortunately, the ice conditions in the general area turned out to be practically 10/10 with no lead systems, and only an occasional open patch. It was decided to try an icebreaking run along the profile before the equipment was deployed. Progress along the line starting at 88°13’N, 147°E had been less than a mile with frequent interruptions and narrow escapes when the hydrophone cable finally got stuck. This initiated a seven hour long fight trying to recover the equipment. In the end, the last active section was suddenly caught in the propeller of the Oden and the cable was destroyed.

Spare streamer sections were available, but helicopter reconnaissance showed tight sea-ice everywhere around us.  At this point, about two days of survey time remained. However, the ice conditions and the expedition schedule implied that further pursuit of seismic data collection by Oden on Lomonosov Ridge would not be feasible during Arctic Ocean 2001. It was agreed that an option would be to spend the remaining survey time on Gakkel Ridge, conditions permitting. This alternative was not exercised due to concern over the equipment status. Kristoffersen and Berge transferred to Yamal and returned to Longyearbyen in order to come up with improved equipment for a subsequent two-week seismic data collection effort from Oden north of Svalbard. In total about 100 line kilometres of relatively good quality seismic data was collected and 10 sonobuoy measurements made. The signals from the sonobuoys had generally poor signal to noise ratio and limited range.

Preliminary results

We were fortunate during Arctic Ocean 2001 to have favourable ice conditions in a limited area that permitted acquisition of the most important seismic lines. Other attempts to collect seismic data carried a heavy penalty in terms of equipment damage and physical labour with little return. It should be recognised that target oriented seismic surveys in the central Arctic Ocean are considerably more of a gamble than the reconnaissance surveys hitherto conducted in leads of opportunity. The new data demonstrate considerable lateral variability of the strata below the main unconformity.