"The survey is an important part of our work to achieve the goal of storing carbon dioxide in the subsurface," research coordinator Odd Magne Mathiassen at the Norwegian Petroleum Directorate (NPD) said in a statement.

Troll operator StatoilHydro has been storing carbon dioxide (CO2) below the seabed at the Sleipner gas field in the North Sea since 1996. But that is CO2 stripped from the gas stream at the field, not emissions from plants on shore.

"Finding the optimal placement of injection wells is important to ensure that the carbon dioxide can be stored and that it will remain in the reservoir in the future," he added.

The "Johansen formation" in which the CO2 could be stored is located below Troll's oil and gas reservoirs, at a depth of approximately 2,500 metres (8,200 feet), the NPD said.

Troll, on stream since 1996, still has more than a billion barrels of oil equivalent of recoverable reserves -- nearly a third of Norway's total reserve estimate.

The 3D-seismic surveys are made by vessels which shoot pressure waves down into the seabed. The waves bounce off the layers of rock to be caught by equipment towed on cables behind the vessel and made into computer images of the subsurface.

Processing and interpretation of the 3D-seismic data, to be carried out by StatoilHydro, will probably be completed early in 2009, the NPD said. (Reporting by Wojciech Moskwa, editing by Anthony Barker)