Shell flags intent to test winter seismic in Beaufort
In a Jan. 12 letter to the Alaska Department of Natural Resources, Shell Exploration and Production Co. gave notice of its intent to carry out a research program, to investigate the feasibility of seismic data acquisition from floating ice in the Beaufort Sea. Shell plans to conduct the research between mid-March and mid-May 2007.
Although the company expects the research to take place entirely within the federal outer continental shelf, Shell has applied for a state geophysical permit, as a contingency against ice conditions forcing the work to be carried out closer to land.
In a fact sheet describing the program, Shell said that it wants to establish whether it can acquire seismic data from floating ice in the company’s shallow water Beaufort Sea oil and gas leases — water depths in some of the leases are too shallow to permit the operation of a seismic vessel, the company said.
In October at the National Marine Fisheries Service Arctic Open Water Peer Review Meeting in Anchorage Shell also said that shooting offshore seismic from sea ice in the winter might provide a means of eliminating any possible conflicts between seismic operations and subsistence hunting by North Slope communities.
Forty-eight inch iceIn its letter to DNR, Shell said that the winter seismic testing would take place on ungrounded ice, at least 48 inches thick, in water less than 60 feet deep. Monitoring of potential sites would start in January, with the actual site being chosen using criteria such as the measured ice thickness.
A key research objective will be to find a seismic technique that can overcome problems associated with what is known as a “flex wave,” the transmission of excessive seismic noise through the ice.
“Noise associated with the flex wave has been found to be up to a thousand times higher in amplitude than the desired seismic signal,” Shell said in its fact sheet.
The survey crew will place seismic sources and receivers both above and below the ice in an area of 16 square kilometers, to test for source/receiver pairings that will best address the flex wave and other noise effects. In addition to receivers attached to the ice surface, some receivers will be suspended in the water column and others will lie on the seafloor. Sources will include standard and lightweight vibrators; accelerated weight drop on the ice; and small-volume air gun arrays deployed through holes augured in the ice.
Veritas DGC Land Inc. will conduct the experimental survey, using a vehicle fleet that includes a variety of sizes of vibrators. A modified track vehicle will deploy sub-ice seismic sources. Seismic receiver deployment will involve 13 tracked vehicles.
A camp of about 120 people about a half-mile offshore the West Dock at Prudhoe Bay will require up to 35 sled trailers, including medical facilities; crew quarters; offices; kitchen and dining facilities; technical work spaces; generators; and fuel storage units.