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Feature Article

Gathering Multibeam Bathymetry Aboard Icebreakers
US-Canada Joint Expedition Deploys Healy and Louis S. St-Laurent For Extended Continental Shelf Mapping in Arctic Ocean

Feature Author
Andrew A. Armstrong
Joint Hydrographic Center
NOAA Office of Coast Survey
Durham, New Hampshire

Feature Author
Larry A. Mayer
Professor and Director
Center for Coastal and Ocean Mapping
University of New Hampshire
Durham, New Hampshire

Feature Author
David C. Mosher
Research Scientist
Geological Survey of Canada'Atlantic
Natural Resources Canada
Dartmouth, Canada

Multibeam swath track line for the 2011 U.S.-Canada joint Arctic Ocean bathymetric mapping expedition overlaid on the international bathymetric chart of the Arctic Ocean.
In August and September 2011, the United States and Canada combined resources for a joint extended continental shelf survey in the Arctic Ocean. Article 76 of the Convention on the Law of the Sea details the criteria for establishing the limits of a nation's continental shelf beyond 200 nautical miles from the shoreline. These criteria involve the key data requirements of bathymetry, sub-bottom profiles and deep seismic profiles.

Obtaining bathymetry and seismic profiles in the Arctic Ocean presents the challenges of remoteness and ice cover. Vessels, equipment and strategies must be tailored to overcome these difficulties. At some times of the year in certain parts of the Arctic, a single suitably equipped icebreaker can successfully carry out bathymetric or seismic surveys. However, even though recent summer ice cover in the Arctic has been at or near historic lows, much of the Arctic Ocean where the U.S. and Canada need bathymetric and seismic data remains covered with heavy multiyear ice that prevents a single icebreaker from simultaneously breaking ice and acquiring high-quality data.

To overcome the limitations of a single icebreaker, the summer 2011 Arctic cruise was a combined expedition of the U.S. Coast Guard cutter Healy and Canadian Coast Guard icebreaker Louis S. St-Laurent, led by scientists from NOAA Office of Coast Survey, the University of New Hampshire, the Geological Survey of Canada and the Canadian Hydrographic Service.

Icebreaker Equipment
In this expedition, Healy's primary mission was multibeam echosounding. Healy was equipped with Kongsberg Underwater Technology Inc.'s (Lynnwood, Washington) EM 122, a 12-kilohertz, 288-beam multibeam echosounder. Multibeam echosounding data was processed aboard ship using CARIS (Fredericton, Canada) HIPS software for data cleaning and QPS (Portsmouth, New Hampshire) Fledermaus software for visualization and analysis. Healy also operated a Knudsen Systems Inc. (Ogdensburg, New York) 320B shallow-penetration chirp sub-bottom profiling system, operating at approximately 3.5 kilohertz.

Louis S. St-Laurent's primary mission was seismic reflection profiling. In addition to its seismic system, the vessel was equipped with a Knudsen Engineering Ltd. (Perth, Canada) 320 B/R Plus 12-kilohertz single-beam echosounder and a Knudsen 3260 chirp high-resolution (approximately 3.5 kilohertz) sub-bottom profiler.

Mapping Offshore Alaska
Healy departed Barrow, Alaska, in August 2011. Before the rendezvous with the Canadian icebreaker, Healy spent five days mapping the continental slope of Alaska's Barrow margin. Seafloor mapping operations began with a CTD cast, using a Sea-Bird Electronics Inc. (Bellevue, Washington) SBE 911plus to establish a sound-speed profile and a multibeam patch test to calibrate any timing or attitude offsets.

On the second day of mapping, an isolated conical-shaped feature measuring 500 meters across and 80 meters high was located at 2,000 meters depth. Although similar in size and shape to the ice-cored pingos often found on shallower water in the nearby Mackenzie River delta of Canada, this feature is much deeper than expected for a pingo. Pingos have not been charted off the Alaskan coast, but the presence of this feature suggests that their existence should not be ruled out.

Dual Icebreaker Operations
After completing 18,070 square kilometers of mapping on the Barrow margin, Healy transited northwest to meet Louis S. St-Laurent, which departed from Kugluktuk, Canada. The ships met at the northern end of the Chukchi Plateau and began a long northward line of seismic profiling.

Healy led to break ice, and Louis S. St-Laurent towed seismic gear in the open path Healy created. This line extended more than 1,300 kilometers, beginning on the Chukchi Plateau, crossing the Mendeleev Plain, the Alpha Mendeleev Ridge complex and the Makarov Basin, and ending nine days later on the Lomonosov Ridge, only 172 kilometers from the North Pole.

The science team had been concerned that the Healy's GPS-aided inertial positioning system might have difficulty at these high latitudes. Although one of Healy's two Applanix Corp. (Richmond Hill, Canada) POS MV 320 units did temporarily lose track at nearly 88 degrees north, the second unit maintained track throughout the mission. To continue this article please click here.

Andrew Armstrong has been actively involved in the United States project to map the extent of the continental shelf beyond 200 nautical miles, serving as the project team lead for the Arctic and as co-chief scientist on the project's six Arctic mapping cruises.

Larry Mayer received a Ph.D. from the Scripps Institution of Oceanography in marine geophysics in 1979. He has participated in more than 70 cruises and has been chief or co-chief scientist of numerous expeditions, including six mapping expeditions in the ice-covered regions of the high Arctic.

David Mosher specializes in marine geophysics and is head of the marine and petroleum geoscience section at the Geological Survey of Canada'Atlantic. He has led Canadian efforts in seismic data acquisition for the Arctic Ocean as part of Canada's extended continental shelf mapping program.

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