Multibeam Bathymetry Mapping for U.S. UNCLOS Concerns: A Gold Mine for Marine Geology

Since 2003, the University of New Hampshire's Center for Coastal and Ocean Mapping-Joint Hydrographic Center has conducted mapping of several U.S. continental margins in areas where a potential exists for an extended continental shelf as defined under Article 76 of the United Nations Convention on the Law of the Sea. UNH was directed by Congress, through funding to NOAA, to map the bathymetry in areas in the Arctic Ocean, Bering Sea, Gulf of Alaska, Atlantic, Gulf of Mexico, Philippine Sea, and slopes of Kingman Reef and Palmyra Atoll. These new data can be used to accurately locate the 2500-m isobath and to determine the location of the maximum change in gradient at the base of the continental slopes. To achieve these objectives, the area between ~1000 m and ~5000 m isobaths are mapped. The program has mapped >900,000 km2 as of September 2007. The bathymetry data are collected with multibeam echosounders navigated with inertial-aided DGPS and are fully motion compensated. An integral part of the data collection is measurements of the sound-speed profile in the water column to correct for refraction. The data are fully processed at sea. Most cruises also collect 3.5-kHz high-resolution profiles and some have included gravity measurements. All processed bathymetry and associated acoustic backscatter data are immediately available one the web and the raw multibeam datagrams and processed gravity data are archived at NOAA/NGDC. The new data provide a wealth of new information on the geomorphology of the continental margins. The mapping discovered many new features on the U.S. margins, as well as better defined features known to exist but either poorly mapped or mapped with obsolete mapping technology. New features discovered during the surveys include an undiscovered seamount, christened Healy Seamount and a series of huge sediment ridges striking normal to the Barrow margin in the Arctic Ocean; a series of plateaus and ridges north of Bowers Ridge in the Bering Sea; deep-sea channels incising two submarine fans and a meandering channel on a channel levee in the Gulf of Alaska and an almost completely buried seamount of the Bowie-Kodiak Seamount chain; dendritic channel systems trending west from the summit of the West Mariana Ridge, as well as 10 seamount summits <500-m deep also on ridge. Other surprises include what appear to be plunge pools and cascades at the end of steep canyons in water depths of several thousand meter found in the Gulf of Alaska, Bering Sea, Atlantic margin, Gulf of Mexico and the eastern Philippine Sea. Other common features seen in the new bathymetry are intricate channel systems resembling subaerial drainage systems but that have never been subaerial. These have been mapped in the Gulf of Alaska and Atlantic margins and the eastern Philippine Sea. Better-definition of known features include the offshore trace of the Transition Fault and details of Surveyor and Baranoff Fans and Channels in the Gulf of Alaska, a 1200-m high seamount SW of New Jersey on the Atlantic continental rise that was named Knauss Knoll in 1967 but has remained uncharted, 6 bends in Hudson Canyon channel that approach 90 degrees, and a 150-km wide collapsed section of the lower continental slope off New Jersey, and a 20 km wide zone of furrows that run for 200 km roughly parallel to the isobaths of the northern Sigsbee Abyssal Plain adjacent to the Sigsbee Escarpment.