NOAA ocean explorers used an advanced multibeam sonar mapping system on NOAA Ship Okeanos Explorer last month to discover and map the first deepwater gas seeps found off the U.S. Atlantic Coast north of Cape Hatteras. The seeps were found at water depths greater than 1,000 meters (3,300 feet). Based on preliminary information, scientists believe the seeps are likely emitting methane gas.
Locating seeps with this advanced technology will expand opportunities for researchers to study how seeps in the deep ocean environment affect ocean chemistry.
"Finding and mapping deep ocean seeps is vitally important but has been limited by technology," said Stephen Hammond, Ph.D., acting chief scientist in NOAA's Office of Ocean Exploration and Research. "With advanced multibeam sonar, it may become routine to discover seeps while we systematically explore our poorly-known ocean."
NOAA's use of advanced technology to discover seeps will benefit other agencies, including the U.S. Geological Survey (USGS).
"It's important to find and understand such seeps because they have global significance for the transfer of methane carbon from long-term storage in ocean-floor sediments into the ocean and atmosphere," said Carolyn Ruppel, Ph.D., chief of the USGS' Gas Hydrates Project. "Methane released into the water column is often oxidized to carbon dioxide, leading to changes in ocean chemistry, such as ocean acidification."
The seeps were mapped between Nov. 2 and 20 at three locations with water depths of 1,000 to 1,600 meters (3,300 to 5,250 feet). Approximately 25 distinct seafloor gas seeps were identified based on plumes rising into the water column as high as 1,100 meters (3,600 feet). The sites are between 147 and 163 kilometers (91 and 101 miles) off shore, with one site east of Cape Henry, Va., and two sites south and southeast of Nantucket Island, Mass.
"NOAA tested the ship's multibeam sonar last year in the Gulf of Mexico and confirmed its advanced signal processing made it a highly capable new tool to detect gaseous seeps at great depths and over wide areas," said Robert Detrick, Ph.D., assistant administrator of NOAA Research. "This technology and the information it delivers is extremely valuable to researchers and ocean resource managers in NOAA, in other agencies, and across the nation."
Further descriptions of the deepwater seep expeditions are on the Ocean Explorer website for the Northeast and Mid-Atlantic Canyons Expedition and 2011 Gulf of Mexico Expedition.
Single beam sonar systems have been used extensively to map gas seeps but do not provide as much coverage as typically collected by multibeam systems. Multibeam sonar obtains information from a fan-shape of beams, mapping a wider area more quickly and efficiently. Most multibeam sonars cannot process sonar signals from water-column seeps but the multibeam sonar on Okeanos Explorer is one of the few that is specially configured to do so.
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