Technology Spotlight: Multibeam Sonar

The Inner Space Center (ISC) works with a lot of different technologies to get the job done. This week we are going to take a deeper look into multibeam sonar!

ISC partner vessels, the Okeanos Explorer, operated by the National Oceanographic and Atmospheric Administration (NOAA), and the E/V Nautilus, a private exploration vessel operated by the Ocean Exploration Trust (OET), are both equipped with hull-mounted multibeam sonar systems. Using multibeam sonar, the Okeanos Explorer and the Nautilus collect important data about the seafloor and ocean waters they explore.

Multibeam sonar is a technology system used to create three-dimensional maps of the seafloor. Ships generate maps one line at a time by sailing back and forth across an area of interest, like a person walking a lawn mower back and forth across a yard to cut grass. The lines overlap to make sure no points on the seafloor are missed. As the ship sails, the sonar system emits sound waves from underneath the ship in a fan pattern. These “pings” of sound bounce off the seafloor and back up to the ship, where they are detected by a receiver, usually mounted on the hull, or bottom, of the ship. The sonar system then calculates water depth using the speed of sound in water and the time it took the pings to return to the receiver. The formula, distance equals speed multiplied by time (distance = speed x time), gives the distance from the ship at the water’s surface to the locations where the sound waves struck the seafloor. The sonar system sends this raw depth data to a computer that constructs a three-dimensional picture of the seafloor, called a bathymetric map, like the one shown in the photo above.

Bathymetric maps show the depth of the seafloor, much like topographic maps show elevation on land. Bathymetric maps show depths using a rainbow gradient of color: from shallow depths shown in bright red down to yellow, green, blue, and purple, with purple representing very deep locations. The map above represents a seamount, or underwater hill. The red area indicates the top of the hill, the most shallow point. As the depth increases, the color changes down the hill from red to green down a steep slope. At the base of the seamount, the depth increases more gradually and the colors of the map change from green to light blue to dark purple.

Bathymetric maps of the seafloor are used for several different things. Scientists involved in ocean exploration refer to bathymetric maps to determine target sites to investigate with remotely operated vehicles, or ROVs.  Without maps, the ROVs would be searching a vast, dark area with no direction. Since the maps provide depth data, they are also very important for letting boaters know about potential obstacles or hazards on the ocean bottom. Maps can also provide visuals of important long-term coastal changes, like sea level rise, erosion, and land sinking, also called subsidence. By comparing bathymetric maps of the same area made at different times, scientists and coastline management officials can observe these trends over time and interpret their implications for the future. For example, if a beach has been eroding steadily and has experienced considerable land loss, scientists may advise public officials to begin a replenishment project to return sand to the beach. Scientists also use multibeam sonar to determine what kind of sediment, or underwater soil, makes up the seafloor. This information, called seafloor backscatter data, can tell scientists what the seafloor is comprised of, from fine sediments and sand to hard rock or even the metal of a sunken ship.

Multibeam sonar can also determine the makeup of seawater using water column backscatter data. Changes in the density of the water usually indicate the presence of bubbles, meaning natural gas, often methane, is seeping out of underground deposits in the seafloor. Methane gas can react with seawater to form carbon dioxide, which can affect the pH, or acidity, of the water in a process known as ocean acidification. Underwater organisms can only survive in water within specific pH ranges, so major changes in pH can kill many species in an area. Methane gas can also escape from seawater into the atmosphere, where it becomes a harmful greenhouse gas that contributes to global warming. Monitoring and managing gas seeps is key to understanding and lessening these harmful oceanic and atmospheric changes. In 2012, NOAA used water column backscatter data to discover the first Atlantic underwater methane seeps ever found north of Cape Hatteras, North Carolina. Found at twenty-five sites, as deep as 1600 meters, or 5200 feet, the plumes rose as high as 1100 meters, or 3600 feet, in some places. The discovery implies there are likely more seeps throughout the Atlantic and that multibeam sonar will be key in discovering and monitoring these sites.

Featured Image by NOAA Okeanos Explorer.

http://oceanexplorer.noaa.gov/okeanos/explorations/ex1301/media/mb_mapping_guide.pdf

https://en.wikipedia.org/wiki/Multibeam_echosounder

http://oceanservice.noaa.gov/facts/bathyuses.html

http://www.noaanews.noaa.gov/stories2012/20121219_gas_seeps.html

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