SOund NAvigation and Ranging—SONAR—is used to find and identify objects in water. It is also used to determine water depth (bathymetry). Sonar is applied to water-based activities because sound waves attenuate (taper off) less in water as they travel than do radar and light waves.
Prior to the advent of sonar, mariners used lead lines to take systematic 'soundings' of the seafloor, which enabled them to produce early depth charts and bathymetric maps. Sonar was first used during World War I to detect submarines. By the 1920s, the U.S. Coast and Geodetic Survey—the precursor to NOAA’s National Ocean Service—was using it to map deep-water areas. The technology steadily improved, and by World War II, was used once again for military purposes.
In the 1960s, the development of digital computer technology made plotting of sonar data much easier, but this technology was not available to the civilian scientific community until the U.S. Navy declassified it in the 1970s.
The ROV Little Hercules' scanning sonar imaged this 19th century wooden-hulled shipwreck in March 2012. Click image for larger view.
A view of multibeam sonar water column backscatter data used aboard the Okeanos Explorer to detect gas seeps. Click image for larger view.
Scientists use two general types of sonar—active and passive. Active sonar transducers emit an acoustic signal or pulse of sound into the water. If an object is in the path of the sound pulse, the sound bounces off the object and returns an “echo” to the sonar transducer. If the transducer is equipped with the ability to receive signals, it measures the strength of the signal. By determining the time between the emission of the sound pulse and its reception, the transducer can determine the range of the object. (Range = sound speed x travel time / 2).Scientists use both high and low sound frequencies, depending on the needs of the mission. Both have advantages and disadvantages. For example, higher sound frequencies (up to 1Mhz) provide better image resolution, but the acoustic energy pulses can travel only a short distance. Lower sound frequencies (50kHz to 100kHz) provide a lower picture resolution, but the energy pulses can travel greater distances.
Active sonar transducers can be mounted on the keel of a ship or the hull of a submarine or lately on Remotely Operated Vehicles (ROV) and Autonomous Underwater Vehicles (AUV). They also may be towed beside the ship (towfish) at a predetermined water depth.
Passive sonar systems are used primarily to detect noise from marine objects, such as submarines, ships, and marine animals like whales. Unlike active sonar, passive sonar does not emit its own signal, which is an advantage for military vessels that do not want to be found or for scientific missions that concentrate on “listening” to the ocean. Rather, it only detects sound waves coming towards it. Thus, passive sonar cannot measure the range of an object unless it is used in conjunction with other passive listening devices. Multiple passive sonar devices may allow for triangulation of the sound source. (Read more on passive ocean acoustic monitoring.)
Recovery of a DF 1000 sidescan sonar towfish. Click image for larger view.
Computer software is used for acquisition of sidescan sonar mapping data. Click image for larger view.
Marine researchers commonly use side-scan sonar technology to search for and detect objects on the seafloor. Side-scan sonar requires three components—a towfish that sends and receives acoustic pulses, a transmission cable attached to the towfish that sends data to the ship, and the ship’s processing computer. The computer plots the data points so that scientists can visualize the found object or seafloor feature. The side-scan beam is oriented to the side of the ship, and usually slightly downward. As the ship moves along its path, the towfish, which is dragged usually near the sea floor, searches the water along the ship’s side. Side scans search at constant speeds and along straight lines, allowing the ship to map the ocean bottom as it travels.Side scan sonar continuously records the return echo, thus creating a “picture” of the sea floor. This picture is made up of dark and light areas. Hard objects protruding from the bottom send a strong echo and create a dark image. Shadows and soft areas, such as mud and sand, send weaker echoes, thus creating a light image. Studying these dark and light images, scientists can create accurate maps of the sea floor, and locate seafloor features and possible obstructions to navigators. For example, shipwrecks are commonly found and mapped using side-scan sonar. Read about how the wreck of the USS Monitor was located, and shipwrecks were found in the Thunder Bay sanctuary.
Side-scan sonar, however, does not usually provide bathymetric data. To obtain this information, scientists use ship-mounted multibeam sonar systems, often in concert with side-scan systems. Like side-scan, multibeam sonar provides a fan-shaped coverage of the seafloor, But instead of recording the strength of the return echoes, multibeam systems measure and record the time elapsed between the emission of the signal from the transducer to the seafloor or object, and back again. And instead of a line of soundings, multibeam sonars produce a “swath” of soundings. Search patterns usually resemble overlapping parallel lines to ensure full coverage of an area.
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