Side Scan Sonar and Multibeam Sonar
Sidescan sonar or multibeam sonar or a combination of both are used for searching for wrecks and obstructions
Hull-mounted multibeam sonar (left) and towed side scan sonar (right).
Side scan sonar is a specialized sonar (SOund NAvigation and
Ranging) system for searching and detecting objects on the seafloor.
Like other sonars, a side scan sends out sound energy and analyzes the
return signal (echo) that has bounced off the seafloor or other
objects. In a side scan the transmitted energy is formed into the
shape of a fan that sweeps the seafloor from directly under the
towfish to either side, typically to a distance of 100 meters. The
strength of the return echo is continuously recorded creating a
"picture" of the ocean bottom
where objects that protrude from the bottom create a dark image
(strong return) and shadows from these objects are light areas
(little or no return). While the shape of the seafloor and objects
on it can be well- depicted, most side scan systems can not provide
any depth information. For more information on the operation of
side scan sonar click on these sites maintained by Klein or the Marine
Group.
Side Scan Sonar Image of Sunken Barge
Multibeam sonar systems provide fanshaped coverage of the seafloor similar to side scan sonars, but the output data is in the form of depths rather than images. Instead of continuously recording the strength of the return echo, the multibeam system measures and records the time for the acoustic signal to travel from the transmitter (transducer) to the seafloor (or object) and back to the receiver. Multibeam sonars are generally attached to a vessel, rather than being towed like a side scan. Therefore, the coverage area on the seafloor is dependent on the depth of the water, typically two to four times the water depth.
Gridded Multibeam Sonar Depths in the Approaches to New York Harbor
Search patterns are usually run as a series of parallel lines that ensure overlapping coverage of the sidescan or multibeam sonar. Undercertain circumstances 200% coverage is obtained by running a second search pattern perpendicular to the first over the same area.
The NOAA Ship RUDE utilized both of these systems in surveying the TWA Flight 800 crash site south of Moriches Bay, New York.
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Information
For information regarding content, please contact Jerry Mills
Jerry.Mills@noaa.gov
Phone: 301-713-2780, Ext.116
Last Updated September 8, 2000