Side-scan sonar

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Diagram of sidescan sonar

Submerged bridge in 160 ft of fresh water seen on sidescan sonar imagery using a Humminbird 981c Side Imaging system

Side-scan sonar (also sometimes called side scan sonar, sidescan sonar, side looking sonar, side-looking sonar and bottom classification sonar) is a category of sonar system that is used to create efficiently an image of large areas of the sea floor. This tool is used for mapping the seabed for a wide variety of purposes, including creation of nautical charts and detection and identification of underwater objects and bathymetric features. It may be used to conduct surveys for maritime archaeology; in conjunction with seafloor samples it is able to provide an understanding of the differences in material and texture type of the seabed. Side scan sonar imagery is also a commonly used tool to detect debris items and other obstructions on the seafloor that may be hazardous to shipping or to seafloor installations by the oil and gas industry. In addition, the status of pipelines and cables on the seafloor can be investigated using side scan sonar. Side scan data is frequently acquired with bathymetric soundings and sub-bottom data which provides a glimpse of the shallow structure of the seabed. Side scan sonar is also used for fisheries research, dredging operations and environmental studies. It also has military applications including mine detection.

Side scan uses a sonar device that emits fan-shaped pulses down toward the seafloor across a wide angle perpendicular to the path of the sensor through the water, which may be towed from a surface vessel or submarine, or mounted on the ship's hull. The intensity of the acoustic reflections from the seafloor of this fan-shaped beam is recorded in a series of cross-track slices, which when stitched together along the direction of motion, become an image of the sea bottom within the swath (coverage width) of the beam. The sound frequencies used in side-scan sonar usually range from 100 to 500 kHz; higher frequencies yield better resolution but less range.

One of the inventors of side scan sonar was German scientist, Dr. Julius Hagemann, who was brought to the US after WW II and worked at the US Navy Mine Defense Laboratory, Panama City, FL from 1947 until his death in 1964. His work is documented in US Patent 4,197,591 which was first disclosed in Aug 1958, but remained classified by the US Navy until it was finally issued in 1980. Experimental side scan sonar systems were made during the 1950s in laboratories including Scripps Institution of Oceanography and Hudson Laboratories and by Dr. Harold Edgerton at MIT.

Military side scan sonars were made in the 1950s by Westinghouse. Advanced systems were later developed and built for special military purposes, such as to find H-Bombs lost at sea or to find a lost Russian submarine, at the Westinghouse facility in Annapolis up through the 1990's. This group also produced the first and only working Angle Look Sonar that could trace objects while looking under the vehicle.

The first commercial side scan system was the Kelvin-Hughes "Transit Sonar", a converted echo-sounder with a single-channel, pole-mounted transducer introduced around 1960. In 1963 Dr. Harold Edgerton, Edward Curley, and John Yules used a side scan sonar to find the sunken Vineyard Lightship in Buzzards Bay, Massachusetts. A team led by Martin Klein at Edgerton, Germeshausen & Grier (later E.G. & G., Inc.) developed the first successful towed, dual-channel commercial side scan sonar system from 1963 to 1966. In 1967, Edgerton used Klein's sonar to help Alexander McKee find Henry VIII's flagship Mary Rose. That same year Klein used the sonar to help archaeologist George Bass find a 2000 year old ship off the coast of Turkey. In 1968 Klein founded Klein Associates, Inc. and continued to work on improvements including the first commercial high frequency (500 kHz) systems and the first dual-frequency side scan sonars. In 1985, Charles Mazel of Klein Associates produced the first commercial side scan sonar training videos and the first Side Scan Sonar Training Manual.

For surveying large areas, the GLORIA sonar was developed by Marconi Underwater Systems for NERC. This operated at relatively low frequencies to obtain long range. It was used by the US Geological Survey and the Institute of Oceanographics in the UK to obtain images of continental shelves world-wide.

Manufacturers of higher frequency side scan sonar systems include Raytheon, Northrop Grumman (formerly Westhinghouse), EdgeTech (formerly E.G. & G.), L-3/Klein Associates, J.W. Fishers Mfg. Inc., Imagenex Technology Corp., Reson A/S, Sonatech, Inc., Benthos (the sonar formerly produced by Datasonics), WESMAR, Marine Sonic Technology, Kongsberg Maritime, Inc., Geoacoustics, EDO Corp., Ultra Electronics and Humminbird (Techsonic Industries, Inc).

Up until the mid 1980s, commercial sidescan images were produced on paper records. The early paper records were produced with a sweeping plotter that burned the image into a scrolling paper record. Later plotters allowed for the simultaneous plotting of position and ship motion information onto the paper record. In the late 1980s, commercial systems using the newer, cheaper computer systems developed digital scan-converters that could mimimic more cheaply the analog scan converters used by the military systems to produce TV and computer displayed images of the scan, and store them on video tape. Now data is stored on computer hard drives.