Acoustic location in air was used before radar. Sonar may also be used in air for robot navigation while SODAR (an upward looking in-air sonar) is used for atmospheric investigations. Acoustic emission measurements can also be made for crack detection.
The term sonar is also used for the equipment used to generate and receive the sound. The frequencies used in sonar systems vary from infrasonic to ultrasonic. The study of underwater sound is sometimes known as hydroacoustics.
Principle of an active sonar - send and return pings.Sounder and Sonar in the fisheries
The use of sonar and sounding equipment is commonplace in many fisheries vessels. Modern sonar gives the ability to locate schools of fish in the water column and then to position the fishing gear accurately to target the school. The technology is now commercially available to classify the bottom type (e.g. rock, sand, silt) by processing the signal from a depth sounder. This, in conjunction with Global Positioning Systems (GPS), digital charts and personal computers, allows an operator to build a highly detailed three-dimensional model of their fishing grounds over time. When further integrated with information on catches, weather, time and tide, such systems can incorporate much of the information and experience an operator gains over time. Improvements in commercially available sounder and sonar technology will likely continue with the incorporation of more frequencies, higher levels of resolution and a better ability to detect and discriminate fisheries resources for targeting.
For some decades acoustic techniques have been used as a survey tool in estimating the abundance or biomass of fish stocks. Only recently has it been possible to determine, with some confidence, the identity of the fish schools being detected. The CSIRO's multi-frequency towed device (MUFTI) technology is capable of discriminating both fish species and size, and has been used in trial surveys of deep-water species such as orange roughy and blue grenadier. Acoustic biomass-surveys are particularly suited to species were the population aggregates into a small area, such as during spawning. In the case of orange roughy the multi-frequency system has significantly reduced the uncertainty surrounding biomass estimates, and generally improved the assessment of the stocks' condition. Whilst not directly applicable at present, this sort of technology is under continuous development and will eventually flow through to the fishing industry in the future.
The other main acoustic device that will have an increasingly marked affect on fisheries development is the sidescan sonar with its ability to produce detailed maps of the seabed. Sidescan sonar maps the seafloor in a swath either side of the vessel—hence the term 'swath mapping'. Despite the comprehensive look of broad-scale bathymetric charts, most Australian waters are very poorly mapped, with large areas virtually unknown. The directed use of high-resolution swath mapping over the past few years has had spectacular results in the waters around, and to the south of, Tasmania. Examination of swath mapping of the Chatham Rise (east coast of New Zealand) almost doubled the estimated area suitable for orange roughy in the area. Geoscience Australia (GA) is currently undertaking the Seabed Mapping and Characterisation project, which is expected to provide detailed seabed mapping of the entire Exclusive Economic Zone (EEZ). The project will provide data to support Australia's UNCLOS claim to seabed beyond the EEZ and assist in Regional Marine Planning.
LINKS and REFERENCE:
* Fisheries Acoustics Research (FAR) at University of Washington http://www.acoustics.washington.edu/
* "ACOUSTICS IN FISHERIES AND AQUATIC ECOLOGY" http://www.ifremer.fr/sympafae/
* http://www.daff.gov.au/brs/fisheries-marine/info/technology
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