Image of side-scan sonar and multibeam sonar. This image belongs to the NOAA
and is used solely for academic purposes. It can be found at http://chartmaker.ncd.noaa.gov/HSD/wrecks.html.
What is Sonar?
Sonar, or Sound Navigation and Ranging, is a method used to detect objects using sound waves. A sound wave is sent out, and once it reaches an object or a hard surface it is reflected back. The time that it takes for the sound wave to make the whole trip, as well as other factors, help determine the where the object is, what its shape is, and can help in the creation of a map of the area.
Sonar is used in
seafloor mapping. The sound waves that are sent out can determine the landscape
of the seafloor. Ultimately sonar can be used to map the seafloor’s depth and
elevations. It can also map out any objects in the sea such as submarines or
underwater cables. Sonar was very useful in World War II, since it helped the
There are two main types of sonar used in seafloor mapping: side-scan sonar and multibeam sonar.
Side-scan sonar is mounted on a towfish device that sends out pulses or sound waves, in the shape of a fan. The sonar can usually cover an area of about 100 meters on either side of the towfish and measures the strength of the pulse that is returned. A strong pulse is depicted lighter in the images, and a weak pulse would result in a darker color (NOAA, 2002). Side-scan sonar is usually only a few feet above the seafloor when emitting the pulses. This is due to the fact that “sound frequencies used in side-scan sonar usually range from 100 to 500 kHz; [and] higher frequencies yield better resolution but less range” (Wikipedia, 2007).
With this is information it is easy to see that in order for side-scan sonar to cover as much area as possible, it must be close to the seafloor. Another issue with this type of sonar is that it can only be useful in mapping out objects in the seafloor, but not depth. (NOAA, 2002).
Diagram of side-scan sonar. This image belongs to the United States Geological Survey
and is used solely for academic purposes. It can be found at http://en.wikipedia.org/wiki/Image:Side-scan_sonar.svg.
Multibeam sonar measures the time it takes for a pulse to be reflected back instead of the strength of the pulse. This results in multibeam sonar being more useful in determining the depths of the seafloor instead of objects. Unlike side-scan sonar, multibeam sonar is attached directly onto a vessel, and can emit sound pulses “typically two to four times the water depth.”(NOAA, 2002)
Image of USS Monitor using multibeam sonar. This image belongs to Klein Associates Inc.
and is used solely for academic purposes. It can be found at http://www.l-3klein.com/image_gallery/5000_images/monitor/monitor.html.
Multibeam sonar contains many beams that emit pulses at the same time. This allows for more accurate results and at a greater speed. For side-scan sonar results to be accurate the towfish must be traveling at a speed of 5 knots or less. For multibeam sonar, the vessel can travel at high speed, and it will still yield accurate results with more coverage. (Klein Associates Inc.)
Optic Cable Mapping
When mapping an undersea transcontinental fiber optic cable, it would be more functional to use multibeam sonar. The cable would have some height compared to the seafloor, and therefore the multibeam sonar would be able to detect it. The other issue is that side-scan sonar would take longer to map out a fiber this long and with accurate results. When using multibeam sonar the cable can be mapped out much faster and still yield accurate results.
Image of underwater cable using multibeam sonar System 5000. This image belongs to Klein Associates Inc.
Obtaining Sonar Data and Technology
Some companies such as Thales Underwater Systems provide sonar technology to the military. TUS is a manufacturer of sonar systems for submarines and surface ships. It is a “subsidiary of Thales Naval” (Wikipedia, 2007). Klein Assocites Inc, also has images of sonar available on their site. Sonar products from their site can be obtained through the U.S. General Services Administration (GSA). The National Oceanic and Atmospheric Administration (NOAA) also has sonar maps of the seafloor available at: http://www.csc.noaa.gov/benthic/data/data.htm.All of these companies use sonar in different ways. Some use it to better the
Habitat Mapping: Spatial Data.” NOAA.
of Coast Survey: Side Scan and Multibeam Sonar.” NOAA.
Sensing for Coastal Management: Multibeam Sonar.” NOAA Coastal
sonar" Wikipedia: The Free Encyclopedia.
Wikipedia: The Free Encyclopedia.
“System 5000 Sonar System.” Klein Associates