Image showing the basic method of active sonar. (Ibid.)
SONAR (SOund Navigation And Ranging) is a method of identifying the location of underwater objects through the emission and reception of sound waves.
Sonar comes in two varieties, active and passive.
Active sonar works by emitting a sound of known frequency and energy underwater, called a "ping". When the ping hits an object, its sound is scattered in many directions, including back towards its source. A hydrophone is used (basically an underwater microphone) to detect the reflection of the "ping". The distance and location of the relfecting object are then determined based on the time between emition and reception of the ping and the energy and frequency of the received sound. (Wikimedia Foundation, Inc., 2006)
Image showing the basic method of active sonar. (Ibid.)
Passive sonar works by listening to any incoming sound waves and using their frequency and energy to identify the source. The source could be an active sonar system or anything else that makes noise in water, such as ship engines or rudders, or even fish. (Ibid.)
Several attributes of water have to be taken into account due to their effects on the propogation of sound waves, both on the speed of sound and the direction in which it travels. These include:
Distinct changes in temperature or pressure result in refracted waves. Refraction is based on Snell's Law, which states that λ1sinθ1 = λ2sinθ2, i.e. the wavelength of the sound immediately prior to hitting the change times the angle at which it hits equals the wavelength of the sound after passing this barrier times the new angle at which it travels. The speed of sound is affected in the same way as the wavelength. (Ibid.)
"The speed of sound in sea water is roughly 1500 meters/second.
(The MB-System Cookbook, 2003)
Side scan sonar is a method of using sonar to map the the ground below by emitting continuous sonar pulses while moving. The sound continuously reflects back to the system's receiver, which creates an image based on the energy of the signal received, where darker points in the image match things protruding from the ground and lighter spot indicate were there is nothing. It does not measure the depth. Side scan sonar is usually towed behind the vessel using it. (Office of Coast Survey, 2002)
Multibeam sonar is similar to side scan sonar, except that it is attached to the vessel (rather than being towed) and measures the time difference between sound emission and reception, rather than the energy of the incoming sound waves. Thus, it measures depth instead of making a picture. (Ibid.)
Multibeam sonar incorporates GPS data and an Inertial Motion Unit (IMU) to track exactly where the vessel is many times a second. Knowing where the vessel is and how much it has moved allows it to more accurately measure "the timing of the sonar echo." (Remote Sensing for Coastal Management, 2006)
Multibeam sonar would produce data most suitable for mapping an undersea transcontinental fiber optic cable route. Its high precision depth measurements allow planning around all of the various aspects of the ocean, from mountain peaks to trenches, which would have significant effects on placement of the cable.
The MB-System Cookbook (2003). How Sound Travels Through Water. Retrieved December 19, 2006 from
http://www.ldeo.columbia.edu/~vschmidt/mbcookbook/x145.htm
Office of Coast Survey (2002). Side Scan and Multibeam Sonar. Retrieved December 19, 2006 from
http://chartmaker.ncd.noaa.gov/HSD/wrecks.html
Remote Sensing for Coastal Management (2006). Multibeam Sonar. Retrieved December 19, 2006 from
http://www.csc.noaa.gov/crs/rs_apps/sensors/multi_beam.htm
Wikimedia Foundation, Inc. (2006). Sonar. Retrieved December 19, 2006 from
http://en.wikipedia.org/wiki/Sonar
Wikimedia Foundation, Inc. (2006). Image:Sonar Principle EN.svg. Retrieved December 19, 2006 from
http://en.wikipedia.org/wiki/Image:Sonar_Principle_EN.svg
