Sonar

Sonar (SOund NAvigation and Ranging) is used for many purposes. Passive sonars detect noise from marine objects, such as submarines, ships and marine animals. Active sonars emit a pulse of sound or “ping,” into the water and then listen for an echo when the signal is reflected off an object. To measure the distance to an object, one measures the time from emission of a pulse to reception. Some common sonar
devices are:
Side Scan Sonar —as the name implies, is a sonar that looks out sideways. It is used to map seafloor topography or to locate objects on the bottom or in the water column. It consists of a tow fish with a transducer running along each side. Acoustic pulses are transmitted orthogonal to the axis of the tow fish. The receiver measures the time it takes for the pulses to return and their strength. These are combined together to create a “shadow picture” that depicts the shape and texture of the seafloor and any objects laying on it. A good review of side scan sonars can be found at http://inventors.about.com/gi/dynamic/offsite.htm?site=http://www.instituteformarineacoustics.org/SonarPrimer/SideScanSonar.htm
Scanning Sonars —also known as forwardlooking sonars and sector scanning sonars, are used for navigation on subsea vehicles. The transducer sweeps back and forth to paint a picture of what is in front of the vehicle.
Sonar altimeters are underwater echo sounders . They measure height off the seafloor from a subsea vehicle. Altimeters are also used to study sediment transport and to monitor scouring around bridge pilings.