SONAR
Sonar (Sound Navigation And Ranging) is a technology used to identify and locate objects underwater. It is also used to measure water depth (bathymetry). Sonar is used underwater because sound waves taper off less as they travel through water than light or sound waves.
Before the invention of sonar, crews would use lead lines to systematically take soundings of the ocean floor. This allowed them to create early bathymetric maps and depth charts. The first practical use of sonar was during World War I to locate submarines. The U.S. Coast and Geodetic Survey had (later the NOAA’s National Ocean Service) mapped out deepwater regions with sonar in the 1920s. By the 1960s, digital technology enhanced sonar plotting. However, practical applications in the commercial sector became available only after the U.S. Navy declassified the technology in the 1970s.
SONAR helps vessels identify and locate objects in the sea using series of pulses and echoes.
Active and Passive Sonar
Scientists differentiate between passive and active sonar. Passive sonar systems detect underwater acoustics from marine objects, including marine life, ships, and submarines. Passive sonar does not send out its own signal, which is advantageous for the military in preventing detection. It is also preferred for scientific missions that must focus on listening to underwater acoustics. These sonar systems detect only sound waves moving towards the system. Therefore, a passive sonar system must use other passive listening devices in order to measure the range of an underwater object. Using multiple devices makes triangulation possible for determining the source of the sound.
Active sonar transducers send out an acoustic pulse or sound into the water. It that sound hits an object, it bounces off and returns an echo to the active sonar transducer.
Transducers may be equipped with features to receive signals, giving it the capability to measure the strength of the sound. By measuring the time between sending and receiving of the sound pulse, the sonar system can identify the range of the object.
Scientists use both low and high sound frequencies based on the needs of the activities. Both have pros and cons. For instance, higher sound frequencies produce better images; however, the high-frequency pulses travel only short distances. Lower sound frequencies can travel greater distances but produce poorer image quality.
Active sonar systems may be mounted to the hull of a submarine or a ship’s keel. In addition, they may be mounted on Autonomous Underwater Vehicles (AUV) or Remotely Operated Vehicles (ROV).
Side Scan Sonar
A side scan sonar system records a return echo on a continuing basis, thus producing a picture of the ocean floor. The picture depicts light and dark areas, while soft areas like sand and mud send weaker echoes and produce a light image. Harder objects send stronger echoes and produce a dark image. Shipwrecks are commonly located and mapped with side scan sonar systems. However, this method does not produce bathymetric data, although utilizing a multi-beam sonar system in tandem with side scan sonar will provide this data.
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