Sidescan Sonar

A sidescan sonar uses high-frequency sound pulses that are bounced off the seafloor to create an image of morphology and differences in seabed texture and substrate types. Typically, a sidescan sonar consists of two transducers commonly mounted in a towed body or 'fish', or they are located in either side of a ship, remotely operated vehicle or autonomous underwater vehicle. Each transducer generates a fan-shaped sound pulse perpendicular to the vessel track. After the sound pulse emitted by the transducers arrives at the seafloor, some of the sound is reflected back to the transducer and some is reflected away. The returned sound reflections are known as backscatter. Over time, as the sonar moves along the track, the recorded sound refelctions for an acoustic image of the seafloor are created. Typical frequencies used in sidescan sonar systems range from 100 to 500 kHz with higher frequencies producing a higher resolution.

To interpret the acoustic image, the following assumptions are used:

  • a flat seabed to convert the slant range into horizontal ranges
  • a non-refracted ray path
  • a vertical plane of isonification

Usually, the acoustic image is viewed in real time using a ' waterfall display' in which the raw data obtained is displayed as a pulse by pulse record while it is being collected. The raw data are always recorded for later analysis and interpretation. Using a GPS input the sidescan sonar system data can display in geographic space. This allows the surveyor to produce a seafloor image superimposed on other media that contain important survey information or data such as nautical charts.