Sidescan sonar

A sidescan sonar uses high-frequency sound pulses that are bounced off the sea floor to create an image of the sea floor morphology shape) and show differences in seabed texture and substrate types. Typically, a sidescan sonar consists of two transducers mounted in a towed body or 'fish'. Transducers can also be mounted on either side of a ship, on a remotely operated vehicle (ROV) or on an autonomous underwater vehicle (AUV). Each transducer generates a fan-shaped sound pulse perpendicular to the vessel track. When the sound pulse hits the sea floor, some of the sound is reflected back to the transducer and some is reflected away. The returned sound is known as backscatter.

Figure 1. Beam pattern of
sidescan sonar systems

Strong return (high backscatter) typically occur when the sound is reflected off hard and rocky surfaces, while weak return (low backscatter) occur if reflected off softer sediments (e.g. sand). Because of the geometry of the sound pulse sent toward the sea floor, an obstacle rising above the seabed, such as shipwreck or steep hill can cast shadows (no return) in the sonar image. The size of the shadow can be used to determine the size of the feature (Figure 1).

Over time, as the sonar moves along the vessel track, the recorded sound reflections form an acoustic image of the sea floor. Typical frequencies used in sidescan sonar range from 100 to 500 kHz with higher frequencies producing a higher resolution image.

The acoustic image can be viewed in real-time using a 'waterfall display' in which the data obtained are displayed as a sound by sound record while it is being collected (Figure 2, 3). The data are also recorded for future analysis and interpretation. Using a GPS input the sidescan sonar system data can be displayed geographically.

Sidescan sonar data

Sidescan sonar data are used for a range of purposes including:

  • Delineating naturally occurring and man-made sea floor features (reefs, sand waves, rocky outcrops, submarine infrastructures, wrecks).
  • Assessment of environmental considerations for marine geology resource management including the location of ore and aggregate deposits.
  • Resource management for targeting areas of suitability (e.g. feasibility of undersea cable placement), where identification of sea floor type and geohazards, such as underwater landslides and pockmarks, may have considerable impact on the development.
  • Environmental management, including establishing baseline data to support environmental monitoring and habitat mapping.
  • Military applications such as mine detection.
  • Shipwreck detection and size calculation according to its shadow.
  • Sidescan systems are used because of the high resolution they provide and the rapidity at which they can cover the sea floor (wide swath). Therefore, they are used for discriminating small features, geological or man-made origin.

Figure 2. Sidescan sonar image showing
a shipwreck located in the MH370 search
area discovered in December 2015.
Source: ATSB

Figure 3. Example raw 'waterfall'
sidescan record
(Image from Penrose et al., 2005)

Reference

Penrose, J.D., Siwabessy, P.J.W., Gavrilov, A., Parnum, I., Hamilton, L.J., Bickers, A., Brooke, B., Ryan, D.A., Kennedy, P., 2005. Acoustic techniques for seabed classification. Cooperative Research Centre for Coastal Zone Estuary and Waterway Management, Technical Report 32.