Side Scan Sonar

Side Scan Sonar provides sophisticated digital pictures of the sea-floor surface.

This works by transmitting a narrow acoustic beam to the side of the survey track line which spreads out across the seabed. As the acoustic beam travels outward from the side scan sonar, the seabed and other obstructions reflect some of the incident sound energy back in the direction of the side scan sonar (known as backscatter). The travel time of the acoustic pulses from the side scan sonar are recorded together with the amplitude of the returned signal as a time series and sent to a topside console for interpretation and display.

As with any acoustic sonar, side scan sonar only show echoes of objects that reflect sound back to the side scan sonar transducer, such that hard shiny surfaces are sometimes only seen when they are at right angles to the side scan sonar and rough seabed textures can blot out smaller targets completely. Some types of material, such as metals, boulders, gravel or recently extruded volcanic rock, are very efficient at reflecting acoustic pulses (high backscatter). Finer sediments like clay and silt, on the other hand, do not reflect sound well (low backscatter). Strong reflectors create strong echoes, while weak reflectors create weaker echoes. Knowing these characteristics, you can use the strength of acoustic returns from the side scan sonar to examine the composition of the sea floor.

Interpretation of side scan sonar data develops with experience. Side scan sonar reflections of isolated small objects give no indication of shape or attitude. Man made structures, such as platforms or rock walls tend to have regular patterns that are easier to identify.

Using a side scan sonar is like looking at a world made of shiny black plastic, in the dark, with only a narrow torch beam for illumination. When close to large objects, or in a depression in the seabed, that the viewing range of the side scan sonar may be severely limited.

Very strong reflectors may give multiple echoes along a bearing line, and are identified by being equi-spaced in range. The plan view provided by the side scan sonar also does not show how high an object is, unless an acoustic shadow is cast, in which case the length of the acoustic shadow is related to the height of the object, its range, and the height of the side scan sonar.