Sidescan sonar uses sound energy reflected off the seabed to reveal the detailed texture or roughness of the seafloor. Unlike echosounders which only measure depth, a sidescan sonar records an image of the seabed by measuring the strength of returned echoes.
A sidescan transducer is typically mounted inside a long cylindrical pod, called a towfish, which is connected via a cable to a survey vessel and towed slowly behind and close to the seabed. The sound energy is transmitted from either side of the towfish in a wide fan shape.
The returned echo strength is very sensitive to the subtle variations in seafloor hardness and roughness, and the resulting sidescan sonar image is typically represented as a scrolling grey-scale image on a computer screen.
White pixel values in the sidescan image represent hard or rough features, while dark pixel values represent soft or smooth seafloor. Black shadows are found behind objects that mask the sound energy being directed at it.
Shipwrecks are often found using sidescan sonar because their complex shapes are very different to the surrounding flat seabed. The relative variation in texture is also very useful in defining the seabed habitat boundaries. For example, identifying hard ground features such as reefs within soft sediment areas. The marine life found in these contrasting habitats are often quite different.
The Gallery images and Movie below are examples from various sidescan sonar surveys conducted on the Great Barrier Reef and in northern Australian waters. These images also include results of the Geological Long Range Inclined Asdic (GLORIA) deep-water sidescan survey of the Coral Sea in 1989. The GLORIA survey shows vast areas of the Queensland Trough covered in sediment flows sourced from the numerous submarine canyons incising the north-east Australia margin.