A sonar, usually built into a towfish, that sends fan-shaped pulses sideways and records echo strength along each side to build a photo-like image. Higher frequencies, for example 400 to 900 kHz, give finer detail over shorter ranges; lower ones see further. Raw records are in slant range and need correction to ground range before mosaicking. It is the standard tool for finding wrecks, debris and small obstructions, but it gives no reliable depths.
Side-scan picked up a 3 metre contact at 60 metres range on the starboard channel, mark it for development.
Range scale sets everything: halve the range and the image sharpens, but line spacing halves and survey time doubles.
Also heard as
- sidescan
- SSS
- side scan
Related terms
Towfish
A streamlined body towed on a cable behind the vessel that carries side-scan or other sensors close to the seabed.
Acoustic shadow
The dark area behind an object in a side-scan image where the object blocked sound from reaching the seabed.
Nadir gap
The unimaged strip directly under a side-scan towfish, roughly twice as wide as the fish's height above the seabed.
Contact
A target picked out of side-scan or multibeam data that may be an object worth investigating.
Layback
How far astern of its tow point a towed sensor actually rides, measured horizontally.