Side Scan Sonar

Side scan sonar is a method of underwater imaging using narrow beams of acoustic energy (sound) transmitted out to the side of the towfish and across the bottom. Sound is reflected back from the bottom and from objects to the towfish. Certain frequencies work better than others, high frequencies such as 500kHz to 1MHz give excellent resolutions but the acoustic energy only travels a short distance. Lower frequencies such as 50kHz or 100kHz give lower resolution but the distance that the energy travels is greatly improved.

Image of "swath" coverage.

A basic Side Scan Sonar System consists of a topside processing unit, a cable for electronic transmission and towing, and a subsurface unit (towfish) that transmits and receives acoustic energy for imaging.

Image of sonar towfish.

The towfish generates one pulse of energy at a time and waits for the sound to be reflected back. The imaging range is determined by how long the towfish waits before transmitting the next pulse of acoustic energy. The image is thus built up one line of data at a time. Hard objects reflect more energy causing a dark signal on the image, soft objects that do not reflect energy as well show up as lighter signals. The absence of sound such as shadows behind objects show up as white areas on a sonar image.

Sunken shipwreck imaged using side scan sonar.