Sidescan sonar

A sidescan sonar is an echosounder that comprises one or two arrays of sonar transducers mounted obliquely to the vessels direction of travel, which are capable of imaging the seabed in a wide swath on either side of the vessel. The sonar emits a fan-shaped acoustic pulse (or ‘ping’) perpendicular to the path of the sensor through the water. The intensity of sound reflected from the seabed and its time of arrival are recorded in across-track slices for each ping, and when combined produce a swath image. Multiple swaths can also be combined to form an image for a wide area of the seabed, and are commonly used for identification of various substrate types (e.g. sand, seagrass, rock), and feature detection (e.g. presence of navigation hazards). The sound frequencies used in side-scan sonar usually range from 100 to 500 kHz. Sidescan sonars are suitable for assessing the nature (e.g. roughness, presence of objects such as rocks) of the seabed, however most do not provide accurate depth (or bathymetry) information. Higher frequencies enable more efficient object detection but have reduced range. A sidescan may be mounted on the hull of a vessel, or towed behind the vessel (on a torpedo-shaped ‘towfish’) at the desired depth.