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Side-scan sonar is a category of active sonar system for detecting and imaging objects on the seafloor. The multiple physical sensors of the sonar — called a transducer array — send and receive the acoustic pulses that help map the seafloor or detect other objects. This array can be mounted on the ship’s hull or placed on another platform like a tow fish.
How does it work?
As the ship moves along its path, the transducer array sends out signals on both of its sides, sweeping the seafloor like the fan-shaped beam of a flashlight. Side scans search at constant speeds and in straight lines, allowing the ship to map the ocean bottom as it travels. The tow fish will record data at different sound frequencies, depending on the survey goals: a lower frequency (50 kilohertz (kHz) -100 kHz) can cover large swathes of the seafloor at low image resolution. Higher-frequency pulses (500 kHz to 1 megahertz) record smaller areas but in much greater detail.
The resulting “picture” from side-scan sonar data is made up of dark and light areas. Hard objects protruding from the bottom send a strong echo and create a dark image. Shadows and soft areas, such as mud and sand, send weaker echoes and create light areas. These dark and light images help scientists create accurate maps of the seafloor.
Why is it used?
As a specialized sonar system, side scan has particular benefits. This system is often used to map cultural heritage sites like shipwrecks, to characterize the makeup of the seafloor, and can even be used to help biologists identify habitats of marine animals. Side scan is sometimes chosen for an expedition because it is less expensive to run than, for example, a remotely operated vehicle with a high-definition camera. In this way, scientists are able to efficiently cover a lot of ground.
Side scan cannot measure bathymetry (depth), so it is often used in tandem with depth-measuring tools such as single-beam and multibeam sonar in order to create a more comprehensive map of the seafloor.
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