NOAA conducts hydrographic surveys primarily with side scan and multibeam sonar. Sonar (sound navigation and ranging) uses sound waves to find and identify objects in the water and determine water depth.
Side scan sonar is a specialized system for detecting objects on the seafloor. Most side scan systems cannot provide depth information.
Like other sonars, a side scan transmits sound energy and analyzes the return signal (echo) that has bounced off the seafloor or other objects. Side scan sonar typically consists of three basic components: towfish, transmission cable, and topside processing unit.
In a side scan, the transmitted energy is formed into the shape of a fan that sweeps the seafloor from directly under the towfish to either side, typically to a distance of 100 meters. The strength of the return echo is continuously recorded, creating a "picture" of the ocean bottom. For example, objects that protrude from the bottom create a light area (strong return) and shadows from these objects are dark areas (little or no return) as in the image at left above, or vice versa, depending on operator preference.
Side scan sonar is typically used in conjunction with a single beam or multibeam sonar system to meet full bottom coverage specifications for hydrographic surveys. NOAA field units use various models of side scan sonar in both hull-mounted and towed configurations.