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Using Side Scan Sonar to Collect Hydrographic Data

Side scan sonar unit

Side scan sonar units are towed in the water, often behind the vessel. Click image for larger view.

Side scan sonar technology is a specialized sonar system used as a tool to search for and detect submerged objects such as wrecks and obstructions on the sea floor. Side scan sonar is also used to determine the shape of the seafloor. Most side scan sonar systems, however, can not provide any depth information.

A typical side scan sonar consists of a topside processing unit on board a vessel, an electrical/mechanical cable for data transmission and towing the side scan, and a towfish. The towfish transmits and receives narrow bands of acoustic energy (sound) that are reflected back for imaging.

Like many other sonars, a side scan sends out sound energy and analyzes the return signal (echo) that has bounced off the bottom or from other objects. With side scan sonars, the transmitted energy has a fan-shape form that sweeps the bottom from directly beneath the towfish to either side, typically to a distance of 100 meters.

Side scan sonar image shows the wreck of the Herbert D. Maxwell

This side scan sonar image shows the wreck of the Herbert D. Maxwell, a schooner that sank in the Chesapeake Bay on May 16, 1910. Click image for larger view.

Certain frequencies tend to work better than others, depending on the situation. High frequencies (500 kilohertz (kHz) to 1 megahertz (MHz)) can provide good resolution, but the sound only travels through the water column for a short distance. Conversely, lower frequencies in the 50 kHz or 100 kHz range will give a lower resolution, but the distance that sound travels will improve significantly.

The strength of the return echo is continuously recorded, creating an image or picture of the seafloor. A side scan sonar image is produced one line of data at a time. Natural or human-made submerged objects that lie on the bottom create a darker image (strong return) and shadows from these objects are light areas with little or no return.