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Mapping Techniques: Acoustics

Side-Scan Sonar

Research Vessel Illustration

Illustration of a research vessel with a towed side-scan sonar system and a hull-mounted multibeam sonar.
Courtesy: NOAA Coast Survey

Side-scan sonar systems have side-looking transducers that send out sonar signals in pulses across the seabed. The transducers receive the sound that is reflected off the bottom or off objects on the seafloor. The data are transmitted to a graphic recorder that displays an image of the scan. It displays continuous scans and creates detailed pictures of the seafloor.

Advantages and Limitations

Side-scan sonar systems can identify different types of seafloor, such as mud, smooth sand, rippled sand, rock outcrops, and canyons. Dense objects such as rocks, coarse sand, and metal will reflect strong signals. Soft features such as silt, mud, or fine sediments absorb sonar energy and produce lighter acoustic returns.

Side-scan systems are very useful in turbid water conditions, but as with the other acoustic methods, some field verification using physical sampling or imagery is necessary. These systems do not produce any bathymetric information. Additionally, water depth must be at least 2 meters for sampling.

What can you see in a side-scan sonar image? View Image

Uses

Side-scan sonar systems are very accurate for imaging large areas of the seafloor. They are capable of producing continuous characterization information of the seafloor at all depths. Lower frequency systems (around 100 kHz) provide wide swath coverage and are used to create mosaics of the entire survey area. Higher frequency systems (300 kHz and above) can provide higher resolution images. These data reveal detailed information of distinct objects or features on the seafloor. These higher frequencies have shorter ranges and are generally used to image a particular feature or area of interest. Some side-scan sonar is very sensitive and can measure features on the ocean bottom smaller than 10 centimeters (less than 4 inches).

The U.S. Geological Survey uses side-scan sonar in conjunction with multibeam bathymetry to map benthic habitats in Glacier Bay, Alaska.

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