Quarterdeck (6.3) Online -- Texas A&M Oceanography

Texas A&M University
Department of Oceanography

Winter 1998 / Spring 1999 - Vol. 6, No. 3

Side-scan sonar (continued)

DID YOU KNOW?

Side-scan sonar images usually show two things: the geometry and the texture of the seafloor.

In this manner, after many passes, the sonar strips can be pieced together to form a "mosaic," often the final product of a side-scan sonar survey. It is equivalent to a large, black-and-white aerial photo.

Side-scan sonar images usually show two things: the geometry and the texture of the seafloor. First, the geometry of the seafloor is revealed in shadows and highlights. Things that stick up in the water, a coral reef, for example, reflect a lot of sound energy back on the side that faces the sonar and leaves a shadow on the far side. Conversely, a crater has a shadow on the near side and high reflection on the far side. So the sonar interpreter can "see" topography by shadows.

Second, the side-scan sonar images show irregularities in the texture of the seafloor and sediment. Most sound energy arrives at an angle and bounces away from the sonar (remember from physics: the angle of incidence equals the angle of reflection). But irregularities on the surface and in the sediment can reflect back some sound toward the sonar. This reflection is called backscatter.

For example, the small particles in mud and clay give little backscatter. Larger grains of sand backscatter more, and sediments with shell fragments and even larger particles may backscatter much more. Side-scan sonar is sensitive to the type of sediments and textural patterns on the seafloor and so it is frequently used to map variations in such properties.

Many swaths of the towfish create a mosaic image of the seafloor. [137K]

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Copyright 1998-1999, Department of Oceanography, Texas A&M University.
Updated November 24, 1998. (abdw)

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