Fate of Dredge Disposal Material and Polluted Sediment, Offshore Honolulu, Hawaii

Comparison Between Multibeam and Sidescan Backscatter Maps

These images present a comparison of seafloor mapping using two different systems, multibeam and sidescan sonar, to produce acoustic backscatter images. Backscatter refers to the amount of acoustic energy reflected back to a source (a listening device either towed or mounted to the hull of a boat), and is useful for mapping areas of differing sediment character or hardness. Backscatter maps are used to map seafloor geology, disposal materials, and other targets of interest. The image on the left was obtained using a 30 KHz ship-mounted multibeam system in 1998; each point on the map is precisely located using satellite navigation. The image on the right was obtained in 1993 using a towed 60 KHz sidescan sonar vehicle to obtain a backscatter image. Unlike the multibeam data which is precisely georeferenced, sidescan sonar is not precisely georeferenced, therefore adjacent survey lines were matched by correlating recognizable objects to create this mosaicked image.

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For more information, please contact:
Michael E. Torresan

U.S. Geological Survey
345 Middlefield Road MS 999
Menlo Park, CA 94025