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In 2001, scientists used side scan sonar to survey "Shipwreck Alley" in the Thunder Bay National Marine Sanctuary, off the coast of Michigan.

So far, we only know about 40 shipwrecks in the sanctuary. There may be more than 100!


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towfish and side scan sonar echo rollover
Side scan sonar is usually towed behind the survey boat. The equipment is called a towfish. Roll over this image to see how it works.

What is Side Scan Sonar?

Another type of sonar scientists use to map the sea floor is side scan sonar.

Side Scan Sonar

Side scan sonar creates a picture or an image of the sea floor. It measures the strength of how "loud" the return echo is, and paints a picture.

Hard areas of the sea floor like rocks reflect more sound and have a stonger or louder return signal than softer areas like sand. Areas with loud echoes are darker than areas with quiet echoes. Objects or features that rise above the sea floor also cast shadows in the sonar image where no sound hit. The size of the shadow can be used to guess the size of the feature.

Roll over the image above with your mouse to see a diagram of how side scan sonar works.

Did you know?

Press play button to start the movie.

In this movie, a survey ship uses a multibeam sonar and a side scan sonar. Multibeam measures the depth of the sea floor and the side scan identifies items on the sea floor. Scientists use the multibeam soundings to create rainbow-colored maps that depict the depth of the sea floor. The side scan sonar paints a black-and-white image.

Movie Credit: U.S. Naval Oceanographic Office

Click here if you want to see the movie
in Quicktime.

You may need to download: Quicktime | Windows Media

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