How do scientists "see" the shape of the seafloor in the dark? Instead of using light waves to illuminate objects, they use sound waves to image the seafloor using variations of sonar-based techniques. With a picture in hand, scientists can look for evidence of past tsunami-generating landslides, or they can scout out likely areas of future concern

Why use sound instead of light to create images? You can't see to the bottom of the ocean by shining a light from a ship. Light doesn't get very far through the water column. Sound does. In fact, waves similar to sound are able to go through solid rock -- these are earthquake waves.

What is it about light that allows us to see? When we see things, we are actually seeing the light that bounces off objects. When there is no source of light, it's dark because there are no light waves bouncing around for our eyes to detect.

Sound waves can be used in a similar way to "see" things. After turning on a sound source, we can look at the pattern of reflected sound waves that bounce back to us. Our own ears and brain don't process sound into mental pictures. However, there is a way to do this using mathemetical methods, an understanding of physics, instrumentation, and computing tools. This is the basic scheme of all sonar-based imaging techniques.

Large-scale imaging techniques using sound (or other mechanical waves) include:

• Sonar - oldest and least expensive method to measure bathymetry


• Multibeam - more expensive way to get better coverage in measuring bathymetry


• Side-scan sonar - way to produce images of specific objects or ground textures on the seafloor


• Sub-bottom profiling - ways to see underground layering and other structures

At the Ocean Institute
Coming soon.

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Where to find more information:

Sound-based (acoustic) imaging techniques
Mapping Techniques: Acoustics
Summary of major techniques, including sonar, multibeam, side-scan sonar, and sub-bottom profiling.
NOAA Coastal Services Center

Side-Scan Sonar Images
fromVoyage to Puna Ridge

Side-Scan and Multibeam Sonar
Office of Coast Survey, NOAA

Mapping
Ocean Explorer: Technology, NOAA

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Sonar (Echo-sounding)
Sonar
Ocean Explorer: Technology, NOAA

Sounding Out the Ocean's Secrets
About the historical development of sonar-based techniques in studying the ocean.
National Academy of Sciences

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Image examples
Bathymetry of the Los Angeles Margin
Perspective image of U.S.G.S. map data.
Visible Earth, NASA

Multibeam Mapping of the Los Angeles, California Margin
Images that show the shape of the seafloor offshore Los Angeles and Orange Counties, California.
U.S. Geological Survey

Pacific Sea-Floor Mapping Images
Images of the seafloor from many locations on the Pacific Coast including Lake Tahoe and Crater Lake.
Western Regional Coastal & Marine GeologyU.S. Geological Survey

First Image of Historic Shipwrecks Released on 100th Anniversary of Palmer-Crary Loss, December 16, 2002
Side-scan sonar images of the wreck.
Stellwagen Bank National Marine Sanctuary, NOAA

Stellwagen Bank Sanctuary Releases New Shipwreck Images on 104th Anniversary of Portland Loss, November 26, 2002
Side-scan sonar images of the wreck.
Stellwagen Bank National Marine Sanctuary, NOAA

Whales and Walrus: Tillers of the Seafloor
Side-scan images reveal feeding pits scoured by gray whales and walruses.
Hans Nelson, Marine and Coastal Geology Program, U.S. Geological Survey