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Bathymetric Data
Bathymetry is a standard tool used to map the topography of the ocean floor. Exploration of the ocean bathymetry progressed after World War II, and revealed that the ocean bottom was not flat as previously assumed. The ocean bottom consists of a system of diverse topographic features, and can be divided into three large provinces: the continental margins, deep ocean basins and the mid oceans ridges. Surprisingly, much of the ocean remains to be mapped. Bathymetric data is important for monitoring oceanographic features and processes. Bathymetric data can be applied to scientific questions with geological, biological, physical and chemical emphases.
Bathymetric surveys are often correlated with Global Positioning System (GPS) technology to provide geographic data. Some standard techniques to probe the ocean floor are echo sounding, multi-beam sonar, sidescan sonar, seismic reflection, seismic refraction and satellite altimetry. Bathymetric maps are generated by use of an echo sounder which measures the depth of the sea floor. The echo sounder consists of a transducer (sound transmitter) located under the hull of a vessel. The transmitter sends a sound pulse, which reflects off the sea bottom and is recorded by a hydrophone. The water depth is computed by the time it takes the sound reflect off the seafloor back to a transducer. Some methods also measure the strength of the returning signal to identify substrate types.
A modern type of echo sounding is multi-beam sonar. This technique directs sounds down and sideways, to measure a wide swath of ocean bottom. This is more robust than the traditional echo sounder in that it covers more area. Another bathymetric technique is sidescan sonar, in which engineers produce a map by directing a sound from the side of a ship. This technique compliments the echo sounder in that it can provide extra details on relief and irregularities in the sea bottom by the way they reflect the sonar. Side scanning sonar has less signal attenuation, and a lower angle of viewing differences in topography. This technique is valuable for locating sunken ships.
Another technique is seismic reflection, which allows for analysis beneath the surface of the bottom. Geologists employ a stronger seismic pulse at lower frequencies. This can reveal layers, thickness, and sediment types. Seismic refraction allows geologists to study rock layers. This technique is useful when a sound wave travels through the water is bent, or refracted, rather than reflected. Satellite altimetry also measure ocean depth, and are useful for vast areas not yet measured with sonar. Although it is less accurate than sonar, it can provide global coverage, and lead to more detailed investigations.
Gridded Bathymetry in Alaska
Data Access
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