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Bathymetry is foundational data, providing basic infrastructure for scientific, economic, educational, managerial, and political work. Applications as diverse as tsunami hazard assessment, communications cable and pipeline route planning, resource exploration, habitat management, and territorial claims under the Law of the Sea all require reliable bathymetric maps to be available on demand. Fundamental Earth science questions, such as what controls seafloor shape and how seafloor shape influences global climate, also cannot be answered without bathymetric maps having globally uniform detail.
Since one cannot directly map the topography of the ocean basins from space, most seafloor mapping is a tedious process that has been carried out over a 40-year period by research vessels equipped echo sounders. So far only a few percent of the oceans have been surveyed at the 200-meter resolution. It has been estimated that 125-200 ship-years of survey time will be needed to map the deep oceans and this would cost a few billion dollars. Mapping the shallow seas would take much more time and funding. Fortunately, such a major mapping program is largely unnecessary because the ocean surface has broad bumps and dips which mimic the topography of the ocean floor. The extra gravitational attraction of features on the seafloor produces minor variations in the pull of gravity that produce tiny variations in ocean surface height. These bumps and dips can be mapped using a very accurate radar altimeter mounted on a satellite.
The ability to infer seafloor bathymetry from Space was first demonstrated in 1978 using Seasat altimeter data but the spatial coverage was incomplete because of the short three-month lifetime of the satellite. Most ocean altimeters have repeat ground tracks with track spacings of hundreds of kilometers so they cannot be used the infer bathymetry. Adequate altimeter coverage became available in 1995 when the United States Navy declassified radar altimeter data from the Geosat mission flown in 1985. This has been augmented by dense coverage by the ERS-1 altimeter during a geodetic mapping in 1995. With today's technology, a new altimeter mission could achieve a five-fold improvement in global ocean floor bathymetry.
Related links of interest:
Bathymetry for Google Earth
http://www.ngdc.noaa.gov/mgg/bathymetry/relief.html
http://topex.ucsd.edu/marine_grav/explore_grav.html
http://topex.ucsd.edu/marine_topo/
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Figure 1 - Global bathymetry derived from ship soundings and satellite altimeter measurements of the ocean surface topography. Smith, W. H. F., and D. T. Sandwell, Global seafloor topography from satellite altimetry and ship depth soundings, Science, v. 277, p. 1957-1962, 26 Sept., 1997. More information is available here.
Figure 2 - Modern tools for mapping the deep ocean floor. (left) A shipboard multibeam echo sounders uses sound waves to map 10-20 km wide swaths at ~200 horizontal resolution. (right) An Earth-orbiting radar cannot see the ocean bottom, but it can measure ocean surface height variations induced by ocean floor topography. While the resolution of the echo sounder technique is far superior to the resolution of the satellite altimeter technique, complete mapping of the deep oceans using ships would take 200 ship-years at a cost of billions of dollars. Indeed, the shipboard and altimeter methods are highly complementary. When interesting features are discovered in satellite gravity, these can be surveyed in fine detail by ships.
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