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Karissa Oleyanin
Superhero Member
Posts: 4127

« on: February 01, 2009, 02:00:32 am »

The geologic and topographic structures of the ocean floor primarily reflect plate tectonic activity that has occurred over the past 150 million years of the 4.5 billion year age of the Earth. Seafloor geology is far simpler than the geology of the continents because erosion rates are lower and also because the continents have suffered multiple collisions associated the opening and closing of ocean basins (Wilson Cycle). Despite their youth and geologic simplicity, most of this deep seafloor has remained poorly understood because it is masked by 3-5 km of seawater. For example, the Pacific-Antarctic rise, which has an area about equal to South America, is a broad rise of the ocean floor caused by sea floor spreading between two major tectonic plates (see Poster southeast of New Zealand). To the west of the ridge lies the Louisville seamount chain which is a chain of large undersea volcanoes having a length equal to the distance between New York and Los Angeles. These features are unfamiliar because they were discovered less than 20 years ago. The Louisville seamount chain was first detected in 1972 using depth soundings collected along random ship crossings of the South Pacific. Six years later the full extent of this chain was revealed by a radar altimeter aboard the Seasat (NASA) spacecraft. Recently, high density data collected by the Geosat (US Navy) and ERS-1 (European Space Agency) spacecraft data show the Pacific-Antarctic Rise and the Louisville Ridge in unprecedented detail. In an age when we are mapping the surfaces of Venus and Mars, it is difficult to believe that so little is known about our own planet.

The reason that the ocean floor, especially the southern hemisphere oceans, is so poorly charted is that electromagnetic waves cannot penetrate the deep ocean (3-5 km = 2-3 mi). Instead, depths are commonly measured by timing the two-way travel time of an acoustic pulse. However because research vessels travel quite slowly (6m/s = 12 knots) it would take approximately 125 years to chart the ocean basins using the latest swath-mapping tools. To date, only a small fraction of the sea floor has been charted by ships.

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. These bumps and dips can be mapped using a very accurate radar altimeter mounted on a satellite. In this brief report we attempt to answer some basic questions related to satellite measurements of the ocean basins. What causes the surface of the ocean to bulge outward and inward mimicking the topography of the ocean floor? How big are these bumps? How can they be measured in the presence of waves and tides? What are some of the non-military applications of these data? What has been discovered from the new Geosat and ERS-1 data?


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