3.4 Measuring the Depth of the Ocean
The depth of the ocean is usually measured two ways:
The first transatlantic echo soundings were made by the U. S. Navy Destroyer Stewart in 1922. This was quickly followed by the first systematic survey of an ocean basin, made by the German research and survey ship Meteor during its expedition to the South Atlantic from 1925 to 1927. Since then, oceanographic and naval ships have operated echo sounders almost continuously while at sea. Millions of miles of ship-track data recorded on paper have been digitized to produce data bases used to make maps. The tracks are not well distributed. Tracks tend to be far apart in the southern hemisphere, even near Australia (Figure 3.11) and closer together in well mapped areas such as the North Atlantic.
Echo sounders make the most accurate measurements of ocean depth. Their accuracy is ±1%.
The Relationship Between Sea Level and the Ocean's Depth
Let's make the concept more exact. To a very good approximation, the sea surface is a particular level surface called the geoid (see box). By definition a level surface is a surface of constant gravitational potential, and it is everywhere perpendicular to gravity. In particular, it must be perpendicular to the local vertical determined by a plumb line, which is "a line or cord having at one end a metal weight for determining vertical direction" (Oxford English Dictionary).
The excess mass of the seamount attracts the plumb line's weight, causing the plumb line to point a little toward the seamount instead of toward Earth's center of mass. Because the sea surface must be perpendicular to gravity, it must have a slight bulge above a seamount as shown in figure 3.12. If there were no bulge, the sea surface would not be perpendicular to gravity. Typical seamounts produce a bulge that is 1–20 m high over distances of 100–200 kilometers. This bulge is far too small to be seen from a ship, but it is easily measured by satellite altimeters. Oceanic trenches have a deficit of mass, and they produce a depression of the sea surface.
The correspondence between the shape of the sea surface and the depth of the water is not exact. It depends on the strength of the sea floor, the age of the sea-floor feature, and the thickness of sediments. If a seamount floats on the sea floor like ice on water, the gravitational signal is much weaker than it would be if the seamount rested on the sea floor like ice resting on a table top. As a result, the relationship between gravity and sea-floor topography varies from region to region.
Depths measured by acoustic echo sounders are used to determine the regional relationships. Hence, altimetry is used to interpolate between acoustic echo sounder measurements (Smith and Sandwell, 1994).
Satellite-altimeter systems Now let's see how altimeters can measure the shape of the sea surface. Satellite altimeter systems include a radar to measure the height of the satellite above the sea surface and a tracking system to determine the height of the satellite in geocentric coordinates. The system measures the height of the sea surface relative to the center of mass of Earth (Figure 3.13). This gives the shape of the sea surface.
Many altimetric satellites have flown in space. All observed the marine geoid and the influence of sea-floor features on the geoid. The altimeters that produced the most useful data include Seasat (1978), GEOSAT (1985--1988), ERS–1 (1991–1996), ERS–2 (1995– ), Topex/Poseidon (1992–2006), Jason (2002–), and Envisat (2002). Topex/Poseidon and Jason were specially designed to make extremely accurate measurements of sea-surface height. They measure sea-surface height with an accuracy of ±0.05 m.
Satellite Altimeter Maps of the Sea-floor Topography Seasat, GEOSAT, ERS-1, and ERS-2 were operated in orbits with ground tracks spaced 3 km - 10 km apart, which was sufficient to map the geoid. By combining data from echo sounders with data from GEOSAT and ERS–1 altimeter systems, Smith and Sandwell (1997) produced maps of the sea floor with horizontal resolution of 3 km and depth accuracy of ±100 m.
|Department of Oceanography, Texas A&M University
Robert H. Stewart, email@example.com
All contents copyright © 2005 Robert H. Stewart,
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Updated on August 31, 2007