Map Dramatically Deepens Learning About Sea Floor

Until recently, map makers could chart the alien landscapes of Venus and Mars in greater detail than the sea floor that covers 70% of the Earth’s surface.

People have been drawing increasingly accurate maps of onshore geography for about 5,000 years--the age of what many scholars consider humanity’s earliest known map. However, the effort to produce even rudimentary surveys of the mountains, rifts and ocean plains hidden miles beneath the waves began in earnest only a few decades ago.

In recent years, ships have crisscrossed the seas towing echo-sounders that use sonar waves to map narrow bands of ocean bottom. That technique--called bathymetry--accurately measures depth but is so focused and time-consuming that in 30 years of mapping voyages, only 5% of the ocean has been surveyed this way.

With the advent of satellites, scientists have surveyed the entire ocean bottom, but they still could not directly sense the sea floor itself. These orbiting altimeters detect the effect on the sea surface of minute variations in gravity caused by the rise and fall of sea floor topography. The resulting gravity maps, while covering much more extensive ocean terrain much more quickly, are not by themselves very accurate, and the best of them--prepared by the U.S. Navy for use by its nuclear submarine commanders--were secret.

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Now, two researchers have overcome the shortcomings of each mapping technique by combining information from ship-borne sonar instruments with newly declassified satellite data to create the most detailed global map yet of the ocean floor.

The new digital sea floor panorama is 30 times more precise than any previous global gravity map and, in large areas of the ocean, 30 times more accurate than earlier maps based on depth-sounding data alone, the researchers said.

It is not the sort of map that a weekend sailor would use, but for research purposes it is so valuable that no scientist interested in the sea is likely to be without it.

“It is a basic measurement of the planet that allows you to do many kinds of science,” said David T. Sandwell of the Scripps Institution of Oceanography in La Jolla. He prepared the new ocean floor map with Walter Smith, a geophysicist at the National Oceanic and Atmospheric Administration.

For oceanographers, the digital global map reveals new detail about the shape of ocean basins as well as major new canyons, ridges and seamounts that help control currents and tides worldwide. It doubles the number of known underwater volcanoes. With it, climate experts can refine computer models of global weather patterns by better understanding the behavior of large ocean currents that affect temperature.

For oil companies, it suggests clues to fossil fuel deposits. For biologists and commercial fishermen, it reveals previously unknown fish habitats. For geophysicists, the digital atlas of the ocean offers a broader glimpse of the forces that shape the Earth, detailing the scars left by sea floor convection patterns and plate tectonics. “The most surprising feature for everyone was that the ocean floor geology was so simple and agrees so well with plate tectonic theory,” Sandwell said.

He and Smith have been working on the map since 1989, and they released a preliminary version, based on gravity data alone, two years ago. They have been refining the map ever since to improve its accuracy.

“The old charts are essentially three-quarters wrong,” Smith said. “You can go from being three-quarters wrong to being about three-quarters right by using our method.”

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The heart of the map is the data collected by the European Space Agency’s ERS-1 satellite and the U.S. Navy Geosat satellite, which together fill hundreds of CD-ROMs with digital information about variations in the ocean gravity field. The Geosat data were collected from 1985 to 1986 but were not declassified until 1995, after the ERS-1 data had been made public. More recently, Sandwell and Smith have incorporated data from NASA’s Topex/Poseidon satellite.

“The gravity field reflects things on the ocean floor pretty precisely, but it doesn’t reflect the broad variations in ocean depth,” Sandwell said. In some areas, the gravity data can be misleading because they may indicate seamounts or rifts that could, in fact, be buried deeply beneath layers of sediment.

The pair’s map overcomes the limitations of gravity data by using the sonar information amassed during 30 years of sea voyages to calibrate the satellite measurements and correct depth errors.

“The value of what we have done here is not that it is the most detailed view of any particular spot but that we have covered the entire world at a uniform level of clarity and precision,” Smith said. “. . . The more we study the oceans, the more we see that topography has an enormous influence on how the oceans circulate.”