Scientists at the University of California, San Diego's (UCSD) Scripps Institution of Oceanography and the National Oceanic and Atmospheric Administration (NOAA) have used satellite measurements to develop the most detailed picture to date of the global seafloor.
      The computerized image of the seafloor opens new doors to underwater exploration, including identifying previously unknown volcanoes, studying the tectonic history of the oceans, and searching for untapped petroleum reserves.
      "This data provides us with the first overall view of 70 percent of the Earth in very fine detail," said David Sandwell, a professor in the Institute for Geophysics and Planetary Physics at Scripps who developed the new map with Walter Smith of NOAA. "It opens up whole new areas of marine geology and geophysics and research into other oceanographic processes."
      Sandwell and Smith created the new map by combining newly declassified measurements from a Navy satellite, Geosat, with data collected by the European satellite, ERS-1.
      The ocean basins hold valleys and mountains that dwarf any found on land. In fact, Earth's largest geophysical feature is a 30,000-mile-long submerged volcanic range called the mid-ocean ridge that spans the globe much like the seam of a baseball. A substantial amount of Earth's seismic activity as well as most of its volcanism is found along the crest of the mid-ocean ridge. Here, new crust is created by upwelling magma and the newly formed seafloor pulls away from the ridge in a process called seafloor spreading. The gigantic tectonic plates slide across the mantle and are ultimately subducted back into the Earth, gradually changing the face of our planet.
      Scientists have been attempting to map the Earth's rugged underwater terrain using shipboard sonar since the 1920s. But to date, they have managed to chart only about 5 percent of the global seafloor from ships. Sandwell estimates it would take approximately 125 years to chart the ocean basins using the latest sonar tools.
      Fortunately, such a major undertaking is no longer necessary because scientists have devised a way to explore the ocean bottom from space. Although satellite sensors are incapable of imaging the seafloor directly, by mounting a radar altimeter aboard satellites such as Geosat and ERS-1, scientists can bounce a microwave signal off the ocean and measure the height of the sea surface to fractions of an inch.
      One of the primary forces creating the broad bumps and dips that mark the sea surface is the gravitational attraction generated by geological structures such as mountains and valleys on the seafloor. A typical undersea volcano, which is about 6,500 feet tall, for example, produces a bump on the ocean surface that is a little over 6 feet high. By analyzing the surface gravity fields such structures create, Sandwell and Smith were able to infer what the topography of the seafloor looks like. To be accurate, however, they first had to use computer algorithms to filter out sea surface changes caused by such things as tides and currents.
      Sophisticated satellite data on the sea surface has only recently been made available to oceanographers. Early satellite missions, such as Seasat, revealed new landmarks in the global ocean but were limited in resolution and the amount of detail they could provide. In 1985, the Navy launched Geosat, which acquired high-resolution profiles of the global ocean on tracks that averaged about three miles apart. The satellite, which gathered data until it failed in 1990, was able to detect structures greater than 3,200 feet tall and 6 miles across.
      The Geosat data began to be declassified in response to requests from scientists at Scripps and other institutions. More recently, a panel called the Environmental Task Force recommended that the seafloor measurements collected by Geosat be declassified. The task force was established by Vice President Al Gore when he was a senator to evaluate whether certain classified military data and technologies would be of greater value if released to the scientific community at large.
      The Navy declassified the first set of Geosat data in 1990 that covered a doughnut-shaped area of ocean that surrounds Antarctica between 60 and 72 degrees south latitude. In 1992, they released data south of 30 degrees south latitude. The Geosat data for the entire global sea surface was declassified in July 1995.
      The ERS-1 satellite was launched in 1991 by the European Space Agency as a multi-use satellite. It began gathering similar data to Geosat in 1994. That data was made available to scientists last year. While Geosat and ERS-1 took similar measurements of the sea surface, Sandwell and Smith were able to develop a map of the ocean bottom of unprecendented detail by combining data gathered by both satellites. The new map will give marine geologists an unprecedented ability to study the tectonic plate activity that has occurred on the ocean floor over millions of years.
      The theory of plate tectonics explains how continents form and move about the Earth's surface. As the tectonic plates spread apart, transform faults are created that run perpendicular to the ridge system and allow the plates to slide past one another. These transform faults leave behind scars called fracture zones that remain frozen in the plates over time. Because fracture zones record the history of plate motion, scientists can use them to reconstruct the ancient positions of the continents.
      "In the Atlantic, you can follow one fracture zone from Africa all the way over to North America, which provides evidence of how these plates have moved apart over the last 100 million years or so," Sandwell said.
      Of most interest to oil companies, however, is the information the seafloor maps provides on the topography of sediment basins that may hold large petroleum reserves. The seafloor map is of particular use in providing detailed information on remote areas of the globe, such as the Caspian Sea.
      The Geosat data already has led to the discovery of new undersea volcanoes. Data declassified in 1992 on the oceans south of 30 degrees south latitude, for example, revealed a chain of undersea volcanoes about half the length of the Hawaiian Emperor Chain. Called the Foundation Seamounts, the volcanoes are located south of Easter Island in the Pacific Ocean. The site is the most difficult place to reach on Earth by ship because it is the farthest point from any port.
      "We didn't know this chain of volcanoes existed before," Sandwell said. "but the map allows you to see them quite clearly. It's like being able to drain the oceans and look at the Earth from space - that's the kind of view you get."
      Such a view of the ocean bottom is a boon to researchers such as Sandwell and Peter Lonsdale, a professor of oceanography at Scripps, because it gives them precise information on which areas of the seafloor they should explore next by ship.
      "For many places, the satellite altimetry data provides the most detailed maps that exist," said Lonsdale, who maps the structural patterns of the seafloor to interpret the geological history of the oceans. "So they are the best guides available for planning your survey."
      While the satellite data provides a good overall map of the ocean floor, scientists still must rely on sophisticated acoustic echo sounder equipment to investigate an area of the seafloor in detail. An instrument used by Lonsdale and Sandwell, for example, called Sea Beam offers a picture of structures on the ocean floor that is about 100 times greater in detail than that generated by satellite measurements.

About the Authors:
Janet Howard and Cindy Clark work at SIO Communications, the media branch of UCSD. They may be reached at 619-534-3624.

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