Seafloor Still About 90 Percent Unknown, Experts Say
National Geographic News
|February 17, 2005|
The U.S. nuclear submarine San Francisco crashed into an uncharted underwater mountain in the South Pacific last month, killing one submariner and injuring dozens of others.
The incident remains under investigation, but it spotlights a troubling nautical realitywe may know more about the geography of the moon than that of the ocean floor.
Geophysicist Walter Smith is with the National Oceanic and Atmospheric Administration's Laboratory for Satellite Altimetry in Silver Spring, Maryland.
He says that even a compilation of all historical data gathered by ships, no matter how primitive, would leave much of the ocean floor uncharted.
"If you make an estimate using all the historical data in a place like South Pacific, and compare it to the United States at the same scale, it looks a bit like the interstate highway system," he explained. "It's like sending surveyors out and saying that every few miles they can measure the height of the ground, but [they can] never leave the interstate highway systemthen asking them to come back and make a map showing all the geographic features of the United States."
In such a scenario the Grand Canyon, among thousands of other features, would be unknown.
Estimates vary, but the amount of properly mapped seafloor in the public domain is likely around 2 or 3 percent.
Classified military information could boost that figure to as high as 10 percent, although even the percentage tends to be obscured in layers of secrecy.
The result is that current navigational charts are a mixed bag, often recycling data from primitive surveying techniques of past decades or even past centuries.
Surveys Are Accurate, but Oceans Are Vast
Survey ships equipped with sound-based systems can map the seafloor with striking accuracy.
"We can measure in the deep ocean with accuracies to 0.5 percent of the water depth with a really good system," said Jim Gardner, a research professor at the University of New Hampshire's Center for Coastal and Ocean Mapping/Joint Hydrographic Center. The Durham, New Hampshire-based center is operated in partnership with the U.S. National Ocean Service.
"The spatial resolution is down to sub-meter [less than three feet] accuracy. In shallow waters on the [continental] shelf we can measure five-centimeter [two-inch] bed forms. But we can't do [these things] all over the world."
Survey ships drop a "beam" below them to map narrow swaths of the seafloor. Mapping all the oceans this way might take a thousand years of ship time and cost tens of U.S. billions of dollars.
"Technology has improved dramatically in the last ten years, but the deep oceans are just so vast," Gardner said. "Something else has to be done."
The Naval Oceanographic Office (NAVOCEANO) operates seven survey ships from the John C. Stennis Space Center in south Mississippi.
NAVOCEANO has also taken to the air in an attempt to more efficiently map the shallow waters along coastlines.
The Compact Hydrographic Airborne Rapid Total Survey (CHARTS) system uses laser technology and digital imaging to quickly map the ocean floor from the air.
"The system uses two lasers," explained Barbara Reed, NAVOCEANO's hydrography department director. "One laser penetrates to the bottom, while another is reflected off the surface. The difference between them is the water depth."
CHARTS cannot penetrate murky waters, such as those near some river mouths, but it has proven effective over large stretches of coastline.
"It works to a depth of about 50 meters (165 feet)," explained NAVOCEANO Commanding Officer Captain Jeffrey Best.
"Along many coastlines we can chart at 125 knots versus 12 knots in a ship. It's significantly more efficient."
Ocean Surface Reflects Seafloor Topography
At the National Oceanic and Atmospheric Administration, Walter Smith works from even higher in the sky with satellite altimetry images able to create a wide, general view of ocean-floor topography.
The system uses an indirect technique, mapping the shape of the ocean surface to indicate the mountains, trenches, and other features that lie below.
"If a mountain is on the seafloor, [its] gravity field pulls extra water to that point and causes the [surface] water to bulge," Smith explains. Conversely [the ocean surface] is sunken toward the Earth over a trench."
The satellite images present a "smoothed out" picture of ocean-floor topography rather than a detailed picture.
"People say, Gee, if isn't good enough to pinpoint a reef or to navigate, why bother?" Smith said. "Most people don't realize that almost all of the ocean is unmapped by traditional means."
Satellite data can be helpful to surveying ships. "Satellite altimetry can be a good indicator of where you need to survey," Best said. "It's rough, it's not the fidelity needed for navigation, but in helping to prioritize where you need to survey, it can be of great use."
A diverse group of scientists could also benefit from an update of the satellite data, which is 10 to 20 years old.
The images paint what Smith calls "a pretty clear picture" of the Mid-Ocean Ridgea view quite different from what might have been assumed from scattered, detailed measurements at points along the system of underwater mountains that snakes its way between the continents.
"The big picture provides enough detail to say if you don't understand a certain aspect of seafloor spreading or plate tectonics, you might want to work here," Smith said.
Such maps could also be critical for tsunami-preparation efforts.
"No matter how deep the ocean is, [a tsunami] still feels the bottom, and its path is still influenced by what's on the bottom," Smith said.
Thus, understanding the location of trenches, seamounts, and other features is essential to calculations of how a tsunami will move and where and in what force it will come ashore.
Other studies that could benefit from mapping include marine animal habitat and ocean mixing rates, which are essential to absorption of greenhouse gases. All are dependent on more detailed knowledge of the "other" 70 percent of Earth's surface.
"The uninitiated think that we've mapped the whole world," said the University of New Hampshire's Gardner. "But we really do know more about the back side of the moon than we do the ocean floor."
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