A systematic sounding of the seafloor.

In March 1983, President Reagan added more than 3 million square miles to the United States' jurisdiction by declaring the waters 200 miles offshore as the nation's Exclusive Economic Zone (EEZ). Last year scientists on the British ship Farnella began a six-year mapping project of this region. The primary economic thrust of the project, called EEZ-SCAN, is to draw the road maps for finding future petroleum and mineral resources. But the Farnella -- now charting the Gulf of Mexico with its unique side-scan sonar system GLORIA II -- has already bestowed a wealth of scientific treasures on researchers, who have never had a comprehensive mapping of coastal regions on this great a scale.

"The difference between what we've been able to use in the past and using GLORIA is like the difference between exploring the Rocky Mountains in a jeep versus looking at them from the space shuttle and getting the overall view -- where th major faults lie, how the geological features relate to one anotehr," says Gary Hill, coordinator of the Marine Geology Program at the U.S. Geological Survey (USGS) in Reston, Va.

The wide view of EEZ-SCAN also enables scientists to see features that might have been overlooked by smaller-scale studies. Last summer, for example, on Farnella's first main leg along the West Coast, researchers discovered dozens of new seamounts and a few new earthquake faults in the 250,000 square miles mapped.

The Farnella just finished the first of three excuursions in the Gulf of Mexico, mapping 50,000 square miles on the western side. Most of the sonar data have yet to be processed, but USGS reports that hundreds of features -- including salt domes, landslides and submarine channels -- are visible. According to Hill, the images help to confirm an earlier theory that growing salt domes block old seafloor canyons, trapping organic sediments and creating basins that either generate or cap oil and gas reservoirs. They also found evidence suggesting that sediments are still actively squashing an underlying layer of salt, a somewhat unexpected result. In addition, Farnella's crew discovered large sand dune fields in 3,000 meters of water -- similar to dunes found in the Pacific last summer. "There's something going on in deep water that people just aren't aware of," says Hill.

The Farnella is currently mapping the eastern side of the Gulf of Mexico. Future legs are planned around Florida, Puerto Rico, Alaska, Hawaii and the Atlantic coast. Then the USGS will go back to these areas and sample the seafloor in order to help interpret the sonar images. As for the Farnella and GLORIA, Britain's Institute of Oceanographic Sciences, which designed and built the one-of-a-kind mapping system, is now discussing the possibility of using it to survey the EEZs of other countries.