Satellite Mapping Takes To The Oceans

Sometimes the best way to understand something really big is to stand away from it to have a look. Some Scripps Institution of Oceanography scientists want to do just that to understand earthquake activity across Southern California.

They will rely on satellite eyes in the sky. Radar mapping from a satellite can reveal tiny variations in the topography that could lead to a better understanding of how and when earthquakes occur, says geophysicist David Sandwell.

Such information could contribute to better estimates of when a quake might occur on a specific fault, he said. And a team of scientists led by Scripps' researcher Bernard Minster has proposed that NASA build a $90 million satellite mapping that would suit such earthquake studies. From a vantage 350 miles overhead, the satellite would emit a radar beam that would strike the ground across a 60-mile-wide swath and reflect back to the satellite. Computers would construct a map showing variations in height or tilt of as little as half an inch.

When quakes occur, the radar could show fault lines and how much the surface lifted, sank or tilted, Sandwell said. "We want to measure the tectonic motion of the Earth in Southern California," he said. "And we're most interested in the change in the topography." Sandwell is an author on an article in Friday's issue of the journal Science describing the merits of such synthetic aperture radar measurements -- those made by a moving radar source, in this case a satellite. Radar makes maps by measuring the time it takes radio waves to strike the Earth and reflect back.

Radar also has the advantage of being able to peer through clouds and bad weather. The United States does not have such a satellite for civilian science, although the Europeans, Japanese and Canadians do, Sandwell pointed out. These monitor ice floes and oil spills on waterways, among other uses. In 2000, a space shuttle is scheduled to carry a radar system that will make the most detailed topographic map of Earth ever created, one that will distinguish features 10 feet across. But Minster, responding to a request from NASA for projects that study Earth, has proposed a radar satellite that could monitor tectonic activity in Southern California as well as activity affecting volcanoes, and even the advance or retreat of great ice floes at the South Pole. The radar also could survey sites after disasters, such as major earthquakes. The satellite would include scientific contributions from NASA's Jet Propulsion Lab in Pasadena, Stanford University and other institutions.

It is under review by NASA, and Minster said a preliminary decision on the project is expected next month. He said such a satellite would yield more coherent images than currently available. Part of the reason for this is that it can work in conjunction with other satellites, called global positioning satellites (GPS), that keep a precise watch on the position of the radar satellite. GPS maps are being made by scientists at Scripps and elsewhere, but they show only horizontal movement between points on the ground.

 

Satellite Maps