Until recently, scientists studying the degradation of the Amazon's forests haven't been able to see all the damage for the trees. With a computer-aided look, researchers report today that twice as much of the basin has been disturbed by tree-cutting as previously thought.

Satellite-imaging to measure deforestation has been capable of detecting only clear-cut swaths of land, where all the trees have been removed or burned to allow farming or cattle grazing.

Now, a new satellite-imaging system developed by researchers at Carnegie Institution and Stanford University can spot the loss of forest canopy on a finer scale, allowing them to take into account areas where trees have been thinned.

"With this new technology, we are able to detect openings in the forest canopy down to just one or two individual trees," said Greg Asner, a Carnegie scientist and lead author of the report on selective logging in the journal Science.

"Selective logging" -- cutting one or two high-value trees, like mahogany, in an area and leaving the rest intact -- has been considered by some a sustainable alternative to clear-cutting.

Brazil's Space Research Institute's photos weren't sharp enough to spot locations where only a few trees had been cut and hauled away to saw-mills. So Asner and colleagues developed a new computer-aided system to analyze the images and then worked with Brazilian scientists doing ground surveys for three years to confirm on the ground what was detected from space.

The scientists found selective logging leaves a much bigger footprint than had been thought, creating big gaps in the forest canopy that disrupt the local environment.

The scientists found that, from 1999 to 2002, selective logging added 60 percent to 128 percent more damaged forest area -- 4,685 to 7,973 square miles -- than was reported using standard imaging.