Scientists have discovered that previous satellite photographs of the Amazon have missed a form of surreptitious logging that is equally destructive, but not as apparent from space.

Now a team of American and Brazilian specialists have for the first time been able to assess from space the damage done by "selective logging", when one or two trees are removed leaving surrounding trees intact.

They found that selective logging of mahogany and other valuable hardwood trees, which is often illegal, is destroying an area of the Amazon equal to that razed by conventional logging.

Gregory Asner, one of the leaders of the study published today in the journal Science, said that the new satellite technique has provided a shocking insight into the true scale of Amazonian destruction.

"People have been monitoring large-scale deforestation in the Amazon with satellites for more than two decades, but selective logging has been mostly invisible until now," said Dr Asner, a researcher at the Carnegie Institution in Washington and Stanford University in California.

"With this new technology, we are able to detect openings in the forest canopy down to just one or two individual trees," he said. Conventional satellite images have revealed that an area of about 5,800 square miles of the Amazon rainforest is burnt or cleared each year to make way for cattle ranching, farming or other development.

However, when the scientists used the new satellite technique to estimate the area being destroyed by selective logging they found it was comparable - indicating that the overall rate of destruction was twice as high as previously thought.

The total volume of carbon released into the atmosphere as a result of selective logging between 1999 and 2002 is between about 10 and 15 million tons, the scientists estimated. This represents a 25 per cent increase in the overall flow of carbon from the Amazonian forests into the atmosphere.

Dr Asner said: "This was totally surprising to us and alarming to our colleagues, especially those interested in conservation, climate change and the ability of governments like Brazil to enforce environmental laws."

Selective logging occurs when loggers go into an area of pristine forest to cut down the most valuable individual trees, such as mahogany, causing enormous damage in the process. "On average, for every tree removed, up to 30 more can be severely damaged by the timber harvesting operation itself. That's because when trees are cut down, the vines that connect them pull down the neighbouring trees," Dr Asner said.

"Logged forests are areas of extraordinary damage. A tree crown can be 25 meters. When you knock down a tree it causes a lot of damage in the understory. It's a debris field down there," he said.

Studies of area subjected to selective logging have revealed that light penetrates to the understory and dries out the forest floor, making it vulnerable to fires.

Selective logging also involves bringing in heavy equipment using makeshift dirt roads that allows other people to come in later and change the landscape even further, fuelling the process of deforestation.

The satellite technique developed by the scientists at Stanford University and the Carnegie Institution allows the scientists to peer through the dense forest canopy to find out what is happening underneath.

The signals they have exploited show how much green vegetation is in the canopy, how much dead material is on the forest floor and how much bare soil there is, Dr Asner said.

Over the course of four years, the scientists amassed the first full survey of selective logging across the Amazon basin. "We found much more selective logging than we or anyone else had expected - between 4,600 and 8,000 square miles every year of forest spread across five Brazilian states," said Dr Asner.

To make sure their assessments were correct, the scientists went out into the field to compare their satellite data to what they could observe from the ground.

The findings confirmed their worst suspicions - that conventional satellite photography has missed about half of the damage caused by illegal logging.

"The Brazilian government has laws against these logging operations, but they can't enforce them over the enormous geography we're talking about," Dr Asner said. "They can't have a cop on every corner, so our idea is to give them these results in hopes that it might help their law enforcement effort," he said.