In this satellite image, the yellow line left of center represents a main logging road in central Africa, while the perpendicular lines denotes secondary logging roads.

The yellow dots mark the areas affected by skid trails and tree-felling.

"It's pretty striking," said Nadine LaPorte, a habitat ecologist who led the satellite survey. "There's clearly a lot of disturbance in the forest."

The researchers were not able to establish whether the land grants, called concessions, given to the loggers, are legal.

"In the Democratic Republic of Congo there's an area that we mapped with logging roads that should not have had a logging concession," LaPorte said.

"There are two possibilities: Either the logging is illegal, or it may have been legal at some point. But it's impossible to tell."

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