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If trees are rotten inside, loggers often leave them standing for wildlife to
nest in. So researchers at MIT and the University of Alaska are jointly
patenting a non-destructive way of checking the health of trees (WO 99/44050).
Several belts containing ultrasonic transceivers are slung around the tree, at
spaced intervals. The transceivers generate pulses in a known sequence, and
sensors detect when they arrive. The resulting pulse pattern gives a 3D image of
the interior of the trunk.