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Lidar System Finds Fault with Trees

Photonics Spectra
Aug 1999
Diedtra Henderson

Would that Washington state geologists had it as easy as their counterparts in California, where faults, such as the San Andreas, show up in aerial photography as long, deep furrows in the Earth's brow. But geologists can't see Washington's faults for the trees: The evidence that whispers of "blind" faults below ground is hidden beneath dense undergrowth and towering evergreens.

Scientists at the US Geological Survey have tapped aerial laser terrain mapping to reveal a dramatic scarp in southern Bainbridge Island, Wash., raised by earthquakes more than 1200 years ago. Lidar images show a sharply etched cliff that lies parallel to a strand of the Seattle Fault zone.

"It gives you new eyes," said Robert Bucknam, a geologist on the survey's Geologic Hazards Team. "This is a really potentially valuable new tool."

This month, EagleScan Inc. of Boulder, Colo., will finish another Washington mapping begun in October. US Geological Survey scientists wanted to use lidar to make aerial maps, to detect other hidden faults and to create floodplain maps in the Snoqualmie/Fall City area.

To make room for the 250-lb time-of-flight Nd:YAG lidar system, EagleScan stripped a Cessna of all but its front seats. The proprietary charge-coupled device array digital camera sees through a peephole in the fuselage to record panchromatic 2024 x 2044-pixel images.

Like a machine gun firing a wide swath, the laser launches 1053-nm beams of light at 4000 pulses per second. The system measures the time it takes for the IR beams to return. Depending on aircraft speed, it can record a point measurement every 3 to 5 m, said Rob Eadie, sales and marketing manager for EagleScan.

The laser bounces off fringy canopy, wispy tree branches and solid ground, making the initial three-dimensional model look rough, he said. But then a software program subtracts tree points, creating maps that look as sharp as a photograph.

Eadie is quick to point out that lidar is not an ideal solution for all terrain at all times. The system is reliable when clouds are higher than the plane. EagleScan's technology can erase up to 95 percent of the vegetation, he said, leaving behind stubble such as low shrubs and trees that hug steep slopes.

"In theory, it works. But in practice, we do have a lot of trouble with trees," he said. "It's not, by any means, a panacea to dealing with the vegetation problem."

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