Digital Cameras Help Solve Mystery of Leaking Aqueduct
Brent D. Johnson
When New York's water supply began turning up in lakes and pools around Ulster and Orange counties, it was assumed that the 45-mile, 900-ft-deep upstate water tunnel had sprung a leak. But Water Department engineers didn't know how to pinpoint the source. They considered it to be too dangerous to drain the tunnel for a manual inspection, which hadn't been attempted since 1958, because the absence of pressure from water flowing through the tunnel might cause the aqueduct to collapse.
The city turned to Woods Hole Oceanographic Institution in Massachusetts and to one of its robotic deep-sea submersibles for a solution. The vehicle had to swim down a long 13.5-ft-diameter tube and stay centered so that the entire circumference could be imaged. The autonomous torpedo-shaped submarine, dubbed Persephone (the Greek goddess of the underworld), has internal navigation.
A robotic deep-sea submersible was put to work in a less exotic setting, capturing digital images inside an aqueduct to pinpoint the source of a leak.
Initially, the US Department of Environmental Protection wanted video cameras on the submarine to photograph the length of the aqueduct, but Robert G. Goldsborough III, who integrated the imaging system on the Woods Hole vehicle, said that the power requirement for video would have been far too high for a battery-powered autonomous vehicle. Instead, the crew decided to use still shots and strobe lighting.
To solve the imaging problem, the Woods Hole engineers needed a digital camera that met their stringent requirements; they selected the DVC-1412M from DVC Co. in Austin, Texas.
Five of the cameras were positioned around the diameter of the vehicle, affording a 360° view of the tunnel for complete coverage. Each camera was configured with a 4.8-mm Schneider lens with a field of view covering 85° around the tunnel, which allowed good overlap between images.
Goldsborough said that a major benefit of this camera is its unique form factor. Its size and shape allowed it to be packaged alongside a small and compact PC-104 computer for image acquisition. The computer included a PC-104 version of the Matrox Meteor II digital interface board that coupled directly to the camera via a short ribbon cable. The camera assembly was designed to fit into a pressure housing that measured 13 in. long by 5 in. in diameter and that could store up to 40,000 12-bit tiff images.
The 4.8-mm lens and 2/3-in. CCD produced a pixel resolution of 1/10 of an inch on the tunnel wall, with the vehicle flying down the center of the tunnel. With a camera-to-subject distance of about 6.1 ft and a light-to-subject distance of about 8 ft, most of the illumination was absorbed by the water. But the high quantum efficiency of the DVC-1412M at the region of interest in the blue/green part of the spectrum allowed the imaging system to capture good images even in the optically dense water. "With three to four pixels, you can see fine detail," Goldsborough said. "They have even observed changes on the tunnel wall as small as 1/4 in."
The engineers also were able to make small changes in the camera without spending a lot of time or money because of the modular concept in its design. For example, the infrared filter was easily removed from the camera, which enabled the camera to gain all the light filtered through the water.
In addition, they used such camera operations as HDO, a triggered one-shot high-speed shutter mode that is part of the feature set of the standard DVC-1412M, to simplify the integration of the camera into the imaging system. In HDO mode, the host computer generates a pulse that controls the instant of exposure, while software sets the duration of exposure. In this manner, the five cameras and four strobe lights could be synchronized. The single pulse simplifies the logic that is required, eliminating the need for complex circuitry to synchronize the cameras, thus reducing power consumption.
The camera can operate at a rate as fast as 10 fps with 13.92 x 10.40-pixel, 12-bit resolution, but the complete imaging system was limited by the image acquisition computer to 1.2 seconds per frame. The recycle time of the four 50-W/s strobe lights is 1 fs.
During the 15-hour trip through the aqueduct, the submarine collected a staggering 180,000 digital photographs. Now begins the grueling process of analyzing each photo for evidence of a leak.
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