Submersible imaging system inspects NYC water supply
After becoming aware of questionable streams along a 900-ft underground tunnel traveling from a reservoir in the Catskill Mountains to a reservoir north of New York City (NYC)...
After becoming aware of questionable streams along a 900-ft underground tunnel traveling from a reservoir in the Catskill Mountains to a reservoir north of New York City (NYC), the city's Department of Environmental Protection (NYCDEP; Flushing, NY, USA; www.nyc.gov/html/dep) determined that the Delaware Aqueduct was the source of the leaks. Flowing at an estimated rate of 30 million gallons a day, the lost water represents only 3% of the total flow but has the potential to create structural damage. As the city's most important water-supply system, approximately 65% of the city's water flows through the 45-mile aqueduct.
City officials chose the Woods Hole Oceanographic Institution (WHOI; Woods Hole, MA, USA; www.whoi.edu) to design and build a 9.5-ft-long autonomous underwater vehicle, the Tunnel Inspection Vehicle (TIV), to inspect the tunnel. Since the team required images for the full 360° in the tunnel, they installed five DVC-1412M cameras from DVC (Austin, TX, USA; www.dvcco.com), each equipped with a 4.8-mm Schneider lens, to provide an 85° field of view. A PC-104 CPU from Advanced Digital Logic (San Diego, CA, USA; www.adlogic-pc104.com) and two 60-Gbyte IDE hard drives per camera were installed and configured with a Meteor-II /Digital PC-104 Plus frame grabber from Matrox Imaging (Dorval, QC, Canada; www.matrox/imaging). Four strobe lights illuminated the surfaces and were synchronized with the cameras and frame grabber using the Matrox Imaging MIL-Lite imaging library.
Once the craft was launched, the voyage took 15 hours for the 45-mile trip. In addition to using a Doppler-shift-based sensor to measure the distance along the bottom of the tunnel, the strobe lights and the five cameras were triggered once per second, capturing more than 180,000 images. "Image acquisition was driven by the acoustically measured distance along the tunnel," explains Roger Stokey, a senior engineer at the WHOI. "We knew the field of view of the cameras, so every time a certain amount of distance was moved another image was taken." After the voyage, the 12-bit TIFF images were off-loaded and stored onto 150 DVD disks. The images from the five cameras are viewed on a six-screen, 21-in.-diagonal flat-panel monitor system built by 9X Media (Los Gatos, CA, USA; www.9xmedia.com).
The duration of the mission and the number of cameras and strobes on-board required a large on-board battery. The NYCDEP required triple housings around the battery to prevent accidental leaks into the potable-water supply system; this made the vehicle larger than it otherwise would have been. The TIV also had to be small enough to fit into the tunnel's openings for the launch and retrieval and had to function at depths between 600 and 2400 ft.
The TIV had to follow the center of the 13.5-ft-diameter tunnel to capture images at the same distance to the tunnel walls throughout the length of the tunnel, which made the system's navigational system more complicated than other vehicles from the WHOI. Engineers used an Acoustic Doppler Current Profiler from RD Instruments (San Diego, CA, USA; www.rdinstruments.com) with signal-processing algorithms to find the range from the submarine to the tunnel wall at several points. This information controls and centers the vehicle.
The WHOI has sent the data to subcontractor ASI-Group (St. Catharines, ON, Canada; www.asi-group.com) for image analysis and reporting. With no other feasible solution, the robotic submarine enabled the NYCDEP to inspect 100% of the tunnel's surface and to assess the integrity of the tunnel and determine possible repair alternatives.