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www.sea-technology.com October 2014 / st 35 inspection was planned and conducted using sonar technology since no useful visual feed- back could be obtained with the system cam- eras. ASI proposed a multibeam sonar system to collect the tunnel profle sections. Since 360° coverage in a confned environment was re- quired, multiple heads would need to be in- tegrated since no single 360° sonar head was available at the time. ASI selected the Teledyne RESON (Slangerup, Denmark) SeaBat 7125 multibeam sonar system since it had a good track record and had previously been operated as a dual-head system, although never confgured to operate with three heads, which ASI's inspec- tion plan called for. The manufacturer also provided data acquisition and processing software, making them a "one-stop-shop" for development of this sensor package into one integrat- ed system that could be applied in the tun- nel. To provide full imaging of the tun- nel surfaces, ASI se- lected the Teledyne BlueView (Bothell, Washington) M900 multibeam imaging sonar, using three units again to provide coverage of the full in- terior. As with the Sea- Bat 7125 units, three sonar heads had never been integrated onto a single platform for simultaneous op- eration. Accurate positioning in the tunnel was critical for devel- opment of the georeferenced data that the client required. T he La Higuera hydro- electric project is a 155-megawatt, run-of-river development in the Andes Mountains on the Tinguirir- ica River, located approxi- mately 150 kilometers south of Santiago, Chile. The proj- ect is managed by Tinguiriri- ca Energía, a company that is co-owned by Pacifc Hydro and SN Power. Construction of the project was completed in 2009 and in- cludes an 18-kilometer-long headrace tunnel that delivers water to the power plant. In August 2010, a short section of that tunnel collapsed, forcing a 20-month out- age while repairs were undertaken. As part of the follow-up monitoring program, the owner installed a variety of sensors for real- time monitoring of geotechnical parameters in the tunnel while it is operational. To gain a more comprehensive overview of the tunnel condition, the owner wished to inspect the tunnel during short outages without having to de-water it. The owner was familiar with long tunnel inspections using ROVs and issued a tender calling for proposals that would provide high level point-cloud data and sonar imaging that could be used to verify the general condition of the tunnel, look for deformation of the tunnel cross-section and detect cracking, displacements or separations of the tunnel lining. ASI Group Ltd. of St. Catharines, Canada, was awarded the contract to provide such a vehicle. ASI has been provid- ing long tunnel inspection services for more than 20 years and has the longest fully tethered systems for each size class of ROVs in the world, with the longest tether being 10 kilo- meters in length. Tunnel Inspection System Due to the known condition of extremely poor visibility caused by the high sediment loads in the water, the entire High-Resolution Tunnel Surveys by ROV ASI ROV Multibeam Sonar System Inspects Hydroelectric Infrastructure By Bob Clarke • Rick Fletcher Three ROVs customized for long ex- cursions and inspections of fooded tunnels: ASI's Mohican (10 kilome- ters), Falcon (5 kilometers) and vLBV (2.5 kilometers).