Geohazard Monitoring of the Ituango Dam Using Radar Satellite Based InSAR Technology

摘要

The Ituango Dam is a 237-meter tall embankment earth-fill dam located in northwestern Colombia. The $3.8 billion USD project has been under construction since 2011 and, once complete, will provide more than 17% of the country's electricity needs through an underground powerhouse containing eight generators producing 2.4 Gigawatts of power. In April 2018, with the dam still 15 meters short of its final design height, a sudden closure of the operating diversion tunnel occurred, causing the reservoir to fill prematurely. The surface expression of this event was the formation of a 60 - 80 meter diameter sinkhole. The reservoir induced two separate landslides in May 2018 in the right abutment of the dam, leading to the evacuation of more than 25,000 people living downstream of the dam. Ultimately, the dam was never breached and the embankment remained intact, however, the landslides highlighted the need for regular geohazard monitoring around the entire reservoir, and especially on the critical slopes located directly above the dam where the landslides occurred. Since July 2018, 3v Geomatics (3vG) has provided ongoing geohazard monitoring of Ituango's reservoir slopes using radar satellite images acquired by the Japanese Advanced Land Observing Satellite 2 (ALOS-2). Using a technology called Interferometric Synthetic Aperture Radar (InSAR), the phase information of satellite radar images is compared over time to determine which specific areas around the reservoir are moving, when, and by how much. InSAR is a remote sensing technology that provides surface displacement measurements for millions of data points around a reservoir with millimeter to centimeter precision. Displacement severity and extents are updated every time the satellite passes over Ituango, which for ALOS-2 is every 14 days [3]. Because InSAR is a remote sensing technology, no site visits are required, nor does any in-situ equipment or reflectors need to be installed. 3vG began delivering displacement information for the entire reservoir after the third ALOS-2 image was acquired (6 weeks from start). Once 10-13 satellite images were acquired from each of the two satellite orbit directions (ascending and descending), a comprehensive analysis was performed, providing full displacement histories for each data point around the reservoir. Based on the first 6 months of monitoring, the InSAR analysis identified over 10 areas of movement located within 9km of the dam, with the most critical slope above the dam showing consistent displacement at a rate of more than 50 centimeters per year. In total, over 600 km2 of reservoir slope is being monitored for geohazards, and over 7.4 million data points were generated that each have time series charts of displacement over time. The charts provide cumulative displacement over time, and can indicate whether any areas are accelerating, which can be a precursor to a larger-scale slope failure. Integral Geotechnical is using 3vG's InSAR results to mitigate risk and ensure the safety and stability of the Ituango Dam and the areas surrounding the Cauca River valley.