Paris, 04 Jan 2006
The European Space Agency (ESA) is working with engineering firm AMEC to keep an eye on possible subsidence in the world's largest artificial hole, the Palabora copper mine, located 360 km north-west of Pretoria, South Africa.
The vast open cast mine was excavated for 38 years, and is said to be visible from space. The resulting hole is 2,000m in diameter and 762m deep. Open-cast mining was abandoned by owners Rio Tinto in 2002, when the sheer depth of the mine made extraction uneconomic.
However, the mine has remained open thanks to underground mining using the 'block caving' technique, where thin strips of rock are removed to encourage large cave-ins. Although this technique is highly efficient, subsidence has been detected at the pit-bottom, a kilometre above the shaft. Some 60 million tonnes collapsed into the pit from the north wall, and shifts of up to two metres have been discovered.
AMEC has worked in partnership with the ESA's Earth Observation Market Development (EOMD) programme to test a new technique known as Synthetic Aperture Radar Interferometry (InSAR).
The InSAR technique compares multiple satellite radar images, taken from almost-identical points in space but at different times. The radar interference patterns or 'interferograms' are corrected for subtle topographic and atmospheric or barometric differences. Changes on the ground are reflected in the differences in the interferograms to give measurements accurate to a few millimetres over areas tens of kilometres across.
'InSAR provides continuous data coverage over large areas to sub-centimetre accuracy,' said AMEC project manager Stu Anderson.
InSAR images were compared for two consecutive 24-day periods in 2004, and so far the news is good for Palabora - the area affected by subsidence is restricted to the northeast sector, and the magnitude of the subsidence is reducing. During the two measurement periods, the differences detected in the north-eastern sector were 5cm and then 2cm respectively, tapering to zero in the east of the mine on both occasions.
The next phase of the project will bring in the ESA's 14-year archive to try to see how subsidence may have affected other very large-scale engineering projects, such as Germany's Bad Reichenhall salt mine.
A test applied to the abandoned Hollinger mine in Ontario, Canada found a subsidence of 25-55mm in an area previously found to have no subsidence using traditional surveying techniques. The InSAR is also essential for 'Acquiring information at remote sites or areas considered unsafe for personnel to enter,' according to AMEC's vice-president and managing director of environmental operations in Europe, Timothy Conley.
Further applications for the technique are planned or underway with rail operators in Germany and the UK, and Teck Cominco's road-building project in the Peruvian Andes. Further projects have been discussed with Terasen Gas for a pipeline in British Columbia, DYWIDIG Bau for a tunnelling scheme in Germany and a landslide-warning system at Turtle Mountain, Alberta, Canada.