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Stability of Africa

International engineering firm AMEC is working with ESA to improve monitoring of ground subsidence linked to mining activity. Radar satellites in orbit 800 kilometres away can reveal millimetre-scale elevation shifts across wide areas of land. The largest man-made hole in Africa is located 360 kilometres north-east of South African capital, Pretoria. The Palabora copper mine was excavated open-cast for 38 years: the end-result is easily visible from space: a yawning pit approaching 2000 metres in diameter and 762 metres deep (more than 230 metres beneath sea level).

Mining at Palabora uses a highly-efficient method called ’block caving’. It is based on extracting thin blocks of rock to induce large-scale cave-ins in overhanging ore. However, even though the excavations are being carried out more than a kilometre underground, last year they induced dramatic surface instabilities. More than 60 million tonnes of mass collapsed into the pit from its north wall, with movements of up to two metres in the vicinity and cracking as far as 300 metres away from the pit rim. There was concern that further subsidence might threaten mine infrastructure on the pit’s east rim. AMEC’s Earth & Environmental Division were called in to evaluate the threat posed – who then turned to an innovative deformation-measuring method based on imagery from radar satellites flown by ESA and the Canadian Space Agency (CSA).

AMEC is working within the ESA Earth Observation Market Development (EOMD) programme to evaluate the commercial possibilities of a technique known as Synthetic Aperture Radar Interferometry (InSAR). Through InSAR, multiple satellite radar images of the same site – acquired from as near as possible to the same point in space though at different times – can be combined together to highlight slight changes in surface elevation that occurred between the image acquisitions. For this mining problem at Palabora AMEC designed test cases to see what InSAR could deliver and how it could be used. Various SAR images were acquired for two consecutive 24-day periods in 2004. The news turned out to be good, the affected area was restricted to the northeast sector of the pit, away from the east-side infrastructure, and the size and magnitude of the deformations was reducing. Based on the products developed during this investigation, InSAR will also be performed over Palabora to serve as a means of early warning of future subsidence. AMEC is an international project management and services company with office networks across Europe, Asia and the Americas.