Radar satellite service checks stability of Africa's largest artificial hole
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).
In 2002 owner Rio Tinto declared the pit's depth meant surface extraction was now uneconomic, instead developing an underground mine to work the copper ore beneath the pit bottom. The decision was good news for the regional economy, as it should extend the life of the mine and associated copper smelter and refinery by at least another 20 years.
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.
Rio Tinto was concerned 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.
Think of it as a space-based version of 'spot-the-difference', which sees down to a few millimetres. Even the tiniest vertical surface shift changes the distance it takes for the radar signal to travel down to the surface of the Earth and back to the satellite, and so alters the phase of the signal. Just like sets of ripples meeting in water, the combination of radar signals that have different phases sets up interference patterns – so these combined images are known as 'interferograms'.
Such interference 'fringes' can be thought of as resembling contour lines, but on vastly reduced scales. Once topographic and atmospheric effects are accounted for, the fringes remaining on an interferogram can be used to precisely measure the extent of any ground shifting over the entire area covered – usually tens of kilometres across.
"While traditional survey techniques and geotechnical instrumentation can provide detailed information at specific points of interest, InSAR provides continuous data coverage over large areas to sub-centimetre accuracy within a particular timeframe of interest," said Stu Anderson, AMEC Project Manager. "These data are often available back to the early 1990s for many locations."
For this mining problem at Palabora, AMEC sat down with Rio Tinto and together they designed test cases to see what InSAR could deliver and how it could be used by Rio Tinto. 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.
During the first period the maximum measurable subsidence was found to be five centimetres nearest the large north wall failure, tapering to zero in the east. During the second period the maximum value was two centimetres, and the affected area was reduced, with the east wall again proving unaffected.
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.
Regulators typically compel most mining companies to regularly check for deformation. In a bid to see if InSAR can bring benefit to other companies in the mining industry, AMEC has been consulting with more of its extensive list of clients. To this end, AMEC also completed satellite-based subsidence tests over Germany's Bad Reichenhall salt mine on behalf of owners Südsalz, a slip-prone mine waste rock dump at the closed Hayden Hill gold and silver mine in northeast California in the US for the company Kinross Gold and the closed Hollinger gold mine near Timmins in Ontario, Canada, for Placer Dome.
In this case, an extension of the InSAR method, based on historical available satellite data such as the 14-year archive from ESA's ERS archive, was utilised. Known as Coherent Target Monitoring (CTM), this method provides detailed quantification of land motion. Results were compared to in-house survey findings, with promising results.
For Hollinger Mine, AMEC employed satellite data between 1992 and 2003 to detect subsidence in the range of 25 to 55 millimetres within an area previously identified as stable using traditional geological instrumentation and survey techniques, showing that CTM provides additional insight into deformation occurring in the vicinity.
"Another advantage of using SAR interferometry is that it is not dependent on sending crews into the field," commented Timothy Conley, Vice President and Managing Director of AMEC's Earth & Environmental operations in Europe. "It is therefore valuable for acquiring information at remote sites or areas considered unsafe for personnel to enter."
InSAR and CTM subsidence monitoring have many other commercial applications beyond the mining sector. Testing is also either already under way or being finalised by AMEC for railways in Germany for Die Bahn and in the UK for Network Rail, as well as for a mine access road in the Peruvian Andes for Teck Cominco.
Future tests are proposed for a pipeline corridor in British Columbia for Terasen Gas and tunnel construction in Germany for DYWIDIG Bau (formerly Walter Bau). AMEC is also using InSAR as part of a landslide warning system at Turtle Mountain in Alberta, Canada.