Satellite surveys stability of Africa's largest
artificial hole
International engineering firm AMEC is working with the European Space Agency
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
Pretoria in South African. 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." - ESA
More information:
 To find out
more about using Earth Observation for hazard mapping or further opportunities
with EOMD, please contact eomd@esa.int.
Or Pierre-Philippe Mathieu Pierre.Philippe.Mathieu@esa.int
European Space Agency
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