Mapping mining-induced subsidence from space in a hard rock mine: example of SAR interferometry application at Kiruna mine
CIM Bulletin, Vol. 97, No. 1083, 2004
E. Henry, C. Mayer and H. Rott
LKAB’s Kiruna mine is a large-scale sublevel caving iron operation, located in northern Sweden. Sublevel caving leaves intrinsically no support between the undercut hangingwall and the footwall, so that the hangingwall deforms, fractures, and then caves.The limits of the deformation, fracture, and caving zones widens as the mine deepens.
LKAB has made significant efforts to monitor the deformation zone and predict when it will reach the town of Kiruna, which is located on the hangingwall side of the mine. LKAB carefully maintains a 3 km by 2 km benchmark network, which is surveyed at least once a year with a total station system from reference points. These reference points have to be frequently replaced to cope with the widening of the deformation zone; the increasing size of the network makes it difficult to increase or even maintain the measurement precision level. Guaranteeing reference points are located outside the deformation zone is also a difficult task.
In the quest for more effective monitoring techniques, LKAB decided to evaluate satellite-borne synthetic aperture radar interferometry (InSAR). InSAR is a technique developed in the late 1980s allowing for detailed target-free subsidence monitoring caused by earthquakes, water or oil pumping, slope instability, etc. Two pairs of radar images taken from dedicated satellites (European Space Agency’s ERS-1 and ERS-2 satellites, for instance) with specified repeat orbits are compared and temporal displacements of the surface are detected at a high accuracy (
The test proved successful and the width of the hanging-wall deformation zone could be assessed (see figure) with a precision in the order of one centimetre. Total station lines on ground were used to validate InSAR results, and the agreement was excellent.
InSAR surpasses classical total station surveying methods by its unique ability of generating continuous deformation maps, rather than giving punctual information. It also proved interesting in terms of processing time and cost. However, this technology is sensitive to the temporal stability of ground conditions. Rapid changes in snow cover and vegetation, for example, destroy the radar signal coherence, and best results were obtained with SAR images acquired during spring and fall.
InSAR would be particularly suitable for subsidence monitoring at mines located in arid areas or any area with low vegetation cover: steppe, prairie, alpine, and arctic vegetation.
This paper describes the basic principles of InSAR and evaluation results at Kiruna mine.
Interferometry, InSAR, Kiruna, Mining, Subsidence, SAR, Satellite