Abstract

Surface subsidence associated with coal mining activities in the Mpumalanga Province, South Africa, changes the natural environment in several ways and current challenges for mining companies lie in rehabilitation of the natural environment and the prevention of further degradation. To monitor the spatial and temporal evolution of surface subsidence, traditional field-based monitoring approaches, including GPS and spirit levelling, are employed at a number of locations. However, the resulting measurements are point-based and frequent visitations are necessary to map the evolution of surface subsidence basins over time. To address these limitations, differential interferograms derived from repeat-pass satellite-borne synthetic aperture radar (SAR) systems were tested for their ability to measure and monitor surface deformation. The resulting interferograms revealed several features indicative of surface subsidence. Ground truth data confirmed the presence of a subsidence basin detected using differential interferometry techniques during the 35 day period between April 12, 2008 and May 17, 2008, with a maximum vertical deformation of 3.2 cm being recorded. Interferometric monitoring revealed an eastward migration of the subsidence basin between June 2, 2008 and September 15, 2008, with an additional 4.7 cm of subsidence being observed. This migration coincides with the advance of the working face of the mine during this period. The results demonstrate the ability of interferometric synthetic aperture radar techniques to measure surface subsidence as well as the monitoring of the evolution of subsidence basins over time. This implies that the technique could be included, together with traditional field-based surveying techniques, in an operational monitoring system.

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