Underground » Environment - Subsidence and Mine Water
Mine subsidence is the lowering or collapse of the land surface, caused by underground mining activity. Mine subsidence is of major concern to the coal mining industry, government regulators, and environmental groups.
Subsidence is currently monitored by repeated ground survey using automatic/digital levels (in line levelling), total stations (in EDM height traversing) and GPS receivers (in static and kinematic surveys). Both digital levels and total stations can deliver 0.1mm height change resolution, while GPS can claim 5mm in static and 2-3cm in RTK (real-time-kinematic) height determination accuracy. These current techniques monitor ground subsidence on a point-by-point basis and are, therefore, relatively timeconsuming and costly.
Differential radar interferometry (DInSAR) can deliver ~1cm height change resolution. The combination of regular radar beam scanning and movement of the satellites carrying the radar sensor enables imaging of the mining region in seconds, from which subtle ground movements can be detected. For example, subsidence with a maximum amplitude of 1cm over 24 hours has been detected with a resolution of 3mm using a pair of satellite images acquired by the European Space Agency. In another example, a DInSAR result has been generated using a pair of satellite images acquired 44 days apart by the Japanese Space Agency. Quantitative validation comparing the DInSAR-derived subsidence profile against ground truth shows a best RMS error of 1.4cm. However, atmospheric disturbances such as tropospheric inhomogeneity (differential tropospheric delay) can lead to misinterpretation of DInSAR results. Differential corrections as much as several centimetres may have to be applied in order to ensure sub-centimetre accuracy for the DInSAR result. A methodology has been developed to use GPS (the Global Positioning System) observations to measure atmospheric disturbances so that the DInSAR results can be corrected.
A Geographic Information System (GIS) has been extensively used to post-process InSAR results throughout this project and. GIS can be used to present the final results in various formats, for example, profiles for validating with ground truth, subsidence contour maps, and three-dimensional views. Professional looking thematic maps can be generated based on these analyses, lining up with the practice within the mining industry to deliver drawings/maps in a GIS format. Multi-temporal DInSAR results can be analysed using GIS, and the final results compiled into an animation, showing the subsidence region moving as time passes. A virtual reality image ("fly-through") has been generated in the GIS, combining DEM, aerial photography, and DInSAR subsidence results.
The UNSW InSAR-GPS-GIS Integration Software has been developed to support the seamless flow of data among the three technologies, DInSAR, GPS, and GIS.
Several radar satellite missions, some especially designed for InSAR, are scheduled for launch in the near future. Therefore radar data of global coverage with weekly or even daily revisit will be made available at multiple radar bands. With atmospheric disturbances properly accounted for, DInSAR will be a cost-effective, reliable, and operational tool that complements traditional ground survey methods.