Mine Subsidence Monitoring Using Multi-source Satellite SAR Images

Ground subsidence due to underground mining has posed a constant threat to the safety of surface infrastructure such as motorways, railways, power lines, and telecommunications cables. Traditional monitoring techniques like using levels, total stations and GPS can only measure on a point-by-point basis and hence are costly and time-consuming. Differential interferometric synthetic aperture radar (DINSAR) together with GPS and GIS have been studied as a complementary alternative by exploiting multi-source satellite SAR images over a mining site southwest of Sydney. Digital elevation models (DEMs) derived from ERS-1 and ERS-2 tandem images, photogrammetry, airborne laser scanning, and the Shuttle Radar Topography Mission were assessed based on ground survey data using levelling as well as GPS-RTK. The identified high quality DEM was then used in the DINSAR analysis. Repeat-pass acquisitions by the ERS-1, ERS-2, JERS-1, RADARSAT-1 and ENVISAT satellites were used to monitor mine subsidence in the region with seven active mine collieries. Sub-centimeter accuracy has been demonstrated by comparing DINSAR results against ground survey profiles. The ERS tandem DINSAR results revealed mm-level resolution. Introduction Mine subsidence is the lowering or collapse of the land surface, caused by underground mining activity. The rocks above mine workings may not have adequate support and can collapse from their own weight either during mining, or long after mining has ceased. Factors affecting subsidence include (Nesbitt, 2003): • Depth of cover, • Overlying strata properties, • Seam thickness, • Panel width, • Chain pillar size, and • Surface topography. The need for subsidence monitoring of underground mining operations is multi-fold: • Legislative requirement, • Subsidence prediction, • Maximize coal extraction, • Structural design, • Risk management, and • Environmental monitoring. For example, proposals to enhance coal recovery through the use of the longwall mining method will receive critical review and assessment as to the likely environmental impacts Mine Subsidence Monitoring Using Multi-source Satellite SAR Images Linlin Ge, Hsing-Chung Chang, and Chris Rizos of subsidence. This assessment considers all impacts on infrastructure such as roads, houses, buildings, pipelines, bridges, as well as public utilities such as schools, water supply and sewage systems, and dams. In addition, the possible subsidence impact upon natural features such as streams, rivers, lakes, cliff lines, rock formations, and archeological sites are also considered. Recommendations for ongoing monitoring and review are included as approval conditions, with the results or reporting of such programs being included in the annual environmental management report. In addition, in the established coalmine fields of eastern Australia, it is becoming increasingly difficult to select underground mine sites which avoid major engineering structures, both on the surface and underground (highways, bridges, buildings, and abandoned underground workings). The Tower Colliery, an underground longwall mine southwest of Sydney, is a representative example where the surface topography overlying the mine consists of several steep-sided river gorges. The surface is traversed by a freeway which crosses one of the gorges. Consequently, a major surface subsidence monitoring program has been in place for several years, including intensive conventional line leveling, GPS and EDM surveying, plus real-time monitoring of critical components of the bridge structure. Therefore, ground subsidence due to underground mining is of major concern to the coal mining industry, government regulators, and environmental groups, to name just a few. Subsidence is currently monitored by repeated ground survey using automatic/digital levels (in line leveling), total stations (in EDM height traversing) and GPS receivers (in static and real-time-kinematic (RTK) surveys) (Schofield, 1993). Both digital levels and total stations can deliver 0.1 mm height change resolution, while GPS can produce 5 mm in static and 2 to 3 cm in RTK height determination accuracy. These current techniques monitor ground subsidence on a point-by-point basis and are, therefore, relatively timeconsuming and costly. Hence, the monitoring is usually constrained to very localized areas, and it is very difficult to monitor any regional deformation induced by underground mining. In addition, even in the localized area, the monitoring points are not usually close enough to assist in understanding the mechanisms involved in ground subsidence. New techniques must be developed that both are accurate and give a fine spatial characterization of the ground deformation. Synthetic aperture radars look to the cross-track (or range) direction (direction perpendicular to the direction of motion) or along-track direction and use coded waveforms to PHOTOGRAMMETRIC ENGINEER ING & REMOTE SENS ING March 2007 259 Cooperative Research Centre for Spatial Information & School of Surveying and Spatial Information Systems, The University of New South Wales, Sydney NSW 2052, Australia ([email protected]). Photogrammetric Engineering & Remote Sensing Vol. 73, No. 3, March 2007, pp. 259–266. 0099-1112/07/7303–0259/$3.00/0 © 2007 American Society for Photogrammetry and Remote Sensing CARRS-2 03/02/2006 5:26 PM Page 259