Mining Subsidence Monitoring Using the Combined Insar and Gps Approach

Compared to opencut mining, underground coal mining is more common in Australia due to its lesser impact on the environment. Although there are several underground mining methods, such as the longwall, pillar, bord, and miniwall, longwall mining is the most commonly used method due to its high efficiency. However, longwall mining does lead to significant ground subsidence. Mining subsidence is so common that in order to ensure the safety of surface transportation and local residents, current monitoring programs in many mines have included GPS surveys, conventional precise levelling and theodolite surveys, EDM surveys, and remote electronic monitoring (Hebblewhite et al, 2000). These techniques are costly and inefficient compared with remote sensing techniques such as that afforded by Synthetic Aperture Radar Interferometry (InSAR). However, InSAR results may not be accurate enough for this application due to the uncertainties in the SAR satellite orbit and the signal disturbances due to the atmosphere. GPS surveys can be used to account for these biases. The Satellite Navigation And Positioning (SNAP) group at The University of New South Wales, with its strength in GPS deformation monitoring, and the Radar Interferometry Group at Stanford University, with its strength in InSAR data processing, have developed a joint project to monitor ground subsidence due to underground mining, by combining the techniques of InSAR and GPS. In order to test the combined technique, both radar corner reflectors and GPS receivers are collocated in the mining region whenever the SAR satellite (ERS-2) passes over. The reflectors can be seen as bright spots on the SAR image to facilitate precise image co-registration during InSAR data processing, while the GPS measurements are used as ground control points to mitigate errors in the InSAR results. Technical details on how to collocate the radar reflectors and GPS receivers are discussed. Two components of the project, the ‘soft’ and ‘hard’ densification schemes, are described.