Published in Australia's Longwalls
Funded by the Australian Coal Association Research Program (ACARP) and the Australian Research Council (ARC), a group of researchers from the University of New South Wales has been studying the possibilities of monitoring subsidence over the whole mining area in a cost-effective fashion by combining interferometric synthetic aperture radar (InSAR), GPS and geographic information system (GIS).
The combined system has the potential to offer cost savings to both mines and government. For example, government or mines can set up a centre to monitor mine subsidence and validate it with the ground survey conducted by mines. Remote sensing results will be useful in assessing a subsidence damage claim.
Traditional monitoring uses optical/digital levels, total stations or recently, GPS receivers. Because these measurements are taken on a point-by-point basis on the ground, a limited number of survey marks are established in a straight line and surveyed regularly to monitor the subsidence in a one-dimensional profile.
Project co-coordinator and University of NSW senior lecturer Dr Linlin Ge said the new technology uses traditional GPS and GIS, along with InSAR data processing. “In the InSAR data processing, two radar images acquired over the same region from two repeated satellite visits are combined to form an interference pattern (the initial interferogram), which consists of contributions from the topography, subsidence, satellite orbit error, atmospheric interference and radar noise,” said Dr Ge.
“An external digital elevation model (DEM) is used to simulate the terrain and take the topography away from the initial interferogram. Orbit error is also simulated and removed, which produces the differential interferogram. This differential interferogram is then converted to a subsidence map, which is contaminated by radar noise and possibly also atmospheric interference, depending on whether the weather conditions are similar to each other during the two radar acquisitions. Mine subsidence is represented as bright spots in the two-dimensional map with a pixel resolution of about 20m,” he said.
The GPS measurements are then used to estimate the difference of atmospheric conditions between the two radar acquisitions. The University of NSW InSAR-GPS-GIS integration software combines this estimation with the subsidence map derived from InSAR and outputs a GPS-corrected InSAR result in the GIS format. GIS software is used to analyse the result together with other GIS information available for the mine site such as mine plan and aerial photo.
The final mine subsidence result can be delivered in many different ways, including contour map, profiles, three-dimensional perspective view and multiple results generated for the same mine site can be composed into an animation.
Currently InSAR, GPS and GIS are all commercially available as separate software packages. However, mines will have to wait one or two years to obtain an integrated system specifically designed for subsidence monitoring. The integrated program will be made available through the newly funded Cooperative Research Centre (CRC) for Spatial Information.
