Mine Subsidence Monitoring

August 11, 2008
ERSDAC

When underground cavities are formed by mining activities in subsurface coal mines around the world, the ground above the cavities sometimes subsides and as a result can damage surface structures. To monitor this sort of anthropogenic ground subsidence, surveys using techniques such as levelling and GPS are conducted on the ground surface. However, such surveys require considerable time, labour and cost, and only the local ground deformation at the survey point can be determined.

The attached image shows the ground subsidence process at an underground coal mining site, as revealed by applying "DInSAR" (Differential Interferometric SAR) processing to the data acquired by PALSAR on board the ALOS (Daichi). The repeat cycle of ALOS/PALSAR is 46 days. PALSAR has repeatedly observed this site during consecutive repeat cycles and the time-series changes in ground deformation (subsidence) has therefore been determined. The image displays the subsidence accumulated from June 29, 2007, as well as the location of underground tunnels (represented by red and white lines) shown on the aerial photograph.

In "DInSAR" processing, the phase difference of backscattered and reflected waves, which are acquired at different times but at almost the same satellite position, is firstly obtained. By removing the effect of differences in the satellite orbit and the terrain from the obtained phase difference, the remaining phase difference can be converted into the deformation signature. This deformation monitoring method doesn't need any ground survey points, and can therefore monitor possible ground subsidence over a wide area. This example in the attached image presents ground subsidence caused by large-scale mining at a depth of about 300-500m. It can be seen that ground subsidence of 70cm, at a maximum, has occurred during the 276 day period.

PALSAR uses an L-band microwave of 23.6cm wavelength, which has a great advantage in providing good DInSAR results regardless of the ground vegetation cover.

This result was obtained by the research team of the Cooperative Research Centre for Spatial Information and the School of Surveying & Spatial Information Systems, the University of New South Wales, using PALSAR data provided by ERSDAC, in partnership with the Consortium for Mine Subsidence Monitoring under Sub-Commission 4.4 of the International Association of Geodesy.

[Bibliography] A.H. Ng, H. Chang, L. Ge, C. Rizos, M. Omura (2008): RADAR INTERFEROMETRY FOR GROUND SUBSIDENCE MONITORING USING ALOS PALSAR DATA, Proceedings of The XXI Congress, The International Society for Photogrammetry and Remote Sensing 3-11 July 2008 Beijing, China..