RADAR INTERFEROMETRY FOR GROUND SUBSIDENCE MONITORING USING 
ALOS PALSAR DATA 
A.H. Ng a '*, H. Chang 3 , L. Ge a , C. Rizos 3 , M. Omura b 
a Cooperative Research Centre for Spatial Information & School of Surveying and Spatial Information Systems, The 
University of New South Wales, Sydney NSW 2052, Australia - (alex.ng, hsing-chung.chang)@student.unsw.edu.au, 
(l.ge, c.rizos)@unsw.edu.au 
b Department of Environmental Science, Kochi Women's University, 5-15 Eikokuji-cho, Kochi 780-8515, Japan - 
omura@cc.kochi-wu.ac.jp 
Commission VII, WG VII/2 
KEY WORDS: Remote Sensing, Land, Monitoring, SAR, Radar 
ABSTRACT: 
This paper describes the results using data from ALOS and ENVISAT satellites for the purpose of subsidence monitoring over 
underground coal mine sites in the state of New South Wales, Australia, using the differential interferometric synthetic aperture 
radar (DInSAR) technique. The quality of the mine subsidence monitoring results is mainly constrained by the noise due to the 
spatial and temporal decorrelation between the interferometric pair and the phase discontinuities in the interferogram. This paper 
reports on the analysis of the impact of these two factors on the performance of DInSAR for monitoring ground deformation. The 
ALOS L-band PALSAR DInSAR results have been compared to DInSAR results obtained from ENVISAT C-band ASAR data to 
investigate the performance of ALOS PALSAR for ground subsidence monitoring. Differential interferograms from SAR data 
acquired using different operating frequencies, for example, X-, C- and L-band, from the TerraSAR-X, ERS-1/2, ENVISAT, JERS-1 
and ALOS satellite missions, were simulated. The simulation results showed that the new satellites ALOS, TerraSAR-X and 
COSMO-SkyMed perform much better than the others. ALOS PALSAR and ENVISAT ASAR images with similar temporal 
coverage were searched. The two-pass DInSAR technique with a 25m DEM was used to measure the location and amplitude of 
ground deformation. Strong phase discontinuities and decorrelation have been observed in almost all ENVISAT interferograms and 
hence it is not possible to generate the displacement maps. However these problems are minimal in ALOS PALSAR interferograms 
due to its spatial resolution and longer wavelength. Six successive subsidence maps are generated with eight ALOS PALSAR 
images from both ascending and descending orbits. The results are compared with ground survey data at two sites with RMS error of 
1.7cm and 0.6cm being achieved. The accumulated subsidence can be estimated by adding up all subsidence maps; however the 
error in each DInSAR result, such as the geocoding error between each result, will also accumulate. An approach for minimising 
geocoding error in order to calculate the accumulated subsidence from a series of SAR images is described. 
1. INTRODUCTION 
1.1 Underground Mining in Australia 
Ground subsidence is the lowering or collapse of the land 
surface which can be caused by either natural or anthropogenic 
activities. Most ground subsidence in Australia is human 
induced, and in non-urban areas is usually related to 
underground mining, especially for coal. The magnitude (areal 
extent and amount) of subsidence due to underground mining 
depends on a number of factors, including the depth of cover, 
overlying strata properties, seam thickness, panel width, chain 
pillar size and surface topography (Nesbitt, 2003). The rocks 
above the mine workings may not have adequate support and 
can collapse from their own weight either during mining or long 
after mining has been completed. Therefore ground subsidence 
due to underground mining is a major concern to the mining 
industry, government, environmental groups and others. In 
Australia most underground coal mines employ the longwall 
mining technique, where a long ‘wall’ of coal is mined in a 
single slice in order to maximise the recovery of coal. The 
subsidence caused by this technique can be very large, occur 
immediately after or during mining, and can therefore cause 
serious problems, for example, changing the river courses and 
damaging building foundations. The subsidence induced by this 
mining technique can have a spatial extent of several hundred 
metres. 
Several methods are currently used for mine subsidence 
monitoring, including levelling, total station surveys, and GPS 
(Schofield, 1993). However these techniques have limitations, 
primarily because they measure subsidence on a point-by-point 
basis. Spacebome differential radar interferometry (DInSAR) is 
a technique which can measure the ground movement (or 
deformation) of an entire area. It is quicker, less labour 
intensive and hence less expensive compared to the 
conventional ground-based survey methods. 
1.2 Test Site 
Two test sites were chosen for this study: Westcliff and Appin 
(Figure 1). The two underground mine sites are very close to 
each other and are therefore imaged in the same radar 
acquisition. The width of each longwall panel in the 
underground mines is about 200-300m, which is 100-150m 
from the edge to the centre of a longwall panel. The depth of 
the coalmines at these test sites is between 300-500m. The 
ground subsidence at the test sites have typical peak amplitudes 
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