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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