RADAR INTERFEROMETRY FOR SAFE COAL MINING IN CHINA 
L. Ge a , H.-C. Chang a , A. H. Ng b and C. Rizos a 
Cooperative Research Centre for Spatial Information 
School of Surveying & Spatial Information Systems, The University of New South Wales, Sydney, Australia 
a (l.ge, hc.chang, c.rizos)@unsw.edu.au; b alex.ng@student.unsw.edu.au 
Commission VII, WG VII/2 
KEY WORDS: DInSAR, Mining, Ground Deformation Monitoring, SAR 
ABSTRACT: 
Land surface deformation due to underground mining is always a concern to the local communities and environment. The 
underground coal mines at the study site in Northern China are located quite closely to each other. In order to ensure safe mining as 
well as to prevent illegal mining activities, differential radar interferometry (DInSAR) was introduced for monitoring mine 
subsidence. The spacebome SAR data acquired by ENVISAT (C-band) and ALOS (L-band) were tested in this paper. The limits of 
radar interferometry were discussed by addressing the issues of decorrelation and deformation phase gradient. The ALOS results 
show it is more suitable for monitoring the rapid and large land deformation due to underground mining. However, the data acquired 
in summer season (June - September) have higher noise, which may be caused by the local agriculture. 
1. INTRODUCTION 
Land deformation caused by underground mining becomes 
more and more a concern for many areas in China. Especially 
the population density in China is very high by comparing with 
many other countries. In order to ensure safe mining as well as 
to prevent illegal mining activities, the implementation of an 
effective mining monitoring scheme is important. Conventional 
subsidence monitoring measurement is done by precision digital 
levels, total stations and GPS receivers. Both digital level and 
total station can deliver 0.1 mm in height resolution while GPS 
can sense 5 mm changes in static mode. However, applying 
leveling and GPS surveys over a large area are not just labour 
intensive, but also very time consuming. Ideally, the mine 
subsidence should be carefully monitored with a surveying 
network. However, surveying over the entire network may take 
a few weeks. The coverage of the surveying network may also 
be limited due to site accessibility. The maintenance of the 
survey marks are another important and yet difficult issue. 
Differential radar interferometry (DInSAR) technique has been 
demonstrated its capability of monitoring land subsidence (Ge 
et al., 2007). DInSAR can monitor the land deformation at sub 
centimetre accuracy over a large area (at the radar swath width 
of 50~100km). D-InSAR method is a land deformation mapping 
technology, which is complementary to other surveying 
methods. 
The test site in this study locates in the middle of Huabei plain, 
China. It covers about 570km 2 . The mine has been excavated 
since early 20th century. Today, the region is one of the most 
important coal mining areas in China, producing about 2,700 
million tones of coal per year. There are about 500 collieries 
built up in this region, and some new collieries are planned to 
be constructed in the near feature. As a result, the region has 
been environmentally degraded. And the subsidence is expected 
to become more severe in the future. The land-use is a mixture 
of villages and farmlands of wheat and corns. 
2. METHODOLOGY 
Repeat-pass spacebome DInSAR is used here to derive ground 
displacement maps. The basic theory of radar interferometry 
can be found in great details in (Bamler and Hartl, 1998; 
Gabriel et al., 1989; Rosen et al., 2000; Zebker and Goldstein, 
1986). In short, two SAR images acquired from two slightly 
different positions, at different times, are used to measure the 
phase difference, or so-called interferogram, between the two 
acquisitions. Interferogram consists of topographic information, 
land deformation occurred between the two acquisitions, 
atmospheric disturbances, orbit errors and noise. DInSAR is the 
process to measure the phase variation due to land deformation 
by eliminating or minimising the other components. The 
topographic phase contribution can be simulated by introducing 
a digital elevation model (DEM). A 3 arc-second SRTM DEM 
(approximately 90m resolution) is used in this paper to simulate 
the topographic phase, which can be removed from the 
interferogram. The atmospheric component is primarily due to 
fluctuations of water vapour in the atmosphere between the 
satellite and the ground. The atmospheric delay can be 
identified using the fact that its fringe structure is independent 
over several interferograms, or can be modelled by using a GPS 
network (Ge et al., 2003 & 2004). As the volume of the water 
vapour in the atmosphere varies with low spatial frequency, it is 
sometimes negligible in the applications such as mining 
subsidence monitoring where the spatial frequency is much 
higher. 
In the differential interferogram a complete 2n phase change is 
equivalent to a height displacement of half of the wavelength of 
the radar signal in the slant range direction. That is 11.75cm for 
JERS-1 and ALOS L-band data. Since the measured phases in 
the interferogram are wrapped in modulo of 2n, the height 
displacement map can be derived by ‘phase unwrapping’ the 
interferogram. Finally the unwrapped phase can be converted to 
height change.