The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part B7. Beijing 2008 
68 
of 20 to 50cm about 1-2 months after the mine process has 
ceased, and can be up to 90-100cm over a full year. 
As the peak subsidence at the mine site is much greater than the 
maximum subsidence that the European Space Agency satellite 
systems ERS SAR and ENVISAT ASAR can detect, the phase 
fringes in the ERS and ENVISAT interferogram corresponding 
to the ground surface displacement are expected to be saturated. 
One of the aims of this research was to investigate the 
capability of the Japanese ALOS PALSAR data for monitoring 
ground deformation caused by mining activities. The ALOS L- 
band PALSAR differential InSAR results have been compared 
to differential results obtained from other satellite SAR data 
such as ENVISAT C-band ASAR. 
Figure 1. Location of the test sites (Appin & Westcliff) on a 
LANDSAT image. 
2. METHODOLOGY 
Spacebome repeat-pass differential interferometric SAR 
(DInSAR) has already proven its value for ground deformation 
monitoring in many applications due to its high precision and 
high spatial resolution (Goldstein et al., 1993, Camec et ah, 
1996, Ge et ah, 2007, Chang et ah, 2005). When the SAR 
system images the ground, both amplitude (strength) and phase 
(time) of the backscattered signals are recorded by the receiving 
antenna. By computing the phase difference from two SAR 
images acquired at different times it is possible to generate a 
radar interferogram, which contains information about the 
(static) topography and any displacement in the slant range 
direction that may have occurred between the two SAR image 
acquisition dates. However, the effect of atmospheric 
disturbance, orbit error and decorrelation noise should also be 
considered. Hence, the interferometric phase can be written as: 
&Ф - Фтора + Фое/о + ФAtmos + ФOrbit + ФNoise 
topographic phase is removed using an independently derived 
digital elevation model (DEM). Since the spatial extent of 
subsidence is expected to be only of the order of several 
hundred metres, the atmospheric disturbance can be assumed 
insignificant (Camec et al., 1996). The orbit error contribution 
can be corrected during DInSAR analysis, and the phase noise 
can be reduced by applying an adaptive filter. Therefore the 
phase due to geometric displacement of the point is given by 
(Zebkerand Goldstein, 1986): 
^Фdefo ~ Л ^ 
Я (2) 
where X = wavelength of the radar signal 
acquisitions in the line-of-sight (LOS) direction 
The displacement vector along the LOS of the radar system is a 
composite of the vertical, easting and northing displacement 
components. However, due to the lack of SAR data acquired 
from different viewing angles and orbit heading at a similar 
time period, it is not possible to derive the 3-D displacement 
vector. The deformation due to underground mining activity is 
most likely in the vertical direction, with the horizontal 
deformation being much smaller (Peng, 1986) and hence the 
horizontal displacement is assumed to be negligible for the 
purposes of this study. Under this assumption the LOS 
displacement can be converted into vertical displacement: 
cos (^„ c ) 
where ^ = surface displacement in the vertical direction 
^ mc = incidence angle 
3. SIMULATIONS 
In order to avoid aliasing in phase-unwrapping process, the 
phase difference between any two adjacent pixels in the 
interferograms should be less than one-half cycle (71) (Chen et 
al., 2002), otherwise the wrapped phase in the interferogram 
becomes ambiguous and cannot be unwrapped. Therefore the 
maximum deformation of a whole subsidence bowl due to 
underground mining that can be detected without phase 
discontinuity can be written as: 
where ^ = phase difference between the two images 
< h° po = phase due to the topography 
^Defo = phase due to the geometric displacement of the 
imaged point 
$ Atmos = phase due to atmospheric disturbance 
Qorbit = phase due to orbit error 
^Notse = phase due to decorrelation noise 
In order to estimate the displacement all the other components 
should be carefully removed or accounted for. In this study, the 
max,LOS 
W 
S resolution 
Я 
4 
(4) 
where S max LO s = maximum deformation of the subsidence bowl 
in the LOS direction that can be detected without phase 
discontinuity 
gresolution ~ ground resolution of the SAR sensor 
w = radius of the subsidence bowl 
A/4 = distance corresponding to a one-half cycle of the 
interferometric phase