The international Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part B7. Beijing 2008
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For the same amount of ground deformation, the induced
DInSAR phase fringes will be denser in the results generated
using shorter radar wavelength (e.g. C-band) than the ones
generated using longer wavelength (e.g. L-band). Figure 2 (a)
shows the high phase gradient within the subsidence bowl
causing the phase ambiguous. As a result, the phase fringes
cannot be fully recovered, even after being filtered.
Figure 1. Histograms of coherence values of (a) - (c) ALOS
interferometric pairs: 14/6/2007 - 30/7/2007 Bperp = 515m,
30/7/2007 - 14/9/2007 Bperp = 143m, and 15/12/2007 -
30/01/2008 Bperp = 351m, and (d) ENVISAT pair: 3/1/2008 -
7/2/2008 Bperp = 361m, respectively. The mean coherence
value of these 4 pairs are 0.355, 0.461, 0.586 and 0.379.
5. RESULTS AND DISCUSSIONS
The study area is about 24km x 25km. The differential
interferograms of the selected four interferometric pairs (3 from
ALOS and 1 from ENVISAT) are shown in Figure 3. In the
results, over 21 mine subsidence spots are identified using
DInSAR technique. The magnitude of ground deformation due
to underground mining can be more accurately measured or
recovered if the interferometric pair illustrates high coherence
over the scene and at the same time the deformation phases are
not ambiguous. In this study, the ALOS pair with the mean
coherence value of 0.586 shows the deformation fringes quite
clearly as shown in Figure 2 (b).
Figure 4 shows the final deformation map derived from the
ALOS pair, 15/12/2007 - 30/01/2008. The result is overlaid on
a high resolution optical image with the aid of geographic
information systems (GIS). The maximum subsidence detected
due to active underground mining activities over a period of 46
days is about 36cm.
There are several coal collieries in the local area and they are
closely located to each other. Even though the proposed mining
location and mining schedule are recorded, it is still hard to
track the precise locations of mining activities. But it is
extremely important for ensuring the safety of the miners as
well as the local residents when the mining progress at the two
collieries are close to each other. The results of DInSAR
derived from successive acquisitions were post-analysed against
other supporting spatial data in geographic information systems
(GIS). By comparing the locations against the proposed mining
schedule, any illegal mining activities can also be identified.
6. CONCLUDING REMARKS
This paper demonstrated the capability of differential radar
interferometry (DInSAR) for monitoring the ground surface
deformation caused by underground mining activities. The
study area chosen in this paper is in the Northern China.
ENVISAT and ALOS SAR data acquired in C- and L-band
respectively were tested in this paper. Within an area of
approximately 24km x 25km, more than 20 subsidence spots
are identified using DInSAR. The results show ALOS data are
more suitable for mine subsidence monitoring. The L-band
radar signal and the high imaging resolution (8m) of ALOS
reduce the problems caused by high deformation phase gradient
and improve the scene coherence. It is also found that the local
agricultural areas may introduce noises into the interferometric
results, especially during summer season. Nevertheless, the area
of impact of mine subsidence at multiple collieries can be
monitored at the same time using DInSAR. The DInSAR
deformation map is complementary to the ground surveys and
assists to identify any abnormal subsidence behaviour. Together
with other spatial data, it helps to ensure the safety of the local
communities as well as the mine workers.
Figure 2. Enlarged DInSAR phase fringes induced by ground
deformation of (a) ENVISAT (C-band) pair: 3/1/2008 -
7/2/2008 Bperp = 361m, and (b) ALOS (L-band) pair:
15/12/2007 - 30/01/2008 Bperp = 351m.