128 
Figure 3 Map of the vertical displacement of the urban areas of Tangshan measured with ERS differential SAR interferometry 
between 1997 and 1998. The results were exported to and post-processed in the GIS. The subsidence regions can now be seen clearly 
and the color coding indicates the magnitude of subsidence. After overlapped with the Mine Area map, we can see it has largest 
displacement in Tangshan Mine area, in Gangyao Mine and Majiagou Mine, it has average 10 cm displacement, in west of Tangshan 
Mine, and it has average 15 cm displacement due to the Kast collapse. 
From Pos: 39* 37'38.3(To Pos: 39* 35' 49.13" N,118* 18'59.38" E 
0.25 m 
0.20 m 
0.15 m 
(a)Position profile #1 
Figure 4 Profiles of the vertical displacement velocity obtained from 
directions over the subsidence area of Tangshan Mine. 
From Pos: 39* 36'26.3To Pos: 39* 36'09.26" N, 118* 12’02.79" E 
(b) Position profile #2 
SAR interferometry between 1997 and 1998 along two 
4.2 Envisat 
The interferogram formed with the two Envisat ASAR images 
contains both the topography and subsidence, the SRTM-DEM 
with 90m resolution was used as the reference DEM. The 
coherence and the differential interferogram are presented in 
figure 5, the data was filtered and multi-look processed to 
improve phase statistics, we can see from the figure that only 
the urban areas and Guye district has high coherence. 
To date, ten ENVISAT images have been acquired near the 
same site during 17 Jan. 2004 and 17 May 2007. Unfortunately, 
the baseline distance of some pairs is over 700m, and it failed 
to give sufficient spectral overlap between the two images. The 
characteristics of the local vegetation cover make it difficult for 
C-band images to conserve sufficient coherence over only one 
year. In near future, the Permanent Scatterers (PS) technique 
will be used to improve the accuracy of InSAR and D-InSAR 
in monitoring the surface subsidence.