133
determined by ground objects’ texture characteristics with little
influenced from water (ice). The vegetation on the alpine aim is
denser than on the alpine meadow, and conglomerates on the
alpine aim have higher density, so the coherence of alpine aim
is higher than of alpine meadow.
Figure 4. Influence on coherence of part area season thawing.
The images, composing pair C or D, are obtained at the time
between the end of January and the beginning of March and
April respectively, and this period belongs to the process of
temperature rising in spring. In the pair C, when the
temperature is rising in March and April, alpine aim firstly
begins to thaw because of more density vegetation and energy
exchange, and the scattering characteristics correspondingly
take place change that leading to decorrelation. However, alpine
meadow still keeps in lower temperature, so the coherence of
ground objects on alpine meadow varies little. The phenomena
presented above results that alpine aim has little lower
coherence than alpine meadow. In April, with the temperature
rising further, frozen ground in the region of alpine aim thaw
strongly and the coherence decrease continually. However,
frozen ground in the region of alpine meadow has not began to
thaw and the coherence in alpine meadow still keep high value
relatively. So, there is a big difference on conherence between
alpine aim and alpine meadow in the pair D.
Table 1. Comparison of coherence for different process of
frozen earth changing
ID
Date
Whole
Railwa
y
A
041118-041223
66
0.431
0.340
0.206
0.157
D
050127-050407
-185.4
0.335
0.335
0.175
0.133
H
040108-040527
-14.4
0.283
0.176
0.152
0.091
K
040527-041014
18.6
0.174
0.176
0.112
0.097
O
050407-060216
53.8
0.198
0.143
0.115
0.090
The two images, composing the pair H, are acquired at the
beginning of January and the end of May. At the end of May,
all near-surface of permafrost in Beiluhe area enter upon the
process of thawing with the temperature rising, and the change
from thawing phenomenon is same to all the ground objects, so
ground object characteristic becomes the main factor that
influences the coherence again in Beiluhe area.
To analyze the coherence characteristics for different
thawing/ffeezing process at all permafrost area, we select 5
interferograms to compare the coherence coefficients. Tab. 1
list the coherence coefficients and perpendicular baseline for
the 5 pairs.
Comparing the pair A and H, it can be found that the pair H has
shorter spatial baseline but lower coherence than the pair A for
all kinds of ground objects. The two images, composing the pair
A, are both acquired in winter. On the contrary, the two images,
composing the pair H, are not acquired in the same season. One
is acquired in the stable winter, and the other is acquired at the
active process on April. In other words, the pair H experiences
the process of frozen ground thawing, so the ground objects’
dielectric characteristics have an evident change and coherence
falling correspondingly. This means that the coherence is
affected by the thawing process.
With regard to the pair H and the pair K, the value of two pairs’
spatial baseline is very close, but their coherence has a great
difference and the coherence of the pair H is much higher than
that of the pair K. Comparing the time of image acquired for
two pair, we can find that the pair H is from a stable statue to an
active statue and the pair K goes through two active process
which are thawing and freezing, so in the pair K ground surface
change a lot and the effect of decorrelation is more strong.
Both the pair O and the pair D experience the process of
thawing and freezing, but the pair O goes through all
thawing/freezing circle in almost one year interval. The
coherence of the pair O is lower than that of the pair D, which
means that ground surface has seriously decorrelated after the
process of thawing and freezing, even the ground surface goes
back to the stable status again.
5.2 Deformation Analyzing of PS
Whenever enough images are available, DInSAR limitations
can be overcome by adopting a multi-interferogram framework.
The PS technique takes advantage of long temporal series of
SAR data, acquired over the area of interest along the same
(nominal) satellite orbit, to filter out atmospheric artefacts and
to identify a subset of image pixels where high-precision
measurements can be carried out. These pixels, almost
unaffected by temporal and geometrical decorrelation (usually
but not necessarily corresponding to man-made objects) are PS
(Colesanti, 2003; Bu'rgmann, 2006).
There are 385 points were selected as PS in experiments area
with 12 interferograms between Jan. 8, 2004 and June 1, 2006,
a lot of PS are around the Qinghai-Tibetan railway and highway
passing Beiluhe area. Figure 6 showed the PS candidates
distribution around the railway and highway. There are two
lines interlacing with each other in these images, the left one is
the Qinghai-Tibetan railway, and the right one is the highway.
The deformation history of PS are derived from 12
interferograms after removed the topographic phase and the
effect from atmosphere and orbit errors. Fig.7 showed the
deformation value for different time period compared with Jan.
27, 2005 around Qinghai-Tibet railway area. The maximum
