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Mapping without the sun
Zhang, Jixian

computed as the quotient of the coherence coefficient of the
pre-seismic interferogram ÿ M and the coherence coefficient
of the co-seismic interferogram y M ( see equation 3).
r U ". - r Mco
Y Mp „ + y Meo
Acquisitions suitable for interferometric analysis of the Bam
earthquake were acquired by Envisat-ASAR before and after
the earthquake on ascending and descending passes (Figure 1).
Unfortunately, only the descending acquisitions cover the
complete displacement pattern of the event. Therefore, the
descending acquisitions were employed in the paper.
Two data pairs, [Jun. 11, 2003 - Dec. 3, 2003 (pair-1)] and [Jun.
11, 2003 - Jan. 7, 2004 (pair-2)] were used to derived the
coherences of the pre-seismic interferogram and the co-seismic
interferogram, respectively. In practice, the coherence change
index is estimated in local windows. 3x3 windows were used
to calculated the coherence change index in the cities of Bam
and Baravat and along the faults. 9x9 windows were used to
calculated the coherence change index in other areas.
g S
3* 1
Descending passes
Bam locate in the center of the images
35/Bf =.
-581 Dt=35
Ascending pas**»
Bam locate in the margin of the
Figure 1 Envisat ASAR Bam Dataset
The magnitude of the interferometric coherence depends on a
variety of factors, including imaging geometry, topography,
signal-to-noise ratio and temporal changes of the scattering
properties of the imaged land surface [7]. Temporal reduction
of coherence in interferometric images is often referred to as
temporal decorrelation. Effects of temporal decorrelation have
proven to be very difficult to understand in detail and no
general models exist that can explain the loss of coherence over
time quantitatively. However, temporal decorrelation can be
understood qualitatively as a measure of change in the
backscattering processes in the two SAR acquisitions used to
form the interferogram. Geometrical changes at the scale of
radar wavelength (5.6 cm for Envisat- ASAR) can therefor
significantly lower interferometric coherence. Our intention
here was to investigate whether this can be exploited to map
and quantify the destruction caused by the Bam earthquake
using the magnitude of the interferometric coherence.
The coherence image of the coseismic interferogram (see figure
2(b)) shows a generally high coherence across the image as is
expected for unvegetated terrain over a time period of only 35
days. Note that the coherence values are decreased markedly in
the city of Bam and on the slopes of the surrounding mountains.
Also, the trace of the ruptured fault itself clearly stands out as a
sharply defined elongated region of low coherence.
Nevertheless, we did not attempt to interpret the coherence of
the coseismic interferogram alone. Incoherent regions in the
image occur in different parts of the image and clearly are not
all related to structural damages caused by the earthquake.
Furthermore, the coherence map for an another interferogram
created for the same area spanning the time from June 11, 2003
to December 3, 2003 (see figure 2(a)), not including the time of
the earthquake, also shows relatively low coherence in the city
of Bam itself.
The coherence change index map is shown in fig. 3. Areas of
strong coherence decrease are - as expected - the city of Bam
itself and the fault trace, but also extended regions outside the
city limits, particularly to the east, where no significant changes
related to the earthc
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