Full text: Mapping without the sun

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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