4.3. Building damage extraction
Dujiangyan city, located near the Yingxiu-Beichuan fault, was
severely damaged in Wenchuan earthquake. Many old buildings
were completely damaged and some of them collapsed. The radarbackscattering characteristics of a collapsed building and an uncollapsed
building were quite different, which results in the difference
of backscattering coefficients. The backscattering mechanisms
of different types of building were illustrated in Fig. 5.
Double bounce reflection will happen between the building walls
and ground and produce a strong backscattering (Fig. 5a and b).
Specular reflection will happen on flat roof and produce a weak
backscattering (Fig. 5a). Single bounce reflection will happed on
tilted roof faced the radar beam and produce the strongest backscattering
(Fig. 5b). However, diffuse reflection will happen on
the debris of collapsed buildings and produce a low backscattering
(Fig. 5c). Double bounce reflection seems dominant in undamaged
urban areas, but microwaves will be scattered in different directions
with collapsed buildings. So, the collapsed buildings should
have lower backscattering coefficients and were shown as darker
than un-collapsed buildings. Furthermore, the orientation of the
buildings relative to the radar beam has effects on the backscattering
coefficients.
Fig. 6 shows the building damages of sample blocks in Dujiangyan
city center. The ancillary GIS data of blocks and buildings were
extracted from a pre-earthquake Quickbird image acquired on July
22, 2005. Fig. 6a shows the aerial photograph acquired on May 18,
2008 of Hehuachi residential area, Dujiangyan city. The building
damages were visually interpreted from the aerial photograph,
where red polygons represent collapsed buildings, purple polygons
represent semi-collapsed buildings and yellow polygons represent
un-collapsed buildings (Fig. 6a). Fig. 6b shows the backscattering
coefficient image of TerraSAR-X acquired on May 15, 2008 of
Hehuachi residential area. The collapsed buildings were shown
comparatively darker than the un-collapsed buildings (Fig. 6b).
Fig. 6c shows the building damages of some sample city blocks extracted
from the backscattering coefficient image of TerraSAR-X.
Fig. 6d is the index map which shows the location of sample blocks
in Dujiangyan city.
The backscattering coefficient sigma nought r0 (dB) of collapsed,
semi-collapsed and un-collapsed buildings was calculated
from TerraSAR-X image. Fig. 7 shows the difference between the
statistic values such as minimum, mean and maximum. For both
near west-east and near north-south orientated buildings, the
mean backscattering coefficient of collapsed buildings is 1.75 dB
lower than that of un-collapsed buildings. For semi-collapsed
buildings, the mean backscattering coefficient partly depends on
how much is the collapsed part. The backscattering coefficients
of near north-south orientated buildings are slightly higher
(0.7 dB) than that of near west-east orientated buildings, both for
collapsed and un-collapsed buildings. However, the backscattering
coefficients of un-collapsed buildings ranges from 11.57 dB to