STUDY ON RELATION BETWEEN INSAR COHERENCE AND SOIL MOISTURE
Ting Zhang 3 ’ b , Qiming Zeng a ’ b , Ying Li a ’ b , Yun Xiang 3 ’ b
institute of RS and GIS, Peking University, Beijing, P.R. China, 100871
b Key Lab of Spatial Information Integration and Engineering Application of Beijing
qmzeng@,pku.edu.cn zhangting4012@163.com.
KEY WORDS: Coherence; Soil moisture; InSAR Interferometry; De-correlation;
ABSTRACT:
Coherence is an important parameter to measure InSAR data quality in repeat-pass interferometry and soil moisture change can
cause de-correlation. Relation between coherence and soil moisture was examined in this paper by using the ERS-1 interferometry
pairs on Cangzhou area for which there are some heavy rain between the acquisition dates of the two images. The soil moisture is
obtained based on the distributed hydrological model and the coherence is calculated by reducing the other de-correlation factors as
much as possible. Correlation and regression analysis are carried out on the two images. It is found that the relation between the two
may satisfy exponential distribution and is different with the area. Different soil types and land use appear different patterns.
1. INTRODUCTION
Interferometrical SAR (InSAR) as a powerful mean for
topographical mapping, measurement of subtle earth surface
shift, was chosen to monitor the ground surface subsidence in
CangZhou, southeast of HeiBei province. However, due to the
decorrelation between the interferometric pairs, we couldn’t get
the accurate subsidence measurement of the study area. This
made us quite puzzle. First, the interferometric pairs were
acquired on Apr 1995 and May 1995 with about one month
interval over CangZhou area; then the perpendicular baseline of
these two images was very small, and the precise orbits data
were also used to reduce the co-registration errors during
InSAR processing. In addition, the ground surface subsidence
in Cangzhou area is quite serious. So all these condition should
be quite good for Interferometry, but why we got an unqualified
result. For further research, we find that during the period, there
was a heavy rainfall in the research area, which might cause the
soil moisture changed. If this is true, the soil moisture could
change the surface scattering rate which will promote temporal
de-correlation and cause an adverse effect on interferometric
coherence. At present, the specific influence relation is seldom
studied at home and abroad, so, we carry out the study on the
relation between InSAR coherence and soil moisture.
This paper shows the method of calculating the soil moisture
and obtaining the coherence and the comparison and analysis
results with the two, and also presents an approach to reduce the
effect that soil moisture makes on coherence. 2
2. STUDY AREAS AND DATA
Cangzhou is situated in the east of North China Plain and is one
of the areas which is short of water most seriously around the
Bohai area. The climate is usually dry in Cangzhou area, but
during our study periods, heavy rain happened for several times.
The study area covered by the SAR images is located on the
quadrate area with longitude from 116.143°E to 117.474°E and
latitude from 37.715°N to 38.831°N.
The data used in the model contain SRTM elevation data
provided by NASA, daily meteorological data (rainfall,
temperature, wind speed, vapor pressure, radiation of sunshine,
etc) from April to middle of June in 1995 provided by Hebei
Meteorological Bureau, soil map and land use map. The rainfall
data of nine meteorological stations are used and the location of
these stations is shown with green dots in figure 4. The rainfall
distribution can be seen from figure 1 and the numbers in the
legend represents Station Number. Besides, two ESA-1 images
with approximate pixel resolution of 5mx 15m were acquired on
Apr. 6, 1995 and Jun. 15, 1995 over Cangzhou areas to obtain
coherence. The perpendicular is 122.53m, which is short and
very suitable for interferometry.
Apr. 6. 1995 Jun. 15, 1995
Figure 1 The rainfall distribution between Apr. 6, 1995 and Jun.
15, 1995.for different stations.
3. METHODS
For the study on the relation between coherence and soil
moisture, we need coherence and soil moisture data. Coherence
can be derived in the process of InSAR interferometry from the
image pair. Because very precise measurement data of soil
moisture can not be acquired directly, we calculate it through
meteorological data by building hydrologic model. Our work is
based on the raster data model of GIS because the distributed
hydrologic model is popular at present and applied widely. The
model is built according to water balance principle. All point
data, such as rainfall, is interpolated to surface raster maps.
This work has been sponsored by National Science Foundation of China (40571098) and National High Technology
Program (2006AA12Z150).