AN ANALYSIS OF SURFACE SUBSIDENCE IN CHIBA USING PSInSAR TECHNIQUE

Currently, surface subsidence has become an important problem what we are facing. Because of complex topography, uneven distribution of rainfall, and the fast development of urbanization, many cities of the world have undergone surface subsidence disaster, such as Chiba, Paris, Tokyo, Beijing. The surface subsidence has occurred in Chiba since the early twenty-first century. The surface subsidence seriously threatens the safety of human life and property. In order to monitor surface subsidence, people have done a lot of research, and time-series InSAR technique with its better coverage, lower cost and high measurement accuracy advantages shows great potentiality for monitoring surface subsidence. Time-series InSAR technique can be applied for analysis of subtle surface subsidence which occurred consistently for a long term period. This paper uses time-series InSAR technique, Permanent Scatterers Interferometric SAR (PSInSAR), to monitor surface subsidence of Chiba. The used dataset consists of thirty-four Envisat ASAR images from September 2006 to August 2010. For the experimental results, this paper uses GPS data to verify the reliability of the results, and the results can provide information for local government to prevent the occurrence of

In this paper, we adopt PSInSAR technique to carry out surface subsidence monitoring experiment, then analysis the results of the experiment, and verify the reliability of the results.Because of the more images, we will acquire the more reliable results.One of the images is selected as the public master image, others are slave images to match the public master image space and carry out interference treatment.And then a selection model is adopted to elect a sufficient number of PS points, which have highly steady radar scattered characteristics.

Principle of PSInSAR technique
By making analysis about phase of points, the PS point deformation results are obtained.Finally an interpolation method is used to get deformation results throughout the study area.Currently, PSInSAR technique has been widely applied to urban surface subsidence monitoring [6], [7].

The study area and experimental data
Chibal is located in Japan's Kanto region, and it contains the Kanto Plain and Boso Peninsula.It is a long and narrow peninsula that is about 96 km east-west and 129 km north-south.Table 1 shows the SAR data information.In order to verify the reliability of the experimental results, this paper uses settlement information of GPS points to compare with the experimental results.Because GPS data is original data, GPS data needs to be related to treatment to obtain GPS deformation rate corresponding to the SAR image time series.
Then, it needs to find a PS point near the GPS point, compare GPS deformation rate with PS deformation rate and obtain rate line chart (Figure 6).It can be found from figure 6   Finally, this paper uses GPS data to verify the reliability of the results.
subsidence has  become an important problem what we are facing.Because of the frequent occurrence of natural disasters, such as earthquakes, volcanoes, and the rapid development of urban construction, the formation of surface subsidence phenomenon increases , and the urban ground surface deformation is more prominent , the surface subsidence seriously threat to the safety of human life and property.As an earth observation, synthetic aperture radar interferometry (InSAR) has opened up a new research direction for monitoring surface subsidence [1].And InSAR has some advantages, such as better coverage and lower cost, and it can work for al-weather condition to monitor surface subsidence.It has become a hot international research.With the in-depth study of InSAR technique, differential interferometry technique (DInSAR) was born [2].DInSAR technique shows amazing advantage in monitoring earthquakes, volcanoes, landslides and  * Corresponding author other surface deformation, but in monitoring the slow surface deformation, such as the urban surface subsidence, it is impacted by time decorrelation, space decorrelation and atmospheric delay.To solve this problem, researchers proposed time series InSAR technique [3], such as the Permanent Scatterers Interferometric SAR technique (PSInSAR) and Small Baseline Subset technique (SBAS) [4].And PSInSAR technique achieved better results in urban surface subsidence monitoring applications.
PSInSAR technique is the innovation and development of DInSAR technique.PSInSAR technique is a time series technique, and it adds the amount of time on the basis of conventional DInSAR technique [5].PSInSAR technique uses many SAR images, at least 20 scenes, covering the same area.

Figure 1
Figure 1 shows the position of the study area, where is in the red box.Chibal is a coastal city, which is near the Pacific Ocean.Since the unconsolidated soil is the main basement of Chibal area, there are some possibilities of potential subsidence risk.Along with the rapid development of the city and the increase in population, the city need more and more water resource.Over-exploitation of water resource caused the surface subsidence of Chibal, and the surface subsidence has occurred in Chiba since the early twenty-first century.

Figure 1
Figure1The study area

Figure 4 Figure 5
Figure 4 The subsidence rate and distribution of Envisat ASAR's PS that the PS point has a more consistent trend settlement with the GPS point, and the surface deformation obtained by PSInSAR technique is reliable.

Figure 6
Figure6The subsidence rate of GPS and PS

Table 1
The parameters of Envisat ASAR