THE XIONG’AN NEW AREA DEFORMATION MONITORING USING SENTINEL-1 SAR DATA BASED ON SBAS-INSAR TECHNIQUE

: In order to study the land subsidence trend since the construction of the Xiong’an New Area, 125 images of Sentinel-1 from January 2017 to September 2020 was obtained in this paper, and the average land subsidence rate and cumulat i ve land subsidence of Xiong’an New Area is obtained by the SBAS-InSAR technology of multi-main image coherent target by GAMMA and the Self-developed software, and the causes of land subsidence from the geographical space in the Xiong’an New Area were analyzed by the divisional timing analysis method. The results showed that about 80% of Xiong’an New Area had slight land subsidence with the rate less than 1cm/yr and the cumulative subsidence less than 3cm, and the maximum subsidenc occurred in the northern of Xiongxian County with rate 7cm/yr and the cumulative subsidence was 30cm. Furthermore, the average subsidence rate and the cumulative maximum subsidence in the north and the northwest of Xiongxian County, the Baima Village and the Beifeng Village were studied. The research results showed that the land subsidence in Xiong’an New Area was related to the over exploitation of geothermal resources and groundwater, and the results could provide important reference values for keep abreast of the land subsidence status of the Xiong’an New Area, as well as the healthy development and long-term planning of the New Area.


INTRODUCTION
On April 1, 2017, the government of China decided to establish a national new area of Xiong'an in the hinterland of North China around Xiongxian County, Anxin County and the Baiyangdian area of Rongcheng. With the goal of relieving noncapital functions of Beijing, the basic construction of Xiong'an New Area includes external transportation, urban transportation, construction of above-ground and underground buildings, as well as the development and utilization of groundwater and geothermal resources. However, due to the relatively flat and low-lying terrain, the hinterland of North China, where the new area located, has a sedimentary layer of great thickness and an aquifer holds rich water (ZHANG Y H et al,2011). Moreover, the shallow surface is soft, with large pores and strong compressibility (ZHANG Ling et al,2020. As a result, huge engineering construction in a short time, especially the largescale exploitation of underground resources and deep utilization of underground space, will definitely lead to a wide range of subsidence and affect the construction of the new area and its sustainable development. Synthetic aperture radar interferometry (InSAR) measurement technology is a new remote sensing technology that can effectively monitor slight land deformation (Wang Xiaoqing et al,2018). Compared with traditional monitoring technology, InSAR technology has the advantages of wide coverage, allweather and low cost, and it has been applied to urban land subsidence monitoring. Foreign scholars Ferretti et al. proposed time-series analysis techniques such as PS-InSAR and SBAS-InSAR to improve the reliability of deformation monitoring (GAO Ertao et al,2019). SBAS-InSAR technology selects images with short time baselines in multiple images acquired and combines them into several small sets, conducting deformation monitoring even with a small number of images. Furthermore, it can greatly reduce the de-coherence between interfering images and take all coherent point information into account when remove the atmospheric phase, leading to better atmospheric phase removal effect.
With the rapid development of cities and the increase of population, the geothermal and groundwater resources in the North China Plain such as Beijing (Yang Q et al,2018), Tianjin and Hebei have been exploited continuously (Jia Sanman et al,2019), resulting in large-scale land subsidence. For example, based on SBAS-InSAR technology, Ge Daqing and others obtained the information of ground subsidence in the North China Plain from 2012 to 2015 with RADA-SAT-2 data. However, the land subsidence in Xiongxian County, Rongcheng County, Anxin County and Gaoyang County has not been described specifically. Before the construction of the New Area, Zhang Yonghong et al. got the data of land subsidence of Xiong'an from 2012 to 2016 with the same technology. The results show that during this period, about 78% of the area of Xiong'an New Area was in the land subsidence area with the subsidence rate of about 10mm per year, and the area where the risk of land subsidence was evaluated as relatively serious and serious were concentrated in Xiongxian County. DAIKR et al. monitored the land subsidence in the first year of construction (2017-2018) of the Xiong'an New Area, and pointed out that the distribution of land subsidence in this area was closely related to the spatial distribution of thermal well mining.
To sum up, a large range of land subsidence is founded in the Xiong'an New Area according to the existing research results of land subsidence, and its development trend and its relationship with the distribution of underground resource exploitation are worthy of further exploration with the deepening of resource exploitation during the construction of the New Area. Therefore, based on the SBAS-InSAR technology, this paper monitors the land subsidence in the area with the Sentinel-1 data from 2017 to 2020, obtains the change rate and cumulative subsidence of the four key areas since the construction of the New Area, analyzes the relationship between land subsidence and geothermal resource exploitation, agricultural irrigation and industrial water consumption, providing important reference values for keep abreast of the land subsidence status of the Xiong'an New Area, as well as the healthy development and long-term planning of the New Area.

Research Area
Xiong'an New Area is a national New Area in Hebei Province and locates in the North China Plain, and adjacent to Beijing, Tianjin and Baoding. It covers a total area of 1576.6 km2, and still lacks of fresh water resources though with Baiyangdian, Xiaoyi River, Fuhe, Caohe and other water systems. After the establishment of the Xiong'an New Area, a large number of infrastructures have been built, and due to the long-term overexploitation of groundwater, the North China Plain is truly troubled with land subsidence and the research area with relatively serious problem is shown in Figure 1.

Experimental Data
The situation of land subsidence of Xiong'an New Area during 2017-2020 was monitored based on SBAS-InSAR technology and with Sentinel-1 (1W) data to keep abreast of land subsidence in the area. The technology and tools used in the paper includes: Sentinel-1 interferometric image of the European Space Agency; C-band (wavelength 5.6cm) with resolution of 5m×20m; single-polarization (VV) observation method with the interference mode width of 250km×250km and the resolution of 30m(XIONG Peng et al,2020); TOPS mode imaging, which could provide high-quality and wide-range interference phase values(XIONG Jiacheng et al,2019). The basic parameter information of Sentinel-1 image data is shown in Table 1 The Sentinel-1 satellite is a two-constellation satellite with a revisit period that can be shortened to 6 days. In this research, the Sentinel-1 data of 125 scenes that covering the research area was used, and its time span was from January 8, 2017 to September 1, 2020. The number of data sources and time distribution are shown in Figure 2.

Figure 2. Number of data sources and time distribution
In this paper, the precise orbit ephemerides (POD) of the Sentinel-1 satellite was used for orbital error correction; NASA 30-meter resolution Shuttle Radar Topography Mission (SRTM) data was used as an external reference DEM to remove terrain effects.

Process Flow
Small baseline subset interferometry (SBAS-InSAR) , proposed by P. Berardino, is a new time-series analysis method for monitoring small land deformation and it can effectively overcome the limiting factors (space-time de-coherence, atmospheric effects, etc.) in the data processing of the differential interferometric synthetic aperture radar (D-InSAR) measurement technology (Solari L et al,2016), and continuously monitor the microdeformation of and with accuracy of millimeter level. The technical flow of SBAS-InSAR is shown in Figure 3.  Extract the average subsidence rate and cumulative subsidence of the Xiong'an New Area and its surrounding areas with obtained radar image data processed by GAMMA and SBAS-InSAR technology that based on multi-main image coherent targets, and extract average subsidence rate and cumulative subsidence with self-developed deformation analysis program.
In addition, crop the SAR image in sub-areas to improve calculation speed and accuracy of monitoring results since the coverage area of Sentinel-1 interferometric wide image is (250×250) km, which is much larger than the research area in this paper. Then the data processing and deformation analysis of sub-areas were completed under the same standard. A unified deformation reference benchmark was established for different sub-areas to conduct overall integration analysis that across tracks and multiple areas, then the deformation rates and cumulative deformation were analysed overall and sub-areas.

Interference Processing
Select any scene of the Sentinel-1 interferometric wide images as the super main image, register the rest images with the super main image, and set the temporal baseline threshold (TBT) and perpendicular baseline threshold (PBT) . N+1 SAR images can generate at most M differential interferograms, and satisfy the following relational equations: , Analyze the temporal baseline and perpendicular baseline distance, and set the maximum temporal interval baseline threshold as 36d and the perpendicular baseline threshold as 149, forming 197 interference image pairs. The baseline spatiotemporal distribution formed by the interference pair is shown in Figure 4. Conduct interference processing to main and auxiliary images according to the combination of small baseline pair to generate differential interference phase (as shown in Figure 5 and Figure  6). There is a lot of noise in the original differential interferogram, which seriously interferes with the efficiency and accuracy of the two-dimensional phase unwrapping algorithm and the accuracy of extracting deformation information. While the noise is significantly reduced by filtering, and the fringes are relatively clear.

Subsidence time-series Analysis
The surface deformation can be decomposed into linear deformation (main deformation) and nonlinear residual deformation (Zhu L et al,2015), and the interference phase difference formed by two coherent points (set as m, n) on the ith interferogram can be expressed as (WANG Xiaoqing et al,2012): where ( ) is the differential interference phase difference of adjacent highly coherent points, is the time baseline of the interferogram, is the model phase difference of adjacent highly coherent points, including the shared phases of difference in elevation error between bright spots ( ) and the difference in linear phase transition rate ( ), ( is the difference between nonlinear deformation phases, ( is the atmospheric influence phase difference, ( is the noise phase difference, ℷ is the radar wavelength, 、 、 respectively represents The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XLIII-B3-2022 XXIV ISPRS Congress (2022 edition), 6-11 June 2022, Nice, France perpendicular baseline, oblique distance, and radar wave perspective at point m on the interferogram.
A local Delaunay triangulation network was established according to the obtained stable high-coherence points, and the initialization parameter solution and refinement parameter solution were performed (ZHANG Ling et al,2014). The relative linear deformation rate and relative elevation error between two points were alculated by the model coherence coefficient. Integrate the relative linear deformation rate and relative elevation error between the two highly coherent points by set a certain high coherence point with known deformation and DEM error as the reference point to obtain the linear deformation rate and nonlinear deformation of each point target.
Estimate the corresponding nonlinear deformation of each SAR imaging moment (with the first phase image as time reference) through filtering method of time and space, and accumulate together the linear deformation and the nonlinear deformation to generate the time-series of cumulative deformation at 3month intervals from January 2017 to September 2020 (see Figure 7). It shows that the subsidence continued to expand from January 2017 to September 2020, while the subsidence development was slow in earlier stage and became obvious until 2020, and the maximum cumulative subsidence reached nearly 30cm.

Characteristic Deformation Analysis
The deformation rate figure (Figure 8) and the cumulative deformation value figure (Figure 9) about Sentinel-1 data from January 8, 2017 to September 1, 2020 show that obvious subsidence area existed in the northern part of Xiongxian County. Among which, most area of the county (about 70% or more) had slight land subsidence with rate of less than 1cm/yr and cumulative subsidence of less than 3cm. While the most significant subsidence occurred in the northern area, mainly distributed in Daying Town, Xiongzhou Town and other areas.
The maximum subsidence rate of those areas was nearly 7cm/yr, and the maximum cumulative subsidence reached 30cm. The comparison of the monitoring results in this paper with relevant literature shows that a clear subsidence trend occurred in the northern area of Xiongxian County with the maximum subsidence rate of 7.5cm/yr during 2012 to 2016 (see Figure  10). It found that the subsidence trend was continued in most area of Xiongxian Couty (70%) in 2017-2020 while put the relevant literature and the research results of this paper together, and the subsidence rate and cumulative subsidence continued to expand. Therefore, Xiong'an area kept a slow and stable subsidence trend during 2012-2020, which was clearer in geothermal fields of Niutuo The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XLIII-B3-2022 XXIV ISPRS Congress (2022 edition), 6-11 June 2022, Nice, France  According to the monitoring results from Hebei Surveying and Mapping Department, the annual subsidence of each reference station area are -3.15cm, -1.21cm and -1.16cm, respectively. While the annual subsidence of surrounding areas of those three reference stations are about -3.37cm, -1.01cm, and -1.44cm, respectively in accordance with the subsidence rate monitored in this paper (see Figure 8), and the subsidence trend is basically consistent with the subsidence.
Extract four areas with obvious deformation characteristics (area #1, #2, #3 and #4 in Figure 12), such as the northern part of Xiongxian, the southern part of Anxin and other areas with severe subsidence, and conduct subsidence time-series analysis respectively (see Figure 13 -20). From the perspective of the spatial distribution of subsidence areas, three areas (#1-3) in the northern part of Xiongxian locate in the area with rich geothermal resource, and #4 area is covered by farmland and needs underwater for irrigation for a long time, which results in obvious subsidence.
The average subsidence rate of the center point of #1 area is 3.24cm/yr, and the maximum cumulative subsidence is nearly 12cm; the average subsidence rate of the center point of #2 area is 3.43cm/yr, and the maximum cumulative subsidence is nearly 15cm; the average subsidence rate of the center point of #3 area is 4.17cm/yr, the maximum cumulative subsidence is nearly 15cm; the average subsidence rate of the center point of #4 area is 3.8cm/ yr, and the maximum cumulative subsidence is nearly 13cm.

CONCLUSION
This paper extracts the average subsidence rate and cumulative subsidence of Xiaong'an New Area and its surrounding areas from 2017 to 2020 with the 125-scene Sentinel-1 images and with the SBAS-InSAR technology that based on multi-main image coherent targets. It also compares the monitoring results from three continuously operating reference stations with the subsidence rate obtained by InSAR, and those two monitoring results are basically consistent. More than 70% of the area in the Xiongan New Area has slight subsidence with rate less than 10mm/a. Small land subsidence occurs in Rongcheng County and its surrounding area, while significant land subsidence occurs in some areas of Xiongxian and in the south of Anxin County. On the whole, the areas with relatively serious land subsidence are concentrated in Xiongxian, in where subsidence continues to develop compared with the monitoring results during 2012-2016. The large-scale subsidence in Xiong'an New Area is mainly caused by multiple factors such as groundwater extraction, geothermal energy exploitation, and underground space construction. With the construction of the new area, more geothermal wells have been dug and mainly located in the northern part of Xiongxian, which is also an important reason for the rapid land subsidence and large cumulative subsidence in the northern part of Xiong'an New Area.