# Ground Subsidence Analysis in Tianjin (China) Based on Sentinel-1A Data Using MT-InSAR Methods
## 3 Results
### Comparison of Corresponding Points
Statistics for the absolute value of velocity difference suggested that 75.12% of the corresponding points' difference is smaller than 5 mm/year, while only 3.56% is larger than 10 mm/year. The corresponding points with a velocity difference greater than 10 mm/year are mainly distributed in the high subsidence area. Besides, combined with remote sensing optical images, it was found that almost all these corresponding points appeared in the emerging industrial parks in the suburbs (Figure 9B). Newly built factories are densely distributed in the industrial park, requiring a large amount of freshwater, mostly extracted from groundwater. Due to the excessive exploitation of groundwater in the industrial park, uneven ground subsidence occurs. Considering the different sensitivity of PS and SBAS methods to such unstable subsidence, it is likely that the subsidence rates gained at the same location will eventually be slightly different. Comparatively, the corresponding points' velocity difference is less than 5 mm/year in areas where the subsidence rate is relatively stable, such as high-rise buildings in urban area and transportation facilities (highways and railways, for example). Figure 8: Regression analysis between surface subsidence rates gained from PS and SBAS in (**a**) the whole study area; (**b**) the high subsidence area; (**c**) the medium subsidence area; and (**d**) the low subsidence area. It is necessary to analyze the cumulative displacement of the corresponding points with different velocities for knowing the different performance of the two MT-InSAR techniques. In our research, to compare the subsidence results of the two methods, six corresponding points (points a, b and c in Figure 9) are selected for time series analysis. Points a1 and a2 fall on two plants in the emerging industrial parks. Point b1 is located on a new high-rise building, and point b2 on Jinba Railway. Points c1 and c2 are located in the old residential area in the suburbs. The velocity difference of point a is more than 10 mm/year, of point \(b\) is between 5 and 10 mm/year and of point \(c\) less than 5 mm/year. At the same time, points a and c are located in the high subsidence area, while b in the medium area. From the displacement trend during time for the corresponding points (Figure 10), it can be seen that in general, the displacement of the points exhibits a sinking trend with a nonlinear behavior over the past two years. It is worth noting that the data fluctuation, for all the six points, is larger than for SBAS data, indicating that to a certain extent, the method of PS is more sensitive than SBAS for ground subsidence detection in our study area. In the high subsidence area, from March 2017 to March 2019, the cumulative displacement of point a1 detected by PS is \(-\)102.54 mm, larger than the displacement of  
Figure 9: (**A**) Distribution of the corresponding points in the classes 0–5, 5–10 and \(>\) 10 mm/year, according to the velocity difference between PS and SBAS values, superimposed on the Sentinel-2A images. Six corresponding points (a1–c2) located in the high and medium subsidence areas have been indicated; (**B**) zoom on corresponding points with a large velocity difference (\(>\) 10 mm/year); most of these points are distributed in the high subsidence area. \(-\)71.82 mm detected by SBAS. Similarly to the point a1 case, the displacement of other points obtained by PS is always larger than the SBAS result (Figure 10).