# Ground Subsidence Analysis in Tianjin (China) Based on Sentinel-1A Data Using MT-InSAR Methods
## 3 Results  
### Comparison of Results Obtained by the Two MT-InSAR Techniques and Field Validation  
After the processing of 20 Sentinel-1A SLC images based on PS and SBAS methods, the annual mean subsidence velocity maps from March 2017 to March 2019 were obtained, respectively (Figure 4a,b). The InSAR technique measures the projection of 3D ground displacement in the LOS direction [38]. Negative values correspond to an increase of the sensor-to-object slant range distance [12], revealing the ground is sinking, whereas positive values indicate surface uplift. As shown in Figure 4a,b, the surface displacement results derived from SBAS have a relatively consistent spatial distribution with PS, and the Tianjin study area has exhibited prominent uneven subsidence characteristics in the analyzed two years. The severe settlement area is mainly distributed in the northwest of the study area, concentrated at the junction of the Wuqing, Xiqing and Beichen districts. The subsidence velocity gradually increases from the urban district of Tianjin to the suburbs, roughly in the SE-NW direction. The extension of the total area affected by high subsidence (in red) is 40.84 km\({}^{2}\) according to PS analysis and 72.13 km\({}^{2}\) according to SBAS analysis (Figure 4a,b).  
According to the surface deformation results of InSAR inversion, the study area can be divided into three levels of subsidence, namely low subsidence area, medium subsidence area and high subsidence area (Figure 4a,b). Based on Google Earth images and field investigations, it is found that high subsidence areas (24.75% of the study area) are mostly concentrated in emerging suburban industry areas. There are many industrial parks in the high subsidence area, where the mean subsidence rates are -52.08 mm/year (PS) and -56.97 mm/year (SBAS).  
In sites TJ10 and TJ11 near Wangqingtuo town, in the high subsidence area, some old buildings have experienced serious sinking (Figure 5b,c), compromising the security of local residents. Remote sensing images and a field survey showed that a number of factories existed near the town and subsidence caused the uneven deformation of the ground, even inducing the inclination of the lampposts (Figure 5d). Low subsidence areas (35.61% of the study area) are mostly distributed in the urban area of Tianjin, and the surface subsidence rate is at a low level, with mean rates of -3.23 mm/year (PS) and -1.87 mm/year (SBAS). The medium subsidence area (39.64% of the study area) is distributed in the \"buffer\" zone between the Tianjin urban area and suburban industrial zone. There are basically no industrial parks or high-rise buildings in this area, while shopping malls, playgrounds, hospitals and schools are widely distributed. The mean subsidence rates are \(-\)25.40 mm/year (PS) and \(-\)19.76 mm/year (SBAS). In the medium subsidence area, we investigated the settlement of Hebei University of Technology and its surrounding area (TJ05-TJ08). There are obvious ground subsidence phenomena in the dormitory and teaching buildings on the campus (Figure 5f). As shown in Figure 5g,h, visible road sink and wall cracks have appeared around the university.  
Figure 4: Annual line of sight (LOS) mean subsidence velocity map from March 2017 to March 2019 from (**a**) PS-InSAR and (**b**) SBAS-InSAR data. The deformation value is displayed with different colors. H stands for the high subsidence area, M means the medium subsidence and L for low subsidence area. The PS results are superimposed on the Sentinel-2A images and field investigation points (TJ01-TJ15) are shown in (**b**).