# Ground Subsidence Analysis in Tianjin (China) Based on Sentinel-1A Data Using MT-InSAR Methods
## 4 Discussion
### Comparison of the Two MT-InSAR Techniques
Unlike the PS-InSAR method requiring the targets with high coherence values, SBAS is a powerful means which can identify targets with sufficiently high coherence values [12; 14]. The combination of small baselines relays on the vector involving values of the unwrapped differential interferometric phase [19]. Previous studies have shown that when comparing PS and SBAS methods, the method of SBAS can obtain considerably more coherent points than PS [23; 42; 43]. In our study, the annual LOS mean subsidence velocity maps obtained based on SBAS were composed of 912,385 coherent pixels, with a density of 1981 units/ km\({}^{2}\), more than PS-InSAR returning 728,903 PS points with 1582 units/ km\({}^{2}\). Especially in the high subsidence area (i.e., the suburbs of Tianjin), the density of coherent points acquired from SBAS is 1.32 times more than that of PS, and the extension of the subsiding area detected by means of SBAS is 43.38% larger than that of PS (Figure 4a,b). On the whole, SBAS can obtain the settlement of low coherence points in the high subsidence areas, which makes up for the shortcomings of the PS algorithm to a certain extent. Our research showed that both the PS and SBAS methods can achieve good results in the subsidence inversion process of key zones such as emerging industrial parks, residential areas (old or new) and transportation facilities (railways, for example). Further, the subsidence value obtained by PS is generally greater than the SBAS value (Figure 10). For example, the subsidence rate at point b1, located in a newly built high-rise building area, is \(-\)40.56 mm/year (PS) and \(-\)35.32 mm/year (SBAS); meanwhile, the subsidence rate at point b2, situated on the railway, is \(-\)41.33 mm/year (PS) and \(-\)35.82 mm/year (SBAS). In addition, in the areas with the same subsidence level, the subsidence rates gained from the two methods are slightly different due to their different sensitivity to ground objects with different structures. Both points a and c are in the high subsidence area. Point a is located in the emerging industrial parks, where the groundwater is unevenly extracted, and uneven subsidence occurs, resulting in a velocity difference between PS and SBAS greater than 10 mm/year. Point c is in the old residential area in the suburbs, where the land subsidence is more widespread and uniform with respect with the industrial area; here, the velocity difference between the two methods is less than 5 mm/year. The difference in subsidence rates obtained by PS and SBAS seems to depend on the greater capability of the PS technique to discriminate ground objects with different velocities. Figure 11: (**a**) Distribution of main roads and railways in the study area, superimposed on the Sentinel-2A images. (**b**) PS points’ distribution. (**c**) The standard deviation of mean subsidence velocity derived by SBAS. In our research, the standard deviations of the average subsidence rate (Figure 11c), calculated from every temporal subsidence based on ENVI SARscape, represent the precision of the mean subsidence rate for the SBAS method. It is worthy of note that the spatial distribution of PS points is in good agreement with the high-precision value of the SBAS inversion results (Figure 11b,c) and 68.45% of PS points are distributed in the area with a standard deviation value of smaller than 0.5 mm/year. In addition, in the PS map, there are a few regions with a serious loss of coherence located in the high and medium subsidence areas (I, II and III in Figure 11b,c) and correspondingly, the precision value of these areas in the SBAS results is relatively low, falling in the class 1.5-2 mm/year. Combined with remote sensing optical images, it can be seen that there is a large distribution of farmland in these areas, causing low coherence, as mentioned above. It can be concluded that in urbanized areas with artificial buildings, PS and SBAS can obtain enough coherent points, while in farmland areas, both the two methods suffer from low coherence, being the PS technique more sensitive.