Open Geosci. 2018; 10:678–687 Research Article Open Access Deliang Chen*, Yanyan Lu, and Dongzhen Jia Land deformation associated with exploitation of groundwater in Changzhou City measured by COSMO-SkyMed and Sentinel-1A SAR data https://doi.org/10.1515/geo-2018-0054 Received July 4, 2017; accepted September 13, 2018 1 Introduction Abstract: The Urban Agglomeration in Yangtze River Delta The Yangtze River Delta is located in the eastern part is one of the most important economic and industrial re- of China, with a low and flat ground surface. It occu- gions in China. The City of Changzhou is one of the most pies an area of nearly 210 700 km2, including part of important industrial citys in Yangtze River Delta Urban Ag- Jiangsu Province, Zhejiang Province, and Shanghai City. glomeration. Activities here include groundwater explo- The Yangtze River Delta is one of the most developed areas ration. Groundwater overexploitation has contributed to in China. The developed economics and a rise in popula- the major land deformation in this city. The severity and tion cause the domestic and industrial water supply to in- magnitude of land deformation over time were investi- crease rapidly. Groundwater is extracted from the aquifer gated in Changzhou City. A Small Baseline Subset Interfer- system underlying the Yangtze River Delta to handle the in- ometric Synthetic Aperture Radar (SBAS-InSAR) technol- creasing water demand. This area has suffered from land ogy, provides a useful tool in measuring urban land defor- subsidence since the early 1920s due to the increasing mation. In this study, a time series of COSMO-SkyMed and amount of the groundwater usage [1]. Changzhou City is lo- Sentinel-1A SAR images covering Changzhou City were ac- cated in the south Yangtze River Delta area, with its north quired. SBAS-InSAR imaging technique was used to sur- adjacent to the Yangtze River and south to Taihu Lake. vey the extent and severity of land deformation associated Changzhou City is one of the most economically developed with the exploitation of groundwater in Changzhou City. cities in the Yangtze River Delta, and it is characterized by Leveling data was used to validate the SBAR-InSAR pro- a dense population and numerous towns. Changzhou City ductions, the error of SBAR-InSAR annual subsidence re- was a typical area in Yangtze River Delta with serious land sults was within 2 mm. The results showed that three main subsidence due to excessive groundwater withdrawal [2]. land subsidence zones were detected at Xinbei, Tianning In 2000, the provincial government announced a regula- and Wujin District. Four subsidence points were selected tion to comprehensively ban deep groundwater extraction to analyze the temporal and spatial evolution character- by 2005 to mitigate the widespread subsidence and earth istics of land subsidence. The subsidence rate of P1 to P4 fissures in the Changzhou City. was −2.48 mm/year, −12.78 mm/year, −18.09 mm/year, However, up until now there has been no large-scale and −12.69 mm/year respectively. Land subsidence over high-precision measuring work conducted in Changzhou Changzhou showed a trend of slowing down from 2011 to City Especially how the subsidence and rebound evolve 2017,especially in Wujin District. SBAR-InSAR derived land spatially and temporally before and after the implementa- deformation that correlates with the water level change in tion of the policy limiting groundwater exploitation. Only six groundwater stations. Indicated that with groundwater some local leveling measurements have been taken. Un- rebound, the land rebound obviously, and the maximum fortunately, the present leveling data set is not very reli- rebound vale reached 9.13 mm. able due to lack of historical measurements and incom- plete records. In order to investigate the scope, magnitude, Keywords: Index Terms-land deformation; groundwater distribution, and temporal change of land deformation in level change; COSMO-SkyMed; Sentinel-1A *Corresponding Author: Deliang Chen: College of Geographic Yanyan Lu: Institute of Natural Resources and Environment audit, and Biologic Information, Nanjing University of Posts and Telecom- Nanjing Audit University, Nanjing, 211815, Jiangsu Province, China munications, Nanjing, 210023, Jiangsu Province, China, E-mail: Dongzhen Jia: School of Earth Sciences and Engineering, Hohai [email protected] University, Nanjing, 211100, Jiangsu Province, China Open Access. © 2018 Deliang Chen et al., published by De Gruyter. This work is licensed under the Creative Commons Attribution- NonCommercial-NoDerivs 4.0 License. Land deformation associated with exploitation of groundwater in Changzhou City Ë 679 Changzhou City and the possible causes, synthetic aper- Staufen area in southwestern Germany. They obtained ex- ture radar interferometry (InSAR) would be ideal technol- perimental data that was consistent with the results of the ogy to employ. InSAR is an advanced remote sensing tech- leveling survey [15]. Calò et al. monitored the landslide nology that has been developed over the last decades, and in the Ivancich area in central Italy based on the X-band it features multiple prominent technological advantages, COSMO-SkyMed SAR data and SBAS-InSAR [16]. Hu et al. such as 24-hour all-weather capability, high-deformation used C-band ENVISAT ASAR data and SBAS-InSAR to sur- sensitivity, and high-resolution. Therefore, InSAR comple- vey land subsidence in Beijing, and they obtained results ments other techniques such as precise leveling and global that were comparable with the leveling data [17]. positioning system (GPS) for subsidence monitoring. Cur- In this study, we utilized the SBAS-InSAR technique rently, differential InSAR (DInSAR) has been widely ap- based on COSMO-SkyMed and Sentinel-1A SAR data to de- plied in monitoring surface deformation [3–6]. Atzori et al. rive the land deformation time-series in Changzhou City to used X-band COSMO-SkyMed and C-band ENVISAT SAR study the dynamic evolution of land deformation before data with DInSAR to examine the seismic deformation and after the implementation of subsidence control, and of L’Aquila in central Italy, and they integrated DInSAR to analyze the causes of subsidence. Section 2 presents an data with 30 GPS site displacements to constrain inversion overview of the study area and datasets. Section 3 presents models for fault dislocation [7]. The best fit fault plane was the methodology involved. Section 4 and 5 present the in very good agreement with the results of CMT (centroid results and land deformation analysis work, respectively, moment tensor) solutions. Tomás et al. used TerraSAR-X and concluding remarks are given in Section 6. data and DInSAR to investigate the deformation of La Pe- drera Dam deformation in Spain, and their results showed a good agreement with other studies [8]. Ge et al. used the 2 Study area and datasets C-band ENVISAT ASAR and L-band ALOS PALSAR data to monitor the land subsidence of Bandung Basin, West Java province, Indonesia. Good correlations were observed be- 2.1 Study area tween DInSAR results and GPS survey data [9]. Traditional DInSAR focuses on single pair of InSAR Changzhou City is the study area located on the deltaic de- images for deformation detection. Factors such as tempo- posits of the Yangtze River Delta on the east coast of China ∘ ∘ ral/spatial decoherence and atmospheric phase interfer- (Figure 1(a)). Its geographic location is 31 09’N-32 04’N ∘ ∘ ence often affect the formation of interferograms. X-band and 119 08’E-120 12’E, with Taihu Lake to the south and SAR data and DInSAR are affected by temporal and spa- Wuxi city to the east. The urban region of Changzhou tial decoherence, which could lead to unreliable deforma- contains Xinbei, Zhonglou, Tianning, and Wujin districts. tion results [10]. To overcome these problems, advanced (Figure 1(b)) The ground elevation is less than 10 m in the DInSAR technologies, such as persistent scatterer (PS) and plain area, higher in northwest, lower in the middle and small baseline subset (SBAS) have been developed. The southeast. There are several bedrock hills spotted in south PS method generally requires a large quantity of SAR im- and west part of this city, and the groundwater elevation ages; otherwise, severe decoherence, originated from over- is about 80 to 178 m [18]. Sedimentary and igneous rocks stretched temporal and spatial baselines is likely to oc- constitute the basement of Changzhou City. This city ex- cur. Therefore, the nonlinear deformation phase cannot perienced seawater intrusions four times and deposited a be separated from the noise and atmospheric delay phase set of Quaternary sediments since middle Pleistocene. The and reliable deformation results cannot be obtained [11]. thickness of Quaternary deposits strata varies, which af- However, a small quantity of SAR images is not a prob- fected by the bedrock [19]. There are four aquifers in the lem for the SBAS method. Moreover, SBAS limits the ge- Quaternary alluviums, namely, one unconfined and three ometric decoherence resulted from long baselines; there- confined aquifers, which are denoted as U, I, II and III (Fig- fore, the processing of SAR data can utilize an unlimited ure. 1(c)). Conventionally, groundwater with in 50 m deep amount of SAR interferometric pairs to enhance the sam- is shallow, i.e., U and I, otherwise is deep, i.e., II and III pling rate in temporal space [12]. The SBAS-InSAR tech- [20]. nique has been successfully applied to studies of surface Changzhou City is within a temperate climate zone of deformation events caused by geological disasters, such abundant rainfall and dense river network. Over the past as volcanic deformations, landslides, and urban ground several decades, its urban and rural areas have witnessed deformation [12–14]. Lubitz et al. applied TerraSAR-X data phenomenal economic growth, which has been accompa- and SBAS-InSAR to monitor the surface deformation of the nied by severe pollution of the surface water and general 680 Ë Deliang Chen, Yanyan Lu, and Dongzhen Jia was about 26∘, and the imaging mode was Stripmap.