Surface Deformation Evaluation In Chengdu Using Time-series InSAR Technique

Surface subsidence is one of the major disastrous causes in urban areas.It is also one of the important factors restricting urban development and threatening urban security.With the acceleration of urbanization,cities now become more and more crowded than ever before.The rapid urbanization is often accompanied by the massive construction of high-rise buildings and transportation infrastructure,and excessive groundwater mining.The urban surface subsidence rate is accelerated.Interferometric Synthetic Aperture Radar(InSAR)is a unique tool for analyzing the ground deformation and is a widely-used method for studying the surface deformation.Compared with the in situ accuracy level and GPS measurement techniques,InSAR technique has the advantages of the wide measurement coverage,high level of accuracy,short data acquisition cycle,and low cost.After decades of the advancement,InSAR technology has evolved from the traditional DInSAR technology to the time-series InSAR(TS-InSAR)technique,which further improves the measurement accuracy.In this TS-InSAR analysis,the PS-InSAR and small baseline subset(SBAS)InSAR(SBAS-InSAR)techniques coupled with the multi-temporal spaceborne SAR observations were used to evaluate the surface deformation of Chengdu,Sichuan,China.Ninety Sentinel-1 images and twenty Envisat/ASAR images covering the urban areas of Chengdu were analyzed.The time span of the Sentinel-1 images was from October 2014 to November 2018.The Envisat ASAR images were from May 2008 to July 2010.Results of the surface deformation were overall consistent when the PS-InSAR and SBAS InSAR techniques were employed.The uplift was observed in the northwestern part of Chengdu,whereas the subsidence in the southeastern area.The central area of the city was basically stable except for isolated locations where there are large population flow and/or heavy traffic.In those locations,the subsidence occurred.The followings were attributed to the causes/reasons for the observed surface deformation.The Qinghai-Tibet plate from the west pushes eastward to the Chengdu plain tectonically causing the uplift in the northwestern area.The recent and rapid construction of numerous high-rise buildings at Longquanyi District located in the southeastern Chengdu might cause the subsidence locally.The groundwater level declined relatively slowly and might be uniformly at the citywide scale.The citywide surface deformation was uniform roughly.Large subsidence funnels were not found.The base of the Chengdu plain is hard,and the overlying soil is loose.The urban area should be less affected by major earthquakes.However,Dujiangyan City located in the northwest of Chengdu is near the feet of large mountain ranges.The city was close to the epicenter of the 2008 earthquake and was seriously damaged by the quake.Massive recover activities were carried out after the 2008 earthquake.Thus,the surface deformation of the newly reconstructed city was of great interest.From October 2014 to November 2018,the surface deformation was very small even though noticeable subsidence occurred near the mountainous areas in the north and the downtown areas of the city.In addition,the deformation of the Zipingpu dam,Dujiangyan was studied.The results show that from June 2008 to July 2010,the dam experienced a large subsidence rate due to the dam's own subsidence.From October 2014 to November 2018,the dam showed periodic deformation.This is mainly due to the change of reservoir water pressure with time and the thermal expansion and contraction caused by the change of temperature in the four seasons,but the annual subsidence rate of the dam is basically within 5 mm.