Monitoring Of Urban Ground Collapse Using Multi-temporal InSAR Technique

Ground collapse is a sudden geological disaster.Recently,the collapse events frequently happened in urban areas,such as Foshan collapse in 2018 and Xining collapse in 2020,which have caused serious economic and life losses.It is urgent to monitor and make early warning for the urban ground collapse.Relevant research showed that the phenomenon of ground subsidence and fissures appeared before the collapse,which can be used to predict the collapse event.However,current ground-based monitoring methods have the disadvantages of high cost and low efficiency,particularly for a wide range of areas.Interferometric Synthetic Aperture Radar(InSAR)technology has been widely used in various surface deformation monitoring due to its advantages of high spatial resolution,wide range of coverage,all-day hours and all-weather conditions.An intensive research has showed that InSAR technology can capture the deformation signals before the ground collapse,which is used to make early warning of the collapse event.However,there are still some problems that have not been well solved for the current studies,such as the problems of interferometric decorrelation due to the maximum detectable by InSAR technology,the surface deformation characteristics after the collapse and the formation mechanism of the collapse.Based on this background,multi-temporal InSAR techniques were applied to two typical collapse events-Foshan and Wink collapse accident,to obtain the time series deformation and subsequently discuss the forming mechanism of collapse.Our result can provide technical support for the monitoring and early warning of urban collapse in the future.The main research contents and achievements of this paper are summarized as follows:(1)The technologies of InSAR,Differential InSAR,Small Baseline Subset InSAR,the Stacking,Multidimensional Small Baseline Subset InSAR are reviewed in this study.(2)For two collapse sinkholes occurred in Wink of the United States in 1980 and 2002,18 sentinel-1A images with ascending orbit and 18 sentinel-1A images with descending orbit spanning from 2017-11 to 2018-06 were processed by MSBAS technology to obtain the twodimensional time series deformation.The results showed that there were 5 prominent deformation regions over the study area,among which the maximum deformation was about-55cm/a in vertical direction and 12cm/a in horizontal direction.Considering the local geological documents,the land subsidence of the study area was caused by the oil and groundwater extraction,as well as water flooding since 1926.In 2011,the subsidence was accelerated due to the phenomenon of drought.After 2017,the deaccelerated subsidence was observed since the empty soil covering force equilibrium development.Aiming at the problem of deformation gradient which is larger than the maximum detectable deformation gradient of InSAR technology,the subsidence was retrieved through the method of “remove-restore”.Comparing of the annual deformation rates before and after using the “remove-restore” method showed that the deformation magnitude is greatly increased after using the “remove-restore” method,proving the effectiveness and reliability of our proposed method.(3)For the collapse accident of subway line 2 in Foshan city,Guangdong province on February 07,2018,56 Sentinel-1A images spanning from 2017-03 to 2019-01 and ALOS-2 images spanning from 2014-11 to 2018-10 were collected to obtain the spatio-temporal deformation information by using SBAS-InSAR technology.The results showed the continuous subsidence in the vicinity of collapse sinkhole,where the maximum deformation rate reached more than 30mm/a.Based on the field investigation and analysis of geological conditions,the mechanism of formation of the subsidence was discussed: The uneven settlement of the soft soil under the water supply pipeline leads to cracks in the water pipe,which leads to the leakage of the water inside the pipeline,resulting in cracks in the pipe segments that had not reached the freezing point,and finally leading to the collapse of the tunnel and the ground.