Land Subsidence Monitoring And Analysis Of Influencing Factors In Su-Xi-Chang Area Based On Multi-source SAR Data

Located on the eastern coast of China,Yangtze River Delta is an alluvial plain formed by long-term sediment deposition from the Yangtze River,entering sea through this region.Tectonic movement,natural compaction of sediments,and intense human economical activities are having a combined effect on the unique Quaternary geological evolution background of this region,causing frequent occurrence of land subsidence disasters,damaging urban infrastructures and underground pipeline networks,posing a direct,serious threat to the lives and properties of local people.In the center of Yangtze River Delta,the Su-Xi-Chang(which means Suzhou,Wuxi,and Changzhou)are is experiencing serious land subsidence disasters.With the acceleration of regional urbanization,demand from urban and economic development has led to excessive exploitation of deep groundwater,causing severe subsidence disaster.Since 2000,polices that prohibit groundwater exploitation were officially announced across this area.With the implementation of corresponding measures in Su-Xi-Chang are,groundwater level began to rebound by various degrees,regional surface subsidence was effectively controlled,and process of subsidence has been slowing.However,surface subsidence has not disappeared entirely,its threat to urban development still exists.Therefore,under a scenario that prohibits groundwater exploitation,large-scale monitoring of land subsidence and analysis of influencing factors of subsidence will provide scientific support to decision makings of subsidence prevention for corresponding government branches.In addition,such research will also have a significant implication in the promotion of sustainable development of social-urban economy.In this study,Small Baseline Subset-Interferometric Synthetic Aperture Radar(SBAS-InSAR)method is combined with multi-source SAR data to conduct large scale,long-term surface deformation monitoring in Su-Xi-Chang area.Based on this information,influencing factors in regional subsidence are also explored by combining groundwater level change data geological data,and information about local buildings.Main research results are listed as follows:(1)Land subsidence rates are measured and calculated based on co-located ENVISAT ASAR and ALOS PALSAR data at the same time,then the results are cross validated.Results of comparison showed that the overall difference is relatively low,which is<15 mm/year,with a Spearman rank correlation coefficient of 0.97 and RMSD of 7.11 mm/year.Thus,it can be validated that the multi-source SAR based land subsidence monitoring is feasible and reliable in Su-Xi-Chang area.(2)Land deformation in Su-Xi-Chang area during period from 2003 to 2017 are observed by processing ENVISAT ASAR,ALOS PALSAR,COSMO-SkyMED and Sentinel-1A data with SBAS-InSAR method.Results showed that land subsidence in Su-Xi-Chang area are mostly occurred in Wujiang District of Suzhou,Jiangyin City of Wuxi,Wujin District of Changzhou.Land subsidence in Wujiang District of Suzhou is the most severe,with an annual subsidence rate of-90 mm/year.Based on the monitoring during 2015?2017,areas with>-15 mm/year subsidence rate is 500 km2 in Wujiang district of Suzhou.For Jiangyin District,maximum land subsidence rate is-56 mm/year.There is a series of subsidence funnels,which are interconnected with each other,to form a subsidence belt with a vast area,and maximum subsidence rate is this belt has reached-60 mm/year.(3)Effect of groundwater level change on surface deformation is analyzed with groundwater level observations and InSAR observations.Results showed that in most areas,with the rise of groundwater level,tendency of surface rebound is obvious,with a maximum rebound of 323 mm.In areas with serious subsidence,such as Wujin District of Changzhou,land surface of these region will not rebound with the rise of groundwater level due to the hysteresis of the formation compression.The trend of subsidence will persist,until formation consolidation is completed.According to Terzaghi-Jacob theory,water release coefficients in the surface rebounding areas ranged between 1.05×10-3?9.30×10-5.By the theoretical empirical value,these coefficients are elastic water release coefficients,which indicate that these areas are dominated by the elastic deformation.Water release coefficients in the surface subsidence areas ranged between 1.23 × 10-2?2.50 × 10-1.By theoretical empirical values,these coefficients are plastic water release coefficients,which indicate that these areas are dominated by plastic deformation.The above analysis show that the rise of groundwater level is one of the factors that has caused surface rebounding.In areas with t groundwater level rising and surface rebounding,ground deformation there is mostly elastic;on the other hand,in areas with groundwater level rising and persistent land subsidence,ground deformation there is mostly plastic.(4)Deformation analysis with InSAR measurements along geological section lines show that in areas not supplied with Yangtze River water,with serious land subsidence,clay layers there are thicker.InSAR measurements at boreholes in the Su-Xi-Chang region,the thickness of the clay layer overlying the second confined aquifer,and the thickness of the clay layer at the aquiclude roof show that the surface subsidence value is higher with the increment of thickness of these two types of clay layers.The thicknesses of these two types of clay layers at the location with fastest land subsidence(-17 mm/year)are 136 m and 25 m,respectively,and at the location with fastest land rebound(+4 mm/year),the thicknesses of these two types of clay layers are 26 m and 21 m respectively.InSAR surface measurements in areas with fault distribution show that there is slight subsidence in such areas,and the annual deformation rate varies from-6 mm/year to 4 mm/year.At present,faults have not caused major geological disasters yet,but the long-term effect of faults on surface deformation should not be ignored.(5)Based on Landsat TM/OLI optical images,building area and Normalized Difference Building Index(NDBI)from 2001 to 2007 are extracted,to analyze the influence of buildings on surface deformation.In Su-Xi-Chang,rapid growth of built-up areas has occurred in three areas which are Wujiang District of Suzhou(total increment of 190.89 km2),Jiangyin City of Wuxi(total increment of 313.23 km2),and Wujin District of Changzhou(total increment of 148.07 km2).These distrcits are also the areas that have been experiencing the most severe land subsidence disasters.Wujin District of Changzhou was selected to study how land subsidence is influenced by urbanization,for this area is least affected by groundwater exploitation and geological structure.Results showed that when the Spearman rank correlation coefficient between NDBI and maximum land subsidence rate is 0.3,land subsidence rate is-50?-40 mm/year,and NDBI is 0.2818?0.3221;when minimum Spearman rank correlation coefficient is 0.10,land subsidence rate is-14?-6 mm/year.This indicates that weight loading of buildings has more influence to land subsidence in areas with higher subsidence rate;and areas with lower subsidence rate are less affected by such influence.If the above results are overplayed on a traffic route network.It can be found that high traffic volume are also amplifying the subsiding tendency.Area are affected by both static(buildings)and dynamic(traffic volume)weight loading are having a much severe land subsidence.