Spatiotemporal Characterization of Land Subsidence and Uplift (2009–2010) over Wuhan in Central China Revealed by TerraSAR-X InSAR Analysis

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Spatiotemporal Characterization of Land Subsidence and Uplift (2009–2010) over Wuhan in Central China Revealed by TerraSAR-X InSAR Analysis

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Spatiotemporal Characterization of Land Subsidence and Uplift (2009–2010) over Wuhan in Central China Revealed by TerraSAR-X InSAR Analysis
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Remote Sensing
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Bai, Lin
Jiang, Liming
Wang, Hansheng
Sun, Qishi
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MDPI
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Sinkholes ( lcsh )
Deformations (Mechanics) ( lcsh )
Karst ( lcsh )
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serial ( sobekcm )
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Wuhan

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The effects of ground deformation pose a significant geo-hazard to the environment and infrastructure in Wuhan, the most populous city in Central China, in the eastern Jianghan Plain at the intersection of the Yangtze and Han rivers. Prior to this study, however, rates and patterns of region-wide ground deformation in Wuhan were little known. Here we employ multi-temporal SAR interferometry to detect and characterize spatiotemporal variations of ground deformation in major metropolitan areas in Wuhan. A total of twelve TerraSAR-X images acquired during 2009–2010 are used in the InSAR time series analysis. InSAR-derived results are validated by levelling survey measurements and reveal a distinct subsidence pattern within six zones in major commercial and industrial areas, with a maximum subsidence rate up to −67.3 mm/year. A comparison analysis between subsiding patterns and urban developments as well as geological conditions suggests that land subsidence in Wuhan is mainly attributed to anthropogenic activities, natural compaction of soft soil, and karst dissolution of subsurface carbonate rocks. However, anthropogenic activities related to intensive municipal construction and industrial production have more significant impacts on the measured subsidence than natural factors. Moreover, remarkable signals of secular land uplift are found along both banks of the Yangtze River, especially along the southern bank, with deformation rates ranging mostly from +5 mm/year to +17.5 mm/year. A strong temporal correlation is highlighted between the detected displacement evolutions and the water level records of the Yangtze River, inferring that this previously unknown deformation phenomenon is likely related to seasonal fluctuations in water levels of the Yangtze River.

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Article SpatiotemporalCharacterizationofLandSubsidence andUpliftoverWuhaninCentralChina RevealedbyTerraSAR-XInSARAnalysis LinBai 1,2 ,LimingJiang 1, *,HanshengWang 1 andQishiSun 1,2 1 StateKeyLaboratoryofGeodesyandEarth'sDynamics,InstituteofGeodesyandGeophysics, ChineseAcademyofSciences,Wuhan430077,China;bailin112@mails.ucas.ac.cnL.B.; whs@whigg.ac.cnH.W.;sunqishi14@mails.ucas.ac.cnQ.S. 2 UniversityofChineseAcademyofSciences,Beijing100049,China Correspondence:jlm@whigg.ac.cn;Tel.:+86-027-8677-8612 AcademicEditors:ZhenhongLi,RobertoTomas,ZhongLuandPrasadS.Thenkabail Received:10March2016;Accepted:14April2016;Published:20April2016 Abstract:Theeffectsofgrounddeformationposeasignicantgeo-hazardtotheenvironmentandinfrastructureinWuhan,themostpopulouscityinCentralChina,intheeasternJianghanPlainattheintersectionoftheYangtzeandHanrivers.Priortothisstudy,however,ratesandpatternsofregion-widegrounddeformationinWuhanwerelittleknown.Hereweemploymulti-temporalSARinterferometrytodetectandcharacterizespatiotemporalvariationsofgrounddeformationinmajormetropolitanareasinWuhan.AtotaloftwelveTerraSAR-Ximagesacquiredduring2009areusedintheInSARtimeseriesanalysis.InSAR-derivedresultsarevalidatedbylevellingsurveymeasurementsandrevealadistinctsubsidencepatternwithinsixzonesinmajorcommercialandindustrialareas,withamaximumsubsidencerateupto67.3mm/year.AcomparisonanalysisbetweensubsidingpatternsandurbandevelopmentsaswellasgeologicalconditionssuggeststhatlandsubsidenceinWuhanismainlyattributedtoanthropogenicactivities,naturalcompactionofsoftsoil,andkarstdissolutionofsubsurfacecarbonaterocks.However,anthropogenicactivitiesrelatedtointensivemunicipalconstructionandindustrialproductionhavemoresignicantimpactsonthemeasuredsubsidencethannaturalfactors.Moreover,remarkablesignalsofsecularlandupliftarefoundalongbothbanksoftheYangtzeRiver,especiallyalongthesouthernbank,withdeformationratesrangingmostlyfrom+5mm/yearto+17.5mm/year.AstrongtemporalcorrelationishighlightedbetweenthedetecteddisplacementevolutionsandthewaterlevelrecordsoftheYangtzeRiver,inferringthatthispreviouslyunknowndeformationphenomenonislikelyrelatedto seasonaluctuationsinwaterlevelsoftheYangtzeRiver. Keywords:multi-temporalInSAR;grounddeformation;Wuhancity;Urbandevelopment;karstgeology 1.IntroductionAccompanyinglarge-scaleurbanizationandindustrializationduringthepast35years,morethan95megacitiesinChinahaveundergonerapidlandsubsidence[1].Thepotentialconsequencesoflandsubsidencemainlyincludedegradationoftheaquifersystemanddamagetotheutilityinfrastructures,buildings,railroads,highwaysandbridges[2,3].Inthiscontext,intensiveeffortsandinvestigationsrelatedtolandsubsidencemonitoringhavebeenuntakeninfourmajorsubsidingregionsinChina:theYangtzeRiverDelta[4,5],theNorthChinaPlain[6,7],theFenweiBasin[8]andthePearlDelta[9].AsoneofthemostpopulouscitiesinChina,Wuhancityhasexperiencedarapidurbanexpansionoverthepastdecades.Consequently,varioustypesofgeohazardsrelevanttogrounddeformationRemoteSens. 2016 8 ,350;doi:10.3390/rs8040350www.mdpi.com/journal/remotesensing

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RemoteSens. 2016 8 ,350 2of14havebeenfrequentlyoccurringinWuhanmetropolitanareas,duetoanthropogenicactivitiese.g.,groundwaterpumping,tunnelexcavationand/ornaturalgeologicalprocessese.g.,alluvialsoilconsolidation,karstcollapseandsurfacewaterloading[10,11].Priortothisstudy,however,ratesandpatternsofregion-widegrounddeformationinWuhanwerestilllittleknown.Consequently,thereisanincreasingdemandforregularandlarge-scalemonitoringofgrounddeformationtosupporttheintegratedsustainabledevelopmentofWuhan.Overpastdecades,space-borneSyntheticApertureRadarInterferometryInSARhasbeenprovenasaneffectiveremotesensingtechniquetodetectgrounddeformationassociatedwithearthquakes[12,13],volcanoeruptions[14,15],permafrostdegradation[16],glaciermovement[17],landsidesactivity[18,19]andsubsidenceinvestigation[2022].Comparedwithpoint-measurementgeodetictechniquessuchasGlobalPositioningSystemGPSandlevellingsurveys,InSARprovidesdeformationmeasurementsatasignicantlyimprovedresolutionoverlargeareas.However,itisstillachallengeasapracticaltoolformonitoringsubtlegrounddeformationduetotemporalandspatialdecorrelationaswellasatmosphericdisturbances.ToovercomethelimitationsofconventionalInSARmethods,inrecentyearssomeadvancedInSARapproacheshavebeendevelopedbasedontime-seriesinterferometricanalysisofmulti-temporalSARacquisitionsoverthesameareas,hereinreferredtoasmulti-temporalInSARMT-InSARtechniques.TheMT-InSARtechniques,mainlyinvolvingpersistentscattererInSARPSImethods[2325]andsmallbaselinesubsetSBASmethods[2628],identifyandexploitasubsetofimagepixelswhichmaintainahighcoherenceleveloverthestudyperiod,allowingtheaverageratesandtemporalevolutionsofgrounddeformationtobeestimatedwithmillimeter-levelaccuracy.TheseadvancedInSARtechniqueshavebeenwidelyusedinmonitoringurbangrounddeformation,suchasinMacao[3],Shanghai[29],Guangzhou[30],Lisbon[31]andMexico[32].Inthispaper,wepresentresultsfromtherstapplicationofmulti-temporalInSARingrounddeformationmonitoringinWuhan.TheStaMPSStanfordMethodforPersistentScattererapproach[24,33]isappliedto12TerraSAR-XimagesacquiredfromOctober2009toAugust2010toretrieveandcharacterizespatiotemporalvariationsofgrounddeformationinmajormetropolitanareasinWuhan.Moreover,acomparisonbetweenInSAR-basedresultsandinsitudataiscarriedouttovalidatetheInSARobservations.Finally,thepotentialcausesoftheobservedgrounddeformationarediscussedthroughanintegratedanalysisofmultidisciplinaryinformationrelatedtokarstgeology,surfacehydrology,urbandevelopmentandgeotechnicalengineering. 2.StudyAreaandDataUsed 2.1.GeologicalSettingofStudyAreaWuhan,thecapitalofHubeiprovinceandthemostpopulouscityinCentralChina,liesintheeasternJianghanPlainattheintersectionofthemiddlereachesoftheYangtzeandHanrivers.Thecitycontainsmanylakesandparks,includingexpansiveEastLake,thelargesturbanlakeinChina.ThegeologyofWuhanispartoftheYangtzegeosynclinesagwiththecharacteristicsofparaplatform,anddevelopedwellinaalluvialplainwithmonadnock.Climatically,Wuhanisinasubtropicalmonsoonclimatezone,characterizedbyhightemperatureinsummer,lowtemperatureinwinterandabundantprecipitation.Theaverageannualprecipitationreachesupto1261.2mm,concentratedintheoodseasonfromJunetoAugust[34].ThestudyareainthisworkislocatedatmajormetropolitanareasinWuhan,withatotalareaofapproximately28kmby20.5km,andmainlycontainsHankouandWuchangpartsFigure1.TheHankouregionischaracterizedwithanalluvialplainformedbyoodsiltationofYangtzeandHanriversaswellaslakes,andtheWuchangregionisdenudedhillylandexceptthealluvialplainalongtheYangtzeRiver.SoftsoilsarewidelydistributedthroughoutmostareasofHankouandintheriversideareaofWuchang.Thereareoneormorelayersoflensformedbysiltclayandsiltinthearea5~30mbeneaththegroundsurface,andthethicknessofthelensreachesabout10~20m.Thematerialsofthelens,presentingaplasticowstateandstronghydrophilicandalterationfeatures,easilyleadto

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RemoteSens. 2016 8 ,350 3of14groundinhomogeneoussubsidenceundertheinuenceofpumpinggroundwaterandvibration[11].Inaddition,thereareinsidiouscarbonaterocksinthesecondbottomalongYangtzeRiverandthesouthofChangjiangBridgeandwell-developedkarstgeologyintheWuhanurbanareasseeFigure1.Alargenumberofkarstcollapsescausedbyhumanactivities,suchaspumpinggroundwaterforwatersupplyandminedewatering,havebeenrecordedsince1931andposeaseriousthreattobuildingsandhumansafety[35]. Figure1.Thegeographiclocationandsimpliedgeologicalsettingofthestudyarea.ThedistributionsofcarbonatebeltsareredrawnfromLuo[36].L1,L2andL3representthreemajorcarbonaterockbeltsnamedTianxingzhou,DaqiaoandBaishazhou,respectively.Theredrectangleisthestudyarea'sextent.Theredtrianglerepresentsthelocationofthelevellingpoints,situatedaroundXunlimenStationinHankoudistrictandMingduStationinHongshandistrict. 2.2.DatasetsUsedInthisstudy,12TerraSAR-XscenesatHHpolarization,acquiredbetweenOctober2009andAugust2010alongascendingtrack142,areusedtoinvestigatethegrounddeformationinWuhan.MoreparametersoftheTerraSAR-XdataaresummarizedinTable1.ThecoverageoftheTerraSARscenesisshownastheredrectangleinFigure1.

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RemoteSens. 2016 8 ,350 4of14 Table1. TheparametersoftheTerraSAR-Xdata. ParametersDescription Trackno.142 ImagingModesStripMap PolarizationHH OrbitdirectionAscending LookingdirectionRight Centralincidenceangledegree34.9 Rangeresolutionm2.0 Azimuthresolutionm3.3 No.ofimages12 Dateofearliestimageused7October2009 Dateoflatestimageused11August2010 SRTMShuttleRadarTopographyMissionDEMwitharesolutionof30mareusedtoremovethetopographicphaseandgeocodeinterferograms.This30-mDEMwasreleasedin2015byUSGSU.S.GeologicalSurveyandcanbedownloadedathttps://lta.cr.usgs.gov/SRTM1Arc.Inaddition,dailywaterleveldataontheYangtzeRiveracquiredbytheHankouhydrologicalstationfromOctober2009toOctober2010,providedbytheHubeiAdministrationofhydrologyandwaterresources,areusedtoanalyzepossiblecausesforgroundupliftalongbothbanksoftheYangtzeRiver.Finally,insitumeasurementscollectedrepeatedlybypreciselevellingcampaignsbetween2008and2010areutilizedtoevaluatetheInSAR-derivedresults.ThelevellingcampaignswerecarriedoutovertwoconstructionareasofMetroLine2seeFigure1,situatedaroundXunlimenStationinHankoudistrictandMingduStationinHongshandistrict,respectively.Moredetaileddescriptionsonthelevellingsurveyswerereportedin[37,38]. 3.Multi-TemporalInSARDataProcessingTheStaMPSapproach,animplementationofpersistentscattererPSInterferometrytechnique,isemployedinthisstudytocarryoutmulti-temporalInSARanalysisofthe12TerraSAR-Xdata.TheStaMPSapproachusesspatialcorrelationoftheinterferometricphasetondPSpixelswithoutpriorknowledgeoftemporalvariationsinthedeformationrate.Thisstrategycandetectlowamplitudepixelswithphasestabilityinmostterraintypes,withorwithoutbuildings,andisapplicableinareasundergoingtemporallyvariabledeformationwithnopriorknowledge[24].DuetothelargesubsidenceratesofthestudyareaandthelimitationofthequantityofSARimages,weappliedtheSBsmallbaselinemethodimplementedintheStaMPSapproach[39]toderivegrounddeformation.ThefundamentalproceduresoftheStaMPS-SBmethodarebrieyintroducedbelowandmoredetaileddescriptionsofthismethodcanbeconsultedin[24,33,39,40]. 3.1.InterferogramFormationTherstkeystepoftheStaMPS-SBmethodistogeneratetheinterferograms.Acoregistrationalgorithmisappliedtoreducethecoregistrationerrorsowingtolongperpendicularbaselines[33].Inordertominimizethetemporalandspatialdecorrelation,interferometricpairswithsmalltemporalandperpendicularbaselinesareselectedtogenerateinterferograms.Decorrelationisfurtherreducedbyspectrallteringinrangeanddiscardingofthenon-overlappingDopplerfrequenciesinazimuth[39].Specically,weimposed33interferogramsgeneratedfromthe12TerrSAR-XdatasetsbyusingDORISDelftObject-orientedRadarInterferometricSoftware,withaperpendicularbaselineconstraintof180mandamaximumtemporalbaselineof110daysFigure2.Aftertheinterferogramsgeneration,weused30mresolutionSRTMDEMtoremovethetopographicphasecontributionfromtheinterferometricphaseandgeocodeinterferograms.

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RemoteSens. 2016 8 ,350 5of14 Figure2.Temporal/perpendicularbaselineplotsforTerraSAR-Ximagesandinterferogramsusedfortheanalysis.Thebluecirclesrepresentthe12TerraSAR-Ximagesandthegraylinesrepresentthe33individualinterferograms. 3.2.SDFPPixelIdenticationStaMPS-SBmethodperformstime-seriesanalysisonthepixelswhoselteredphasedecorrelateslittleovershorttimeintervals,referredtoasslowly-decorrelatinglteredphaseSDFPpixels[39].Fortheconsiderationofthecomputationalcost,initialSDFPpixelscandidatesareselectedbysettingathreshold.6fortheamplitudedifferencedispersion,whichisthestandarddeviationoftheamplitudedifferencebetweenthemasterandslavedividedbythemeanamplitude[39].Then,thespatialcorrelationofinterferometricphaseanalysisisappliedtoidentifySDFPpixelsfromthecandidates.ThewrappedphaseF int x i,ofthexthpixelintheithattenedandtopographicallycorrectedinterferogramcanbeexpressedasthesumofthephasechangeduetodeformationinthesatelliteline-of-sightLOSdirectionF def x i,thephaseduetothedifferenceinatmosphericretardationbetweenpassesF atm x i,theresidualphaseduetoorbitinaccuraciesD F orb x i,theresidualphaseduetolookangleerrorD F q x i,andthenoisetermduetovariabilityinscattering,thermal noise,coregistrationerrorsanduncertaintyinthepositionofthephasecenterinazimuth F n x i : F int x i W F def x i )]TJ/F267 9.9626 Tf 9.808 0 Td [(F atm x i )]TJ/F273 9.9626 Tf 9.809 0 Td [(D F orb x i )]TJ/F273 9.9626 Tf 9.808 0 Td [(D F q x i )]TJ/F267 9.9626 Tf 9.809 0 Td [(F n x i whereW{}isawrappingoperator.TheSDFPpixelsarethoseforwhich|F n x i|issmallenoughthatitdoesnotcompletelyobscurethesignal,soF n x iisrequiredtoestimateaccuratelytoidentifySDFPpixels.Forthispurpose,therstfourtermsontheright-handofEquationmustbeestimatedandsubtractedfromthewrappedphase.Abandpasslteringofsurroundingpixelsinthefrequencydomainisadoptedtoestimatethespatially-correlatedlookangleSCLAerror,includingthephaseduetogrounddeformationF def x i,variationinatmosphericdelayF atm x i,orbitalinaccuraciesD F orb x iandspatiallycorrelatedDEMerrorD F c q x i.ThespatiallyuncorrelatedlookangleSULAerrortermD F u q x iwhichismainlycausedbyspatiallyuncorrelatedDEMerroranddeviationofthepixel'sphasecenterfromphysicalcenterofthebackscatteringobject,isthenestimatedbyaninversionassociatedwithitscorrelationwithperpendicularbaseline.Subtractingtheseestimatesfromtheinterferometricphase,wegetanestimateofF n x iwhichisthencharacterizedwithameasuresimilartocoherence,referredtoasg x,andthecandidatepixelswithlowg xarerejected.Theprocedureisadoptediterativelyuntil g x isconverged,andasetofSDFPpixelsarenallyidentied[33,39].

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RemoteSens. 2016 8 ,350 6of14 3.3.3-DPhaseUnwrappingandTime-SeriesDeformationRetrievalThethree-dimensionalphaseunwrappingalgorithmproposedbyHooper[40,41]isappliedonthesetsofSDFPpixelstorecovertheunambiguousphasevalues.Inordertounwrapcorrectly,thespatiallyuncorrelatedcontributionneedstobesubtractedbeforeunwrapping.Afterphaseunwrapping,spatialandtemporalbandpasslteringisappliedtoremovethespatiallycorrelatedcontributions.Finally,thedisplacementtimeseriesforeachSDFPpixelisobtainedbyleast-squaresinversion[39].DeformationrateanddisplacementtimeseriesforeachSDFPpixelarenallycalibratedtoastablereferencepointwhichcorrespondstotheWUHNInternationalGNSSServiceIGSstationlocatedinWuhanUniversity.357259 E,30.531654 N. 4.ResultsandInterpretations 4.1.InSAR-DerivedResultsandValidationAtotalofmorethanonemillionSDFPpixelswereultimatelyidentiedandexploitedbymeansoftheStaMPS-SBtime-seriesanalysisforthe12TerraSAR-Ximages,withanaveragedensityof~1933SDFPpixels/km2.Figure3illustratesthespatialdistributionoftheSDFPpixelsandthecorrespondinglineardeformationvelocityintheradarline-of-sightLOSdirection.TheannualdeformationratesofSDFPpixelsrelativetothereferencepointrangefrom 67.3mm/yearto+17.5mm/year.HeterogeneouslandsubsidencepatternsarewidelyfoundinmajorurbanareasofWuhan,mostlyconcentratedincommercialzonesinHankouZone1,industrialzonesaroundWuhanIronandSteelCorp.inQingshanZone3andtheDonghuNewTechnologyDevelopmentZoneZone5inWuchang,aswellasoccurringinkarstcarbonateareasinWuchangandQingshandistrictsZone2,4and6,respectively.CausesfortheobservedsubsidencecouldbegenerallydividedintotwocategoriesofurbandevelopmentforZone1,3and5andkarstgeologyforZone2,4and6.Moredetailedanalysisofpossiblecausesforlandsubsidenceinthesezoneswillbepresentedinnextsub-sections.Itisworthnotingthatexceptionalland-upliftsignals,withdeformationratesrangingmostlyfrom+5to+17.5mm/year,aredetectedalongbothbanksofYangtzeRiverespeciallyalongthesouthernbankZone7,andalsoinurbanareaslocatedononesectorofacarbonaterockbeltbetweenYangtzeRiverandEastLakeZone8.ApossibleexplanationforthisanomalousphenomenonintwoupliftzoneswillbediscussedinSection5.AnaccuracyassessmentoftheInSAR-deriveddeformationresultswasperformedbycomparingthemagainstthepreciselevellingmeasurementsasdescriptedinSection2.Toenablethecomparison,thelevellingmeasurementshavebeenprojectedalongradarLOSdirectionandinterpolatedviaalinearfitwithintheacquisitionperiodoftheInSARobservations.AcomparisonmethodusedinanInSARvalidationprojectofEuropeanSpaceAgencynamedasPSIC4[42]wasappliedtotheaccuracyassessment.InthisstudythecomparisonwascarriedoutbetweeneachlevellingpointandallSDFPpixelslocatedwithinacirclewith50mradiusaroundthecorrespondinglevellingpoint.ResultssuggestthattheInSAR-deriveddeformationratesagreedwellwiththelevellingsurveymeasurementswithanaverageabsolutedifferenceof3.1mm/yearandstandarddeviationof1.8mm/year.

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RemoteSens. 2016 8 ,350 7of14 Figure3.ThemeanLOSdeformationvelocitymapoverthestudyareaduringtheperiodfromOctober2009toAugust2010.Theredstarrepresentsthelocationofthereferencepoint.WhiterectanglesindicateZone1,3and5wherelandsubsidencewasmainlycausedbyurbandevelopment.WhiteellipsesindicateZone2,4and6wherelandsubsidencewasmainlycausedbykarstgeology.ReddottedpolygonsindicateZone7and8wheretheupliftoccurs.ThisresultissuperimposedonaLandsat8image. 4.2.SubsidenceCausedbyUrbanDevelopment AsshowninFigure3,majorcommercialzonesinHankou,industrialzonesaroundWuhanIronandSteelCorp.andtheDonghuNewTechnologyDevelopmentZoneexhibitwideandcontinuouscoverageoflandsubsidence,whicharemostlikelyattributedtogroundwaterover-exploitationduetogrowingdemandforintensivemunicipalconstructionsandindustrialproductionintheseregions.Inparticular,adistinctsubsidencebowlwithamaximumsubsidencerateof67.3mm/yearisdetectedinthecentralurbanareainHankouZone1asshowinFigure3.Thus,weselectthissubsidingregionforamoredetailedanalysisoftherelationshipbetweenurbandevelopmentandresultinglandsubsidence.Figure4illustratesazoomeddeformationratemapofthisregionwhiterectangle1inFigure3.AserioussubsidencewasdetectedintheseareasinhighdensitiesofSDFPpixelsfromtheInSARresults.InZone1,extensiveurbanconstructionactivitieswereundertakenduringtheperiodofSARimageryacquisitions,typicallyincludingtheMetroLine2tunnelingaswellaslarge-scalebuildingconstructionssuchastheoutpatientbuildingofWuhanUnionHospitalandWuhanInternationalPlazaseeFigure4b.Furthermore,Figure4cpresentsthedeformationtimeseriescorrespondingtothreepixelslabelledinFigure4aasPointA,BandC,locatedonbuildingsnearthe

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RemoteSens. 2016 8 ,350 8of14WuhanUnionHospital,theWuhanInternationalConference&ExhibitionCenterandinthevicinityoflevellingbenchmarksintheXunlimenStationofMetroLine2,respectively.ThesubsidenceofPointBisalsotestiedbyaninsituphotographofthebuildingshowninFigure4d,andthesubsidencerateofPointCconsistswithmeasurementsoflevellingsurveysattheXunlimenStation14.7mm/yearvs. 11.6mm/year. Figure4.MeanLOSdeformationratemapsuperimposedonGoogleEarthimageoveraZone1;btheareahighlightedinFigure4abluebox,engineeringprojectsunderconstructionaremarkedwithlightgreenlines;cDisplacementtimeseriescorrespondingtotheSDFPpixelslabeledasPointA,PointBandPointCinFigure4a;dAphotographofstructuraldamageonWuhanInternationalConference&ExhibitionCentercausedbygroundsubsidence.Moreover,mostsubsidingareasofZone1asshowninFigure4aaresituatedatanalluvialplaneformedbysoftsedimentsoilsofYangtzeandHanriversaswellaslakes.Thealluvialdepositsinthisregionarecharacterizedbyahighlycompressiblelayerofsiltandclaytypicallywithadepthof5~30mbelowthegroundsurface[11],andthereforecaneasilyleadtoheterogeneoussubsidenceasaresultofexternalloadonthesoftsoilsduetointensiveconstructionofinfrastructureandbuildings.4.3.SubsidenceRelatedtoCarbonateKarsticationInFigure3,wecannoticeastrongspatialcorrelationbetweenlandsubsidencevariationsanddistributionsoftheknownkarstinventoryseeFigure1.SubsidingareasassociatedwithcarbonatekarstgeologyaremainlyidentiedinZone2,4and6seeFigure3whichareclosetotheYangtze

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RemoteSens. 2016 8 ,350 9of14RiverandlocatedinthreemajorcarbonaterockbeltsnamedTianxingzhou,Daqiao,andBaishazhou,respectively.Inthesecarbonatebelts,variouskarsttypesofsolutionssures,groovesandsinksaswellassmallcavesareextensivelydeveloped,wherethekarstcaveencounteringrateofboreholesisabout46.0%~50.1%,andabout30%ofkarstcavesareunlledorhalf-lled[43].Thisshallowcarbonatekarsticationcanprovideachannelforlossoftheuppersandysoilifhydraulicvariationsofgroundwaterfrequentlyoccurintheverticalandhorizontalkarstseepagezones[44].Inparticular,thereisaclosehydraulicconnectionbetweenthekarstsubsurfacewaterandtheYangtzeRiver,andconsequently,theactivewaterlevelvariationsoftheriver,especiallyduringwet/oodseasons[35],promotethedissolutionofcarbonaterocksandamplifykarstsubsidenceorcollapses.Figure5a,billustratethezoomeddeformationratemapsofZone2andZone4,tworepresentativekarst-impactedsitescorrespondingtoresidentialandindustrialcontexts,respectively.InFigure5a,heterogeneoussubsidencesignals5~15mm/yeararedetectedinZone2,whichincludesseveralurbanblocksnearYellowCraneTowersituatedonSheshanSnakeHill,oneofthemostrenownedculturaltowersinChina.Moderatesubsidencerecodedinthedeformationratemapisrestrictedtothewesternsectorofthekarstarea,withmaximumLOSdisplacementratesof10mm/year.Thissubsidingareahassufferedseverekarstcollapsesinrecentyears,triggeredbygroundwatervariationsduetobuildingconstructionintheneighboringJinduhangongcommunity[45].Someofthedamageobservedinthefacadesofthebuildingsandpavedsurfacesduringthecollapseeventin2011areillustratedinFigure5c,d. Figure5.MeanLOSdeformationvelocitymapsuperimposedonGoogleEarthimageoveraZone2andbZone4.Thelightgreenpolygonsrepresentthedistributionsofcarbonatebelts.ThesubsidenceregioninZone2isoutlinedbyareddashedline.TheredtrianglerepresentsthekarstsurfacecollapsethatoccurredinDecember2011.TheredcrossrepresentstheYellowCraneTower;c,dillustratethephotographsofstructuraldamagecausedbykarstcollapsesthatoccurredin2011.

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RemoteSens. 2016 8 ,350 10of14ComparingwiththeheterogeneoussubsidencepatternintheZone2,arelativelysignicantandspatiallycontinuousgroundsettlement,withLOSdeformationratesfrom10to30mm/year,canbefoundinZone4seeFigure5b.Thesekarstication-relatedsubsidingareas,locatedinamajorindustrialregionofWuhan,containsomeofproductionbasesofSINOPECWuhanCompanyandWuhanIronandSteelCorp.Wuhan,China.Therefore,wesuggestthatdissolutionofshallowcarbonaterocksisamajorcauseforlandsubsidenceintheZone4thatmightbecompoundedbypossiblegroundwaterover-extractionforindustrialproductiondemands. 5.DiscussionItisnotedthataremarkablegroundupliftphenomenonhasbeendetectedinbothbankareasofYangtzeRiver,especiallyalongthesouthernbankseeZone7inFigure3,withdeformationratesrangingfrom+5to+17.5mm/year.HerethisexceptionalupliftsignalishighlightedinazoomeddeformationvelocitymapinFigure6aforarepresentativesiteoftheZone7thatislocatedonthebanksectorbetweentheRiverandtheShaLake,andisfurtherillustratedinFigure6bbythedeformationtimeseriescorrespondingtotheSDFPpointslabelledinFigure6a. Figure6.aMeanLOSdeformationvelocitymapsuperimposedonGoogleEarthimageoverthebanksectorbetweentheYangtzeRiverandtheShaLake;bDisplacementtimeseriesrelevanttotheSDFPpixelslabeledasPointD,EandFinFigure6a vs .waterleveltimeseriesoftheYangtzeRiver.Duetoagoodspatialcorrelationbetweenthedeformationsignalsandtheriverlocations,themostlikelycauseforthegrounddeformationobservedintheseareasisgroundwaterdischargesandrechargesrelatedtoseasonaluctuationsinwaterlevelsoftheYangtzeRiver.Infact,thisstrongrelationshipbetweenmassmovementsofriverbankse.g.,grounddeformations,bankerosionsorlandslidesandhydrologicalprocessesinvolvingboththemagnitudeandfrequencyrainfalleventsandtheseasonaluctuationsofriverwaterlevelshavebeenemphasizedbypreviousstudiesinChina[46]andworldwide[4750].Consequently,comparingtheLOSdisplacementhistoriesofthesementionedSDFPpointsandthewaterlevelchangesofYangtzeRiverduringthestudyperiod,wecanobserveacoherenttemporalcorrelationofvariationsandamplitudesbetweenthemseeFigure6b.Forthisstudy,wenoticea5mdecreaseoftheriverwaterlevelmeasuredinthedryseasonbetweenautumn2009andspring2010,whichcorrespondstoameancumulativedeformationofabout10mmatthreeSDFPpoints.However,asignicantchangeofthedeformationdirectionoccurredaroundthelowestwaterstandinearlyMarch2010,andanacceleratingupliftfromthebeginningofthewetseasoninWuhanlasteduntil11August2010whenthemaximumvalueofcumulativeupliftreachedapproximately15mm,followingthepeakofwaterlevelsamountingto27.3mon30July2010.Inaddition,asimilardeformationbehaviorwiththesouthernbankoftheYangtzeRiverisfoundintheZone8nearHongshanSquareseeFigure3,whichissituatedinasectorofthecarbonaterockbeltnamedDaqiaobetweenYangtzeRiverandEastLake.Wesuggestthattheseasonaluctuationsof

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RemoteSens. 2016 8 ,350 11of14riverwaterlevelsmightpartiallycontributetogrounddeformationvariationsinthisarea.However,itisdifculttoprovideacomprehensiveexplanationforthisdeformationphenomenonduetorelativelycomplexhydrological-conditionsinthisarea,becausegroundwaterandporewaterpressuredataarenotavailableforthisstudy. 6.ConclusionsInthiswork,weappliedStaMPS-basedmulti-temporalInSARmethodologytohigh-resolutionTerraSAR-Ximagesfrom2009to2010todetectgrounddeformationinWuhan,andprovideddetailedinformationonspatiotemporalcharacterizationoflandsubsidenceandupliftinthisregion,forthersttime.TheresultsofaccuracyassessmentsuggestthattheInSAR-deriveddeformationratesagreedwellwiththelevellingsurveymeasurementswithanaverageabsolutedifferenceof3.1mm/yearandstandarddeviationof1.8mm/year.WefoundthatnoticeablelandsubsidenceoccurredwidelyinmajorurbanareasofWuhanandtheaveragemeasuredratesofsubsidencerangedfromapproximately5mm/yearto67.3mm/yearduringtheperiodofobservation.ThesesubsidingareasweremostlyconcentratedinsixcommercialandindustrialzonesinHankouandWuchangdistricts,aswellasthreekarstcarbonatebeltsinWuchangandQingshandistricts.Themaincontributingfactorstothedetectedlandsubsidenceincludeanthropogenicactivities,naturalcompactionofsoftsoil,andkarstdissolutionofsubsurfacecarbonaterocks.However,anthropogenicactivities,suchasintensivemunicipalconstructione.g.,buildingandsubwaytunnelingandindustrialproduction,havemoresignicantimpactsonthemeasuredsubsidenceratesthannaturalfactors.Moreover,remarkablysecularupliftsignalsweredetectedalongbothbanksoftheYangtzeRiver,especiallyalongthesouthernbank,withdeformationratesrangingmostlyfrom+5mm/yearto+17.5mm/year.AstrongtemporalcorrelationishighlightedbetweenthedetecteddisplacementtimeseriesandthewaterlevelrecordsoftheYangtzeRiver,suggestingthatthispreviouslyunknowndeformationphenomenoninWuhanislikelyrelatedtoseasonaluctuationsinthewaterlevelsoftheYangtzeRiver.Thisstudydemonstratesthepotentialofmulti-temporalInSARanalysisofhigh-resolutionSARdatasetsforgrounddeformationmonitoringinWuhan,whichischaracterizedbyvulnerablehydrologicalenvironmentsandcomplexdeformationregimes.TheInSAR-derivedresultsalsoindicateanurgentdemandforregularandlarge-scalemonitoringofdeformation-relevantgeohazardsinthisregion,whichcouldhelpnotonlytobettercharacterizethedevelopmentofcatastrophichazardsrelevanttogrounddeformationbuttorecognizepreviouslyunknowndeformationproblems,suchasinstabilityandcollapseofYangtzeRiverembankments.Inthenearfuture,moreacquisitionsofSARimagese.g.,TerraSAR-X,Sentinel-1andmoregroundobservations,especiallyhydrologicalandmeteorologicaldata,willbecollected,meaningafurtherjointanalysisofmultidisciplinarydatacouldbecarriedouttothoroughlystudysuchcomplicatedgrounddeformationphenomenainthisstudyarea. Acknowledgments:TheworkinthisstudywassupportedbytheNationalNaturalScienceFoundationofChinaNo.41590854,41274024,41431070,41321063,theNationalBasicResearchProgramofChinaNo.2012CB957702andtheHundredTalentsProgramofTheChineseAcademyofSciencesNo.Y205771077.TheauthorswouldliketothankDLRforprovidingtheTerraSAR-XimagesviatheTerraSAR-XAOprojectNo.MTH1827,TUDelftforprovidingtheDORISsoftware,HooperattheUniversityofLeedsforprovidingtheStaMPSsoftware,andLiGangattheChineseUniversityofHongKongforthehelpfuladviceinthedataprocessing. AuthorContributions:LinBaidesignedthestudyandwrotethemanuscript,LimingJiangsupervisedthestudyandreviewedthemanuscript,andHanshengWangandQishiSuncontributedtothediscussions. ConictsofInterest: Theauthorsdeclarenoconictofinterest. References 1.He,Q.;Ye,X.;Li,Z.;Liu,W.ThestatusandpreventionstrategyoflandsubsidenceinChina.Geol.J.ChinaUniv. 2006 12 ,161.InChinese

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RemoteSens. 2016 8 ,350 13of14 25.Kampes,B.M.DisplacementParameterEstimationUsingPermanentScattererInterferometry.Ph.D.Thesis,DelftUniversityofTechnology,Delft,TheNetherlands,2005. 26.Mora,O.;Mallorqui,J.J.;Broquetas,A.LinearandnonlinearterraindeformationmapsfromareducedsetofinterferometricSARimages. IEEETrans.Geosci.RemoteSens. 2003 41 ,2243.[CrossRef] 27.Lanari,R.;Mora,O.;Manunta,M.;Mallorqui,J.;Berardino,P.;Sansosti,E.Asmall-baselineapproachforinvestigatingdeformationsonfull-resolutiondifferentialSARinterferograms.IEEETrans.Geosci.RemoteSens.2004 42 ,1377.[CrossRef] 28.Schmidt,D.A.;Brgmann,R.Time-dependentlandupliftandsubsidenceintheSantaClaravalley,California,fromalargeinterferometricsyntheticapertureradardataset.J.Geophys.Res.SolidEarth2003,108,1.[CrossRef] 29.Perissin,D.;Wang,Z.;Lin,H.ShanghaisubwaytunnelsandhighwaysmonitoringthroughCosmo-SkyMedPersistentScatterers. ISPRSJ.Photogramm.RemoteSens. 2012 73 ,58.[CrossRef] 30.Zhao,Q.;Lin,H.;Jiang,L.;Chen,F.;Cheng,S.AstudyofgrounddeformationintheGuangzhouurbanareawithpersistentscattererinterferometry. Sensors 2009 9 ,503.[CrossRef][PubMed] 31.Heleno,S.I.;Oliveira,L.G.;Henriques,M.J.;Falco,A.P.;Lima,J.N.PersistentScatterersInterferometrydetectsandmeasuresgroundsubsidenceinLisbon. RemoteSens.Environ. 2011 115 ,2152.[CrossRef] 32.Osmanoglu,B.;Dixon,T.H.;Wdowinski,S.;Cabral-Cano,E.;Jiang,Y.MexicoCitysubsidenceobservedwithpersistentscattererInSAR. Int.J.Appl.EarthObs.Geoinform. 2011 13 ,1.[CrossRef] 33.Hooper,A.;Segall,P.;Zebker,H.Persistentscattererinterferometricsyntheticapertureradarforcrustaldeformationanalysis,withapplicationtoVolcnAlcedo,Galpagos.J.Geophys.Res.SolidEarth2007,112,1.[CrossRef] 34.Xu,Y.ResearchonBuoyancyofGroundwaterBasedonGeologicalConditionsofWuhan.Master'sThesis,WuhanUniversityofTechnology,Wuhan,China,2010.InChinese 35.Wu,Y.;Jiang,W.;Ye,H.KarstcollapsehazardassessmentsystemofWuhancitybasedonGIS.InProceedingsofthe2010InternationalSymposiuminPacicRim,Taipei,Taiwan,26April2010. 36.Luo,X.DivisionofSixBeltsandFiveTypesofcarbonateregionandcontrolofkarstgeologicaldisasterinWuhan. J.Hydraul.Eng. 2013 45 ,171.InChinese 37.Lian,B.;Hu,B.;Wang,X.;Liu,F.;Yu,H.MonitoringandnumericalanalysisonthefoundationpitexcavationforMingduStationofWuhansubway. J.YangtzeRiverSci.Res.Inst. 2014 31 ,34.InChinese 38.Ding,L.;Li,W.;Wu,X.;Zhou,C.AnalysisofmonitoringdeepfoundationpitforXunlimenStationofWuhanMetro. J.Railw.Eng.Soc. 2010 9 ,74.InChinese 39.Hooper,A.Amulti-temporalInSARmethodincorporatingbothpersistentscattererandsmallbaselineapproaches. Geophys.Res.Lett. 2008 35 .[CrossRef] 40.Hooper,A.;Zebker,H.A.PhaseunwrappinginthreedimensionswithapplicationtoInSARtimeseries.J.Opt.Soc.Am.A 2007 24 ,2737.[CrossRef] 41.Hooper,A.Astatistical-costapproachtounwrappingthephaseofInSARtimeseries.InProceedingofthe2010InternationalWorkshoponERSSARInterferometry,Frascati,Italy,30NovemberDcember2010. 42.Raucoules,D.;Bourgine,B.;DeMichele,M.ValidationandintercomparisonofPersistentScatterersInterferometry:PSIC4projectresults. J.Appl.Geophys. 2009 68 ,335.[CrossRef] 43.Luo,X.FeaturesoftheshallowkarstdevelopmentandcontrolofkarstcollapseinWuhan.Carsol.Sin.2013,4 ,419.InChinese 44.Gutirrez,F.;Parise,M.;DeWaele,J.;Jourde,H.Areviewonnaturalandhuman-inducedgeohazardsandimpactsinkarst. Earth-Sci.Rev. 2014 138 ,61.[CrossRef] 45.Zhong,Y.;Zhang,M.;Pan,L.RiskassessmentforurbankarstcollapseinWuchangDistrictofWuhanbasedonGIS. J.TianjinNorm.Univ.Nat.Sci.Ed. 2015 35 ,48.InChinese 46.Jian,W.;Xu,Q.;Yang,H.;Wang,F.MechanismandfailureprocessofQianjiangpinglandslideintheThreeGorgesReservoir,China. Environ.EarthSci. 2014 72 ,2999.[CrossRef] 47.Bnyai,L.;Mentes,G.;jvri,G.;Kovcs,M.;Czap,Z.RecurrentlandslidingofahighbankatDunaszekcso,Hungary:Geodeticdeformationmonitoringandniteelementmodeling.Geomorphology2014,210,1.[CrossRef] 48.Fox,G.A.;Wilson,G.Theroleofsubsurfaceowinhillslopeandstreambankerosion:Areview.SoilSci.Soc.Am.J. 2010 74 ,717.[CrossRef]

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RemoteSens. 2016 8 ,350 14of14 49.Rinaldi,M.;Casagli,N.;Dapporto,S.;Gargini,A.Monitoringandmodellingofporewaterpressurechangesandriverbankstabilityduringowevents. EarthSurf.Process.Landf. 2004 29 ,237.[CrossRef] 50.jvri,G.;Mentes,G.;Bnyai,L.;Kraft,J.;Gyimthy,A.;Kovcs,J.EvolutionofabankfailurealongtheRiverDanubeatDunaszekcso,Hungary. Geomorphology 2009 109 ,197.[CrossRef] 2016bytheauthors;licenseeMDPI,Basel,Switzerland.ThisarticleisanopenaccessarticledistributedunderthetermsandconditionsoftheCreativeCommonsAttributionCC-BYlicensehttp://creativecommons.org/licenses/by/4.0/.


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