sensors Article A Decade of Ground Deformation in Kunming (China) Revealed by Multi-Temporal Synthetic Aperture Radar Interferometry (InSAR) Technique Wu Zhu 1,* , Wen-Liang Li 1, Qin Zhang 1, Yi Yang 2, Yan Zhang 1, Wei Qu 1 and Chi-Sheng Wang 3,4 1 College of Geology Engineering and Geomatics, Chang’an University, Xi’an 710064, China; [email protected] (W.-L.L.); [email protected] (Q.Z.); [email protected] (Y.Z.); [email protected] (W.Q.) 2 Yunnan Institute of Surveying and Mapping Engineering, Kunming 650033, China; [email protected] 3 The Guangdong Key Laboratory of Urban Informatics, School of Architecture & Urban Planning, Shenzhen University, Shenzhen 518060, China; [email protected] 4 The Key Laboratory of Urban Land Resources Monitoring and Simulation, Ministry of Land and Resources, Beijing 100812, China * Correspondence: [email protected]; Tel./Fax: +86-29-82339261 Received: 18 July 2019; Accepted: 9 October 2019; Published: 11 October 2019 Abstract: Large-scale urbanization has brought about severe ground subsidence in Kunming (China), threatening the stability of urban infrastructure. Mapping of the spatiotemporal variations of ground deformation is urgently needed, along with summarization of the causes of the subsidence over Kunming with the purpose of disaster prevention and mitigation. In this study, for the first time, a multi-temporal interferometric synthetic aperture radar (InSAR) technique with L-band Advanced Land Observation Satellite (ALOS-1) and X-band Constellation of Small Satellites for Mediterranean basin Observation (COSMO-SkyMed) data was applied to Kunming to derive the time series deformation from 2007 to 2016. The annual deformation velocity revealed two severe subsiding regions in Kunming, with a maximum subsidence of 35 mm/y. The comparison of the deformation between InSAR and leveling showed root-mean-square error (RMSE) values of about 4.5 mm for the L-band and 3.7 mm for the X-band, indicating that our results were reliable. We also found that the L-band illustrated a larger amount of subsidence than the X-band in the tested regions. This difference was mainly caused by the different synthetic aperture radar (SAR)-acquired times and imaging geometries between the L- and X-band SAR images. The vertical time series deformation over two severe subsiding regions presented an approximate linear variation with time, where the cumulative subsidence reached 209 mm during the period of 2007–2016. In view of relevant analyses, we found that the subsidence in Kunming was the result of soft soil consolidation, building load, and groundwater extraction. Our results may provide scientific evidence regarding the sound management of urban construction to mitigate potential damage to infrastructure and the environment. Keywords: ground deformation; multi-temporal InSAR; Kunming; urbanization 1. Introduction Kunming, the capital and largest city of Yunnan province in Southwest China, is located at a longitude of 102◦100–103◦400 east and a latitude of 24◦230–26◦220 north. Situated in the central Yunnan–Guizhou plateau in Southwest China, Kunming is surrounded by the Dian Lake to the south and by mountains on the other three sides [1]. As China’s frontier to Southeast Asia and South Sensors 2019, 19, 4425; doi:10.3390/s19204425 www.mdpi.com/journal/sensors Sensors 2019, 19, x FOR PEER REVIEW 2 of 31 Sensors 2019, 19, 4425 2 of 29 and by mountains on the other three sides [1]. As China’s frontier to Southeast Asia and South Asia, largeAsia,-scale large-scale urban urbanconstruction construction has been has implemented been implemented in Kunming in Kunming over the over past thetwenty past years. twenty During years. thisDuring procedure, this procedure, a great amou a greatnt of amount cultivated of cultivated land and gardens land and were gardens occupied were by occupied high-rise by buildings, high-rise roads,buildings, and roads,other structures and other structures[2]. However, [2]. this However, large-scale this large-scale urbanization urbanization brought about brought a series about of a geologicalseries of geological hazards, hazards, such as such landslides, as landslides, subsidence, subsidence, and andground ground fissures. fissures. Particularly, Particularly, ground ground subsidencesubsidence became became one one of of the the most most prominent prominent geological geological hazards hazards and and caused caused substantial substantial damage damage to to homes,homes, roads, roads, canals, canals, pipelines, pipelines, and and other other infrastructure infrastructure [ 3].3]. Figure Figure 1 shows some photos of the effects e ffects ofof subsidence subsidence in in Kunming, Kunming, where where more more than than fifty fifty individual individual damage damage reports reports were were observed observed through through filedfiled investigations investigations organized inin JuneJune 20162016 [[4].4]. ToTo prevent prevent and and mitigate mitigate future future disasters, disasters, it it is is urgent urgent to tocharacterize characterize and and monitor monitor the the spatiotemporal spatiotemporal variations variations ofof groundground deformationdeformation and summarize the the cacausesuses of of subsidence subsidence in in Kunming. Kunming. FigureFigure 1. Ground 1. Ground deformation deformation investigated investigated in in Kunming Kunming. and (a) the the uplift effects of buildingon buildings steps, and (b) ground roads [4]. crack, (c) ground subsidence, (d) wall crack caused by the groud deformation. Early studies initiated a few years ago using ground-based techniques (e.g., leveling) documentedEarly studies the extent initiated and a causes few years of subsidence ago usingground-based in Kunming [5 techniques,6]. According (e.g., to leveling) their observations, documented thethe subsidence extent and causeswas mainly of subsidence focused inin Kunming two regions, [5,6]. where According the subsiding to their observations, rate reached the 30 subsidencemm/y [5]. Consideringwas mainly focusedlocal geological in two regions, conditions where and the human subsiding activities, rate reached it was reported 30 mm/y that [5]. the Considering subsidence local in Kunminggeological was conditions mainly andcaused human by groundwater activities, it was extraction, reported building that the subsidenceload, and natural in Kunming consolidation was mainly of soilcaused [7]. byAlthough groundwater these extraction,observations building provided load, useful and natural information, consolidation they had of low soil [spatial7]. Although resolution, these andobservations therefore, provided more usefuldetailed information, and comprehensive they had low ground spatial resolution, deformation and therefore,was difficult more detailedto see. Interferometricand comprehensive synthetic ground aperture deformation radar was(InSAR) difficult has to demonstrated see. Interferometric its potential synthetic for aperture high-density radar spatial(InSAR) mapping has demonstrated of ground deformation its potential forassociated high-density with earthquakes spatial mapping [8], volcanoes of ground [9], deformation and other geologicassociated processes. with earthquakes Recent adva [8], volcanoesnced InSAR [9], techniques and other geologichave improved processes. our Recent understanding advanced InSARof the processtechniques of haveground improved deformation, our understanding such as the ofdeep the processlearning of groundapproach deformation, [10], artificial such intelligence as the deep techniquelearning approach [11], optimal [10], artificialphase unwrapping intelligence algorithm technique [[12],11], optimalsignal retrieval phase unwrapping for decorrelating algorithm targets [12], [13],signal four retrieval-dimensional for decorrelating filtering approach targets [ 13[14],], four-dimensional improved synthetic filtering aperture approach radar [ 14(SAR)], improved image coregistrationsynthetic aperture algorithm radar [15], (SAR) atmospheric image coregistration delay correction algorithm method [15 [16],], atmospheric and coherent delay point correction selection algorithmmethod [16 [17].], and Additionally, coherent point an increasing selection algorithmamount of [SAR17]. Additionally,satellites have an provided increasing a large amount set of of multiSAR- satellitessensor SAR have images, provided which a large have set been of multi-sensoremployed to SARreconstruct images, the which spatiotemporal have been employedevolution of to groundreconstruct deformation the spatiotemporal [18]. In this evolution field, current of ground research deformation is focused [ 18on]. three In this aspects. field, currentThe first research aspect is is applyingfocused on the three multi aspects.-sensor The InSAR first to aspect derive is applyingthe long- theterm multi-sensor ground deformation, InSAR to derive such theas in long-term Tehran (Iran)ground [19 deformation,], Xi’an (China) such [20], as Baja in Tehran California (Iran) (Mexico) [19], Xi’an [21], (China)Urayasu [ 20(Japan)], Baja [22], California the Italian (Mexico) Peninsula [21], [23],Urayasu Istanbul (Japan) (Turkey) [22], the [24], Italian and PeninsulaSemarang [ 23(Indonesia)], Istanbul [25]. (Turkey) The [sec24],ond and aspect Semarang is resolving (Indonesia) two- [ 25or]. three-dimensional ground motion through the multi-sensor SAR images merging technique, which Sensors 2019, 19, 4425 3 of 29 Sensors 2019, 19, x FOR PEER REVIEW 3 of 31 Thehas secondbeen aspectused in is resolvingan urban two- subsiding