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IEEE JOURNAL OF SELECTED TOPICS IN APPLIED EARTH OBSERVATIONS AND REMOTE SENSING 1

Time-Series InSAR ApplicationsOver Urban Areas in China

Daniele Perissin and Teng Wang

Abstract—In this study, we present the results achieved withinthe Dragon project, a cooperation program between the Eu-ropean Space Agency (ESA) and the National Remote SensingCenter of China (NRSCC), about monitoring subsidences andlandslides in urban areas, analyzing cities growth and measuringthe deformation of big man-made structures. Among the pro-cessed areas, we report here the main results we obtained inthe test sites of Shanghai, Tianjin, Badong, and Three GorgesDam. The techniques that have been used to process the dataare original SAR interferometry (InSAR), Permanent Scatterers(PS-InSAR), Quasi-PS InSAR (QPS-InSAR), and a combinationof coherent–uncoherent analysis. The results show that time-seriesInSAR techniques allow us to extract ground information withhigh spatial density and thus help us understanding the impact ofurban development on terrain movements.

Index Terms—Synthetic aperture radar interferometry(InSAR), Three Gorges Project (TGP), time-series analysis,urban development.

I. INTRODUCTION

A S THE largest developing country in the world, China hasbeen in an era of rapid urbanization since the 1980s. Gen-

erally speaking, urban areas in China are developing in two mainways: urban expansion and new city construction. For the firstmode, many metropolises are developing rapidly in both pop-ulation and extension. The increasing amount of skyscraperspress the alluvial plains along the river deltas in east China.Moreover, water resources are needed more and more, and un-derground water is overpumped in many developed areas. As aconsequence, in China, many cities have been suffering differentkinds of land subsidence since the end of the last century [1].

At the same time, together with huge infrastructures, manynew cities are being built. The Three Gorges Project (TGP) iswithout any doubt the most significant example in China: a damhas been built on the Yangtze river, increasing the water levelover 100 m, flooding huge areas and changing the face of the en-tire region facing the river. The dam and its surroundings formed

Manuscript received October 31, 2009; revised March 16, 2010. This workwas supported in part by the National Key Basic Research and DevelopmentProgram of China under Contract 2007CB714405 and Contract 2006CB701304and the Nature Science Foundation of China under Contract 40721001.

D. Perissin is with the Institute of Space and Earth Information Sci-ence, Chinese University of Hong Kong, Hong Kong, China (e-mail:[email protected]).

T. Wang is with the State Key Laboratory for Information Engineering inSurveying, Mapping and Remote Sensing, Wuhan University, Wuhan 430079,China, and also with the Dipartimento di Elettronica e Informazione, Politecnicodi Milano, 20133 Milan, Italy (e-mail: [email protected]).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/JSTARS.2010.2046883

an urban-like area themselves. Besides many artificial structuressuch as the dam body, navigation instruments, and power plants,many thousands of staff members and tourists are hosted withinthe dam site. Immigrants moved en masse toward the area, andmany cities have been rebuilt at higher locations. The high pres-sure of the reservoir on the riverbed and the water infiltration af-fection cause potential crust instability. Dam stability and land-slides over the cities along the Yangtze have to be monitoredregularly.

Usually, remote sensing of urban areas focuses on changedetection and land use mapping [2], and results represent therapid urban expansions, especially in developing countries likeChina [3]–[5]. However, impacts of urban development such assubsidence, are rarely studied in this field. Synthetic apertureradar interferometry (InSAR) allows us measuring the terrainmovement with radar observations acquired by satellites [6], [7].Main drawbacks of InSAR are geometric and temporal de-cor-relation [8], [9] as well as atmospheric disturbances that affectthe radar signal [10].

Aiming at solving the restrictions of InSAR, the PermanentScatterers technique (PS-InSAR) [11], [12] was invented anddeveloped at the Politecnico di Milano (POLIMI) in the late1990s. Instead of extracting information from the whole SARimage, PS-InSAR exploits long temporal series of acquisitionsto identify point-like stable reflectors (PSs). The electromag-netic stability of PSs allows obtaining around 1-m accuracyDEMs [13] and millimetric estimates of terrain motion [14]. Re-cently, by considering different interferometric combinations,the Quasi-PS (QPS) technique was presented [18] to extract in-formation also from distributed and decorrelating targets.

Since a good knowledge of the PS physical nature is a keystep for the correct interpretation of the measured deformationmechanism, the physical nature of the targets has been inves-tigated in [19]–[21]. The association of each coherent pixel toan actual target is achieved by exploiting all the measures withtime series of InSAR images. Since PSs are the stable artificialtargets, which can be usually found in urban areas, their lifetimeand density can be used to describe the urban development withspatial and temporal context. With a regular revisiting cycle, thebenefits that can be gained from time series of InSAR imagesare in two aspects: 1) since the PS is related to artificial targets,different PS birth time that represents the urban growth can beobtained with the ground displacement and 2) each PS can berelated to the physical characteristics of the single target.

The work presented here has been carried out within theDragon I and II cooperation projects between the EuropeanSpace Agency and the National Remote Sensing Center ofChina (ESA and NRSCC, respectively). In this framework, the“topographic measurement” group has been working on PS

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2 IEEE JOURNAL OF SELECTED TOPICS IN APPLIED EARTH OBSERVATIONS AND REMOTE SENSING

analysis in several urban test sites in China, getting ground de-formation maps of wide areas, till differential movements oversingle constructions. The time-series InSAR images used inthis paper were acqired by the ESA satellites, namely EuropeanRemote Sensing 1/2 (ERS-1/2) and Envisat. For the imagesfrom both satellites, the ground resolution is around 5 m inazimuth and 25 m in range; the bandwidths are around 16 MHz.The main difference between ERS and Envisat ERS-like ac-quisitions is the carrier frequency (5.3 GHz for ERS-1/2 and5.331 GHz for Envisat). Thanks to the increasing amount ofthe archived ESA SAR images, which allow us to measure therapid development of China since the 1990s, examples of urbandeformation monitoring are reported here.

II. TIME-SERIES INSAR ANALYSIS METHODS

Here, we give a very brief review of the core ideas of PSand QPS techniques, neglecting algorithm details. The inter-ested readers may refer to [11], [12], and [18] for more infor-mation.

The PS-InSAR technique first searches for pixels with highamplitude stability, called PS candidates (PSCs), from a longtime series of InSAR images. For each PSC, the different phasecontributions (dependent on elevation, motion, and atmosphericnoise) are separated, exploiting their different spatial and tem-poral characteristics. Atmospheric phase artefacts are then inter-polated for each image to produce the water vapor concentrationat the SAR acquisition time [11]. The final PSs are identified byevaluating the dispersion of phase residuals through the tem-poral coherence [12]. The precision of the measures dependson the coherence and on the deviation of normal and temporalbaselines.

In the PS analysis [11], the interferometric phase is gener-ated by referring all images to a common Master acquisition. Inorder to extract information also from partially coherent targets,various techniques have been developed, including those pre-sented in [15]–[18]. From its side, the QPS technique finds thebest set of image pairs by maximizing the interferometric coher-ence [22] of the graph connecting all images at hand. Since thesubset of coherent interferograms that carry information can bedifferent from point to point within the analyzed scene, a subsetof coherent interferograms for each point has to be considered.To this purpose, the absolute value of the spatial coherence ischosen as a weight in the estimation process, thus only coherentinterferograms for a given point cooperate to determine the re-sult. Moreover, the phase of the spatial coherence is used insteadof the unfiltered pixel phase to enhance the signal-to-noise ratio.Finally, like in the PS technique, topography and displacementcan be jointly estimated from the subset of filtered coherent in-terferograms. The QPS technique can be implemented by op-erating some relatively small changes in the classical PS pro-cessing chain [11], with considerable advantages in partially co-herent areas.

The QPS technique can be considered to be complementarywith the PS technique, and one will be preferred to the other ac-cording to constraints and requests of the case study at hand. InSection III, we show the results of the PS processing in the citiesof Shanghai and Tianjin. In Section IV, the slow landslide mon-itoring results in Badong and the deformation measures over the

Fig. 1. Locations of Shanghai and Tianjin, Badong, and Three Gorges Dam inChina. The map of China comes from the Chinese State Bureau of Surveyingand Mapping.

Three Gorges Dam are obtained with the QPS technique. In thispaper, we present the results together highlighting the poten-tiality for urban applications of the applied methodologies.

III. METROPOLIS DEVELOPMENT AND SUBSIDENCE

MONITORING

In China, the rate of population living in urban areas increasedfrom 18% to 45% in the last three decades [23]. The rapid devel-opment of Chinese urban areas causes environmental problemsand even geologic hazards [1]. In the following two sections,we present the time-series InSAR analysis results over two me-tropolises and two newly formed cities. The locations of thesefour test sites are shown in Fig. 1. All of these selected areasare very typical of both the urbanization and expansion of the1990s. Also, the selection of the test sites depends on the avail-ability of the archived data from ESA satellites.

A. Shanghai

Shanghai is the largest city in China and one of the largestmetropolitan areas in the world, with over 20 million people.At the beginning of the Dragon Project in 2004, Shanghai wasselected as the first PS analysis test site in China for studyingthe subsidence caused by underground water pumping and bythe rapid city development in the 1990s. Thanks to the archivedERS data of ESA, 40 images spanning the interval 1993–2000were processed. The results of the PS analysis allow detectingareas around the urban center of Shanghai. In this area, sometargets was subsiding with a linear trend of even more than 40mm per year. The PS measures were then compared with the de-formation map retrieved by means of leveling data in Shanghai,revealing quite good agreement.

Fig. 2 shows the most significant outcomes of the analysis.In Fig. 2(a), the geocoded linear deformation trend in Shanghaiis reported. Each point represents a permanent scatterer and thecolor scale (online version) indicates the average linear motion,


PERISSIN AND WANG: TIME-SERIES INSAR APPLICATIONS OVER URBAN AREAS IN CHINA 3

Fig. 2. Comparison between deformation rate measured by (a) the PS technique and (b) optical leveling, where the cumulating subsidence map is interpolatedfrom the leveling measured points in the urban area of Shanghai [24].

Fig. 3. Comparison between PS velocity field estimated with (a) ERS data and (b) Envisat data acquired from the parallel tracks 268 and 497. Black circles identifythe two areas with the highest subsidence rate.

spanning between 40–40 mm/year. In Fig. 2(b), the defor-mation rate interpolated from the optical leveling techniquesis shown [24]. The Yangtze River is indicated with the blackcurve for better understanding the comparison. The two mapsare pretty in accordance, highlighting the strongest motions neara branch of the mouth of the Yangtze River, with a maximumsubsidence rate of 40 mm/year. The displacement time seriesof ten leveling benchmarks have been compared with those ofthe PS closest to them. The standard deviation between the twomeasures is less than 2 mm/year [25].

The positive results obtained in Shanghai drew the interest tomonitoring its urban ground stability also after the year 2000.Considering the loss of gyroscopes of ERS-2 and the difficultyin connecting ERS and Envisat data together (not only for thedifferent carrier frequencies, but mostly because of the city de-velopment), the problem of carrying out a PS analysis with fewEnvisat images per track was tackled. The studies on the phys-ical nature of PSs in urban sites suggested then as solution theexploitation of multi-angle targets as dihedrals to combine dataacquired from parallel tracks [25]. In this way, it was possibleto double the number of data samples to estimate height and de-



Fig. 4. About 10 000 PSs detected in the Tianjin test site. (a) The birth date of PS (shown with different colors in the online version), indicating the developmentof the urban area. The colored (online version) contour lines corresponds to the average PS birth date. (b) The measured deformation trends. Color scale (onlineversion): average deformation trend (�30–30 mm/year).

formation trends of dihedrals. The developed method allowedupdating the subsidence monitoring in Shanghai. The results,which identify the same sinking areas of the ERS analysis, re-veal a general decrease of deformation rate.

Fig. 3 brings the main achievements of the study over a lim-ited processed area. In Fig. 3(a), the deformation map estimatedwith ERS data along a descending track (n. 3) is shown in ge-ographical coordinates. In Fig. 3(b), the same area has beenprocessed using Envisat data taken from two ascending paralleltracks (n. 268 and 497), with 14 and 12 available images, respec-tively. As is visible from Fig. 3, the number of detected targetsis strongly reduced than in a classical analysis, since only dihe-dral-like scatterers are coherently imaged. Nonetheless, a rea-sonable agreement is found between the subsiding areas in thetwo datasets (the two reported circles). Moreover, in the rightcircle, it can be noted that the average rate is lower in the mapretrieved with parallel tracks than in the other one.

B. Tianjin

Along with the successful application of the PS-InSAR tech-nique in Shanghai, Tianjin was selected as other testbed. As thethird directly controlled municipality in China, Tianjin devel-oped rapidly in the 1990s and suffered subsidence problemsdue to underground water overextraction and coal mining. Inthis test site, besides estimating the PS height and deformationtrends, an investigation on the urban targets physical nature hasbeen carried out by exploiting 23 ERS images. By analyzing theamplitude history of SAR images, birth and death days of PSswere also extracted to recognize the growth pattern of the city.The subsiding area was detected being confined to the surround-ings of the city center [26]. This is a change detection techniquethat analyzes amplitude time series rather than couple of im-ages. Moreover, taking into account the targets scattering pat-tern, eventual intensity fluctuations due to the acquisition ge-



Fig. 5. History of the different phases of the TGP. Blue line (online version): water level of the Yangtze river. Red (blue) stems (online version): ERS (Envisat)data acquired over the dam.

ometry are compensated for. The consequence is a much higherreliability and robustness of the estimate.

From Fig. 4(a), it is possible to notice the still rural characterof the suburban area of Tianjin at the time of the acquisitionsby looking at the density of PS. The arrows indicate the maindeveloping directions of Tianjin in the 1990s. By calculatingthe average birth date on a regular grid, we can find a generaltrend increasing with the distance from the city center. The con-tour lines with different average birth date describe the trend weobtained from time-series InSAR analysis. On the other hand,the center of the urban area appears to be mostly stable, whilealong the river and, in particular, in the lower right corner of theimage the highest concentration of sinking targets lies as shownin Fig. 4(b). The analysis in Tianjin was performed with a verylow number of images and even more with a sparse temporalsampling. Notwithstanding, the outcome of the work is partic-ularly meaningful and highlights the motions affecting the im-aged terrain.

IV. SLOW LANDSLIDES AND DAM STABILITY MONITORING

OVER THE THREE GORGES REGION

Three Gorges is an overall name given for a series of gorgesfrom Fengjie, Chongqing municipality, to Yichang, Hubeiprovince, along the Yangtze river in China. From west to east,the three most famous gorges are QuTang Gorge, Wu Gorge,and XiLing Gorge. Since Yangtze is a seasonal river, floodshappen each raining season (from May to October); in thedownriver plains, for example, Jianghan plain is endangeredalmost every five years. Actually, the most important objectivefor constructing TGP is to control the seasonal floods of theYangtze by releasing or storing the water. In the meantime,clean electricity power can be generalized by the different waterlevels up- and down-river of the dam.

In Fig. 5, the history of the phases of the TGP is diagrammedtogether with the height level of the Yangtze river. Red and bluestems (online version) are plotted in correspondence of the ac-quisition dates of ERS and Envisat images, respectively. The 40Envisat images used in the following discussions fall in the thirdphase of the project. It is worth noticing that, in the middle of2006, the construction of all of the power plants were finishedand the water level was increased around 20 m to generalizemore hydropower.

A. Badong

Badong is settled on the river side of the Yangtze River, inHubei province. It is located just between the Wu and XilingGorges. The old town of Badong has a history of more than1500 years. Unfortunately, due to the construction of TGP, thistown was going to be under the rising water and the emigra-tion of the whole town was organized in the summer of 1997.Almost all buildings of the old town were demolished. Mean-while, a new town that covers about 7.3 km with more than50 000 residences began to form in 2002. Since ERS and En-visat data were acquired before and after the construction of thedam, the changes of the river in this area can be observed fromthe reflectivity maps of the data sets of two sensors. Fig. 6 showsthe incoherent average reflectivity maps of (a) ERS and (b) En-visat data. The Yangtze river can be seen as much wider in theEnvisat image, and the urban changes due to the population mi-gration are identified by the red rectangle.

Since the town was built along the steep river banks, landslidemonitoring is a very important task for the whole area. Actually,many consolidation works were settled for landslide protectionin Badong. As shown in Fig. 7, two subsidence regions in thesouth river bank of Badong city are identified by the QPS tech-nique. One is in the west part of the city, about 400 m above theYangtze river. Another region is in the east part of the city nearthe river. With the achieved QPS density of measures, the bor-ders of residential areas are easy to be identified. Moreover, thearea in the south east of the city is also found being affected byterrain instability.

B. Dam Site

The Three Gorges Dam is located near San Dou Ping townin the east exit of the Three Gorges region. The main structureof the TGP consists of three parts: the dam, hydrological gener-ator plants and navigation instruments. The body of the dam is2355 m long and 185 m high. The bottom and top of the dam arerespectively 115 and 40 m wide. The spillway is in the middleof the dam where the main river canal was. The sluice gatesinstalled on the bottom of the dam are used to control the ca-pability of the upstream reservoir and wash the alluvial sandsto the downstream area. The power plant houses, flanking thespillway (left and right riverbank), accommodate altogether 26sets of turbine generators.



Fig. 6. Reflectivity maps of Track 75 Frame 2979 from (a) ERS images and (b) Envisat images. The red rectangle (online version) indicates the new city ofBadong, which is brighter in the Envisat image because of the urbanization. The Yangtze river is much wider in the Envisat image because of the TGP.

Fig. 7. Deformation trends obtained from QPS analysis. Two slow landslidsareas are identified.

By looking at the reflectivity map of the analyzed area inFig. 8, the structure of the dam crossing the river can be easilyrecognized. Four matrices are visible, reporting the amplitude

behavior of four scatterers in the whole SAR dataset. Each pixelof a matrix identifies a pair of images. Blue (online version) in-dicates that the amplitude of the scatterer is similar in the twoimages, and yellow and red (online version) mean that a changehappened. Thus, from the amplitude matrix of the ship lift, itis possible to recognize the two states of the lift, up and down,that change the backscattered signal. The left power plant in thereflectivity map is the north plant and was built before 2002.The amplitude matrix in Fig. 8 of a scatterer in the north sideshows a constant pattern in the processed time span. The rightpart of the dam was under construction, the third matrix showsthe birthday of the target in the construction site, and the fourthmatrix shows a target on the cofferdam, which was dismantledin 2006.

The elevation and deformation trends over the dam site are es-timated with the QPS technique and shown in Fig. 9(a) and (b).Since we cannot find any published topographic map in thisarea, the validation of the results are carried out by comparingthe results on some known locations: 1) the tail water platformof the left part of the dam is 83.5 m; 2) the top of the left planthouse is 117.2 m; 3) the top of the dam is 185 m; and 4) thespillway weir is 158 m. As shown in Fig. 9(a), the estimatedelevation from QPS technique fit close to the ground truth. Inaddition, since the downriver part of the dam faces the satelliteLOS, the elevation of the left dam can be represented as threeparts: the tail water platform, the top of the plant house, and thetop of the dam. The lifting equipment on the top of the dam showabout 200 m elevation. Near the permanent ship locks, although



Fig. 8. Reflectivity map of the Three Gorges dam and different scatterer’s signatures derived from the amplitude of SAR images. In each matrix, a pixel identifiesa couple of images. Blue (online version) means no change of amplitude among the couple and yellow and red (online version) mean progressively increasingchanges. The amplitude series for each pixel was shown as the matrix diagonal.

Fig. 9. (a) The measured elevation and (b) deformation trends over the dam site. The numbers on (a) indicate: 1) the tail water platform of the left part of the dam;2) the top of the left plant house; 3) the top of the dam; and 4) the top of the spillway.

not that many QPS points are present, the five levels of the shiplocks can be observed.

From the results shown in Fig. 9(b), over the dam, only slightdeformation trends can be observed. In other words, the Three



Gorges dam is quite stable during the time span of our data set.From the obtained deformation trends, on the top of the leftpart of the dam, the slight deformation trends are apart fromthe satellite, on the contrary, the power plant on the bottom ofthe dam shows the deformation trends towards the satellite. Wepresume that the dam declined slightly on account of the upriverwater pressure over the riverbed crust.

In the dam surroundings, we detected several subsidenceareas in the left riverbank, especially along the axis of the damto the area between the ship lift and permanent ship locks. Wepresume that the construction of the navigation establishmentschanged the distribution of the underground water and causedsuperficial subsidence. In addition, several slight subsidenceareas can be observed near the Yangtze River. Synthetically,superficial subsidence often happens near the water area ofYangtze, the maximum subsidence appears on the upriver em-bankment near Zigui county. The highest subsidence velocityis over 10 mm/year.

V. CONCLUSION

In this paper, we have shown some results obtained within thecooperation Dragon program for what concerns urban terrainmotion monitoring. PS and QPS techniques allow measuring theaverage deformation trend of man-made structures, revealingsubsidences or the stability of single buildings. The validationwork have been carried out in two aspects: 1) the Shanghai sub-sidence monitoring results are cross-validated with the opticalleveling measures and 2) the elevation measured by the QPStechnique is validated with the well-known points on the Dam.The present results show the potentiality of time-series anal-ysis of InSAR images for urban area monitoring in China, andfor example, the time-series InSAR images and techniques havebeen successfully applied in the Shanghai Institute for GeologySurvey for monitoring the subsidence over this city annually. Asfuture work, we are also contacting relative local governmentand institutes for collecting deformation measures from othertechniques (e.g., leveling and GPS) to validate and enhance ourresults. Moreover, the quality and quantity of information thatcan be extracted with such techniques from -band data arousesgreat expectation towards the future availability of time series of

-band images as taken by TerraSAR-X or Cosmo SkyMed.

ACKNOWLEDGMENT

The authors are very thankful to ESA for the Envisat and ERSdata provided under the Dragon project and to T.R.E. TeleRil-evamento Europa for focusing and registering SAR data. Theauthors would also like to thank the SAR group of the Elec-tronic and Information Department of the Politecnico di Milano,Milan, Italy, lead by Prof. Rocca and Prof. Prati.

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[28] T. Wang, D. Perssin, F. Rocca, and M. Liao, “Three Gorges Dam sta-bility monitoring with time series InSAR analysis,” Science in ChinaSeries D: Earth Sciences, accepted for publication.

Daniele Perissin was born in Milan, Italy, in 1977.He received the M.S. degree in telecommunicationsengineering and the Ph.D. degree in information tech-nology from Politecnico di Milano, Milan, in 2002and 2006, respectively.

He joined the Signal Processing Research Group,Politecnico di Milano, in 2002 and, since then, he hasbeen working on the Permanent Scatterers technique(PSInSAR) in the framework of radar remote sensing.Since October 2009, he has been a Research AssistantProfessor with the Institute of Space and Earth Infor-

mation Science (ISEIS), Chinese University of Hong Kong. He is the author ofa patent on the use of urban dihedral reflectors for combining multisensor syn-thetic aperture radar data, which is his main interest.

Teng Wang received the M.Eng. degree in pho-togrammetry and remote sensing from WuhanUniversity, Wuhan, China, in 2006, and is cur-rently working toward the Ph.D. degree in SARinteferometry fields at the State Key Laboratory forInformation Engineering in Surveying, Mapping andRemote Sensing, Wuhan University and the Dipar-timento di Elettronica e Informazion, Politecnico diMilano, Milan, Italy.

His current research interests include the InSARand Persistent Scatterer InSAR techniques in moun-

tainous areas.

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