2. Canada Centre for Mapping and Earth Observation, Natural Resources, Ottawa ON K1A0E4, Canada
Ground deformation (including vertical and horizontal displacements) is prone to occur in many mega cities located in river deltas characterized by flat terrain and soft sediments. Driven by geological and anthropogenic factors, it causes land subsidence and ground rupture. Severe ground deformation would produce undesirable environmental and economic impacts, including damages of buildings, railways, highways, subways and underground facilities (e.g., water supply pipes, gas, electricity installations), affecting half a billion people living on or near it in the world (Wang et al., 2017; Dong et al., 2014; Syvitski et al., 2009). Therefore, monitoring and measurement of ground deformation in two-dimensions (both vertical and horizontal displacement) in order to fully assess the risks and make early warnings, especially in mega cities, gain great attention all over the world.
Shanghai is among those mega cities, which is a well- known subsidence zone and experienced the most severe land subsidence on record in China by excessive groundwater withdrawal and rapid industrialization (Ye et al., 2016; Dong et al., 2014). Land subsidence in Shanghai was firstly reported in the early 1960s. The fast land subsidence rate reaching 83 mm/year was witnessed during 1956–1965 associated with rapid industrialization and population growth, which demanded large amounts of groundwater. After that, a series of restrictive groundwater pumping prohibitions were released, the groundwater level rebounded and the land subsidence rate decreased. Fortunately, land subsidence has been controlled and ground ruptures have not been observed in Shanghai so far. But other mega cities, like Suzhou, Wuxi and Changzhou in South Jiangsu Province, which are located adjacent to Shanghai and have similar subsidence history to Shanghai have already witnessed ground fissures and cracks in the past years. Figure 1 (photographed by Meng Zhu) showed evidences of ground fissures in Wuxi City. Besides, Shanghai has experienced a rapid urbanization in the recent decades, resulting in dense high-rise buildings, sophisticated traffic network (metro lines, underground tunnels, railways, outer rings, etc.) and over 20 million populations. It has already grown into China's largest and most developed city. The ground deformation would raise even more damage to the surface and subsurface infrastructures, as well as human beings than before. Thus it is of vital importance to continuously monitor and measure ground deformation, especially the horizontal displacement in order to fully assess the potential risks and to make early warnings so that appropriate mitigation measurements can be taken to prevent further damages (Luo et al., 2016; Ye et al., 2016; Duzgun et al., 2011; López-Quiroz et al., 2009; Wu et al., 2008).
Although Shanghai has established extensive land subsidence monitoring networks since mid-1980s. The networks (including GPS, extensometer and leveling) can measure three-dimensional ground motions at a subsiding site. Due to its high costs and low spatial resolution, it cannot meet the requirements for detecting and mapping vertical and horizontal ground deformation at continuous spatial coverage and high spatial resolution over a large area (Samsonov et al., 2014a). In the past decades, InSAR has been proved to be a powerful technique to map ground deformation of natural and anthropogenic causes with high resolution and sub-centimeter precision over large areas (Rao et al., 2017; Xu et al., 2016; Chaussard et al., 2014; Samsonov et al., 2014b, 2011, 2010; Dong et al., 2013; Kusky et al., 2010; Wright et al., 2004). To overcome the effects of temporal and spatial decorrelation and atmospheric perturbations, small baseline subset (SBAS) methodology, which can produce time-series of the line-of-sight deformation from a single set of InSAR observation established by Berardino et al. (2002) and Usai (2003). However, only one component of the ground deformation along the satellite's line of sight (LOS) can be measured with the InSAR technique (Zhu et al., 2014; Samsonov et al., 2013; Wright et al., 2004; Fialko et al., 2001). Multidimensional small baseline subset (MSBAS) technique was developed to address this limitation, which is based on SBAS method and modified to allow direct integration of multiple DInSAR datasets and decompose LOS DInSAR measurements into vertical and horizontal ground deformation when two or more InSAR datasets with overlap temporal and spatial coverage are available (Samsonov et al., 2013; Samsonov and d'Oreye, 2012). This method has been already successfully applied in retrieval of 2D ground deformation in the Greater Luxembourgh region caused by coal mining activities (Samsonov et al., 2013), in the Bologna region of Italy (Samsonov et al., 2014b) caused by groundwater-extraction and Campi Flegrei caused by volcano eruption (Samsonov et al., 2014c). Many other attempts have also been conducted to study 3D deformation based on InSAR technology (Liu et al., 2012; Wright et al., 2004; Rocca, 2003). For example, Liu et al. (2012) developed a multi-platform PSI method to extract the three-dimensional ground deformation during 2007–2010 in northwestern part of Tianjin (China). However, this method required more than 30 SAR images acquired from different satellite platforms or tracks over the research area. Such large amounts of images are difficult to guarantee in most of the study, which made this method hard to be adopted widely.
Many previous studies have monitored and measured land subsidence in Shanghai with InSAR and other techniques (Ye et al., 2016; Dong et al., 2014; Perissin et al., 2012; Wu et al., 2012; Zhang et al., 2012; Shi et al., 2008). However, a couple of limitations still remain. Firstly, investigations of ground deformation in the whole Shanghai area occurring in recent years have not been discussed yet. Restriction of groundwater pumping, rapid urbanization and geotectonic might affect the spatio-temporal process and pattern of ground deformation in Shanghai recently. Secondly, almost all of previous researches were only focused on the vertical deformation in Shanghai while little attention was drawn on the horizontal deformation. Among them, only Ye et al. (2016) developed a three-dimensional numerical model to simulate 3D groundwater flow and aquifer-system displacements in Shanghai Downtown area during 1979–1995. Yet, the horizontal ground deformation over a larger area of Shanghai in the past ten years has still not been clearly understood, which hinders the fully understanding of the entire ground deformation field. Thus, accurately mapping and analysis of 2D ground deformation in order to understand the spatio-temporal ground deformation features in recent years and reveal their driving forces are critically demanded for the geo-hazard early warning and sustainable urbanization development in Shanghai.
Therefore in this paper, we adopt both SBAS and MSBAS methodologies for mapping recent ground deformation in most of the Shanghai region and producing two-dimensional ground deformation field in some specific region of Shanghai with two frames of Radarsat-2 InSAR datasets.1 STUDY AREA
Shanghai is situated in the easternmost of Yangtze Delta and at the estuary of the Yangtze River. It is composed of a mainland zone and three islands (Chongming, Hengsha and Changxing islands) in the Yangtze River with a total area around 6 800 km2. It is bounded by the China East Sea to the east, Jiangsu Province to the northwest and Zhejiang Province to the southwest. The geographic extend of Shanghai is marked by longitudes 120°52'E–122°12'E and latitudes 30°40'N–31°53'N.
The topography of Shanghai is flat with an average elevation of 4 m above sea level, due to its location in the Lower Yangtze River Alluvial Plain (Wu et al., 2008; Damoah-Afari 2006). Quaternary unconsolidated sediments (including clay, silt, sandy clay and sand) underlying the alluvial deposits with 200–350 m thick are widely distributed in Shanghai (Zhang and Liu, 2001).
The groundwater withdrawal, compaction of thick Quaternary unconsolidated sediments and rapid urbanization have been considered as the causes of the occurrence of ground subsidence and subsequent soil fissuring in this region (Ge et al., 2017; Ye et al., 2016; Dong et al., 2014; Shi et al., 2008; Zhang and Liu, 2001).2 DATA PROCESSING
In this paper, we use the SBAS and multidimensional SBAS (MSBAS) methods to derive the vertical and horizontal deformation of Shanghai respectively. The SBAS method is proposed by Berardino et al. (2002) and implemented by Samsonov et al. (2011), while the multidimensional SBAS (MSBAS) method is based on the small baseline method (SBAS) and developed by Samsonov et al. (2013). Detailed discussion of SBAS and MSBAS methods and their implementation can be found in Samsonov et al.(2013, 2012, 2011) and Berardino et al. (2002).
For investigation of spatio-temporal evolution of ground deformation in Shanghai recently, we collected twenty-nine Multi-Look Fine 6 (MF6) Radarsat-2 SLC data acquired during March 27, 2011–December 29, 2013, with 50 km×50 km coverage and 2.66 m×2.68 m resolution. For detection and analysis of vertical and horizontal displacement in Shanghai area, we collected the following Radarsat-2 data: (1) Six descending Multi-Look Fine 6 (MF6) spanning from April 11, 2008 to August 9, 2008 with 50 km×50 km coverage and 2.66 m×2.68 m resolution; and (2) four ascending Multi-Look Fine 2 (MF2) spanning April 27 to August 1, 2008 with 50 km×50 km coverage and 2.66 m×3.01 m resolution. The coverage of SAR data and satellite acquisition parameters are presented in Fig. 2 and Table 1.
Standard interferometric processing was conducted to MF6 and MF2 datasets independently with GAMMA software (Wegmuller and Werner, 1997) as follows: (1) A single image was selected in each dataset as the master image and all the slave images were co-registered to that master. Multilook 7 and 10 for MF6 and MF2 images respectively were applied; (2) interferograms with perpendicular baseline less than 600 m and temporal baseline less than 300 days were generated; (3) the 30 m ASTER DEM data were used to remove the ellipsoidal earth and topographic phase; (4) adaptive filtering (Goldstein and Werner, 1998) was used to filter the interferograms; (5) minimum cost flow (MCF) algorithm (Costantini et al., 1999) were used to unwrap the interferograms; (6) interpolation of small incoherent gaps in each interferogram were filled using a maximum interpolation window with radius of 16 pixels and 16 coherent pixels (Samsonov et al., 2013, 2011; Berardino et al., 2002). The interpolated interferograms were geocoded to the DEM grid.
In total, 195 interferograms from MF6 were produced for spatio-temporal ground deformation inversion, and 15 interferograms from MF6 and 6 interferograms from MF2 were produced for the calculation of two dimensional ground deformation. We analyzed the mean coherence for each interferogram and selected only those interferograms that had a mean coherence (after filtering) above 0.6 for further processing.
Firstly, the SBAS method was applied to those interferograms generated by MF6 during 2011–2013 and derived the vertical components of deformation. Secondly, the MSBAS processing was applied to those interferograms generated by MF6 and MF2N during 2008, limiting to a sub-region which was resampled to a common latitude/longitude grid and outlined in blue (Fig. 2) for the calculation of vertical and east-west components of ground deformation. The workflow diagrams of SBAS and MSBAS processing were presented in Fig. 3.3 RESULTS
The vertical and east-west components of ground deformation over interested area are shown in Figs. 4, 5, and 6. For a better visualization, we clipped largest value of vertical and east-west components of deformation rate to 1 and 2 cm/year respectively, while the actual values at some very small locations might reach to 3–4 cm/year.3.1 Vertical Ground Deformation Occurring in 2011–2013
It can be seen in Fig. 4 that most of the area over Shanghai were stable during the observation period. A very slight subsidence around 0.5–1 cm/year was found in some regions. These regions were detected as fast subsidence sections with deformation rate varying around 1–3 cm/year (Dong et al., 2014), while the subsidence rate was obviously decreased during 2011–2013, indicating that the prohibition of groundwater exploitation in the city had significant effect on mitigation of the land subsidence.
The spatio-temporal evolution of the vertical component of the cumulative deformation from March 2011 is shown in Fig. 5. It was observed from Fig. 4 and Fig. 5 that most of the downtown area remain stable during the whole observation period with a deformation rate at 0–0.5 cm/year, except for some very small area near the border with Boshan District, where the deformation rate reaches to 0.5–1 cm/year. The uptown area, including Pudong, Minhang, Jiading and Baoshan districts show slight deformation at a rate of 0.5–1 cm/year. The time-series cumulative deformation map (Fig. 5) indicated that the spatial dimension and the magnitude of the deformation were increasing slowly around Pudong airport, Disney land, Hongqiao Airport and along Zhonghuan Road. The maximun cumulative deformation could reach to 4 cm during the observation period. These places were also the fast subsidence sections detected in the previous research (Dong et al., 2014). Most of the Baoshan District show obvious subsidence at a rate of around 0.5 cm/year, where Baoshan Industrial Park is located, including many large-scale industrial plants.3.2 Two-Dimensional Ground Deformation Occurring in April–August 2008
It is clearly illustrated in Fig. 5 that most of the interested area remain stable over the observation period, especially in downtown and uptown areas. Slight-to-moderate subsidences at a rate of 1–2 cm/year accompanied with near west-east motion at around 1–2 cm/year are not homogeneously distributed over suburbs in the research area. Some places were only detected fast subsidence and were not observed obvious horizontal movements.
In downtown and uptown areas, no obvious widespread vertical displacement was observed except for some slight-to-moderate subsidence with a value of around 1 cm/year (Fig. 6a). However, slight-to-moderate horizontal displacement was observed in this area. Western movement at a rate of 0–1 cm/year were found in the north part of this area and eastern movement at a rate of 0–1 cm/year were found in the middle (Fig. 6b).
In Pudong district, inhomogeneous vertical displacement was distributed with a value of less than 1 cm/year (Fig. 6a). A wide and fast speed of eastward horizontal displacements with a value of 1–2 cm/year was observed in the northeast part of this area and the rest mainly show westward movement at a comparatively slight rate (0–1 cm/year). Very small portion of fast westward movement reaching up to 2 cm/year were observed in the Lujiazui zone during the observation period (Fig. 6b).
In Minhang District, the spatial distribution of horizontal displacements was larger than the distribution of vertical displacement. Fast vertical and westward horizontal displacement at a rate up to 2 cm/year around the Hongqiao International Airport was found. Besides, some moderate eastward motion at a rate of approximately 1–1.5 cm/year were found around the border of Minhang and Downtown District, while some moderate-to-fast westward motion were found in the south part of this area.
In Fengxian and Songjiang District, only very small portion were covered in the observation area. Moderate-to-fast vertical and horizontal displacements were witnessed with a value of around 1–1.5 cm/year in Fengxian District. Moderate vertical displacements accompanied with fast westward and eastward motions were witnessed in Songjiang District, especially with the border of Minhang, around Hongqiao Airport.
In general, in the above deformation areas, moderate- to-fast land subsidence and horizontal east-west deformation are mainly found within suburbs, particularly in densely residential areas, along traffic lines, and at busiest international airport, while rest of the area remain comparatively stable during the observation period.4 DISCUSION AND CONCLUSION
The spatial distribution of the ground deformation are highly related to human activities, including the construction of complex transportation infrastructures and industrial plants, high-rise building and expansion of development zones, implying induced consolidation of the soft soil during and after completion of the construction projects as the cause (Chen et al., 2012). Therefore infrastructure construction, urban expansion and reclamation play a significant role in the ground deformation process and pattern currently, since the magnitude and depth of groundwater pumping over Shanghai, especially in the downtown and uptown areas, are strictly restricted since 2005.
Unfortunately, there are only 4 ascending frames collected in this study. The overlapped regions and periods of ascending and descending frames are so limited that we can only derive horizontal displacement field during a very short observation period. Hopefully, with the launch of ESA Sentinel, CSA Radarsat Constellation and other SAR satellites, more and more datasets with different acquisition parameters would be available.
Land subsidence caused by compaction of aquifer system is a worldwide problem in urban areas heavily dependent on groundwater supplies (Brunori et al., 2015). Previous studies also demonstrated that groundwater over-pumping would create horizontal displacements at local and regional scales. Tensional strain resulting from horizontal displacements can further generate ground ruptures (Ye et al., 2016; Wang et al., 2009; Burbey and Helm, 1999). Although ground ruptures have not been reported in Shanghai so far, those places where horizontal and vertical displacements concur are more prone to develop into ground ruptures so that special monitoring should be arranged in these places.ACKNOWLEDGMENTS
The work in this paper was supported by the China Science National Foundation (No. 41372353). We would like to thank the Canadian Space Agency for providing Radarsat-2 SAR data. Images presented in this paper were plotted with GMT software. The final publication is available at Springer via https://doi.org/10.1007/s12583-017-0955-x.
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