Panpan Tang

Monitoring Cultural Heritage Sites with Advanced Multi-Temporal InSAR Technique: The Case Study of the Summer Palace

Cultural heritage sites are rare and irreplaceable wealth of human civilization. The majority of them are becoming unstable due to a combination of human and natural disturbances. High-precision, efficient deformation monitoring facilitates the early recognition of potential risks and enables preventive diagnosis of heritage sites. In this study, an advanced Multi-Temporal Interferometric Synthetic Aperture Radar (MTInSAR) approach was developed by jointly analyzing Persistent Scatterers (PSs) and Distributed Scatterers (DSs) using high-resolution SAR images. Taking the World Heritage Site of Summer Palace in Beijing as the experimental site, deformation resulting from PSs/DSs showed that overall the site was generally stable except for specific areas and/or monuments. Urbanization (construction and demolition) triggered new subsidence in the vicinity of East and South Gate of the site. Slight to moderate (mm/cm-level) instabilities of ruins and monuments on Longevity Hill were detected, perhaps due to a combination of destructive anthropogenic activities and long-term natural decay. Subsidence was also detected along the Kunming Lakeside and was probably attributable to variation of the groundwater level, excessive visitor numbers as well as lack of maintenance. This study presents the potential of the MTInSAR approach for the monitoring and conservation of cultural heritage sites.

Large-Area Landslides Monitoring Using Advanced Multi-Temporal InSAR Technique over the Giant Panda Habitat, Sichuan, China

The region near Dujiangyan City and Wenchuan County, Sichuan China, including significant giant panda habitats, was severely impacted by the Wenchuan earthquake. Large-area landslides occurred and seriously threatened the lives of people and giant pandas. In this paper, we report the development of an enhanced multi-temporal interferometric synthetic aperture radar (MTInSAR) methodology to monitor potential post-seismic landslides by analyzing coherent scatterers (CS) and distributed scatterers (DS) points extracted from multi-temporal l-band ALOS/PALSAR data in an integrated manner. Through the integration of phase optimization and mitigation of the orbit and topography-related phase errors, surface deformations in the study area were derived: the rates in the line of sight (LOS) direction ranged from −7 to 1.5 cm/a. Dozens of potential landslides, distributed mainly along the Minjiang River, Longmenshan Fault, and in other the high-altitude areas were detected. These findings matched the distribution of previous landslides. InSAR-derived results demonstrated that some previous landslides were still active; many unstable slopes have developed, and there are significant probabilities of future massive failures. The impact of landslides on the giant panda habitat, however ranged from low to moderate, would continue to be a concern for conservationists for some time in the future.

Archeological crop marks identified from Cosmo-SkyMed time series: the case of Han-Wei capital city, Luoyang, China

ABSTRACT The development of spaceborne Synthetic Aperture Radar (SAR) technology declares that the golden era of SAR remote sensing in archeology is approaching; however, nowadays its methodology framework is still lacking due to the inadequate case studies validated by ground-truths. In this study, we investigated the crop marks using multi-temporal Cosmo-SkyMed data acquired in 2013 by applying a two-step decision-tree classifier in conjunction with a spatial analysis in an area of archeological interest nearby the archeological site of Han-Wei capital city (1900–1500 BP), in Luoyang, China. The time-series backscattering anomalies related to the wheat growth cycle were identified and then further validated in two zones by geophysical investigations (Ground Penetration Radar and electrical measurements) and in a third zone by archeological excavations made after the SAR data acquisition. This study provides a new approach for the relic detection, shallowly buried and covered by the crop vegetation, by temporal crop marks on spaceborne SAR images. We also emphasize the necessity to establish a satellite-to-ground methodology framework for the promotion of remote-sensing technology in archeology.

The backscattering characteristics of wetland vegetation and water-level changes detection using multi-mode SAR: A case study

Abstract A full understanding of the backscattering characteristics of wetlands is necessary for the analysis of the hydrological conditions. In this study, a temporal set of synthetic aperture radar (SAR) imagery, acquired at different frequencies, polarizations and incidence angles over the coastal wetlands of the Liaohe River Delta, China, were used to characterize seasonal variations in radar backscattering coefficient for reed marshes and rice fields. The combination of SAR backscattering intensity and an optical-based normalized difference vegetation index (NDVI) for long time series can provide additional insight into vegetation structural and its hydrological states. After identifying the factors that induce the backscattering and scattering mechanism changes, detailed analysis of L-band ALOS PALSAR interferometric SAR (InSAR) imagery was conducted to study water-level changes under different environmental conditions. In addition, ENVISAT altimetry was used to validate the accuracy of the water-level changes estimated using the InSAR technique—this is an effective tool instead of sparsely distributed gauge stations for the validation. Our study demonstrates that L-band SAR data with horizontal polarization is particularly suitable for the extraction of water-level changes in the study area; however, vertically-polarized C-band data may also be useful where the density of herbaceous vegetation is low at the initial stage. It is also shown that integrated analysis of the backscattering mechanism and interferometric characteristics using multi-mode SAR can considerably enhance the reliability of the water-level retrieval scheme and better capture the spatial distribution of hydrological patterns.

Surface Motion and Structural Instability Monitoring of Ming Dynasty City Walls by Two-Step Tomo-PSInSAR Approach in Nanjing City, China

Spaceborne Multi-Temporal Synthetic Aperture Radar (SAR) Interferometry (MT-InSAR) has been a valuable tool in mapping motion phenomena in different scenarios. Recently, the capabilities of MT-InSAR for risk monitoring and preventive analysis of heritage sites have increasingly been exploited. Considering the limitations of conventional MT-InSAR techniques, in this study a two-step Tomography-based Persistent Scatterers (PS) Interferometry (Tomo-PSInSAR) approach is proposed for monitoring ground deformation and structural instabilities over the Ancient City Walls (Ming Dynasty) in Nanjing city, China. For the purpose of this study we utilized 26 Stripmap acquisitions from TerraSAR-X and TanDEM-X missions, spanning from May 2013 to February 2015. As a first step, regional-scale surface deformation rates on single PSs were derived (ranging from −40 to +5 mm/year) and used for identifying deformation hotspots as well as for the investigation of a potential correlation between urbanization and the occurrence of surface subsidence. As a second step, structural instability parameters of ancient walls (linear motion rates, non-linear motions and material thermodynamics) were estimated by an extended four-dimensional Tomo-PSInSAR model. The model applies a two-tier network strategy; that is, the detection of most reliable single PSs in the first-tier Delaunay triangulation network followed by the detection of remaining single PSs and double PSs on the second-tier local star network referring to single SPs extracted in the first-tier network. Consequently, a preliminary phase calibration relevant to the Atmospheric Phase Screen (APS) is not needed. Motion heterogeneities in the spatial domain, either caused by thermal kinetics or displacement trends, were also considered. This study underlines the potential of the proposed Tomo-PSInSAR solution for the monitoring and conservation of cultural heritage sites. The proposed approach offers a quantitative indicator to local authorities and planners for assessing potential damages as well as for the design of remediation activities.

Mapping Thermokarst Lakes on the Qinghai–Tibet Plateau Using Nonlocal Active Contours in Chinese GaoFen-2 Multispectral Imagery

In order to monitor the response of thermokarst lakes on the Qinghai–Tibet Plateau (QTP) to rapid climatic changes and human activities, an automated method for extracting shorelines from Chinese GaoFen-2 (GF-2) imagery is proposed. First, the water index (WI) images and the potential lake areas are calculated from the preprocessed multispectral imagery and digital elevation model data, respectively. Second, the initial segmentation obtained by global thresholding of the WI images and masking in the potential lake areas are used to implement the contour initialization of active contours models efficiently. Finally, the nonlocal active contours (NLAC) approach is applied to refine the initial segmentation of the WI images, and the final shoreline vector files are produced by some simple and automatic postprocessing steps. Experiments on the GF-2 imagery demonstrate that 1) by exploiting the capability of WI to locate the approximate shoreline effectively around the evolving contour, the processing time of the proposed method can be saved significantly; 2) the NLAC approach can efficiently identify the shoreline by integrating the nonlocal interactions between pairs of patches inside and outside the lake; and 3) the proposed method can conveniently adapt to the multitemporal and multifeature image analysis. Using the manual digitized shorelines as the reference data, an average error of less than one pixel with standard deviation of 0.1320 can be obtained. These results prove that the proposed method is feasible for the identification and monitoring of thermokarst lakes on the QTP.

Permafrost environment monitoring on the Qinghai-Tibet Plateau using time series ASAR images

The permafrost in Qinghai-Tibet Plateau (QTP) has long been the focus of many researchers. In this study, we first use the method that integrates synthetic aperture radar (SAR) intensity and phase information to monitor permafrost environment in the Beiluhe Region, using time series advanced SAR images. The backscattering coefficients (σ0) and deformation were extracted for the main features, and the influences of meteorological conditions to them were also quantified. The results show that both the change in σ0 and surface deformation are closely related to the active layer, and the deformation is also affected by the permafrost table. First, over meadow and sparse vegetation regions, σ0 rose about 6.9 and 4 dB from the freezing to thawing period, respectively, which can be mainly attributed to the thaw of the active layer and increased precipitation. Second, seasonal deformation, derived from the freeze-thaw cycle of the active layer, was characteristic of frost heave and thaw settlement and exhibited a negative correlation with air temperature. Its magnitude was larger than 1 cm in a seasonal cycle. Last, significant secular settlement was observed, with rates ranging from –16 to 2 mm/a, and it was primarily due to the thaw of the permafrost table caused by climate warming.

Quantification of Temporal Decorrelation in X-, C-, and L-Band Interferometry for the Permafrost Region of the Qinghai–Tibet Plateau

Over the permafrost region of the Qinghai–Tibet Plateau, serious decorrelation has restricted the inteferometric synthetic aperture radar (InSAR) techniques in monitoring ground deformation and thaw–melt hazards. Improved understanding, quantification and prediction of the coherence evolution with time are key prerequirements for choosing the optimal observation time and band in such interferometric applications. In this letter, considering the main processes of degradation and the freeze–thaw cycle, a temporal decorrelation model including both the long-term and seasonal coherence factors is proposed. Multitemporal L-band PALSAR, C-band ASAR, and X-band TerraSAR were collected to validate the model’s applicability. Coherence matrices of several typical features were estimated for the parameter inversion. Root-mean-square errors ranging from 0.048 to 0.152 showed good fitting results between the model predictions and the estimated interferometric coherences. The temporal decorrelation characteristics of the permafrost region at the three frequencies were successfully revealed. This letter could also facilitate applications of InSAR techniques in similar areas in other parts of the world.

Use of Intensity and Coherence of X -Band SAR Data to Map Thermokarst Lakes on the Northern Tibetan Plateau

In order to monitor the response of thermokarst lakes on the Tibetan Plateau to rapid climatic changes and human activities, an automated shoreline extraction approach from the high-resolution TerraSAR/TanDEM-X imagery is proposed. First, a preprocessing scheme is applied to calibrate, coregister, and estimate the intensity and coherence of the imagery. Second, the statistical distribution of intensity and coherence are used to refine the data term of a region-based level set model. A distance regularized term and a variable weighting parameter are also added to improve the algorithm efficiency. Finally, a postprocessing is applied to remove false edges related to spurious segments and generate a vector map by geocoding the detected edges. Experiments on the intensity and coherence images demonstrate that: 1) the Fisher distribution can improve the usability of the level set method for extraction of shorelines using the intensity feature, because it can flexibly model the speckle fluctuations of many land classes; 2) the coherence information can significantly improve the efficiency and accuracy of segmentation by suppressing the misclassification caused by the sands around the lakes and the local winds over the lakes; 3) using a variable parameter to weight the data term and a regular term in a level set model can produce more robust and fast segmentation; and 4) using of intensity and coherence in unison can locate the exact position of most of the shorelines and reduce the relative uncertainty within 3 pixels with 90% confidence level.

Coherence based analysis of distributed scatterers in the Qinghai-Tibet Plateau

In Qinghai-Tibet Plateau, permafrost is usually full of ice underground and sensitive to the temperature. So due to the global warming and seasonal change of temperature, great changes of surface in physical characteristics would occur in the temporal dimension. This could lead to the decorrelation between two SAR images, and hinder the application of Interferometric Synthetic Aperture Radar (InSAR) technique in this area. Additionally, distributed scatterers (DS) spread all over the tundra. In this paper, we first introduce DS analytical method, and use 45 ENVISAT-ASAR images to analyze the coherence of several typical ground features. The result shows that DS analytical method could improve the accuracy of coherence estimation, mainly railway and road in our study area.