Abstract
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.
Highlights
As irreplaceable sources of life and inspiration, heritage is the witness to past civilizations and past social evolution, and can hold lessons for modern society
In order to understand the impact of urbanization and the thermodynamics of materials on the vulnerability of Ming Dynasty City Walls in Nanjing, the developed two-step Tomo-PersistentScatterers (PS) SAR interferometry (PSInSAR) approach was utilized to quantitatively assess motion trends on the monument itself, as well as its surroundings
To assess the vulnerability of cultural heritage sites, jointly induced by geological change and anthropogenic activities, a two-step Tomo-PSInSAR approach was developed in the present study to extract motion anomalies at regional-to-monumental observation scales
Summary
As irreplaceable sources of life and inspiration, heritage is the witness to past civilizations and past social evolution, and can hold lessons for modern society. Spatio-temporal changes in motion patterns, either at a regional scale or instabilities at a structural level, could be typical precursors implying the potential vulnerability of heritage sites. Scatterers (PS) SAR interferometry (PSInSAR) [2,3,4,5], Small Baseline Subset (SBAS) [6] and SqueeSAR [7], have been proved effective tools for the monitoring of regional displacements with up to few millimeters accuracy. The accumulation of long time series of SAR data hastened a new era of MT-InSAR for risk monitoring and preventive diagnosis of monuments as well as their surroundings, such as the Historic Centre of Rome, Italy [8,9], Angkor site, Cambodia [10], Mexico City, Mexico [11]
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