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.

Affected by natural and human-induced factors, cultural heritage sites and their surroundings face threats of structural instability and land displacement. Accurate and rapid identification of the key areas facing existing or potential deformation risks is essential for the conservation and sustainability of heritage sites, particularly for huge archaeological regions. In recent years, the successful application of differential radar interferometry techniques for the measurement of millimeter-level terrain motions has demonstrated their potential for deformation monitoring and preventive diagnosis of cultural heritage sites. In this paper, we review the principles of advanced differential radar interferometry approaches and their applicability for structural and ground deformation monitoring over heritage sites. Then, the advantages and challenges of these approaches are analyzed, followed by a discussion on the selection of radar interferometry systems for different archaeological applications. Finally, a workflow, integrating space-borne and ground-based differential radar interferometry technologies for deformation anomaly monitoring and preventive diagnosis of cultural heritage sites, is proposed.

Architectural heritage has important historical, artistic, and technological value. However, these sites are subject to long-term degradation and are sensitive to disturbances from the surrounding landscape, resulting in the potential deterioration of the architecture. Deformation is a quantitative indicator of the vulnerability of architectural heritage. The extraction of three-dimensional (3D) deformation via existing geodetic approaches, such as global navigation satellite systems (GNSS) and interferometric synthetic aperture radar (InSAR), proves to be a challenging task. In this study, we proposed an adaptable methodology for the preventive monitoring of architectural heritage by integrating differential SAR tomography (D-TomoSAR) and a finite element method (FEM) to overcome the limitations of current geodetic approaches in cultural applications (i.e., sparse measurements in space; time- and labor-intensive). The Angkor Wat Temple, a well-known monument at the Angkor World Heritage site, Cambodia, was selected as the investigation target. We used D-TomoSAR to calculate the deformation, height, and thermal dilation of the structure using 46 TerraSAR-X images acquired during 2011–2013. InSAR deformation measurements were used as the constraint for the 3D deformation simulation and structural monitoring using an integrated approach to diagnose the soundness of the architectural structure. The experimental results demonstrate the following: 1) the integration of InSAR measurements prevented the underestimation of structural instabilities and linked the simplified physical model and mechanical parameters to the FEM simulation. 2) The Angkor Wat Temple appeared stable; however, structural instabilities were detected with an absolute deformation rate of up to 3 mm/yr. 3) Daily changes in thermal-related deformations caused by incoming frontal and posterior solar radiation-induced the bending of the architecture and the risk of structural defects. Our study reveals the defect mechanism of the Angkor Wat Temple and demonstrates the potential of the proposed method for the health diagnosis and preventive safeguarding of architectural heritages across the globe.

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.

As a World Cultural Heritage site with sacred landscape featuring an exceptional range of Buddhist art and architecture, much attention has been focused on the sustainable development of Bagan (Myanmar). Particularly, the monitoring of landscape surface subsidence and monument instability is of great importance to the protection and development planning of the Bagan heritage site. In this study, we applied high resolution TerraSAR-X imagery acquired from 2019 to 2020 for deformation monitoring based on the small baseline subset (SBAS) and persistent scatterer synthetic aperture radar interferometry (PSInSAR) approaches. We identified several hotspots and pagodas with displacement anomalies linked to land cover change and previous earthquakes. The cross comparison between SBAS and PSInSAR and the precision of height estimates derived by PSInSAR indicated a millimetric precision of the derived deformation products. The combination of the two multi-temporal SAR interferometry approaches satisfies the two-scale monitoring requirements from landscapes to monuments, particularly for large-scale World Heritage sites. The non-contact monitoring method has potential when traditional methods using field accessibility and surveillance are constrained.

In this study, an integrated monitoring and evaluation approach was proposed to fix the systematic safeguarding gap of the Great Wall corridor using space technologies. Two representative sections of the Great Wall located in Qingtongxia County and Zhangjiakou City in China were selected for a preliminary comparative investigation to ascertain the coupling mechanism and spatiotemporal characteristics of the driving forces for the heritage damage. The surface deformation rates were estimated by synthetic aperture radar interferometry (InSAR), and the interaction between the deformation rates and the normalized difference vegetation index (NDVI) and meteorological and topographic data was calculated using the correlation matrix. The results showed the following: 1) the surface motions along the observed landscape corridor of the Great Wall were decreased in 2015-2018. 2) The correlation coefficients between the deformation rates and the elevation, slope, annual wind speed, and NDVI were 0.524, 0.115, 0.582, and 0.522, respectively, indicating the dominant influence of surface runoff and high winds on the degradation of the rammed-earth wall in the western arid regions. In contrast, 3) the correlation coefficients between the deformation rates and the aforementioned factors were 0.065, 0.027, 0.025, and 0.052, respectively, indicating negligible effects of natural processes on the degradation of the wall in the eastern section. This study not only provides new insights into preventive monitoring and risk assessment of the entire Great Wall but also highlights the potential of space technologies and a geographical perspective for the sustainable conservation of large-scale heritage sites.