Radar interferometry technologies for ‘early stage’ warning of structural instability on monuments at risk

In the field of the conservation of cultural heritage, the new tendency of preventing the occurrence of instability events potentially damaging monuments and historic remains requires the development of innovative procedures allowing an effective warning, to avoid partial or total loss of elements of inestimable cultural value. Approaching this theme means, among others, to think a different concept of ‘early warning’, both due to the particular nature of cultural heritage and the specific relationship existing between preventive diagnosis and the planning of risk mitigation interventions. As the main aim is the preservation of the former quality and structural integrity of the cultural artefact, any warning activity can be actually effective only if it warns about the evolution of instability mechanisms at an ‘early stage’, allowing the execution of appropriate countermeasures to safeguard the exposed elements. Such conservation issue equally concerns cultural heritage as single monument or entire site located within an urban settlement or distributed over a huge suburban area. In regard with the extension of the site to monitor and the typology of landsliding processes affecting the monuments, the choice of the technologies to be adopted should combine the non invasive mode in measuring and monitoring the phenomena with the properties of rapidity, high resolution and cost-effectiveness. In the present work we discuss the results of the experimental implementation of Interferometric Synthetic Aperture Radar (InSAR) technologies on the archaeological heritage of Rome (Italy), carried out in the framework of co-operation with the Commissioner for the archaeological areas of Rome and Ancient Ostia, Italian Ministry of Cultural Heritage and Activities. High sampling frequency and spatial resolution of Ground-Based InSAR (GB-InSAR) instrumentation allowed us to perform real-time monitoring of the structural behaviour of Domus Tiberiana, built on unstable foundation ground affected by a long history of local displacements and collapses. The analysis of constantly updated displacement time series for selected control points, located over the observed scene in correspondence of areas with homogeneous structural behaviour, was exploited to real-time detection of displacement anomalies as deviations from the normal trend. Such monitoring at ‘single monument scale’ was successfully supported by a feedback procedure. It involved the monitoring unit in sending the alerts and the local authority, responsible for the monument, in verifying the presence of any deterioration effects directly on the structures in response to each warning. Same procedures were tested on the central archaeological areas of Palatino and Oppio Hills, as well as on the southern Aurelian Walls and the complex of Roman Aqueducts, adopting Persistent Scatterer Interferometry (PSI), as satellite multi-image technique particularly suitable for monitoring campaigns at ‘archaeological site scale’. High detailed time-series analysis showed localized ground motions and deformations of the monitored structures, even highlighting the occurrence of acceleration phases within the displacement trend of some points, assumable as potential precursors of future deformations. In such cases, the ‘early stage’ warning provided useful information for local conservators, suggesting further on site investigations and guiding the design of specific mitigation interventions to be carried out in the areas confirmed as critical. The main outcome of these experimentations consisted in combining the capacity of performing multiple scale warning activities, by exploiting the different potentials of ground-based and satellite radar techniques, with effective feedback procedures in a productive synergy with the end-users and decision-makers.