Condition Monitoring of Ancient Rome Monuments with ERS and RADARSAT PSI Data
Tapete, Deodato¹; Casagli, Nicola^2
1National Research Council (CNR) of Italy - ICVBC, ITALY; ²Earth Sciences Department, University of Florence, ITALY

Conservators and archaeologists responsible for the preservation of archaeological monuments need non-invasive tools to set up procedures of 'strategic condition monitoring'. Preventative identification of the key areas of concern for structural stability of the ancient buildings and walls (i.e. 'preventive diagnosis') is essential to prioritize mitigation measures and restoration interventions (i.e. 'planned maintenance').
This paper offers a critical discussion of the results obtained from a set of site-specific tests that we performed in Rome, Italy, to address the above operational demand of heritage community by exploiting the well-known capabilities of Persistent Scatterer Interferometry (PSI) for deformation analysis.
Structural health of the Roman archaeological heritage was assessed by combining PSI data from ESA ERS-1/2 and CSA RADARSAT-1/2 imagery covering the last 20 years and processed by Telerilevamento Europa (TRE) s.r.l. with PSInSAR and SqueeSAR algorithms (Ferretti et al., 2001; 2011). The multi-spatial/temporal radar-interpretation was performed following the PSI-based method according to Tapete et al. (2012) and Tapete & Cigna (2012). A wide spectrum of cultural heritage categories was selected jointly with the Commissioner for the archaeological areas of Rome and Ancient Ostia, Italian Ministry of Cultural Heritage and Activities, who funded this research.
We explored the actual potentials of PSI techniques for: (i) condition monitoring of manmade structures chronically affected by ground instability, landslides, rock toppling and cavities collapses (Palatine Hill and Roman Forum); (ii) monitoring and warning of structural stability of partially/totally buried structures (Oppian Hill and Nero's Golden House); (iii) early-stage warning to prevent structural collapses of monuments historically stratified (Aurelian Walls) and long linear architectural complexes (Ancient Aqueducts); (iv) surveillance for public safety (pedestrian passages in the Park of Aqueducts).
The multi-temporal comparison between historical displacements and recent deformation imaged by ERS-1/2 in 1992-2000 and RADARSAT-1 in 2003-2009 respectively, led to a complete zoning of the unstable sectors of Palatine Hill and Roman Forum, thereby producing a 'map of conservation criticalities'. LOS estimates up to -20 mm/yr away from the satellite recorded in the last year of monitoring for the Temple of Magna Mater were correlated to instability of the tuff substratum. Whereas relative stabilization observed over the north-western sector of the hill in RADARSAT-1 data compared with ERS-1/2 ones suggested enormous potentials of this SAR approach to assess indirectly the beneficial/neutral/detrimental effects of consolidation works. The good agreement found with the complementary monitoring campaign carried out by means of ground-based SAR (Tapete et al., 2013) further confirmed the PSI reliability for purposes of condition monitoring.
On the other side, improvements brought by SqueeSAR algorithm to overcome detection limits due to vegetation coverage were enhanced through the back-monitoring campaign over the neighbouring Oppian Hill and Nero's Golden House. Superficial displacements of the soil covering the buried walls were assumed as indicators of potential instability of the inner structure of the monument, and allowed us to warn about hazardous sectors potentially damageable by collapses such as that occurred in March 2010. The latter was indeed back-monitored with RADARSAT-1/2 images covering from 2003 to June 2010, and a reliable explanation of its triggering was hypothesized. RADARSAT-1/2 PSI data also were used to evaluate the analytical and operational advantages (and drawbacks) of SqueeSAR through the case studies of the Ancient Aqueduct and Aurelian Walls, with specific regard to the actual and effective density of measurement points (both persistent and distributed scatters, i.e. PS and DS), as well as to the height information associated to each of them. To exemplify the direct benefits achievable for a better organized and more scientifically sound management of cultural heritage, we discuss the cases of: 1) the medieval tower known as Torre del Fiscale, for which the LOS estimates proved the occurrence of progressive displacements and suggested the execution of targeted consolidation works and deeper ground investigations; and 2) a peripheral sector of the Southern Aurelian Walls, over which the discovery of a curtain detachment confirmed the usefulness of PSI data to preventively identify critical sectors.
The main added values of this research were actually the involvement of the stakeholders (i.e. archaeologists, architects and conservators) throughout the tests and the shared radar-interpretation of PSI data thanks to a bidirectional feedback procedure. A parallel activity of dissemination to the end-users was undertaken to demonstrate how to use the information retrieved from these satellite reservoirs to face real-world situations and issues quite common in the field of heritage conservation.

References
Ferretti A., Fumagalli A., Novali F., Prati C., Rocca F., Rucci A. 2011. A new algorithm for processing interferometric data-stacks: SqueeSAR. IEEE Trans. Geosci. Remote Sens. 49: 3460-70
Ferretti A., Prati C., Rocca F. 2001. Permanent scatterers in SAR interferometry. IEEE Trans. Geosci. Remote Sens. 39: 8-20.
Tapete D., Casagli N., Luzi G., Fanti R., Gigli G., Leva D. 2013. Integrating radar and laser-based remote sensing techniques for monitoring structural deformation of archaeological monuments. Journal of Archaeological Science 40 (1): 176-189. DOI: 10.1016/j.jas.2012.07.024
Tapete D., Cigna F. 2012. Rapid mapping and deformation analysis over cultural heritage and rural sites based on Persistent Scatterer Interferometry. International Journal of Geophysics, Article 618609, pp. 1-19. DOI:10.1155/2012/618609, 2012 Tapete D., Fanti R., Cecchi R., Petrangeli P., Casagli N. 2012. Satellite radar interferometry for monitoring and early-stage warning of structural instability in archaeological sites. Journal of Geophysics and Engineering 9 (4): S10-S25. DOI:10.1088/1742-2132/9/4/S10