Geophysical and geotechnical investigations to support the restoration project of the Roman 'Villa del Casale', Piazza Armerina, Sicily, Italy

A multidisciplinary geophysical and geotechnical study, including some non-invasive geophysical applications, was carried out during the restoration of the 'Villa del Casale', a Roman villa discovered near Piazza Armerina (Sicily, Italy) in 1929, famous for its Roman floor mosaics. The project aims were to characterize the geology of the subsoil and provide information for solving the main, building structural problems including the subsidence of some parts of the floor and the detachment of the tesserae (i.e., the tiles) of the mosaics. Another goal was the detailed study of the underground structures of the Corridor of the Great Hunt, a part of the villa strongly affected by subsidence and detachment of mosaics. The acquisition of geotechnical and geophysical surveys, including core drillings, time-domain electromagnetic (TDEM) soundings, downhole seismic logs and a seismic refraction profile, allowed the reconstruction of the geometry of geological formations below the villa to be ascertained, especially with regard to the top of the bedrock. The results suggest the presence of a buried riverbed that has been identified as the main cause of the villa floor subsidence. The study of the Corridor of the Great Hunt involved full 3D electrical resistivity tomography (ERT) and 3D ground-penetrating radar (GPR) data acquisition using 100 MHz, 400 MHz and 1600 MHz antennas, as well as the above mentioned refraction seismic profile. Data acquisition required the use of special non-invasive sensors to avoid any damage to the mosaics. The integrated inversion of all the data and the comparison between the resulting 3D resistivity model and 400 MHz GPR depth-slices allowed the identification of many shallow anomalies to be identified, including some pipes for water drainage and a reinforced concrete basement placed under the floor during a previous restoration intervention. 100 MHz GPR profiles validated the results of the seismic refraction profile model. Finally, 1600 MHz GPR depth-slices, in the most subsided zone, allowed the location of air bubbles under the mosaic to be identified. This work shows that the integrated use of different geophysical techniques for archaeological purposes, especially if constrained by direct explorations, greatly reduced the intrinsic uncertainties of each method. Moreover, customizing standard geophysical equipment to avoid any damage is essential when working on protected cultural heritage.