GPR and microwave tomography for detecting shallow cavities in the historical area of ‘‘Sassi of Matera’’ (Southern Italy)

The results of a geophysical survey carried out in the historical area of “Sassi of Matera” (Basilicata region, southern Italy) are presented and discussed. Matera is one of the most fascinating and ancient towns of the Mediterranean area and is listed in the UNESCO World Heritage List. It represents an example of a town where the communities developed a cave-life culture in the cavities excavated through the substratum of the hill of Matera. The urban area of Matera, therefore, is characterized by a complex system of interconnected and overlapping ancient shallow cavities: residences, galleries, cisterns, crypts, graves, etc. Consequently, local surface deformation phenomena are often observed. We describe the results from a ground-penetrating radar (GPR) survey carried out in St. Rocco Square, one of the most interesting areas of the historical centre of Matera, that was affected by both spread out and local subsidence phenomena. Great attention was paid to GPR data processing and inversion. In particular, a novel microwave tomography approach was applied to obtain high-resolution images of shallow ancient cavities that caused the local surface deformations. Moreover, the GPR survey was integrated with an electrical resistivity tomography survey in order to prospect those subsurface zones with a high electrical conductivity, where GPR investigation was not possible. The geophysical investigation enabled us to identify the cause of the subsidence phenomena and to reconstruct the geometry of the complex system of shallow cavities partially filled with unconsolidated material.