Research of crypts and/or cavities of the Bronze Age in the Hyblean carbonate soil by means of seismic tomography: a case study

The Hyblean plateau (South Eastern Sicily) consists of a platform carbonate interbedded with volcanic horizons, whose age ranges from the Cretaceous to the Pleistocene and is characterized by the presence of numerous incisions from waterways, known locally as "hollow". The most important is the “Cava Ispica” which crosses for about 13 km the municipalities of Modica, Ispica and Rosolini. It is a collection of archaeological evidence, ranging from prehistoric to medieval low, entirely cut into the rocks, which make this "hollow" one of the most representative historical places of Sicily pre-classical and late antiquity (Trigilia, 2011). The monuments that are within the perimeter of the Regional Archaeological Park have long been excavated, but many still remain below the current floor plans due to the amount of coves that characterizes this part of Modica territory. Non-invasive seismic surveys were carried out in the area between the catacomb of Larderia (IV-V century a.d.) and the area of the Gymnasium’s caves (III century b.c.) in order to identify the possible presence of anthropic cavities in the subsoil. The strong contrast between the low velocity zones such as human cavities, and high seismic velocity waves of limestone rock formation that contains them, strongly favors the use of this geophysical technique (Cardarelli et al., 2010). As witnessed by the numerous excavations carried out in the area during the archaeological surveys the cavities are often filled by material of geological/anthropogenic source that has accumulated in hundreds of years, this means that there is not a sharp contrast between the rocky continuum and the empty. The seismic refraction surveys processed using tomographic method have highlighted low seismic velocity (<400 m/sec) zones distributed evenly in both areas on the site "Gymnasium" and "Terrace", with constant depth of two meters from the surface. These areas represent evidence of the presence of man-made cavities. The size and the shape of cavities have been defined through the isovelocity surface having values of about 400 m/sec. On the basis of knowledge acquired the shapes and sizes of these areas are compatible with paleo-anthropic elements present in the site and visible on the surface, and it is therefore reasonable to expect similar deep structures. The three-dimensional reconstruction doesn’t highlights a net and linear geometry, because, as is expected, can be filled largely by solids. Although the lack of a net velocity contrast between the rock and the filled or partially filled cavities has made difficult to interpret the seismic lines acquired, the numerous informations deriving from surveys carried out are undoubtedly a useful planning tool for the purposes archaeological research.