S. Grassi

Evaluation of the stability of a rock cliff by means of geophysical and geomechanical surveys in a cultural heritage site (south-eastern Sicily)

An integrated geophysical and geomechanical study is proposed for the evaluation of the fracturing condition and the stability of a rock cliff in the cultural heritage site of Ispica (southeastern Sicily).The site comprises several worship constructions such as the Friars Minor Monastery, the oldest construction of the village and a Church. The buildings stand on the edge of a 35 m high calcareous cliff, showing instability conditions related to the presence of deep fracturing and karst features. Due to possible future rockfalls the slope is expected to be subjected to retreat phenomena, which will threat the structures on top.The study of the seismic wave speed and of the environmental noise (microtremors), through active and passive seismic survey, proved useful in highlighting the spatial variation of the degree of fracturing in the rock mass. Moreover, the resonance frequency of the site allowed estimating the frequencies at which the ground motion is amplified by stratigraphic features.The stability analysis pointed out the critical condition of the slope, affected by different potential failure mechanisms with very low safety factors and a high probability of failure.All the considerations are contextualized in the wide Hyblean panorama, where important tectonic structures are responsible of the actual geological setting and of the seismicity of the area, as well as of the geomechanical condition of the slopes.

3D Subsoil Model of the San Biagio ‘Salinelle’ Mud Volcanoes (Belpasso, Sicily) derived from Geophysical Surveys

Mud volcanoes are common in active mountain fronts. At Mt. Etna, located just between the Apennine front in Sicily and its foredeep, there are some manifestations of mud volcanism in the lower border of the volcanic edifice. The activity of these mud volcanoes is characterized by persistent emission of muddy water mixed with salts, which rises to the surface due to the gas pressure in the subsoil. The San Biagio Salinelle is one of the three mud volcano fields located around the Paternò eruptive monogenic apparatus; this old volcanic structure was one of the first subaerial volcanic manifestations that formed in the pre-Etnean phase. It is not fully clear whether and how the activity of the mud fields is connected with the volcanic activity of Mt. Etna. Noninvasive geophysical surveys were carried out in the area of the active cone of the San Biagio Salinelle, in order to identify the probable ascent path of the emitted products. Seismic ambient noise records were collected at the nodes of a specially designed grid and, subsequently, the Vs values were obtained from an active seismic survey. A digital elevation model (DEM) of the area was obtained by a topographic survey, carried out with the GNSS technique (global navigation satellite system), in real-time kinematic mode. The DEM and the topographic benchmark installed will represent the reference surface for future periodic monitoring of the ongoing deformation in the area. Our results provide an accurate and detailed 3D subsurface model showing the shallower feeding system of the investigated mud volcano.

Seismic refraction tomography surveys as a method for voids detection: an application to the archaeological park of Cava Ispica, Sicily, Italy

ABSTRACT Geophysical surveys are commonly used in areas where the presence of ancient civilizations is historically documented. These investigations are able to detect through indirect methods the presence of bodies or structures in the subsoil measuring the variations of some physical parameters, obtaining information about archaeological remains without causing harm to them. In the present study, we investigated the presence of anthropogenic buried cavities partially filled with rubble material, in an area located in the northern part of Cava Ispica, one of the most important archaeological sites located in south-eastern Sicily. The results of seismic refraction prospections, processed with tomographic methods, are presented and discussed. The entire valley contains prehistoric burial sites, Christian catacombs and residential units of various kinds. Despite the numerous searches made over past years, much still remains to be discovered. The seismic refraction tomographic data, analyzed in 3D, revealed the presence of low velocity values (<400 m/s) areas that, considering the lithotypes locally present and the features of other archaeological structures discovered during previous surveys, can be ascribed to the presence of possible cavities. The obtained results show how such geophysical inspections represent an important preliminary tool for archaeological surveys.