Metodi geofisici per la ricerca di cavità: caso di studio alle Cave di Marsala (TP)

Abstract

Many engineering and environmental projects require geophysical exploration to minimize the geological risk connected with the presence of natural or artificial tunnels and cavities (air and/or water filled). In this paper we like to confirm the reliability of combined geophysical methodologies (GPR and ERT) and relative modelling to detect and locate cavities in a particular site (Marsala Western Sicily) where underground dug structures exist. These structures may be used as real artificial physical models, very useful for testing geophysical methodologies. These shallow sandstone caves have, in general, a rectangular shape and are interconnected by tunnels. Most of these structures can be inspected from the inside, due to easy access points. The studies site, a well known area in which abandoned and active caves are presents, due to massive extractive historical and recent activity, seem to be suitable for many kinds of tests. In fact, the area presents relative homogeneity of materials, flat topography and presence of cavities having shapes regular enough, differing only for size and depth. The possibility to inspect cavity and their regular shape allowed us modelling the subsoil to compare synthetic data with experimental data. GPR and Resistivity modelling evidenced the most favourable configuration parameters for the selected acquisition techniques. An air-filled void generally provides a strong dielectric contrast with the surrounding rocks, so that the detectability using GPR is relatively high, but in our case the result seem to bee particularly unambiguous, allowing us recognizing the tops of the cavities. The apparent resistivity data have been processed by a robust constrained inversion technique and compared whit synthetic simulation. Also in this case, the detention of the cavity and its geometry seem to be very clear. GPR data shoved that these underground cavities can be detected also considering diffraction phenomena, which increase the vertical resolution. The used ERT technique seem to be able to recognize correct shapes and dimensions of cavities if their depth is comparable witch their dimensions. The integration of GPR and ERT seem to be very appropriate for cavities detections. In fact, the two techniques are complementary in detecting underground discontinuities (GPR) and volume anomalies (ERT) regarding electrical (resistivity) and electromagnetic (velocity) parameters of the rocks and air, which are generally well correlated.