GNGTS 2017 - 36° Convegno Nazionale

612 GNGTS 2017 S essione 3.2 Statistical approach for cavity detection using seismic refraction and electrical resistivity data P. Capizzi, R. Martorana, A. Carollo, M. Vattano Dipartimento di Scienze della Terra e del Mare (DiSTeM), Università degli Studi di Palermo, Italy Introduction. The presence of cavity in densely populated areas creates serious problems in terms of hazard and risk for the stability of the infrastructure and individual protection. It is therefore essential to identify these areas to minimize the geological risk and implement an optimal plan for the use of the soil. Geophysical techniques are the most efficient methods for obtaining information on the structures and geometries present in the subsoil and constitute an excellent tool for the detection of buried cavities (Maraio et al. , 2015). The determination of dimension and depth of the cavities and of the contrast in physical properties between the cavity and surrounding media are fundamental aspects for understanding the validity of a particular geophysical method (Fasani et al. , 2013). It is also essential that the geophysical survey is designed and chosen on the basis of a priori information of the geological context, making it possible to obtain a rough estimate of depth and size of the voids to evaluate any differences in the properties physico-mechanical properties of the materials involved (Cardarelli et al. , 2010). Today electrical resistivity tomography (ERT) is the most widely method used for the detection of underground voids, however, the combination of experimental data from different geophysical methods is the most suitable approach for the identification and the determination of the buried cavity, as it minimizes the possible ambiguity of geophysical interpretation. In recent years, the technique of electrical tomography has been joined more and more often to the technique of seismic refraction tomography (SRT) in order to obtain more robust interpretations also using a clustering approach (Gallardo and Meju, 2004; Kotyrba and Schmidt, 2014). Synthetic data analysis. �� ���� ����������� ��� ������������� �� ������� ���������� ���� To test limitations and effectiveness of seismic tomography when coupled to geoelectrical technique for cavity detection 2D synthetic models were used. Synthetic models were created with different number of cavity and blocks of highly cohesive lithological material (high seismic velocity and resistivity values). � �������� ������� �� �������� �������� A modified version of multiple gradient (Martorana et al. , 2016) has been used for electrical sequence. A synthetic model with three blocks (6 m x 6 m) is here presented. In particular, two blocks that simulate two cavities (P-wave velocity value of 330 m/s and resistivity value of 100000 Ωm) placed at 4 m and 8 m depth, and a block that simulates the presence of a highly cohesive material (P-wave velocity value of 5000 m/s and resistivity value of 5000 Ωm) located at 6 m depth (Fig. 1). The seismic model has a total length of 102 m and a depth of 30 m, divided in layers with a vertical gradient of velocity (Fig. 1a). The electrical model has been realized in a manner geometrically conforms to the seismic model, with a background resistivity value of 500 Ωm (Fig. 1b). The synthetic seismic coverage (Fig. 2a) and refraction tomographies (Fig. 2b) were created, processed and inverted with Rayfract® software (Intelligent Resource, Inc.), while the synthetic resistivity models (Fig. 2c) have been created and inverted by the RES2DMOD® Fig. 1 - Seismic synthetic model (a) and electrical synthetic model (b).