GNGTS 2017 - 36° Convegno Nazionale

GNGTS 2017 S essione 3.2 613 and the RES2DINV® software, respectively. The electrical resistivity models were constructed using a value of 10000 �� ��� �������� ��� ��� �� ��� ��� ���������� ��������� ����� ��� Ω� ��� �������� ��� ��� �� ��� ��� ���������� ��������� ����� ��� m for cavities and 500 Ωm for the background material. Using the electrical resistivity values, the P wave velocity and the seismic ray coverage, two correlation parameters were calculated for each inversion cell. The first (Fig. 2d) is the product between the normalized logarithm of conductivity and the normalized P wave velocity, “N (Logσ) * N (Vp)”. The second one (Fig. 2e) is the product between the first and the normalized seismic rays coverage, “N (Logσ) * N (Vp) * N (d)”. It was finally applied a statistical approach based on non-hierarchical (k-means) cluster analysis (Fig. 2f) of the P-wave velocity, the density of the seismic rays and the electrical resistivity of the synthetic models. The distance of each element from the initial nuclei and the nuclei obtained after each iteration was calculated by the weighted sum of the euclidean distances of all considered parameters: D = a (dx 2 + dy 2 ) 1/2 + b (dC 2 ) 1/2 + c (dV 2 ) 1/2 + d (dro 2 ) 1/2 , where a, b, c and d are weights, dx, dy dC, dV and dro are respectively the differences between the spatial coordinates x and y, the density of the seismic rays, the P wave velocity and the electric resistivity. Test site data analysis. Experimental data were acquired at the test site of Parco delle Cave of Marsala (Italy, Sicily) to compare theoretical and experimental data, optimize the inversion process and simplify the step of interpretation of the same data. On-site surveys involved a phase of hypogeographical topographic surveys with tools and software used for speleological activities. In particular, the Leica X310 laser distance gauge was used with precision in measurements of 2 mm and 0.5 ° for angular measurements. Cavities in the Park of the Caves are almost all accessible (there are at least 3 entrances), in fact the relief carried out has allowed to determine the various configurations of the voids, the depths and their dimensions, allowing to rebuild the plant of the area and the reference section. The refractive seismic tomography investigation was carried out using 48 geophones with 2 m intervals between the geophones. Along the same alignment, an electrical tomography was acquired with 48 electrodes. Furthermore, the corresponding synthetic modeling of a test site was created to compare theoretical and experimental data. The inversion of the seismic data (after 50 iterations and a misfit value of 0.012 ms) has allowed to obtain the seismic rays density d (Fig. 3a) and of P-waves velocity VP (Fig. 3b). These distribution maps are characterized by a low seismic ray density and low P-wave velocity values ​in correspondence with the cavity blocks. Also the electrical resistivity model (Fig. 3c) identifies three of the four cavities present in the real Fig. 2 - Seismic rays density d (a), P-waves velocity V P (b), electrical resistivity model (c), two correlation parameters (d and e) and cluster distribution (f) for the synthetic model showed in Fig. 1.