GNGTS 2017 - 36° Convegno Nazionale

630 GNGTS 2017 S essione 3.2 Seismic tomography. The seismic tomagraphies were achieved through a 24 channel seismograph with improving signal-to-noise ratio, using 12 vertical geophones (14 Hz). Energizing was obtained through a 8 kg trigger hammer source. The A-A’ profile (Fig. 2) was deployed parallel to the central aisle of the church, using 12 sensors with 3 m interval pitch. A total number of five shots, two of them external to the profile (3m away from the last geophones), 2 intermediate and 1 in the central part of the profile, were done in order to obtain an optimal distribution between recording and energizing points. The test therefore stretched for 39 m shot-to-shot total length. The B-B’ profile (Fig. 2), orthogonal to the previous one, was performed using 2 m inter- geophonic distance. Similarly to the previous profile, a total number of 5 shots, two of them external to the profile (4 m away from the last geophones), 2 intermediate and 1 in the central part of the profile, for a total length of 30 m, were performed. In order to enhance the signal-to-noise ratio, the stacking technique was adopted. The acquired records were processed through the Generalized Simulated-Annealing Optimization (Pullammanappallil and Louie, 1994). The tomographic analysis (Fig. 3c) was performed by using the softwares SeisOpt ® Picker e il SeisOpt ® 2D (Optim, 2003). The first software allowed us to performed the picking of the waves onsets whereas, the second one permitted to get the theoretical model akin to the experimental data, making therefore possible to point out the mechanic parameters of lithotypes characterizing the investigated area. Discussion and concluding remarks. As concerns the georadar investigation, the reflections are more easily detected in the scanned images obtained through 600MHz aerial (Fig. 3a), which allowed us to obtain an higher resolution, with respect to the 200 MHz one, and permitted also to reach a depth of about 3 m, which is suitable for our investigation purposes. The radargrams show the presence of many electromagnetic reflectors having different geometric features whose essence however cannot be easily deduced. Such reflectors can indeed be associated both to the existence of structures due to the anthropic activity (ruins of the old edifice) and to the presence of fractures in the lava basement located underneath the church. Taking into account the good penetration of signal, the detritus level existing under the Fig. 3 - a) Longitudinal section of the area where the main altar is located with a GPR scan, showing the reflections related to the crypt identified beneath this area; b) planimetry of the church with the depth-slice related to 2 m depth; c) southern prospectus of the church with the A-A’ seismic tomographic section.