GNGTS 2014 - Atti del 33° Convegno Nazionale

GNGTS 2014 S essione 3.1 93 a final velocity models that are characterized by an average RMS traveltime error of about 2.5 ms. Larger residuals, between 3 and 4 ms, are occasionally observed on some profiles with a smaller signal to noise ratio, as the profiles near the road. These profiles, contaminated by ���� low- frequency random noise, are consequently characterized by a greater uncertainty on the first arrivals picking. ����� ����������� �������� ������� ��� ���� ������� ����� ���� �������� �� Final tomographic velocity models, for each seismic line, were exported as ASCII files and velocity values were interpolated in the 3D space defined at the surface by the profile location. The ray coverage allowed ����������� ��� ����� �� ��� ���������� ����������� determining the areas of the subsurface effectively resolved by the seismic inversion. All areas of the tomographic models outside the ray coverage have been blanked (Fig. 3). Results. The resulting tomographic models (e.g. Fig. 2) show P-wave velocities that vary from a minimum of about 200 m/s at the surface and increase up to about 1500 m/s at the bottom where the CI basement is reached. It is evident the presence of many velocity lateral variations, which are generally located mainly at depths between 10-20 m from topographic surface. The profiles of group 1 and group 2 (see Fig. 2) show a very articulated CI basement, characterized by an average velocity of 1250 m/s. The velocity anomalies found in the basement, according with the anomalies found on the pilot profile, suggest the presence of underground cavities; in addition, they are consistent on adjacent profiles and appropriate in terms of location. The tomographic models of group 3 (see Fig. 2) were acquired with a different azimuth (approximately WNW - ESE) are oriented approximately 90° to the direction of the profiles of the group 2. The tomographic sections related to these three profiles differ from the others by the absence of major and evident lateral variations of velocity. These profiles, in fact, show a sub-parallel velocity distribution, with a tendency of the faster layer (i.e. the top of the CI) to widen towards the end of the profiles. On this set of profiles, there is no evidence of voids in the CI tuff, as found on the other sets of profiles. Spatial interpolation of the velocity information has allowed us to build a three-dimensional model the top of CI fm. (Fig. 3c), which clearly shows both location and dimensions of the voids. Four core-drillings were done in the area covered by the profiles of group 2, in order to validate the tomography results. These logs explorations are in perfect agreement with our results. Fig. 3 – a) Syntetic geological model of pilot profile site, from core-drilling analysis; b) tomographic velocity model of pilot profile shows an anomalous velocity associated with known cavity (red box); c) 3d model of the top of CI basement from the profile of group 2.