GNGTS 2014 - Atti del 33° Convegno Nazionale

92 GNGTS 2014 S essione 3.1 analysis of a synthetic model made by Cardelli et al. (2010), moreover we acquired a pilot profile, above a cavity with known size and position. The final velocity model resulting from the pilot profile (Fig. 3a) shows that in correspondence of the cavity there is an anomalous negative distribution of Vp velocity associated to the top of the tuff basement (about 1250 m/s) that is coherent whit the size of the known cavity. ����� ���������� �� �������� Model resolution is assessed by a posteriori checkerboard test (Hearn and Ni, 1994), using an input perturbation values of ± 10 m/s in the cell with horizontal size 20 m and vertical size 10 m. Checkerboard test gives information on how data and the method used are spatially sensitive to the velocity variation in the final tomographic model (Improta et al., 2002). The anomalous velocity of Vp associated to the known cavity is in the resolved region of the model. ����� ���� ����� �� �������� �� ������� �������� ���� � ����� ������� ������ �� ����� � After this test, we acquired 20 seismic profiles with a total profile length of about 2 km. The profiles are located as shown in figure 2 in the survey area. The employed acquisition system consists of two Geometric’s GEODE seismographs equipped with vertical geophones with a 40 Hz eigenfrequency, for a total of 48 channels. Seismic data were collected using an 8 kg sledgehammer. At each shot point we stacked 3 recordings to increase the signal and simultaneously to reduce random noise already in acquisition processing. Data were recorded by geophones placed at 2 m intervals, covering a distance of 96 m for each profile. ������ Source move-up was 4 m along all profiles, in order �� ������ � ����� ������� �� ������� ���� ��� to obtain a dense network of seismic rays and consequently a high spatial resolution. In order to increase the turning ray depth the penetration of the refracted waves offline shot were also acquired. Data processing. After installing the geometry into trace header, a simple pre-processing aimed at improving the signal to noise on first arrivals was applied at recording data. This consisted of a frequency filtering, necessary to eliminate low-frequency noise caused by vehicular traffic, and a �������� �������� ����� �� ����� �� ������ ���� �� ���� ����� ������� manually checking trace by trace to remove dead or very noisy traces. The main and longer part of refraction processing was the picking of the first arrivals, performed on the entire dataset, through the commercial Landmark software named ProMAX 2D. The quality control on the readings was performed plotting time-distance curves and using the rules of parallelism and reciprocity described by Ackermann et al. (1986). The readings of the direct and refracted phases, after being checked for consistency, were inverted using a commercial software that is part of the package SeisImager®, distributed by Oyo Coroporation Ltd. and set on the work of Hayashi and Takahashi (2001). The inversion times algorithm is based on a tomographic technique of iterative reconstruction of image, known as “SIRT” [Simultaneous Iterative Reconstruction Technique; Gilbert (1972)]. The SIRT method requires an initial velocity model that was obtained by conventional procedures of refraction analysis (e.g. Burger, 1992). During inversions, ray tracing and SIRT are applied to the velocity model, until the root mean square (RMS) error on the difference between the observed and calculated travel times is minimized. ���������� ���� ���� ��� � �� �������� ��������� ���� ����� ���� ��� Horizontal cell size was 2 m. Velocity increased with depth from 200 m/s at the surface to 1500 m/s at a depth of 30 m. On average 20 to 30 iterations of ray tracing and traveltime inversions were conducted. Running the inversions with 3 nodes, we obtained Fig. 2 – Profiles location on the survey area planimetry.