GNGTS 2019 - Atti del 38° Convegno Nazionale

698 GNGTS 2019 S essione 3.2 maxima that are located at the smallest distance from the reference DC, which was chosen from the two extracted ones (Fig. 2a), depending on the receiver-couple position. A total number of 3381 path-average DCs were picked and are plotted in Fig. 2a. For the inversion, the subsurface was discretized into 100 1D models, and the coverage (Fig. 2b) was measured as the number of receiver paths crossing each model position, as a function of frequency. The plot of Fig. 3b shows that most model positions are illuminated by the data, even though, the coverage decreases with decreasing frequency (increasing depth) and reaches its minimum at the two ends of the line (dashed black lines in Fig. 3b, which show the positions of minimum coverage at the minimum frequency). In Fig. 3c, we show the V S model obtained by the inversion. Compared to the true model, the result shows great similarity, apart from the areas were coverage is minimum (black dashed lines in Fig. 3b). Applying equation 1, the pseudo-2D V S model was transformed into V Sz . V Pz was obtained using equation 2 and the corresponding υ z at each location. The static shift was calculated at the largest investigation depth (18 m), according to equation 3, and is Fig. 2 - a) Reference DC. b) W/D and c) apparent Poisson’s ratio of the two homogeneous areas (area 1 and area 2). Fig. 3 - a) Extracted path- average DCs. b) Plot of data coverage as a function of frequency. c) Inversion result. d) Inverted (green) and true (orange) one-way time (static shift). The black dashed lines in panels b, c and d indicate the positions of minimum coverage at the minimum frequency.