GNGTS 2023 - Atti del 41° Convegno Nazionale

Session 3.3 ______ ___ GNGTS 2023 receivers but from different source location within a certain distance limit to the receiver spread. We pick the fundamental mode of surface-wave as the local maxima on the spectrum. In W/D method a single relationship obtained at a reference location is used for the direct data transformation of the dispersion data (Socco et al., 2017; Socco and Comina, 2017). In laterally varying sites, the DCs are clustered using hierarchical clustering and the method is applied separately to each zone to account for significant lateral variations (Khosro Anjom et al., 2017). For each cluster, a W/D relationship and an apparent Poisson’s ratio is estimated using the method of Khosro Anjom et al. (2019). The W/D relationship is applied to all DCs of the cluster to transform them into time-average VS. Then, sensitivity of the W/D relationship with respect to Poisson’s ratio is analysed and an apparent Poisson’s ratio for each cluster is estimated. The apparent Poisson’s ratio relates the time-average VS and VP and is used to estimate the time-average VP. We use DIX-type equation with regularization (Khosro Anjom et al., 2019) to transform the estimated time-average VS and VP models into interval ones. Finally, we assemble the results from all clusters to create pseudo 2D/3D VS and VP models. Site description and data set The data were acquired in a mining site in the province of Aurignac in south France (Fig. 1a). Between the east and west side of the investigated area, a significant lateral variation exists. The eastern part is characterized by limestone and marl, while the western side contains more recent and looser deposits. The Birdwagen Mark IV off-road equipped with 24-ton vibrator source was used and the wavefield was recorded by 5 Hz vertical geophones that were 50 m apart in the north and 25 m apart in the south. The data were recorded for a duration of 5 s with 2 ms sampling rate. Here, we apply the surface-wave methods to a portion of the data from relatively flat area in the north-west of the mining pits (Fig. 1b). Application to the data set We used a window of 100-by-100 m 2 (squared shape) to select the receivers for each multi-channel DC estimation. To move on for the next DC estimation, we shifted the square by one receiver spacing (50 m). This way, the number of estimated DCs was maximized, aiming at increasing the spatial resolution. In total, we estimated 174 DCs (Fig. 2a) corresponding to the locations shown in Fig. 2b. The clustering of the dispersion data resulted into two sets of DCs, where the ones corresponding to the west side (blue in Fig. 2b) have lower velocities compared to the ones from the east (green in Fig. 2a). In Fig. 2c and 2d, we show the estimated W/D relationship and apparent Poisson’s ratio of the two clusters. The depth range of the W/D relationship and apparent Poisson’s ratio are the function of DC’s wavelengths. For both clusters, lack of short wavelength data prevented the estimation of the W/D relationships for the first 20 m. Higher velocities (larger wavelength) of cluster B enabled the estimation of W/D relationship and apparent Poisson’s ratio up to 140 m of depth, whereas this