GNGTS 2014 - Atti del 33° Convegno Nazionale

GNGTS 2014 S essione 3.1 37 which can be well assimilated to 1D profile, being characterized by deep spatial homogeneous alluvial deposits (Fontana et al., 2008). It is expected that in such environment passive linear arrays retrieve same results for any orientation the operator can choose, being on not laterally variable environment. The acquisition has been performed using an L-shaped array: a linear array of 24 receivers (4.5 Hz natural frequency) along the E-W (x) direction and a perpendicular linear array of 24 receivers (4.5 Hz natural frequency) along the N-S (y) direction. The receiver spacing was 4m for both branches, sampling rate was 2 ms. Controlled source records were acquired besides the passive data, to test the validity of LeMi approach, using a seismic gun as source with record lengths of 2 s. For the passive analysis record lengths was 30 s for each noise window. One of the main advantage of the proposed L-shaped array stays in fact in the possibility to locate a shot point at the vertex of the L-shaped array, in order to simultaneously provide two active tests along the 2 directions with common multichannel analysis of surface wave. Active data results to be used as benchmark are plotted in Fig. 2. Fig. 3 summarizes the advantage of application of the passive LeMi approach in the Badia test site. As it could be seen from data, despite the strictly geological 1D conditions confirmed by several boreholes, the Kx and Ky wavenumbers referred to the two branches of the L shaped passive array present different values. This implies the site was afflicted by Fig. 1 – a) Synthetic example of ReMi spectra on two orthogonal arrays with a concentration of source in a sector. Left: plan view of the simulated geometry, with sources in red, receivers in blue. Centre: ReMi f-k spectrum on the y (N-S) array branch. Right: ReMi f-k spectrum of the x (E-W) array branch. b) Picking of the wavenumber for the case of Figure1a and phase velocity/frequency transform. Kx (black) refers to the x direction and Ky (red) refers to the y direction. In blue the combination to estimate the true wavenumber from LeMi procedure. Note as in this case the blind orientation of a linear array can lead to great overestimation of the phase velocity while the L-shaped array allows to retrieve the true phase velocity.