GNGTS 2023 - Atti del 41° Convegno Nazionale

Session 3.1 GNGTS 2023 ● Attenuation coefficient (Bergamo and Socco, 2014) shows the attenuation of seismic α wave at each frequency and can be computed from the decomposed energy attributes of the two subsequent traces as: . (2) , +1 = , −2α ( +1 − ) Stacking the results (in absolute value) for positive and negative offsets, a clear delineation of the lateral discontinuity position can be obtained. Synthetic model and numerical simulations We created a 3D synthetic model based on the physical and elastic properties of a hypothetical deep mine. The model is 1000 m × 500 m × 500 m in x, y and depth directions, respectively. The tunnel is an 8 m × 8 m rectangular horizontal tunnel in the centre of the model. The model includes 1 m of EDZ around the tunnel and a 5-m thick dipping fault (45° from the horizontal plane) crossing the tunnel. The properties of the background homogenous environment, EDZ and fault are provided in Tab. 1. Tab. 1: The elastic parameters of the synthetic model. VS (m/s) VP (m/s) Density (kg/m 3 ) Background 2700 4450 2800 EDZ 2360 3850 2600 Fault 750 1500 2200 In Fig. 1a, we show the vertical section of the S-wave velocity model along the tunnel at y = 250 m. In Fig. 1b, we show a zoomed view (for better visualization of EDZ) of a horizontal section for S-wave velocity at depth 250 m. We use SOFI3D, an open-source viscoelastic finite-difference algorithm (Bohlen, 2002), to simulate the 3D wave propagation in the model. The method discretizes the wave equation using second-order spatial and temporal finite differences over a staggered grid (Virieux, 1986). The free surface condition along the tunnel is satisfied by imposing the properties of air to the tunnel volume and without explicitly defining free surface conditions across the boundaries of the tunnel (Bohlen and Saenger, 2006). We use grid size of 1 m in each direction, that is small enough to mitigate numerical dispersion. The computations are performed every 0.1 ms to honor the Courant-Friedrich-Lewey criterion and maintain stability. We impose constant quality factors ( and ) equal to 1000, considering spatially invariant intrinsic attenuation. We simulate a seismic experiment with a line of receivers on the floor and one on the ceiling of the tunnel. We define four locations (X = 40 m, 347 m, 654 m and 961 m) on the floor of the tunnel to impose vertical point source (65-Hz Ricker wavelet). The receiver arrays start at X= 41 m up to