GNGTS 2023 - Atti del 41° Convegno Nazionale

Session 3.1 GNGTS 2023 tunnel significantly alters the equilibrium of forces close to the hole and creates the so-called excavation damage zone (EDZ). The EDZ usually includes a very thin portion of the tunnel walls (less than 1 m in depth) in which a reduction in the elastic properties is expected due to the cracks and fractures induced by the excavation. Consequently, seismic data from in-mine surveys include guided waves propagating within the EDZ and surface tunnel waves, which can superimpose and create complex propagation patterns that cannot be handled with traditional surface wave dispersion analyses. Here, we aim at using the surface wave attributes of Colombero et al. (2019) for detection and location of structural discontinuities on in-mine guided and tunnel surface seismic data, without discriminating them. We apply the method to a synthetic data set to verify the possibility of fast fault identification and dipping estimation directly from the raw data. Method We consider four surface wave attributes, highly sensitive to lateral variations, as implemented in Colombero et al. (2019) for multifold data. In summary: ● Energy (Nasseri Moghaddam et al., 2005) is the sum of the squared amplitudes of the seismic traces after geometrical spreading correction. Energy peaks are expected for the seismic traces recorded on low-velocity anomalies, such as faults and shear zones, as a result of the energy trapping and back reflections at the discontinuities edges. ● Autospectrum or autospectral density (Zerwer et al., 2005) is the sum of the squared real and imaginary values of the Fourier transform at each frequency. Consequently, it displays the distribution of the energy content at different frequencies/wavelengths, supporting the previous attribute interpretation and also providing a rough indication on the depth of the detected anomalies. ● Energy decay exponent γ (Bergamo and Socco, 2014) is the exponents that relates the offset and energy ratios of two subsequent recordings as: (1) +1 = +1 ( ) −γ , where and are the energy attribute and the distance from the source for the ith trace. Strong deviation of γ from zero indicates energy decay or concentration caused by the lateral heterogeneity. The energy decay exponent is computed for trace windows having both positive and negative offsets. In presence of lateral discontinuities, they are expected to show marked opposite oscillations at the lateral interface linked to the direction of wave propagation. In particular, energy concentrations can be observed passing from high-velocity to low-velocity materials, whereas energy decays are expected going through the opposite material contrast.