GNGTS 2019 - Atti del 38° Convegno Nazionale

368 GNGTS 2019 S essione 2.2 values ​are divided by a minimum of 0.039 up to a maximum equal to 0.138 recorded in the last meters for P-waves, and a range of 0.005 up to 0.092 for SH waves measurements. We can now obtain intrinsic interval quality factor Q P and Q S estimates from the different receiver pairs and different depths (Fig 3a). A good correlation with the borehole stratigraphy is obtained, with changes in absorption values exhibiting some anomalous peaks at the passages between layers, especially for greater depths of investigation. The rise time method is advantageous because a very short part of the seismogram is required (Fig 2c). We directly isolated each trace with a short 20 ms time windows by using a Blackmann-Harris window function. The rise time exhibits values increasing with the depth of investigation, due to the widening of the first cycle of the signal. Outliers are directly rejected from the processing. By operating a linear regression over an a priori layer division, we can estimates intervals Q in according with Equation 1-2. The interval Q values obtained, when compared to the values ​derived from the SR method, show an excellent correspondence and little uncertainty (Fig 3b). The values obtained by the rise time method are generally higher, as observed in a previous study performed only for Q P (Cercato, Desideri, 2018). Some authors attribute this discrepancy to mode conversions, scattering around the borehole and signal dispersion with depth (Morozov, 2008). Fig. 3 - (a) and (b) Differential attenuation and Interval Q p and Q s values obtained with Spectral Ration method; (c) and (d) Rise time versus travel times and Interval Q p and Q s values obtained with Rise Time method. Conclusions. The estimation of both Q P and Q S values provides an additional data for engineering applications that require an accurate and realistic seismic characterization, for instance realistic seismic response modeling. In addition to that, we have tested the application of the Rise Time at small depth (<100 m). The quality factor assessments between the two methods are consistent although the SR method is more affected by problems related to waveform and frequency content of the seismic signal. References Desideri, F.S. and Cercato M.; 2018: Estimation of near-surface seismic attenuation through VSP surveying, 38° convegno GNGTS , Bologna 12-14 Novembre 2018.