GNGTS 2018 - 37° Convegno Nazionale

GNGTS 2018 S essione 2.2 393 error. Values of Q P from SR method are included from 0.78 to 8.26. In general, the calculated values ​with the Risetime method are higher, considering mean values. The authors attribute this discrepancy to mode conversions, scattering around the borehole (Morozov, 2008), and signal dispersion with depth. For obvious technical issues, the values ​obtained with the RT method can only be relative to depth ranges, as it is not possible to obtain punctual values. The following Table 2 shows the values ​obtained with the two different methods, averaging the values ​obtained for the SR method with respect to the intervals. Tab. 2 - Parameters for field data acquisition. Quality Factor (QP) Depth range (m) Spectral Ratio (mean) Risetime 0-8 (terreno da riporto e sabbie medio-fini) 1.41 1.93 8-32 (ghiaie) 2.69 3.59 32-47 (sabbie ghiaiose) 4.31 6.88 Fig. 3 - a) Spectral Ratio (15 meters depth) obtained using the left side of equation 1-1, b) Risetimes vs depth. c) Risetimes vs depth with Q P values at each interval, d) Comparison of near-surface Q P factors estimates using Spectral Ratio and Risetime methods. Conclusions. The application of the methods for Q estimation using near-surface surveys is essential to provide an additional and important geophysical data for site characterization and seismic modelling. The medium-small depth (<100 m), which is generally correlated with a great seismic attenuation (low Q ), requires an accurate study on the choice of the method to be applied. In this case the dispersive nature of the seismogram suggest to the application of the Rise time method, still not used in the medium-small depth. In order to have a confirmation of the reliability of the data, that show an high attenuation ( Q <5), the results were compared by