GNGTS 2019 - Atti del 38° Convegno Nazionale

GNGTS 2019 S essione 2.2 367 Data processing. In VSP near surface processing, the coherent signal for Q estimates is mainly due to downgoing wavefield and the primary objective is to isolate only this wavefield. A total of 44 records of P and SH waves was used in this analysis (Fig 1a). To increase the S/N for Q assessment, we developed a pre-processing flowchart in Python we selected the traces (stacked up to eight times) after visual inspection. An important step is then the calculation of the time delay between a given pair of receivers (Δ t = ( t – t 0 ). It is performed muting the seismic signal and applying 50-ms window functions for SH waves and 20-ms window functions for P waves. Filtering the frequency of interest is essential and we applied a Blackmann-Harris tapered time window function at each trace in order to improve the quality of the spectrum reducing leakage. Fig. 2 - (a) FFT at 10-15-20 meters depth, (b) Reduced spectral Ratios, (c) Rise time vs travel times. Data results. Spectral evaluation of the signal indicates that the most of energy spectrum lies within 10-100 Hz, with the central frequency at around 50-60 Hz (Fig 2a). To select the frequency content for Q estimation, we consider a logarithm of amplitude spectrum ratio for different frequency band for P and SH waves. The reduced spectral ratios ln[ A ( ω )/ A 0 ( ω )], were calculated for the different receivers in according with equation 1-1. Fig 2b displays the logarithm of amplitude spectral ratio for the bandwidth 0-150 Hz for 3 intervals (43 total intervals). The so called reduced spectral ratio shows a very good linear trend between 15 and 50 Hz for SH waves, especially for deeper pairs of receivers. P waves shows a linear trend between 25 and 55 Hz. We can see that within the frequency band 15-50, these curves can be approximated with a linear function. A linear regression is performed in the selected frequency range to calculate the apparent attenuation γ ( ω ) for each receiver pairs. The results are consistent along the borehole a part from a few outliers, probably due to scattering phenomena, and/or defective coupling at a few receiver depths. We can notice a step increase of the apparent attenuation between 5-10 meter depths, especially for SH waves. The slope