156 Seismic Imaging curve is a diagram of phase velocity versus frequency and Figure 6.7 shows examples of single dispersion curves from shots located at 0 m, 10 m, 50 m, and 60 m. Figure 6.7 Extraction of single dispersion images for a 31 trace window centered at Xmid=30 m, using shots located at (a) 0 m, (b) 10 m, (c) 50 m, and (d) 60 m. On each inset, windowed shot gathers are on the left, corresponding spectrograms are at the bottom right, and computed dispersion images are at the top right. The dashed red lines on the spectrograms and dispersion images correspond to automatic low-cut frequencies defined from the spectrogram amplitude. From Pasquet and Bodet (2017). Through the utilization of multi-shot acquisition setups, a stacked dispersion curve can be obtained, and the S/N ratio improved, as shown in Figure 6.8. The dispersive events were stacked on the frequency–phase velocity panel, and then the interactive picking of events was conducted. Figure 6.8 presents two examples of stacked dispersion curves with picked events for the fundamental and first modes. The associated uncertainties are also defined.
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