135 3. Borehole geophysical methods shown both in time and depth (Figures 3.16c and 3.16d), after depth conversion using the time versus depth conversion law (Figure 3.15b). The synthetic seismogram enables the identification of reflectors in the depth range where the logs have been recorded. The VSP stacked trace allows the identification in the same depth range, but it also enables the prediction of reflectors under the well, particularly in the 90 to 140 m range. (a) (b) (c) (d) Figure 3.15 Processing of a near surface VSP, after Mari and Vergniault (2018). (a) Raw data, (b) vertical time and interval velocities, (c) downgoing P-waves, (d) P-upgoing waves.
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