75 3. Seismic tomography 3.2.2 Data processing Only two preprocessing steps were applied to the multicomponent data set, namely: • a bandpass filter (20-120 Hz); • a re-orientation following the projection along the source-receiver direction R, its normal in the source-receiver plane N and the binormal B, orthogonal to the source-receiver plane. We noted that the component R (Figure 3.10) contains clear direct down-going P-waves, followed by up-going S-wave reflections. The component N has obvious down-going S waves, appearing as a train of quasi-parallel events spread over about 150 ms. Figure 3.10 Filtered and reoriented data. R component along the source-receiver direction; N component perpendicular to R, in the source-receiver plane; B component, orthogonal to R and N, so normal to the source-receiver plane. Adapted from Becquey et al. (1992). The imaging from the cross-hole data set was inspired from a traditional offset VSP. Therefore, based on other previous VSP data acquisitions of P and S-waves at the vertical borehole, a velocity-depth model was built and used by the VSP-CDP time technique to transform the S-S reflected data (Figure 3.11). More details on the VSP-CDP time technique are available in chapter 2 of “Well seismic surveying and acoustic logging” (J.-L. Mari and C. Vergniault, 2018).
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