79 3. Seismic tomography • the separation of up-going and down-going P-waves and S-waves; • the recombination of P-P and P-S up-going waves for the X and Z-components. The following points were noted after the analysis of the processed data (Figure 3.12): • Y-component data present very weak energy compared with that of X and Z-components. For simplicity, this component was disregarded in further processing; • weak up-going S-P and S-S waves; • strong reflected P-P and P-S waves; • some hyperbolic-shaped arrivals, probably due to fault diffractions (see at 4,000 m; 1,700 ms). Figure 3.12 Two-component (X, Z) VSP field data input to diffraction tomography. The Y-component was disregarded due to its weak energy. Adapted from Beydoun et al. (1990). 3.3.1.3 Diffraction tomography processing In this example, the imaging technique deals with the processed X and Z-components of the data, mainly consisting of up-going P-P and P-S waves. The 1D initial elastic model (P and S-wave velocities and density) was created by the combination of geological and geophysical information available for the region. The target zone, covering the reservoir area, is a rectangle extending from 50 m to 550 m east of the borehole with depths from 3,400 m to 4,400 m, discretized by a uniform square grid of 10 x 10 m. The selected field data were 86 VSP levels, ranging from depths of 2,000 – 4,150 m within a time window of 1,400 – 3,400 ms.
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