72 Well seismic surveying and acoustic logging The geology of the well can be summarized as follows: • 0 to 3 m: outer sand layer, • 3 to 6 m: under sand layer, • 6 to 21.5 m: sandy-gravelly alluvial deposits, • 21.5 to 225 m: succession of layers of fine sand, clay and silt; sand layers dominate down to 41 m, • 225 to 280 m: shale, weathered to varying degrees. IFP Energies nouvelles applied the following processing sequence: • normalization of traces, • spectral equalization deconvolution to attenuate the effect of casing-related waves and to increase the resolution, • picking of first arrivals and velocity calculation, • separation of upgoing and downgoing waves by wave number filtering, • deconvolution of upgoing field, • flattening of upgoing field by the application of static corrections, • summation of traces within a corridor. The deconvolved upgoing field (2.17a) is offset in time by the application of a static correction defined at each depth by the vertical time to put the events associated with subhorizontal reflectors in double time and thus enabling a direct comparison with a surface seismic recording near to the well (2.17b). The diffraction hyperbolas are neither flat nor focused in this phase of processing, as shown by the oblique diffractive event visible before 250 ms in the Figure 2.17b. The upgoing waves that have been deconvolved by the downgoing field and flattened can also contain upgoing multiples. To eliminate the effects of the upgoing multiples, a narrow stacking corridor was chosen so as to retain only the reflected signal received immediately after the first arrival. The upgoing waves in the stacking corridor are added to obtain a zero-phase multiple-free stacked trace comparable to the migrated surface seismic trace closest to the well. Figure 2.18 also shows that the diffracting event before 250 ms is eliminated by simple summation on the VSP stacked trace.
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