64 Well seismic surveying and acoustic logging Figure 2.10 Separation of downgoing and upgoing waves by polarization filter (from Mars et al., 1999). Figures 2.11 and 2.12 show the results of the near surface VSP processing, acquired in the 25 to 90 m depth interval. The source is a weight drop (Figure 2.5), the well receiver is an anchored geophone (Figure 2.4a). There is a 5 m distance between 2 measurement points. The listening time is 250 ms. The time sampling interval is 0.25 ms. The previously described processing sequence was applied. The results of the processing are shown in Figures 2.11 and 2.12. The upgoing and downgoing waves were separated by an f-k filter. A reference geophone, sited near the wellhead, was used to check the repeatability of the source in terms of time (TB fluctuation) and amplitude (energy). This process is essential for attenuation measurements (Figure 2.12c).
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