46 Well seismic surveying and acoustic logging Table 1.5 Velocity measurement (Vp, Vs) and tools (DH, sonic, PSSL). Features Vp Vs V stonley => Vs Depth (m) DH Sonic PSSL DH Sonic PSSL Sonic 0-50 Cased hole 55-100 Slow formation, open hole 100-225 225-280 Bedrock, open hole Table 1.5 summarizes the validated measurements, with dark green shading to show those selected to carry out Vs logging, and light green shading to show those that make it possible to verify the continuity between the different techniques applied. Figure 1.31 is a composite that illustrates all of these conclusions for S measurement. 1.5 Conclusion This chapter has detailed the methodology for the acquisition and analysis of invasive measurements in downhole, uphole and crosshole-type boreholes and loggings. It brings a new perspective compared to the ASTM standards, particularly regarding: • the possibility of controlling cementation, • the need to establish calibration procedures for geophysical measurements, and trajectometry and inclinometry tools, • the impact of downhole tube waves, • the possibility of working with 2 crosshole boreholes, providing that certain precautions are taken, • the need to dissociate the P and S acquisition, whether in downhole or crosshole, • the need for a 2-phase analysis to avoid artefacts related to real paths or refracted waves. For deep reconnaissance where an uncased hole can be used, PSSL is undoubtedly the best option. The measurement can be complemented by Stoneley wave FWF sonic logging to improve the vertical resolution. However, it is preferable to have a surplus of data for the first meters, with a downhole or crosshole, because this section is usually cased, outside of the water table, and an invasion of the formation by the sealant, which would distort PSSL measurement, cannot be excluded. For the higher part of the borehole, it is also recommended to carry out a “caliper” as well as a deviation measurement (trajectometry). Finally, the choice of PSSL probe is important because PSSL probe performance does not seem to be uniform. A detailed study of the performances of these probes remains to be carried out.
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