44 Well seismic surveying and acoustic logging Figure 1.30 PSSL, H component, trace collection for 4.5 ms, in two distinct boreholes but within the same homogeneous site at sea, with A and B probes [EDF-EN]. 1.4 Case study of a downhole acquisition, PSSL and acoustic logging, in the same borehole The studied borehole is complex in that it is relatively deep (280 m), it crosses terrain that is outside of the water table as well as underneath it (the water table is at 3 or 5 m), there are unstable formations that require borehole casing (alluvial), and stable ground where the borehole is supported by a bentonite based mud, and finally bedrock at 225 m deep. This high variability in geological and drilling conditions made it necessary to anticipate the implementation of various techniques for measuring the seismic wave velocity in the borehole. Ultimately, the validated results perfectly illustrate the most suited terrain for each method. Namely: • Downhole logging is suitable for a borehole that goes through unstable formations and is fitted with PVC casing,
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