43 1. Shear velocity measurement in boreholes Depth (m) Vs (m/s) Crosshole Vs (m/s) Average ‐20% until 25m, 15% after Average +20% until 25m, 15% after PSSL Vs (m/s) Figure 1.29 Crosshole log vs PSSL in a cased borehole (InterPacific, Grenoble). Finally, it should be noted that the different probes available on the market clearly have different power levels. Indeed, the 2 acquisitions illustrated in Figure 1.30 were made on the same offshore site, in two open boreholes that were different but did however have key similarities. With the first probe the phase oppositions are clear for the S-waves below 10 m in depth, while the results of the second probe are unusable. The performance of all PSSL probes is not therefore comparable and a comparative study needs to be carried out.
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