29 1. Shear velocity measurement in boreholes To ensure the reliability of the analysis for the first 10 meters, a direct modeling or iterative inversion approach is possible (Figure 1.13). That is to say that, the direct calculation of travel times, for a given velocity model, must integrate the direct and refracted paths. Then, depending on the difference between the measured times and the calculated times, the model is adjusted. This iteration is repeated until the error criterion is acceptable. Currently, software packages available on the market are not optimized for borehole measurements and do not directly differentiate between direct and refracted waves but take straight and curved raypaths into account. The straight rays correspond to the transmitted travel paths and the curved rays to the refracted waves. 1.2.1.3 The specific example of offshore It is possible to carry out a P downhole survey at sea in an uncased borehole, using a source such as an air gun or sparker near to the surface. It is, however, much more complicated to find an S source that is operational underwater and a 3-component receiver. Finally, if the ground is unstable, a P downhole survey through an unsealed protective casing can work if the terrain crossed is sufficiently plastic to establish an acceptable coupling with the casing (see Figure 1.14). Figure 1.14 Downhole at sea in an uncased hole and through an unsealed protective casing (Fugro for EDF).
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