37 1. Shear velocity measurement in boreholes is possible to detect refracted waves and confirm their existence solely based on the visual analysis of the signals, crossed with the information from the cores. Figure 1.23 Examples of crosshole recording. a) left, P Crosshole (InterPacific, water table at 3 m), b) right EDF site (water table at 6 m). On the S-wave crosshole recording, on the left of Figure 1.24a, there is a fast S first arrival at 22 and 23 m in depth (shown in red). At greater depths, this S arrival is later. There are therefore two areas for which S velocities can be calculated at 500 and 260 m/s (see velocity model on the right). Directly under the fast formation, at 24 and 25 m deep, we observe a first arrival with little energy (shown in green). This is a refracted arrival, with the transmitted arrival (shown in red) keeping the same phase. The phenomenon can be explained by calculating the distancetime graph for example at 2 m from the interface between the two media (i.e. 25 m deep, see Figure 1.24b). Figure 1.24b shows that at the distance of the first borehole (X = 3 m) the direct wave (P: solid blue line, S: solid green) arrives first, while in the second borehole (X = 6 m) it arrives after the refracted wave (blue dotted line for the P-wave and green for the S-wave). By carrying out this analysis at depths of 24 and 26 m, we can then draw the fastest paths, which are illustrated by the green arrows on the left of Figure 1.24a.
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