31 1. Shear velocity measurement in boreholes results interpretation must be adapted (receiving boreholes at 90° with directions parallel and perpendicular to those of the anisotropy). • Finally, if the test is carried out in a medium with strong vertical anisotropy related to thin beds, the measurement system must also include a specific SH source (sparker P-SH). 1.2.3.1 Prerequisites The quality of the casing sealing has a much stronger impact on the crosshole measurement than it does on the downhole measurement, because the signals are of higher frequency. Velocity calculation requires the precise knowledge of the distance between the boreholes. Human error is possible with inclination measurements (see Figure 1.15) and a calibration defect can affect the trajectory measurements (see Figure 1.16). It is therefore vital to apply a procedure on the surface to control for the directions given by these measurements. Figure 1.15 Example of a result of 2 distinct trajectory measurements and one inclination measurement in the same borehole (InterPacific Project). Figure 1.16 Example of a result of 3 distinct trajectory measurements in the same borehole (InterPacific Project).
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