30 Well seismic surveying and acoustic logging 1.2.2 Uphole An uphole is similar to a downhole, but with the source in the borehole and the receiver on the surface. The practice is more limited because S sources in boreholes generate a signal that fades over a relatively short distance. Even for P-waves, with a relatively powerful source (sparker) it is necessary to perform a large number of stacks. However, Figure 1.13 shows that depending on the context it is possible to obtain interesting results with P-waves (left graph). In addition, it can be useful to carry out an uphole when the measurement is coupled to crosshole acquisition, because this only requires the addition of a 3-component receiver on the surface. 1.2.3 Crosshole The principle consists of measuring the travel time of compressional and shearing seismic waves, transmitted directly between a seismic source in a borehole and a 3-component receiver in a neighboring borehole. The compressional wave (P) propagates through the horizontal plane as well as the horizontal component of the shear wave (SH). The vertical component of the shear wave (SV) propagates in the vertical plane. Therefore, the energy of the P and SH wave is mainly distributed on the horizontal receivers and that of the Sv wave on the vertical receiver. The result of a crosshole measurement is often blindly accepted as a reference, but the method can face the following limitations, it can: • be strongly affected by refracted waves in surrounding indurated strata, • present “spatial aliasing” if bed thickness is less than that of the intervals, • be affected by highly inclined formations, • be strongly affected by poor cementation (casing-sealant-formation connection), • be strongly affected by damage in the vicinity of the borehole. Consequently, it is recommended to perform a downhole and/or uphole recording to complete and validate the crosshole, assuming that the anisotropic attenuation is negligible. Specific cases: • With highly heterogeneous geology, only seismic tomography can provide reliable information for the P-wave profile. Therefore, in an environment with a known heterogeneous nature, it is also recommended to carry out an additional P crosshole acquisition with a hydrophone array so as to exploit the tomography data. For S-waves, the implementation of more than 2 anchored sensors is complex in the geotechnical field and therefore downhole or uphole recording generally has to suffice. • Similarly, if the test is carried out in an environment with high horizontal anisotropy (e.g. beds fractured in a certain direction), the measurement system and
RkJQdWJsaXNoZXIy NjA3NzQ=