32 Well seismic surveying and acoustic logging 1.2.3.2 The acquisition device It is necessary to dissociate the acquisition and the analysis of the compression (P) and shear (S) waves. Until the start of the 1980s (Figure 1.17), the crosshole test was carried out as the source moved forward by striking the head of the drill string. This method has evolved through the development of an emission probe pressed against the borehole wall. This has the major advantage of separating the activities of the borehole drilling group from those of the geophysics measurements team. However, this brings source coupling problems. Figure 1.17 Historical crosshole measurement method, applied in France until the beginning of the 1980s. The typically recommended crosshole method (Figure 1.18) requires 3 boreholes to calculate the velocity between two receiving boreholes without needing precise knowledge of the wave generation time (t0, zero time). Today, this approach is questionable because very often the attenuation is such that the signal of the second borehole is difficult to exploit, and also the shot recording is no longer a technical limitation. Nevertheless, working with only one receiving borehole requires greater rigor, in particular, a calibrated measurement chain is required. As an example, there can be drift between the electric impulse and the generation of a sparker signal (Figure 1.19) due to the wearing of electrodes.
RkJQdWJsaXNoZXIy NjA3NzQ=