26 Well seismic surveying and acoustic logging However, it is not always possible to situate the source on natural ground, but it is possible to make a short borehole to be able to use a borehole source directly under a backfilled area, in addition to that on the surface. However, it is important to use the surface source, because borehole sources are usually less powerful and there can be too much attenuation to make measurements for paths greater than 10 m (limitation on uphole measurement without the addition of a large number of stacks). 1.2.1.1.2 Receivers In downhole surveys, P-wave energy arrives mainly on the vertical component of the receiver and SH wave energy is distributed on the horizontal components. Given that a short “offset” was applied (1 to 3 m), it was necessary to pump the water out of the borehole to avoid tube waves. However, for a deep downhole, the casing should not be emptied to a depth greater than 50 m, otherwise the risk of crushing the tube becomes too great. Consequently, it is not possible to use hydrophones near to the surface to make a P-wave downhole survey. A 3-component receiver anchored to the borehole wall, either a geophone or accelerometer type, is therefore recommended. 1.2.1.2 Analysis of a downhole (DH) The initial step of the analysis involves picking the first P and S arrivals. A good quality signal will ensure that P-waves can be picked without difficulty. For S-waves, it is essential to exploit the polarization property of the wave according to the direction of the strike (Figure 1.10). For this purpose, the phase opposition of shots in the opposite direction makes it possible to unambiguously identify S arrivals (blue and black signals are of opposite sign, in Figure 1.10). Without this opposition, the recorded signal should be viewed with caution, because it may translate tube wave pollution or P-wave interference. Figure 1.10 illustrates that with a conventional source, the phase opposition characterizing the S-waves can be observed down to 50 m deep. In favorable terrain, the same types of signals are observed down to a depth of 100 m. As with seismic refraction, the first step of the analysis is to view the distance-time graph (source-receiver distance curve as a function of time). In the first 10 meters this curve differs significantly from the depth-time curve due to the offset of the source (see Figure 1.5). At this point, the path between the source and the receiver is considered linear. However, this approximation is false if the medium has noticeable velocity variations in the first 10 meters. The impact increases with the offset of the source. From the distance-time graph, a downhole analysis is carried out in depth ranges (slices). The division into slices must be related to the slope break on the distance-time graph, but also and especially in connection with the geological log obtained from cores. On the slices thus defined, the slope between the variations of distance and time of each segment provides the average velocity over the corresponding interval (see Figure 1.11). The slope of the radius linking a point to the origin gives the average velocity of the ground at the corresponding depth. Due to the picking uncertainty,
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