36 Well seismic surveying and acoustic logging 1.2.3.3 Crosshole analysis At first glance crosshole analysis appears simple and robust, and only requires the picking of the first arrival of the selected wave (P or S), and to divide this travel time by the distance between the source and the receiver, to obtain the velocity of the medium. However, several cautionary notes, described below, should be considered. As discussed in the section about receivers, P interpretation using geophones requires the polarization of horizontal traces to optimize the signal amplitude. However, sometimes the result of this operation is less clear than when looking at the results of one of the two components, and sometimes the operation is complex, such as when the receiver has six horizontal components. Moreover, in alluviums, particularly outside of the water table, sometimes the attenuation of P-waves is such that they are no longer distinguishable from noise. Two causes explain this problem: • Firstly, drilling and cementing are delicate in this type of terrain and the result of cementing can be highly variable according to the drilling technique used. For this type of terrain there is a risk of forming a thick cement annular that does not adhere to the formation, which can create a waveguide that filters P-waves more than S-waves, because they are of higher frequency (often 1,000 Hz compared to 200 Hz). To limit this effect, the use of a sonic corer is recommended, to avoid destructuring the terrain. • Secondly, the mechanical source generates insufficient P-wave energy for this type of very attenuating terrain, so a more powerful P-wave source, such as a sparker, should be used. Figure 1.23 illustrates the above comments. The two crossholes were made in distinct sites, but both had sandy-gravelly alluvial cover down to 22 m and 21.6 m respectively, over a clay or marly substratum. In both cases, there is a change in the frequency content between the alluvium (low frequency signal) and the substratum (high frequency signal). However, in the second case the first identifiable arrivals in the first 19 meters are not P-waves, because the times correspond with those of the S-waves probably picked on the horizontal components. On the other hand, in the first case it is possible to identify P arrivals over the entire depth. Attenuation mainly affects distant receivers in crossholes with several receiving boreholes. This justifies making an acquisition with a specific P source that is more powerful than a mechanical source, which is made to maximize the energy of the S-waves. In addition, it is entirely possible to limit the crosshole to two boreholes, but only on the condition that the results are consistent with those of a downhole and/or uphole made in one of the two boreholes. The crosshole method can also be affected by refracted waves (especially when the offset is greater than 6 m). The example presented in Figure 1.24 illustrates that it
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