25 1. Shear velocity measurement in boreholes Seismic source coupling The coupling of the seismic source is also an important point (Figure 1.9). Ideally, the source should be positioned on natural ground (Figure 1.9a). In the presence of backfill, which strongly attenuates seismic waves (Figure 1.9b), it may be advisable to dig out the area until the natural terrain can be accessed. It should be noted that drilling and cementing techniques have a strong influence on signal transmission quality, as shown in the example in Figure 1.9c. a b c Figure 1.9 Effect of source coupling in downhole operation; a: Downhole with a source positioned on natural ground; b: Downhole with a source positioned on backfill (water table at 9.8 m); c: Downhole on the same site as for a and b, but drilling carried out with sonic corer (4 m water table, outside of water table the signal is polluted by refracted waves because the offset is too large).
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