131 3. Borehole geophysical methods geothermal project, in southern Luxembourg. The VSP was recorded with a depth sampling interval of 5 m in the 20–330 m depth interval. The source is a vibrating source emitting a sweep in the 20–120 Hz frequency band. The offset of the source from the borehole head is 8 m. The time sampling interval is 0.5 ms. Figure 3.10 shows the VSP sections, after amplitude compensation, observed on the vertical geophone and on the hydrophone. On the vertical geophone section, we observe a downgoing P-wave, strongly attenuated in the 150–200 m depth interval. We note the presence of both a downgoing Stoneley wave attenuated from 150 m and a fluid wave (with a propagation velocity of 1540 m/s) in the 150–200 m depth interval. On the hydrophone section, we observe the downgoing P-wave with a conversion to a Stoneley wave at a depth of about 200 m. We also observe a strong downgoing Stoneley wave with a set of reflected upgoing Stoneley waves, the strongest of which occurs at the depth where the converted downgoing Stoneley wave is created The processing sequence includes amplitude recovery, picking of the arrival times of downgoing wave fields, wave separation of downgoing and upgoing waves, both for P-wave and Stoneley wave. Figure 3.11a shows the extraction of dogoing and upgoing P-waves. Picking of the arrival times of the downgoing wave fields (P-wave and Stoneley wave) is used to compute time versus depth laws (Figure 3.11b), interval velocity logs, and attenuation logs after flattening of the downgoing wave fields (Figure 3.12). The reconnaissance borehole crosses, after a few meters of landfill and alluvium, that is unconformably underlain by rather similar mainly marly formations dating from the Upper and Middle Liassic, showing slight facies changes towards more silty and sandy or more calcareous facies (units lo4 to lo1a and lm3 to lm1; Toarcian to Pliensbachian, Lower Jurassic). (a) (b) Figure 3.11 VSP vertical geophone processing: Down going and up going P-wave separation (a), Time versus depth law and P-wave interval velocity log (b) (after Mari et al., 2024).
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