90 A new concept of karst development based on hydrogeology and geophysics Fig. 2a) with an in-line extension of 240 m, while a cross-line shot gathers results in a vertical section of only 120 m extension along the in-line direction (the red arrow on the location map of seismic lines, in Fig. 2a). Figures 2c and 2d show examples of an in-line shot (Fig. 2c) and a cross-line shot with a lateral offset of 60 m, respectively (Fig. 2d). The range of offsets was selected to optimize the quality of the seismic image over the reservoir depth interval, i.e., between 40 m and 130 m. A 40 m minimum offset distance was chosen to reduce the influence of the surface waves. The timesampling interval was 0.25 ms, and the recording length was 0.5 s. To perform the time-to-depth conversion of the seismic block, a vertical seismic profile (VSP) was recorded in borehole C1, using a vertical geophone as a borehole sensor. The source is a light seismic source (weight dropper). The sensor was clamped in the borehole, in the 25–120 m depth interval, with a depth sampling interval of 2.5 m. The time-sampling interval was 0.25 ms, and the recording length was 0.5 s. The VSP record is shown in Figure 3a. The vertical axis represents the recording time, and the horizontal axis represents the depth locations of the borehole sensor. The first arrival wave is the down-going P-wave. The picking of its arrival times at the different sensor locations is used to compute both the time vs depth law and the Rms-velocity log (Fig. 3b). Acoustic data were recorded in borehole C1. The acoustic probe (Gaudiani, 1979) is a flexible monopole tool holding a source as a magnetostrictive transducer and a pair of far receivers (3 and 3.25 m offsets beneath the source). The acoustic data were recorded in the 1–20 kHz frequency band. The sampling intervals are 5 µs in time and 5 cm in depth. In a vertical well, monopole tools can enable the recording of five propagation modes including: refracted compression waves (P), refracted shear waves (S, only in fast formations VS > VP fluid, P-wave velocity of the borehole fluid), fluid waves (F), and two dispersive guided modes as pseudo-Rayleigh waves (in fast formations), and Stoneley waves (ST). The acoustic logs associated with the different waves are very high-resolution logs and can be compared with core measurements. The acoustic logs currently obtained for each type of wave are velocity or slowness logs, frequency, and attenuation logs. Full waveform acoustic measurements can be represented as constant-offset sections. A constant-offset section is a set of acoustic records represented as a function of depth and obtained with a fixed source-to-receiver distance. Figure 3c shows the 3m constant-offset acoustic section, with the identification of the refracted P-wave (P), the converted refracted S-wave (S), and the Stoneley wave. One can notice a strong attenuation of the different waves (P, S, Stoneley) in the 50–53 m depth interval, which corresponds to a karstic level (Mari & Porel, 2015). Figure 3d shows the velocity logs computed from the picked arrival times of the refracted P-wave at the two receivers of the acoustic probe. The acoustic velocity log (Fig. 3d left) has been filtered (Fig. 3d right) to have a vertical resolution compatible with the seismic resolution to invert the seismic data to recover seismic velocities from seismic amplitudes.
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