100 A new concept of karst development based on hydrogeology and geophysics in the 1.2- 2 ms time interval, and Stoneley waves in the 2–3 ms time interval. Velocity, amplitude ratio, and karstic index logs are shown in Figures 10b, 11a, 11b, respectively. The acoustic section recorded in borehole M11 shows: • the 60–72 m depth interval, all acoustic waves (refracted P-wave, converted refracted S-wave, and Stoneley wave) are visible with high amplitude, indicating a compact formation; • the 72–75 m depth interval, the refracted P-wave has low amplitude, the S-wave cannot be observed. Stoneley waves are strongly attenuated, and fluid waves appear; • the 75–80 m depth interval is a homogenous formation; • the 80–90 m depth interval, body waves (P and S) and Stoneley waves are not present. Only fluid waves are present, indicating a karstic level; • the 90–120 m depth interval corresponds to a homogeneous formation, all acoustic waves (refracted P-wave, converted refracted S-wave, and Stoneley wave) are visible and of high amplitude, indicating a compact formation. However, in the 110–115 m depth interval, we notice a slight attenuation of the different waves (Fig.11a) and the presence of a fluid wave. The acoustic section recorded in borehole M14 shows: • the 22–38 m depth interval, where only refracted P-waves and Stoneley waves are recorded, indicating a slow formation. We note the presence of fluid waves; • the 38–110 m depth interval of homogeneous profile with a high signal-tonoise ratio; all acoustic waves (refracted P-wave, converted refracted S-wave, and Stoneley wave) are visible and of high amplitude, indicating a compact formation. No fluid waves are visible. However, in the 85–88 m depth interval, we note a very weak attenuation of the various waves, which is confirmed by small decreases of both the P-wave velocity log and the amplitude log (Fig. 10b and 11a). Analysis of the acoustic data (Fig. 11b) shows a small but significant increase of karstic body indicators in the 85–90 m depth interval, suggesting the presence of a karstic feature filled with sediments such as shale. However, no flow is present. The acoustic section recorded in borehole M20 shows: • in the 30–58 m depth interval, an acoustically slow formation where only refracted P-waves and Stoneley waves propagate; • in the 58–80 m depth interval, an acoustically fast formation where refracted P-waves, converted refracted S-waves and Stoneley waves propagate, indicating a compact geological formation; • A karstic unit in the 80–100 m depth interval, characterized by strong attenuation of refracted P-waves (Fig. 11a) and Stoneley waves. In the interval, we can infer the presence of fluid waves, indicating the presence of flow; • in the 100–110 m depth interval, an acoustically fast formation where refracted P-waves, converted refracted S-waves and Stoneley waves propagate, indicating a compact geological formation.
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