102 A new concept of karst development based on hydrogeology and geophysics The acoustic logs (Fig. 10b, 11a and 11b) clearly show that the karstic unit is composed of two karstic levels at depths of 82–87 m and 93–100 m. The acoustic section recorded in borehole M22 shows: • in the 45–80 m depth interval, an acoustically homogeneous formation. Refracted P-waves and Stoneley waves are clearly visible. It is more difficult to identify the converted refracted S-wave; • a karstic unit in the 80–90 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 90–110 m depth interval, an acoustically homogeneous formation; • a second karstic unit in the 110–115 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 115–125 m depth interval, an acoustically homogeneous formation. The amplitude ratio and karstic index logs (Fig. 11a and 11b) clearly show two karstic levels at depth intervals of 80–90 m and 110–115 m. In addition to the analysis of the acoustic sections, acoustic logs are used to identify the karstic levels. A high karstic index value (Fig. 11b) provides an accurate location of a karstic layer. Integration in depth of the karstic index from the bottom to the top of the borehole is computed to mimic a flowmeter (Mari & Porel, 2015). Acoustic flowmeters are shown in Figure 12. Figure 12 Acoustic logs in boreholes M11, M14, M20, and M22: acoustic flow logs.
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