131 5. Contribution of seismic and acoustic methods to the characterization of karstic formations Regarding the data recorded in well M20, the 3 m constant-offset acoustic section shows: • the refracted P-waves between 0.5 and 1 milliseconds • locally, the converted refracted shear waves between 1.2 and 1.8 milliseconds • the fluid waves characterized by very high frequencies in the order of 30 kHz • the Stoneley wave modes An analysis of the acoustic data and the calculations made from the Noise/Signal detector have enabled the detection of karstic levels in two depth intervals between 82 and 88 m and between 95 and 100 m. In these intervals the acoustic section shows a strong attenuation of the refracted P-wave. The presence of the 2 karstic layers was validated by the OPTV logs. The VSP shows a down-going Stoneley mode which is strongly attenuated at 82.5 m deep. The phenomena of a conversion from a down-going P-wave to Stoneley waves can be observed between 82.5 and 100 m. The processing of VSP data and the ambient noise analysis confirmed that the 82.5 to 100 m depth interval is a karstic layer and detect the presence of flow. A PLT log obtained during pumping in well M04 validates the results of the acoustic-seismic experimentation. 5.6 Conclusion This chapter has shown the benefit of combining different geophysical methods to describe geological formations at different scales. The aquifer studied, 20 to 130 m in depth, consists of tight karstic carbonates. In 2004, a seismic reflection and refraction survey was carried out on the study site [1]. An analysis of the seismic refraction tomography was made to calculate the velocity distribution in the karst aquifer cover, and to map the top of this reservoir. The 3D seismic data were processed to obtain, following inversion, the distribution of the seismic velocities in the reservoir. The result was a high-resolution 3D seismic block. This enables the identification of three high-porosity, presumably waterproducing layers, at depths of 35 to 40 m, 85 to 87 m and 110 to 115 m. The 85 to 87 m deep layer is the most porous, with porosities of over 30 %, which represents the karstic part of the reservoir. To locally validate the results obtained by the 3D seismic method, full waveform acoustic data and VSP were recorded in 11 wells, VSP and acoustic methods having a higher vertical resolution than the surface seismic method. A methodology has been developed to detect flow using both ambient noise measurement and VSP data recorded with a hydrophone sensor. The VSP, recorded in water wells, were highly corrupted by Stoneley waves (tube waves). Conversion of the down-going P-waves into Stoneley waves was observed at the level of the karstic bodies. This phenomenon occurs in highly permeable formations. The analysis of the ambient noise shows that the variations of its characteristics (spectral variance)
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