137 Conclusion acoustic logging with those provided by the VSP, as well as the principle of time conversion of acoustic data and the calculation of synthetic seismograms. Chapter 5 presents an innovative example of the application of borehole seismic methods and logging techniques. The example describes the contribution of seismic and acoustic methods to the characterization of karstic formations. For this purpose, it appears that a 3D seismic block can be used in hydrogeology to build a 3D model of karstic aquifers. VSP data characterize karst levels in two ways. Firstly through the conversion of P-waves to Stoneley waves at the top of the most porous levels, and secondly through the analysis of ambient noise which is at its maximum at the level of the water producing layers. Finally, full waveform acoustic logging also enables the characterization of karstic formations, but on a different scale. At the level of a karstic body, we observe a strong attenuation of the refracted P-wave and a distortion of the acoustic signal. The analysis of the acoustic waves recorded simultaneously on the two receivers of a monopole acoustic tool can be implemented to calculate a Singular Value Decomposition (SVD) of the logs, which makes it possible to define acoustic attributes. The attribute, called the Noise/Signal detector, is the product of three normalized terms (velocity coefficients (CV), amplitude coefficient (CA), correlation coefficient (CCor). In karstic zones, a rise in these three coefficients was observed, therefore the analysis of ambient noise (seismic and acoustic) and the conversion of body waves into Stoneley waves can be used to detect and quantify flow circulation, while a 3D seismic block can be used to build a 3D model of karstic aquifers. We can therefore conclude that 3D seismic surveys, full waveform acoustic logging and VSP enable the characterization of karstic formations at different scales.
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