125 5. Contribution of seismic and acoustic methods to the characterization of karstic formations Figure 5.5 Flow detection using both ambient noise measurement and VSP data at well C1. Top left: Ambient noise and ambient noise factor. Top right: VSP data showing a high level of Stoneley waves. Bottom left: Up-going Stoneley waves (instantaneous envelope). Bottom right: P-wave to Stoneley wave conversion factor and VSP flow index. Assuming that flow circulation in the karstic network generates a rise in ambient noise [12], the analysis of the seismic noise was carried out to detect the presence of flows [13]. For this purpose, the average and the variance of the amplitude spectrum of each noise trace was calculated. We noted a significant increase in the ambient noise factor, defined as the relationship between the average and the variance of the spectrum, at the level of karstic bodies. The analysis of the ambient noise therefore shows that the variations of the ambient noise factor correlate with the level of
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