124 Well seismic surveying and acoustic logging The 3D block is composed of elementary cells (2.5 m in the in-line direction, 5 m in the cross-line direction, and 1 m deep), which clearly show the connectivity of the karstic bodies. The local validation of the results obtained by the 3D seismic method was achieved using full waveform acoustic data and VSP, recorded in 11 wells in 2014 and 2015 respectively. Acoustic logging and VSP have higher vertical resolution than the 3D seismic method, however, their lateral investigation was restricted to the vicinity of the well. 11 wells were selected: C1, M03, M05, M11, M13, M14, M20, M22, MP5, MP6 and MP7. In this chapter, we show the results obtained from wells C1, M11, M13 and M20. The well locations are indicated in Figures 5.1 and 5.4. 5.4 Well seismic measurements For VSP acquisition, the seismic source was a lightweight drop and the borehole sensor was a hydrophone. The sampling interval depth was 2.5 m. Before each shot, the ambient noise was recorded. The VSPs were highly corrupted by Stoneley waves (tube waves). The conversion of down-going P-waves into up-going Stoneley waves was observed at the level of the karstic bodies. This phenomenon occurs in highly permeable formations. Figure 5.5 shows the data recorded at well C1. The phenomenon of the conversion of P-waves into Stoneley waves can be identified at a depth of 60 m (Figure 5.5, top right). Indeed, it can be observed that the first arrival, which is the down-going P-wave, is highly attenuated at a depth of 60 m. At this depth, the P-wave is partly converted into a down-going Stoneley wave, which is reflected at the bottom of the well. The VSP data were processed to extract the down-going and up-going Stoneley waves. The down-going Stoneley wave can be extracted using a velocity filter, the velocity being the apparent velocity of the Stoneley wave. A narrow-band wavenumber filter applied after the velocity correction is equivalent to a velocity filter in the f, k domain. The velocity was determined with a velocity scan. For a given velocity value, the Stoneley wave and residue are extracted, the difference between the initial VSP data and the estimated down-going Stoneley wave was calculated. The selected velocity was the velocity that minimized residues. The apparent velocity of the Stoneley mode is about 1,300 m/s. The same procedure was applied to extract the up-going Stoneley wave, but the apparent velocity was negative. A Hilbert transform was applied to the different wave fields to estimate their amplitude (instantaneous envelope). Figure 5.5 (bottom left) shows the increase in amplitude of the Stoneley waves. The instantaneous amplitudes of the up-going Stoneley waves were stacked in a small corridor located after the arrival time of the down-going P-wave, to obtain a body wave to Stoneley wave conversion factor (Figure 5.5, bottom right), which relates to a karstic level at 57 m in depth.
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