127 5. Contribution of seismic and acoustic methods to the characterization of karstic formations The analysis of ambient noise and of the increase in the P-body waves to Stoneley waves conversion is currently the only method available to detect the flow in this C1 well. Due to the small borehole diameter, it was not possible to run BHTV and PLT logs in well C1. However, the karstic level at a depth of 57 m was confirmed by acoustic logging. The same procedure was therefore applied to the dataset recorded in well M13 (Figure 5.6). Figure 5.6 shows: • Top left: the ambient noise and its associated noise factor, which detects a karstic body between 85 and 87 m, and predicts a flow. • Top right: The VSP data. A down-going Stoneley wave reflected at a depth of 85 m can be noted. The conversion of the down-going P-wave into an up-going and down-going Stoneley wave can also be observed at the same depth. • Bottom left: The instantaneous amplitude of the up-going Stoneley waves and the associated conversion factor between the P-wave and the Stoneley wave, which confirms the presence of a karstic body between 85 and 90 m. • Bottom right: the VSP flow index, which predicts a flow between 85 and 87 m. The VSP flow index has been integrated in depth from bottom to top to mimic a flowmeter in order to compare with a PLT log. A good correlation between the 2 logs can be noted, taking into account that the vertical resolution is not the same for the 2 logs (2.5 m for the VSP, and 1 cm for the PLT). The flow variation of the PLT log at 65 m deep is due to a change of the borehole diameter. 5.5 Monopole full waveform acoustic logging The transmission of an acoustic wave through geological formations is used for formation characterization. Monopole-type tools are the most commonly used for this purpose. Monopole sources and receivers are multidirectional. Sources generate compression in the fluid, creating in the formation a compression wave (P-wave) and a shear wave (S-wave) at the refraction limit angles. In a vertical well, these tools are used to record five propagation modes: the refracted compression wave; the refracted shear wave (only in fast formations); the fluid wave; and two dispersive guided modes – the pseudo Rayleigh waves (only in fast formations) and the Stoneley waves. Acoustic logging enables the measurement of vertical propagation velocities and the frequencies of the different waves recorded. The analysis of the acoustic waves recorded on each receiver of the logging tool is used to calculate additional logs, defined as acoustic attributes, useful for the characterization of the formation, such as: amplitude, shape index, wavelength
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