81 3. Acoustic logging adjustment of certain acquisition parameters (gains...). The tool is then lowered to the bottom of the borehole. A control acquisition can be made during the descent (downlog), to ensure that there will be no saturation at acquisition. The measurement operation is then carried out during the ascent (uplog), at constant velocity, according to the sampling interval in depth. A typical ascent speed is 4 to 6 m/min. 3.2.2.2 Acoustic probes Either monopole or dipole tools are used. Monopole tools are the most commonly used. Transmitters and receivers are multidirectional (Figure 3.2a). In the fluid, transmitters generate a compression wave, which creates in the formation a compression wave (P-wave) and a shear wave (S-wave) at the refraction limit angles. Dipole acoustic tools are used to access the S parameters of slow formations and are equipped with polarized transmitters and receivers. Such tools generate polarized compression waves perpendicular to the borehole axis. These compression waves create flexure modes at the well wall that generate pseudo-shear waves in the formation that propagate parallel to the well axis (Figure 3.2b). Figure 3.2 Types of Sonic Array transmitters a) Monopole transmitter emitting a multidirectional pulse. b) Dipole transmitter emitting a directed pulse (modified from Zemanek et al., 1991). When the shear velocity of the formation is lower than the P velocity of the borehole fluid, the flexural wave travels at the S-wave velocity and is therefore the most reliable logging method for estimating a shear velocity log. The difficulty, however, is that these tools from the oil industry are rigid and long (about 10 m long and 10 cm in diameter). Their implementation is not suitable for geotechnical boreholes. Although an adaptation is available for the geotechnical field, namely PS suspension logging (PSSL), which involves a flexible tool. The PSSL method was originally developed in the mid-1970s by researchers at Japan’s Oyo Corporation (Kaneko et al., 1990) with geophones used as receivers. Today, the companies
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