123 3. Borehole geophysical methods 3.3 Full waveform acoustic methods Acoustic tools are currently run to measure velocities (P-wave velocity VP, S-wave velocity VS) of geological formations. The tools used are of monopole or dipole type. Monopole-type tools are the most used. For monopole tools, sources and receivers are multidirectional. In the fluid, sources 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 tools are used to access the shear velocity (VS) of geological formations and are equipped with polarized transmitters and receivers. Such tools generate polarized compression waves perpendicular to the well axis. The compression waves create flexure modes at the well wall which generate in the formation pseudo-shear waves propagating parallel to the well axis. Acoustic tools are built with one source (multidirectional) and 2 receivers (multidirectional) at least, or several sources (multidirectional and polarized) and several receivers (multidirectional and polarized). Acoustic tools are working in wide frequency bandwidths: 1–40 kHz for monopole tool and 1–3 kHz for dipole tool. Consequently, the sampling rates are of several µs in time (5 or 10 µs for monopole tool, 20 µs for dipole tool) and of several centimetres in depth (5 to 30 cm). Full waveform acoustic measurements can be represented as constant-offset sections. A constant-offset section is a set of acoustic records represented as a function of depth and obtained with a fixed source-to-receiver distance. In a vertical well, monopole tools can enable the recording of five propagation modes as: refracted compression waves (P), refracted shear waves (S, only in fast formations VS > VP fluid, P-wave velocity of the borehole fluid), fluid waves (F), and two dispersive guided modes as pseudo-Rayleigh waves (in fast formations), and Stoneley waves (ST). The acoustic logs associated with the different waves are very high-resolution logs and can be compared with core measurements. The acoustic logs currently obtained for each type of wave are velocity or slowness logs, frequency and attenuation logs. In addition to these modes, constant-offset acoustic sections may show coherent slanted events and resonances (R). The slanted events, conventionally named crisscross events, are refracted events reflected on the edges of geological discontinuities (acoustic impedance discontinuities), such as fractures. For their part, the resonances are related to poor cementation between the casing and the formation. A high level of resonances can result in unusable acoustic data. Figures 3.5 to 3.8 show an example of acoustic data recorded in boreholes situated on an experimental site located in the Cher region (France) at the transition from Triassic to Jurassic geological formations, partly overlaid by thin superficial formations. The sedimentary formation is mainly composed of limestone up to 120 m depth and sandstones with some argillite and dolomite intercalations between 120 m and 200 m. The site was investigated from the surface via hybrid seismic imaging methods and from two boreholes (B1 and B2, Figure 3.5a) via FWAL and VSP (Mari et al., 2021, 2023, 2024). A seismic line was recorded at the site with a seismic spread composed of 48 fixed geophones (2 m lag distance between neighbors,
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