39 1. Shear velocity measurement in boreholes Figure 1.25 Crosshole result after inversion with GEOTOMGC software (Terradata for EDF). 1.3 Vs measurement by logging 1.3.1 Acoustic logging with high vertical resolution In geotechnology, the logging tools that can be used are generally of a small diameter (<50 mm) and non-stackable, as is practiced in the field of oil exploration. The conventionally used acoustic probes are therefore short probes (3.5 m), composed of a high frequency monopole-type piezoelectric transmitter (10 to 40 kHz) (see Chapter 3). It is important to remember that this means that the source is omnidirectional and that it generates P-waves in the fluid. These waves are refracted (P, S) at the level of the borehole wall and guided modes also appear. These are, in particular, Rayleigh waves for a solid-air interface and the Scholte-Stoneley wave for a solid-fluid interface. Therefore, the existence of refracted shear waves implies that the P velocity of the fluid is lower than the S velocity of the formation for an uncased borehole. Vs measurement with acoustic logging is therefore recommended for fast formations.
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