41 1. Shear velocity measurement in boreholes In slow formations, the measurement of S velocities cannot be obtained by using monopole-type tools. The oil industry has developed dipole-type tools including polarized transmitters and receivers that generate and record bending waves, the propagation velocity of which is close to the S velocity of the formation (see Chapter 3). The problem is that these oil industry tools are rigid and very long. However, a flexible alternative exists for the geotechnology field: PSSL. 1.3.2 PS Suspension Logging (PSSL) PS Suspension Logging (PSSL) is a flexible logging probe for non-continuous measurement with an electromechanical drill source and two receivers (see Chapter 3). It should be noted that the frequencies used by the PSSL enable measurements to be made in boreholes with PVC casing. Seismic wave velocities are calculated from the difference between the arrival times between the two receivers (Near and Far) spaced 1 m apart. As a result, the tool needs to be centered in the borehole, but since it is flexible (see Figure 1.27) and it is difficult to find centering rings that are adapted to the borehole, this is not always possible. a b Figure 1.27 PSSL tool a) set up of a PSSL tool with a continuously active drill rig (Fugro for EDF), b) typical centering ring component of PSSL tool.
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