18 Well seismic surveying and acoustic logging Moreover, for a given density, the P and S measurements in boreholes also enable the determination of in-situ parameters necessary to define a model that remains elastic under small deformations (Table 1.3). There are essentially two moduli: the Young’s modulus and the shear modulus. 1.1.3 Environmental conditions It is important to note that, in the geotechnics field, drilling for invasive measurements generally crosses unstable terrain. Hence, they are often lined with a casing or PVC tubing that is sealed to the formation with cement grout. Seal quality is paramount to enable a good transmission of seismic signals. Therefore, drilling and sealing must meet the standards described in ASTM D7400 for downhole and D4428 for crosshole. The key points of this phase can be summarized in Table 1.4. Table 1.4 Summary of hole parameters according to ASTM standards for geophysical borehole measurements. Max drilling diameter 175 mm (7 in) Internal casing diameter 50 to 100 mm (2 to 4 in) Tubing type PVC or aluminum, closed at the base Sealing grout Bentonite-cement grout with a density close to that of the surrounding soil to limit the energy loss by this waveguide For PVC-cased boreholes, it is possible to carry out a cementation control through a Cement Bond Log and Variable Density Log (CBL-VDL), even though this tool was initially developed for steel casings (see Chapter 3). The CBL-VDL log is a recording carried out with an acoustic tool centered in the borehole, with a piezoelectric source and receiver that are distinct with a distance between them of 3ft and 5ft for CBL and VDL respectively. If the coupling is good (low acoustic velocity contrast) most of the energy is transmitted and the energy of the refracted waves is low, which visually translates into low CBL and VDL amplitudes (gray rectangles in Figure 1.1). If this is not the case, the amplitude of the recorded waves is high. This signal is somewhat “rectilinear” in nature (red rectangle in Figure 1.1). It should be noted that if the formation has a high velocity compared to that of the casing (2,100 to 2,200 m/s for PVC compared to 5,600 m/s for steel), then the refracted waves in the formation will arrive first, thus masking the refracted waves in the casing (orange rectangle in Figure 1.1). On the other hand, if these waves dominate, it means that the entirety of the casing, grout and formation is adherent.
RkJQdWJsaXNoZXIy NjA3NzQ=