42 Well seismic surveying and acoustic logging However, in a vertical borehole, all logging tools rotate during the ascent and when the wall is inclined, they clamp onto the lower part of the wall. In addition, when a borehole is inclined, there is typically a scraping of the formation related to the friction of the drill string (reaming) and this implies anisotropy of the thickness of the cement grout around the casing. In Figure 1.28, this may account for the differences observed between PSSL and downhole measurements between 75 and 120 m deep, particularly at the level of the clay formation at 75 m, which is typically favorable to the development of “caves”. a b Figure 1.28 PSSL tool and downhole DH; a: Caliper variation depending on ground drilling (O. Serra, 1979); b: Mirandola, comparison of DH S and PSSL results, geology and deviation. This shows that a PSSL analysis also requires control of the diameter (caliper) and the deviation of the borehole (trajectometry), because when its inclination exceeds 30° the PSSL measurement appears distorted. Finally, due to the length of a PSSL (5 to 7 m) and the difficulty to achieve a seal that does not overly invade the formation, especially outside of the water table, in a cased hole, there is a frequently observed gap between the measurement of crosshole or downhole and PSSL in the first 10 meters. Figure 1.29 perfectly illustrates this fact.
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