97 3. Acoustic logging Figure 3.13 is an example of acoustic section imaging obtained in a sandstone reservoir intersected by carbonate beds (Fortin et al., 1991). The acoustic imagery clearly shows the distribution of carbonate beds and their dips relative to the well. This acoustic approach to dip estimation requires a thorough multi-coverage processing. Figure 3.13 Well imaging using acoustic data: reflected waves EVA tool (SNEAP), multiple coverage processing (CGG) (from Fortin et al., 1991). 3.6 Characterization of a formation using Stoneley waves Stoneley waves are used to evaluate the S slowness of slow formations, to study fracturing and to provide an estimation of permeability. The shear waves can only be generated if the S velocity of the formation is greater than the velocity of the compression wave in the mud; we can then say the formation is fast (as opposed to so-called slow formations). In slow formations, the velocity of S-waves can be indirectly estimated in uncased wells from the Stoneley wave dispersion equation (Biot, 1952; Cheng et al, 1981).
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