40 Well seismic surveying and acoustic logging When the formation is slow (Vs formation < Vp fluid), the Scholte-Stoneley waves can enable the estimation of S velocities with an acoustic tool. It is nevertheless necessary to occasionally have the means of tying the conversion between the Scholte-Stoneley velocities and the shear velocities (for example, a downhole log with multi-metric measurement levels). Figure 1.26 is an example of Full Wave Sonic (FWS) logging obtained with a monopole tool. On the right the figure shows the acoustic section obtained with a sourcereceiver offset of 4 ft and on the left the velocity logs of the different wave trains (P refracted, S refracted and Stoneley). Figure 1.26 Example of sonic FW recording (monopole), showing a low energy P first arrival, for which the arrival time decreases significantly after 225 m (entry into the rock), the S arrival is only visible below 225 m where the medium has a velocity that is considered as fast compared to the mud (rock) [SEMM logging for EDF].
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