86 Well seismic surveying and acoustic logging VS = 1,380 m/s ρ = 2.3 g/cm3 QP = QS = 90 – for the fast formation (VS > well fluid velocity): VP = 4,000 m/s VS = 2,000 m/s ρ = 2.3 g/cm3 QP = QS = 90 Figure 3.4 a shows the common transmitter point gather obtained with a monopole tool in a slow formation. Without amplification, the only visible wave is the Stoneley wave. With an amplification of 60 dB, the refracted P-wave clearly appears. The measurement of its dip enables the determination of the VP velocity of the medium. a b c d Figure 3.4 Acoustic logging in slow and fast formations. a): monopole tool in slow formation, b): dipole tool in slow formation, c): monopole tool in fast formation, d): dipole tool in fast formation. Figure 3.4 b shows the common transmitter point gather obtained with a dipole tool in a slow formation. Without amplification, only the flexural mode is visible between 2 and 3.5 ms. After amplification, the refracted P-wave appears between 1 and 1.5 ms. The flexural mode gives a very good estimate of the VS velocity of the formation.
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