87 3. Acoustic logging Figure 3.4 c shows the common transmitter point gather obtained with a monopole tool in a fast formation. After amplification, the refracted P-wave appears in the 0.5 - 1 ms range, the refracted S-wave in the 1.5 - 2 ms range, and the Stoneley wave, which is clearly visible without amplification, in the 2 - 3.5 ms range. Figure 3.4 d shows the common transmitter point gather obtained with a dipole tool in a fast formation. The highly energetic flexural mode is present in the 1.5 - 2 ms range, and the refracted P mode is present in the 0.5 - 1 ms range after amplification. Figure 3.5 a is an example of a 3 m constant offset section, that therefore corresponds to measurements on a receiver that is 3 m away from the transmitter, where these propagations are clearly visible. The source is a magnetostrictive monopole transmitter. The presence of the refracted S-wave indicates that the formation is fast. When there is an impedance contrast between two formations with a dip similar to that of the plane perpendicular to the hole axis, these different waves (refractions and interface waves) can convert and reflect as shown in Figure 3.5b and reveal on the constant offset sections a number of chevron patterns at slow apparent velocity. Such reflections can also occur at casing connections, cavities and any other well wall heterogeneities. a b Figure 3.5 Acoustic sections (SEMM document). a): example of an constant offset section obtained with a monopole source. We observe the refracted P arrival, the refracted S arrival, the Stoneley wave and casing waves. b): Example of refracted-reflected P-waves and reflected Stoneley waves at impedance contrasts in the well. On an constant offset section, these arrivals are in the characteristic form of chevron patterns at slow apparent velocity.
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