91 3. Acoustic logging The acoustic data of the second example (Mari and Porel, 2015) were acquired in a carbonate formation. The results are presented in Figure 3.7. Figure 3.7a shows the 3 m constant offset acoustic section, we can see the refracted P-wave, the refracted S-wave and the associated Rayleigh waves, the very high frequency fluid wave at infinite apparent velocity, occurring just before the Stoneley wave. In the depth range of 80-100 m, the presence of two areas with strongly attenuated waves can be noted. Figure 3.7b shows the velocity log of the refracted P-wave and the associated correlation log used for the quality control of the velocity measurement. In the low velocity and high attenuation zones, this coefficient is low. Figure 3.7c shows the amplitude log of the P-wave, obtained by singular value decomposition (SVD) method, in the same depth interval. At this interval, we note a low formation velocity, a strong attenuation, and a low correlation coefficient which all indicate the presence of karstic levels. These data are part of the case study presented in Chapter 5. 3.5 Acoustic imaging In this section, we provide a simple description of the processes of refraction and reflection imaging. 3.5.1 Refraction acoustic imaging The total transit time Ti,j between a transmitter i and a receiver j of a refracted wave is equal to the sum of the delays in line with the transmitting point (Di) and the receiving point (Dj) and the associated transit time to the refractor (Xi,j/V with Xi,j being the distance between the transmitter i and the receiver j, and V being the refractor velocity): T X V D D i j i j i j , , = + + with, X V t i j k k k i j , , = + = − ∑d 1 1 The quantity dtk,k+1 is the propagation time in the formation between two successive positions of depth k and k+1. Figure 3.8 shows the path of the refracted wave between a source S at position i and a receiver R at position j. The delays D are simple functions of mud parameters (thickness and velocity hm and Vm) and the parameters of the well’s weathering zone (thickness and velocity ha and Va). The delay in S is equal to the travel time between S and B minus the travel time between A and B. The calculation of the delay at each recording depth enables the estimation of the extension of the well’s weathering zone. This parameter is rarely measured.
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