128 Well seismic surveying and acoustic logging and attenuation logs. The acoustic tool used for the field experiment was a flexible monopole tool with two far offset receivers (3 m (R1) and 3.25 m (R2) offsets). At the level of a karstic body, we observed a strong attenuation of the refracted P-wave and a distortion of the acoustic signal that made it difficult to pick the refracted P-wave arrival times. Processing based on Singular Value Decomposition (SVD, [14]) was carried out independently on the 2 offset sections. The refracted wave signal space was given by the first Eigen section obtained by SVD: rsig = λ 1u1v1 T 1 v1 is the first singular vector giving the time dependence, and it is therefore named the normalized wavelet, u1 is the first singular vector giving the amplitude in depth, and is therefore called the propagation vector and λ1 the associated eigenvalue. The amplitude variation of the refracted wavelet over the depth interval is λ1u1. A cost function based on the correlation of normalized wavelets V1 extracted from the two constant offset sections enables the simultaneous calculation of the velocity V of the formation, and the correlation coefficient between the wavelets. SVD processing leads to the calculation of a specific attribute used to detect karstic levels. The attribute, called the Noise/Signal detector, is the product of 3 normalized terms: • A velocity term: CV = 1-(V/Vmax). The lower the velocity, the higher the velocity term. In karstic zones, a high CV coefficient is observed. • An amplitude term: CA = 1-(A/Amax), with A = λ1u1. In karstic zones, a high CA coefficient is observed. • A correlation term: CCor = 1-(Cor/Cormax), Cor being the correlation coefficient between two normalized wavelets. In karstic zones, a high CCor coefficient is observed. Figure 5.7 shows the acoustic data from well M13. On the 3 m constant offset section, we can clearly see: • the refracted P-waves between 0.5 and 1 milliseconds, • the converted refracted shear waves between 1.2 and 1.8 milliseconds, • the Stoneley modes after 2 milliseconds. One can notice a strong attenuation of all waves between 85 and 90 m, highlighted by the Noise/Signal detector log. The BHTV log confirms the presence of a karstic layer. The results are consistent with the seismic measurements (VSP flow index) and PLT flow (Figure 5.6). Figure 5.8 compares BHTV log, VSP flow index, Noise/Signal detector (acoustic logging) and PLT flow at well M11. A karstic body with a flow was clearly detected between 85 and 90 m.
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