109 4. Tying surface seismic data The acoustic sections show: • Synchronization signals in the time interval 0 - 0.5 ms. These electronic signals have no geological meaning. • Resonances in the time interval 0.6 - 0.8 ms, locally in depth. These resonance phenomena are related to poor cementation between the casing and the formation. In the 75 to 90 m depth range, the resonances interfere with the acoustic signals that propagate through the formation. • Formation refracted waves. The first arrival times of these waves vary from 1.8 to 0.7 ms in the depth range 30 m to 80 m. This change in arrival time indicates a gradual increase in formation velocity with depth. There is also a change in the character of the acoustic signal: low frequency in the range 30 to 65 m, high frequency and noise for depths greater than 65 m. • Stoneley waves. These high amplitude waves, guided by the borehole wall, appear after 2.4 ms. They are influenced by casing. The lower part of Figure 4.4 shows the acoustic sections in the time interval 0.6 to 0.8 ms where the resonances are mainly observed. The resonance level can be estimated by calculating the energy of the acoustic signal over the time interval. The normalized acoustic signal energy as a function of depth is an acoustic log that is used to provide a log indicative of cementation quality, referred to here as C index (cementation index). The C index indicates areas of poor cementation, especially in the 30 to 35 m range and at depths greater than 75 m. 4.4 Acoustic logs Acoustic logging is mainly used to provide formation velocities by measuring the arrival time difference of the different wave trains at the different receivers of the tool, in this case R1 and R2 at 3 m and 3.25 m respectively from the acoustic source. Figure 4.5 shows the following logs for refracted P-waves: • The P-wave velocity log (VP) calculated from the time difference δt between the acoustic signals observed on the receivers R1 and R2 • The correlation log between the acoustic signals observed on the receivers R1 and R2, after compensation for δt. A high value of the correlation coefficient indicates a strong similarity between the two signals and a good velocity measurement. In this example, the correlation coefficient is greater than 0.75 for more than 70 % of the measured velocities. It is used to edit the log. • P-wave velocity log after editing • The instantaneous frequency log, which clearly shows the change in the frequency content of the acoustic signal at 65 m.
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