94 Well seismic surveying and acoustic logging Figure 3.10 Example of acoustic logs in a sandstone reservoir (from Coppens and Mari, 1995). Monopole tool: (P wave): a) Slowness; (b) Standard deviation of slowness; c) Delay; d) Residual delay. Dipole tool (bending wave # S wave): e) Slowness; (f) Standard deviation of slowness; g) Extension of the weathering zone (P wave); h) Poisson’s ratio. Knowing the slowness makes it possible to calculate the delays log (Fig. 3.10c). The theoretical arrival times of the refracted wave are then calculated for each shot from the slowness and delay logs. The difference between the calculated times and the actual times represent the measurement dispersion error. This is presented as a residual delay log (Fig. 3.10d) that does not exceed 5 μs, i.e. one half of the time sampling rate in the studied example. The formation studied being a clay-sandstone slow formation (absence of refracted S modes on the monopole sections), the slowness S of the formation was obtained using the bending modes generated by a dipole-type tool. The measurement of the arrival times of the direct bending wave gives access to the slowness S log and its associated standard deviation log (Fig. 3.10e and f). The combination of slowness logs P and S enable the calculation of Poisson’s ratio (Figure 3.10h). The measurement of the arrival times of the refracted P-wave for each transmitterreceiver pair for the whole of the well gives access to the delay at any measurement point and therefore to an image of the extension of the weathering zone of the well.
RkJQdWJsaXNoZXIy NjA3NzQ=