93 3. Acoustic logging Continuity is expressed by a small variation in shape and by a small time difference of the wave from one depth level to another. The use of such a method ensures a pick that is coherent in time, amplitude and frequency. The tool geometry (number of receivers and distance between receivers) and the acquisition geometry (advancement rate) create a redundancy of information at each measurement depth and lead to an estimation of a dispersion or error value for each parameter. Figure 3.9 shows eight constant offset sections (9 to 12.5 ft) recorded with a multireceiver tool. The eight sections are presented on a time window of 1 ms framing the first arrival. The first arrival P picking times were used to calculate the slowness log and its measurement dispersion log (Fig. 3.10a and b). Figure 3.9 Constant offset sections ranging from 9 to 12.5 feet, recorded with a multireceiver tool (from Mari et al., 1992).
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