92 Well seismic surveying and acoustic logging If the velocity of the well’s weathering zone is greater than the formation velocity (Va > V), the measurement of slowness by the acoustic method does not lead to an estimation of formation velocity, but that of the well’s weathering zone. Figure 3.8 Diagram of the first refracted arrival path (from Coppens and Mari, 1995). The average slowness of wave propagation over a given depth interval corresponds to the delay acquired by the wave over this interval. For the same depth position of the transmitter (or reciprocally, the receiver), the delay can be measured by measuring the difference in the arrival time of the wave over each receiver position (or reciprocally, transmitter) located in the depth interval. As a consequence, the slowness of a formation can be estimated by measuring the wave delay by using sorted sonic records, either in common-source point depth gathers or in common-receiver point depth gathers. The average of the two delays then forms a slowness compensated for well effects. Slowness measurement methods based on the picking of first arrival times for each trace provide logs with a resolution equal to the distance separating two receivers (between half a foot and two feet). The most well known method is the minimum energy threshold picking method. If the threshold is poorly selected, or if attenuation or noise problems modify the energy levels, phase jumps of one or more periods (so-called cycle jumps) may occur, distorting the time measurements. Using full waveform tools, it is possible to control the pick quality and avoid phase jumps. Picking by threshold can then be advantageously replaced by picking techniques that take into account other criteria for tracking a wave. Mari and Coppens (1992) have proposed a picking method based on the use of artificial intelligence techniques to track a particular wave from one trace to another, and from one shot point to the other. Wave tracking is done using shape and continuity criteria. The shape of the wave is defined by its amplitude and apparent frequency.
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