111 4. Tying surface seismic data • The attenuation log that shows that these waves are barely attenuated. These waves are sensitive to the borehole wall conditions and therefore to the presence of casing. It is worth using the information provided by the Stoneley waves in an open hole to characterize the geological formation and in particular to detect the presence of fractures. • The frequency log. These waves are dispersive and low frequency (between 4 and 5 kHz). 4.5 Time conversion of acoustic logs and calculation of the synthetic seismogram The velocity log is a measurement made according to depth, sampled at regular intervals (2 cm in this case). It can be used to obtain a time-depth law by integrating the acoustic transit time as a function of depth. At a given depth, the acoustic transit time is equal to the depth sampling interval divided by the formation velocity. The integration of the acoustic transit time provides the integrated time, conventionally known as the Integrated Transit Time (ITT(Z)). If the borehole is vertical, the ITT can be compared to the vertical time provided by the VSP. The integrated transit time ITT(Z) = f (Z) can be used to convert a depth log into time log and to insert these logs on the seismic sections in time. For this purpose, ITT(Z) must be set in two-way time. The depth-time conversion of the log will be done by vertical translation between a position Z in depth, and a time position ITT(Z) with a regular time sampling interval Δt: ITT(Z) = f(Z, Δt). The sampling interval Δt must be chosen to avoid aliasing phenomena between the depth domain and the time domain. Δt must be chosen as a sub-multiple of the seismic sampling interval, to change the scale between the logging measurements and the seismic measurements. Figure 4.6 (a and b) shows the time conversion of velocity and cementation logs measured in depth. The velocity log is used to calculate a reflectivity log (Figure 4.6c) that was filtered in terms of frequency and under-sampled at 0.5 ms (seismic sampling interval in time). On the filtered reflectivity log, a high amplitude reflector can be seen at around 70 ms. The filtered reflectivity log is known as the synthetic seismogram. It is used as the VSP stacked trace to calibrate the seismic horizons observed on seismic sections in time. The synthetic seismogram, duplicated 5 times, is inserted into the in-line section 3 at the CMP position associated with cross-line section 6 (Figure 4.6d). It can be noted that the 70 ms horizon is not present on the seismic section or on the VSP stacked trace (Figure 4.2). At 70 ms, the velocity log increases abruptly. This phenomenon occurs where poor cementation is detected (Figure 4.6b). The presence of the horizon at 70 ms is an artifact related to a velocity anomaly due to the
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