The velocity log in time is used to calculate a log of reflectivity which is filtered in frequency and under sampled at a seismic sampling interval. The filtered reflectivity log is called synthetic seismogram. It is used as the VSP stacked trace to calibrate the seismic horizons observed on the seismic sections in time.

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  Step 1: Reflectivity log before and after frequency filtering
  On the filtered reflectivity log, a high amplitude reflector can be observed at about 70 ms.


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  Step 2: Calibration in time
  The synthetic seismogram trace, duplicated 5 times, is inserted in the in-line section 3 at the CMP position associated with the cross-line section 6. One can notice that the horizon at 70 ms is not present on the seismic section. The presence of the horizon at 70 ms is an artifact linked to velocity anomaly due to the poor cementation of the casing to the formation. The measured acoustic velocity represents the formation velocity for times lower than 70 ms. However, the correlation coefficient between seismic trace and synthetic seismogram at the CMP of intersection is greater than 0.7, showing a good fit of the seismic horizons, in the time interval 38-60 ms. It drops to 0.5 if we consider the interval 38-80 ms.