113 4. Tying surface seismic data 4.6 Integrated sonic time and VSP vertical time Figure 4.7 shows a comparison between the integrated sonic time and the VSP vertical time. The times are set to 0, at 30 m deep, a depth from which the acoustic measurement can be made. In this acquisition the water level was detected at 30 m. It can be seen that the two-time curves are superimposable in the 30 to 77 m range. From 77 m, sonic times are shorter than the VSP times. The drift curve, which is the difference between the sonic times and the VSP times, clearly shows this phenomenon. To reconcile sonic and VSP times, the acoustic velocities must be modified so that the recalculated integrated sonic times are closer to the measured VSP vertical times. a b Figure 4.7 VSP time and integrated sonic time - block shift method; a: comparison of VSP times and integrated sonic times; b top: drift curves before and after velocity compensation by the “block shift” method; b bottom: comparison of VSP and integrated sonic times before and after velocity compensation using the “block shift” method.
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