Near Surface Geophysics > Calibration and Synthetic seismograms
  • Step1: integrated sonic time and VSP vertical time
    Figure shows a comparison between the integrated sonic time and the VSP vertical time. The times are set to 0, at the 30 m 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 interval 30-77 m. From 77 m, sonic times are shorter than VSP times.

  • Step 2: Drift curve and Block shift method
    To reconcile sonic and VSP times, the acoustic velocities must be modified so that the recomputed integrated sonic times are closest to the measured vertical VSP times. For this purpose, the drift curve, difference between sonic times and VSP times, is used to determine the values of velocity correction so that the new integrated times are closest to the VSP vertical times. The drift curve can be approximated by straight line segments. A velocity correction value will be calculated for each line segment. In the present case, the drift curve can be approached by two straight line segments, the first in the depth range 30-77 m, the second in the depth range 77-90 m. In the first interval, it is observed that the time difference is on average constant (0.5 ms). This value corresponds to the precision of the picking. As a result, the sonic velocity curve will not be modified in this range of depth. In the range 77-90 m, the drift curve has a significant gradient which can be compensated for by a constant value velocity correction. The correction method is called "block shift". Figure shows the comparison of the VSP and integrated sonic times before and after compensation of the velocities by the "block shift" method.

  • Step 3: P-wave velocity log and Cementing index logs in time, before compensation of the velocities by the block shift method.

  • Step 4: P-wave velocity log and Cementing index logs in time, after compensation of the velocities by the block shift method.