Near Surface Geophysics > Calibration and Synthetic seismograms

The velocity log is a measure made according to the depth, sampled at regular sampling interval (here 2 cm). 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 called Integrated Transit Time (ITT (Z)). If the borehole is vertical, the integrated transit time 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 log in depth into a log in time. The logs in time can be inserted on the seismic sections in time. To do this, ITT (Z) must be set in two way time. The depth-time conversion of a 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 any aliasing phenomenon between the depth domain and the time domain.

  • Step 1: P-wave velocity and Cementing index logs in depth

  • Step 2: P-wave velocity log and Cementing index logs in time
    One can notice that the acoustic velocity increases abruptly at 70 ms. It is associated with a strong increase of the cementing index.