Generating T-D Pairs from Velocity Model

This process generates checkshots from a chosen velocity model.

Checkshots are generated spaced at regular MD intervals based on a source of velocity data, starting from a manually entered time-depth pair. Some of these checkshots may be removed if they are not required to achieve a specified level of accuracy in TWT.  

Create/resample time-depth

Create/resample time-depth
  1. Set the time-depth survey as the default time-depth pairs in the well.
  2. From the Tool menu, open the Synthetics view.
  3. Select the Current Well.
  4. Right click on the T/D Pair Track and select Create/Resample Time-Depth Survey.
  1. At Mode, select From Velocities.
    • Initial T-D Pair: An initial time-depth pair.  
    • Interval (MD): Minimum MD interval between consecutive generated checkshots.
    • Accuracy (ms): Pairs that affect time-depth conversion below this amount are excluded.
    • Velocity Volume: Velocity volume to use as the input.
    • Velocity Type: The type of data in the selected velocity volume.
    • Velocity Units: The units of the selected velocity volume.  

From Time-Depth Conversion

From Time-Depth Conversion

Time-Depth checkshots can be generated from the project’s pre-configured time-depth conversion model.

  1. To configure the time-depth model, open the File menu and select Configure Time-Depth Conversion. Select a Velocity volume.
  2. Set the time-depth survey as the default time-depth pairs in the well.
  3. From the Tool menu, open the Synthetics view.
  4. Select the Current Well.
  5. Right click on the T/D Pair Track and select Create/Resample Time-Depth Survey.
  6. At Mode, select From Time-Depth Conversion.
    • Initial T-D Pair: An initial time-depth pair.
    • Interval (MD): Minimum MD interval between consecutive generated checkshots.
    • Accuracy (ms): Pairs that affect time-depth conversion below this amount are excluded.

How does this work?

Create time-depth pairs from the project time/depth velocity model.

The initial T-D pair is used, with MD 0, as the checkshot at the top of the well. The remaining checkshots are calculated so that they give a similar well path to the original (but starting from, potentially, a different TVD). Values for TWT are derived from the project time-depth conversion velocity model.

For Vp calculated from deviated well checkshots, the result will not necessarily be the same as the project TD conversion velocity. Although the Vp between two checkshots is calculated from the change in TVD and the change in TWT between them, the change in TWT is due not only to the local velocity near these locations, but also the velocity profile above. If there is significant lateral velocity change above the well then the difference in TWT (and hence, the calculated Vp) may not be very meaningful.