06 Velocity cubes

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Last updated: Sept 22, 2013 by Erlend Kvinnesland
3 min read

Velocity cubes.

Velocity cubes are generated from stacking velocities and check shot wells. The dimension of the cube is in the time domain, while the contents: i.e the cube nodes are velocity values. The cube is thus also called a time velocity cube. The purpose of the velocity cube is to have a fast and reliable method for depth conversion.

There are several methods of generating the velocity cube:

Methods for cube generation

  1. An overall gridding method that transfers the stacking velocities into the cube by one master command. This method is used in Geocaps cube gridding panel.
  2. Gridding individual cube layers and insert them into the receiving cube. This method can be used by users that will script up sophisticated logic for detailed control and quality.

The stacking velocities were collected into one dataset called all_lines in the 06_Velocity folder.

Exercise

Digitize a border line for the cube.

Border line for the cube

  • We need a border line for the interior of the cube. This line should go a little outside _border_line_used for the grids.
  • The cube size should extent the time grids a little bit in order to get no edge effects when depth converting.
  1. Put the screen in 2D mode.
  2. Digitize a closed line just outside border_line.
  3. Transfer it to the Boundary folder and name it cube_border.
  4. Set the screen back into 3D mode.
  5. Perform Zoom to Data on window_aoi (or use one of the grids) to secure the grid window is back.
Exercise

Grid stacking velocity into a cube.

  1. Highlight the stacking velocities all_lines in the Velocity folder. Activate Cube gridding from stacking velocities.
  2. Select correct attribute by clicking Set. The combobox should shift to Scalar. Sometimes the velocity is the field data velocity, but in this case it is in the scalar part.
  3. Click Use Active Window to set the cube window.
  4. Set zmin to 0 and zmax to 6000 which will be the size of the cube in the z direction.
  5. Set the cube increments to 500 500 50 which is an acceptable resolution in our case.
  6. Click Expand to expand the cube window a little bit to avoid edge effects when depth converting.
  7. In our case the size of cube is now 286 194 121.
  8. Go to the Grid Options tab.
  9. Click Use reference grid or outline and browse in cube_border.
  10. Go to the Cube Options tab.

  11. Click Apply base grid and browse in 01Seabed_time from the _06_Velocity folder.

    Note on the base grid _01Seabed_time_

    • A base grid tells where the values from the stacking velocities start. Above the base grid it is only water and the values are the water velocity.
    • The seabed in time is the natural base grid and 01Seabed_time was gridded from time interpretation lines.
    • The grid window and x y grid increments for 01Seabed_time must be exactly equal to the cube.
    • You can do that gridding as an exercise later on. For the moment use the one in the course project.
  12. Go to the Result Cube tab. Observe that the result cube by default is written back to the same folder as the input.
  13. Go to the Testing tab.
  14. Drag the slider bar and set the z value to around 2500.
  15. Click Make velocity grid to perform a test for that level.
  16. If everything is OK click Execute to generate the velocity cube. It will be saved as all_lines time_velocity.


Testing the cube gridding


Velocity cube displayed in transparent mode wit one time layer inside

Check shot data

The velocity cube will usually be checked against check shot wells; i.e. wells that have correct time and depth values downwards.
This project does not have appropriate check shot wells for the area at the moment so here is only the technical procedure.

  • Use the command Deviation velocity cube and wells to see the difference between the cube layers and the crossing with the wells.
Exercise

Using check shot data to adjust the velocity cube.

  1. Highlight all_lines time_velocity and activate Utilities > Adjust velocity cube to checkshots.
  2. Browse in the check shot data.
  3. Specify the correct format of check shot data.
  4. Click Execute to perform the adjustment.
Exercise

Displaying the velocity cube with the manipulator.

There are many ways to display velocity cube data. They are all visible on cubes. Try them out and be familiar with them. The primary one to learn is to use the manipulator.

  1. Highlight all_lines time_velocity and activate Utilities > Manipulator cube display.
  2. Draw the manipulator in all directions by checking X Y or Z.


Using the manipulator to see cube planes

Exercise

Displaying different iso contours from the cube.

Iso contours is a direct way to see the interior of the cube. If too noisy one should consider smoothing the cube.

  1. Highlight all_lines time_velocity and activate Cube contours.
  2. Draw the contours for the values 2000 3000 3800
  3. Evaluate if it is noisy. In that case smooth the cube.
  4. Activate Smoothing Cube and perform smoothing and redisplay the iso contours.


Checking the cube by displaying cube contours