1. Seismic – Domain Conversion Objectives
Quality control time/depth relationships for wells
Define velocity model intervals
Create an uncorrected velocity model using well data
Create a final corrected velocity model using well data
Create a velocity model with uncertainty
Depth convert a 3D Grid
General depth conversion
Optional:
Create a velocity model using a seismic velocity cube
Create velocity model using stacking velocities (xytv point-data)

2. Overview ‘Velocity Modeling’ is separate from ‘Domain Conversion’
Interactively switch between time and depth for converted objects
Define the velocity model by:
Surfaces/constants
Well Time/Depth Relation
Velocity Cubes
3D grids
Convert both ways with the same model
Convert objects independent of the 3D grid
Perform conversions in the process manager

3. Velocity model – bridges the gap between time and depth domain
Time domain
Depth domain
Velocity model
Time -> depth conversion
Depth -> time conversion

4. The Workflow Define velocity model
Define velocity intervals (usually between time interpretation surfaces)
Define velocity functions (automatically calculated or user-defined)
Define output (e.g. velocity cubes, V0 and k for Q.C. purposes)
Domain convert via
Right click on object (uses currently ”active” velocity model)
General Depth Conversion Process for Multiple objects
3D Grid Conversion
Within a workflow in Process manager

5. The Process ZONES Velocity Output Advanced Corrections
Datum
1. Constant
2. Surface
3. Horizon*
ZONES
1. Constant
2. Surface
3. Horizon*
4. Well Tops
Corrections
Or none
A: Auto 1. Correction (Vo) 2. TDR (Vo & k)
B: User 1. Constant 2. Surface
Velocity
Datum
Vo + kz Vo
C: Velocity SEGY (average)
D: 3D Velocity Property (average)
Vo + k(z-z0)
1. Seg-Y
2. Velocity Surfaces
3. Well Data Points
4. T & V logs
5. Residual Points
6. Output Sheet
* Horizon from 3D Grid
Datum
Vo + kz
Output Vo
Vo + k(z-z0)
Saved in Model tab
1. Well trace Md inc
2. Depth & Time tolerance
3. Velocity & k clipping
Advanced
Additional Settings ++
(Gridding & SegY settings)

6. The exercise examples Time/Depth relationship in wells
Velocity Cube (SEG-Y)
Stacking Velocities (Point data)
Velocity Model 1
Velocity Model 2*
Velocity Model 3*
Velocity Modeling examples
Three examples of velocity modeling are given in the exercises section of this chapter.
Velocity Model 1
The first modeling exercise defines the intervals to be velocity modeled. It uses the established Time/Depth relationship from wells to distribute velocitiy trends throughout the area.
Velocity Model 2
The next modeling exercise is based on a loaded SEG-Y Velocity volume that is used to extract velocities from.
Velocity Model 3
The last example is based on Stacking Velocity points that are used to upscale and distribute as a velocity property.
By selecting one of the velocity models, a wide range of objects can be domain converted in accordance with that model. The same object can be converted by different velocity models for sensitivity purposes.
Active (selected) Velocity Model
Domain Conversion of objects
*Optional exercises, see Attachments

7. Define intervals
In the Process pane, open the Geophysics folder and double-click on the Make Velocity Model
Click the ”Append item in the table” icon as many times as available input objects
Change Bottom Interval to desired type
Select the levels to model, from Input or Models tab, click on its name and drop it into the dialog box using the drop in arrow
Keep the Correction set to None 2 1
Observe that domain conversion can be performed from time to depth and depth to time. It is also possible to do time to time and depth to depth conversion. 4 5 3

8. Velocity modeling using well data
Select type of velocity model for each interval. When using the Linvel function for modelling, constants, surfaces or Well TDR can be used as input
Set V0 to Well TDR – Surface/ Constant
Set k to Well TDR – Surface/ Constant 2 3 1
This exercise uses the established Time/Depth relationship in the different wells to derive V0 and k points for the specific intervals (each well with a T/D relation gives V0/k values for each interval). These points are gridded or averaged to make surfaces or constant values for the intervals used in creating the velocity model.

9. Velocity modeling - Output
Open the Output tab
Click on the data types needed as output 1
When creating velocity models, the results can be output in different formats. If a correction is performed when modeling, both original and final velocity models as well as the correction can be output. 2

10. Velocity modeling – Advanced settings
Open the Advanced tab.
Check that ’Well velocity’ tab is open.
Possible to set MD increment, depth and time tolerance and clipping parameters for V0 and k.
4. Activate ’Output settings’ tab 2 1 4 3
Well Velocity settings
These settings are used when using the well time depth relationship (TDR) to calculate V0 and k for the depth conversion.
Well trace MD inc:- the sampling interval for the TDR when calculating V0 and k.
Depth tolerance:- Minimum thickness of a zone (in project depth units) below which it will not be used for calculation of V0 or k.  This option ensures that extreme values in thin parts of the zone do not affect the rest of the model.
Time tolerance:- As above but in project time units.
Clip V0 : - If a TDR is used then the extrapolation between points could lead to extreme velocities away from wells. Enabling this option will truncate the surface at the chosen level.
Clip k : - as above but for k rather than V0.
Clipping: be careful not to clip V0 and k values inside the “normal” data range for the parameter. Even if corrections to Well tops are performed, errors will occur if clipping is to tight.
Output settings
Gridding:- The chosen gridding algorithm will be used for creating all the surfaces made during the velocity modeling process.  For more control over this gridding e.g. using different methods for different zones, create the output as points, grid them using the make surface process and input surfaces instead of the TDR.
Resolution:- Output from the velocity model (SEGY, surfaces and logs) will be created at the resolutions specified here.
5. Defines Gridding algorithms for Surfaces and resolution for Cubes, Surfaces and Logs.

11. Velocity modeling using well data
Open Settings for Wells and go to the Time tab. Select Checkshots, Well Tops or corrected sonic logs as source for time/depth (move the prefered source to the top)
Keep the parameters for the velocity model the same as the values shown
Click Apply or OK to generate the velocity model 1
As previously stated, an alternative to use Well Tops as surce for time/depth relation in all wells is to substitute with Checkshots, velocity logs or sonic logs for each well.
This is done in Settings for each well, Time tab where Override global settings is toggled on and the correct T/D source is selected and put at the top of the list using the up arrow. Toggle Use first item only to avoid using non-QC’ed data as source for TDR.
This setup will generate V0 and k objects found under the appropriate velocity model and can be used for quality control of the result. 2 3

12. Velocity modeling with uncertainty
Create a new velocity model with Well tops correction, based on the previous velocity model
Toggle on Activate uncertainty page icon to insert a Std Dev column under the Velocity model tab and to activate the Uncertainty tab. Fill in appropriate Std Dev. numbers
Go to the Uncertainty tab and fill in appropriate Variogram parameters. Toggle on Iconize uncertainty error surface. Click Apply
A velocity model containing velocity surfaces with uncertainty built into them is generated
Check uncertainty statistics and display the corresponding deterministic and stochastic V0-surfaces 1 4 3 2
Incorporating uncertainty into the velocity modeling enables the user to stocasticly handle the variance in velocities. The principle for including structural uncertainty in Petrel is to identify the possible error representing one standard deviation and to multiply that error with a stochastic surface with values around zero, which again will be added to the base case surface. In other words:
Sr = Sbc + U1s * Usgs
Where Sr is the surface realization, Sbc is the base case surface, U1s is the surface or constant representing one standard deviation error and Usgs is the stochastic error surface.
The standard deviation built into the velocity model can either be given as a constant number or a surface. To insert a standard deviation surface, toggle on the box to the right in the Std Dev. Column, select the surface from the input tab in Petrel Explorer and drop it with the blue arrow.
Deterministic
V0-surface
Stochastic
V0-surface 5

13. Depth convert a 3D grid Make sure the correct 3D Model is active
From the Process tab, under Structural Modeling, open Depth Convert 3D Grid
Select the appropriate Velocity Model
Alternatively change the pillar geometry types for faulted and non-faulted pillars before Applying
A new 3D Model will appear in the Models tab, with the input 3D grid name and an extension [DC] 1 3 5 2 4

14. Depth convert Seismic data
Make sure the correct Velocity Model is active (Bold name).
Right click on the 3D volume and select ’Depth convert by active velocity model’.
Alternatively, from the Process tab, under Geophysics, open the General Depth Conversion process.
Click on the 3D volume name in the Input tab and drop it into the dialog box using the Append selected item to the list icon. Click Apply. 1 2 3
No notes available. 4

15. Attach seismic to active grid
Make sure the correct depth converted 3D Model is active
From the Input pane, right click on the seismic 3D volume and select Attach Seismic Volume to active Grid
Expand the active depth 3D grid under the Model tab, right click on mig.sgy [DC] and insert InLine and Xline.View the result in a 3D Window 1 2 3
This approach to depth stretching seismic data only within a defined model is obsolete since the entire seismic volume can be domain converted (given an appropriate velocity model).
This example is only included as an alternative that has been used up to this date.

16. General depth conversion
Make sure the correct Velocity Model is active (bold text)
From the Input pane, right click on the object to be depth converted and use Depth convert by active velocity model
Clicking the + in front of the depth converted object shows the now available time and depth domains for the object
The Time/depth settings can be changed in the top Toolbar. Alternate between TWT and TVD while observing the effect in a 3D Window 3 2 1 4

17. General depth conversion
Alternatively, or to depth convert several objects, open the General depth conversion process
Select correct velocity model from the pull down menu
Drop in the objects by clicking on their names in the Input pane and then on the Append selected item icon in the process dialog
The Time/depth settings can be changed in the top Toolbar. Alternate between TWT and TVD while observing the effect in a 3D Window 5 6 7 8 9

18. EXERCISE Domain Conversion