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[walid Rudwan Osman]

Loading Sgy velocity cube, how to do it, and display it and QC it.

[Uwe Strecker]

Hello Walid Rudwan Osman,

Would you please let me know, if your velocity cube is in the time domain or is it in the depth domain?  The answer will depend on your reply :)  Thanks for letting me know in advance.

Cheers,

Uwe

On Mon, Dec 22, 2025 at 9:59 AM walid Rudwan Osman <wro...@gmail.com> wrote:

Loading Sgy velocity cube, how to do it, and display it and QC it.

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[walid Rudwan Osman]

it's Time Domain,with migrated cube in the timed omain

and another one is in the depth domain, but 3D migrated cube is depth domain as well

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[Uwe Strecker]

OK, thanks. 

Apparently, your processed velocity data are from a Pre-Stack Depth Migration (PSDM) with corresponding seismic velocity images in two-way travel time (Pre-Stack Time Migration).

If you plan to work with your data in the time domain, please start by loading your Velocity volume like any other seismic volume, i.e., go to:

Survey>Import>Seismic Data>SEG-Y

If uncertain about the data range, please use the icon to Scan the entire input (the one with dual tipped green arrows on it). Then you may also elect to use the looking glass to Examine Input file. Within the new GUI that opens up, you can press the Display Traces icon to view the specified number of traces you elected to view (1,000 is a good number). In the new GUI that opens turn off the wiggle display under Set Display Parameters (green colored mechanical wheel). Select a suited color map, the industry choice is a Rainbow type of color bar. Finalize import.

 

Now that the SEG-Y with the velocity data has loaded, you may still want to create your own time-to-depth model from it.  To do so, please go to:

Processing>Time-Depth Conversion>Seismic

The Time-Depth Conversion 3D GUI pops up. Please select the Direction in which the domain conversion is to take place, i.e., here, Time to Depth. Under Z transform from the different options please choose Velocity Model. In the next line you can actually choose an existing velocity model or build a new one.  Since the velocity model has not been generated yet, you need to create it. To do this, please select Create (to the right of the select button), select your velocity cube again and specify what velocity type the data are in (the options are Vint (interval velocity), Vrms (root mean square) or Vave (average) and hit OK.

You can also generate a velocity model from a project that is in depth, but it is a little bit more tricky. Please find enclosed the outline of a workflow that goes from a velocity volume in depth to time below:

 

1. Create a Depth Project>import Velocity (Depth).sgy
2. Check depth data for possible datum shifts used in migration (for instance, there could be a 100m shift upward in depth, so that your total depth range on import could read -100 to 2900m or something similar)
3. Create time-depth model

1.      create/invert velocity cube with itself (follow procedure outlined under Processing>Time-Depth Conversion>Seismic above

2.      choosing proper velocity type (for instance, VINT)

4. Create inverted volume (Depth-to-Time) and Export (leave depth defaults alone)
5. Import to Time Survey
6. Copy volume in Seismic Manager to rename and under survey range change the sampling interval to the proper sampling interval (for instance, from the default of the calculation to a sampling interval of either 2ms (onshore) or 4 ms (offshore). Sometimes the vertical sample size of the velocity cube is much larger than the sample size of the other volumes and by so using the Make a copy of volume option and then under Volume subselection Range you can set the Time Range to the Step size that your other seismic volumes share (for instance, 4 ms for most offshore data).  Redefining this vertical sampling size is important, if you like to use this velocity as a background model in an inversion performed in the time domain. Shift velocity cube up or down (plus or minus the shift mentioned in the processing report) to compensate for replacement velocity used in equating depth/time migration deliverables. If this became necessary, this could be done under Attribute Set 3D>All>Reference shift>Time (ms). Finally, you can compare this velocity volume that you converted to time yourself to the one you imported previously from your pre-stack time migrated velocity.sgy.  Time shifts notwithstanding, they should essentially be the same. Please note that you can check if available horizon data align with velocity breaks in your velocity profile as a quick QC.

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[walid Rudwan Osman]

Many thanks for the precious comment, God bless you.

If you may, could you please also help with loading CRP  gather both time/depth domain, and to check in the map their location compared to the whole previous survey.

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[Uwe Strecker]

Thank you kindly.  Please know that dGB's special advisor, Dr. Paul de Groot, will contact you shortly regarding your question on prestack gather loading.  Please be advised that there is also a vintage YouTube video  (https://www.youtube.com/watch?v=DRk7tKGrado) on how to manipulate the prestack gathers in OpendTect. 

Also, may I ask that for any type of requests we prefer professionals to use their corporate e-mail instead of their personal e-mail accounts like @gmail, @hotmail, etc. to facilitate our customer support and care & reach. Many thanks!

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[Paul de Groot]

Hello Walid,

Essentially, there is not much difference between SEG-Y data loading of post-stack data and pre-stack data. Select the Survey -> Import -> Seismic Data -> SEG-Y -> Pre-stack 2D ... or Prestack 3D menu. The SEG-Y wizard pops up. If the SEG-Y file follows the standard (Rev-1 / Rev-2), it should load without problems. If the data is Rev-0 or non-standard, you may have to set header file locations for inline / crossline (3D) and/or X,Y positions. As in the post-stack case, use the looking glass tool to examine the file and find the appropriate header fields. 

You have the choice to copy the SEG-Y file into the OpendTect project (under the Seismics folder), or to leave the SEG-Y where it is located and create a link. The latter obviously saves disk space but the former is better when you backup the project. For that reason, I typically copy pre-stack data into a new folder (e.g. RawData) before importing it into OpendTect.

I hope this helps. Have a nice holiday season and best wishes for 2026,

Paul.

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Paul de Groot

Special Adviser

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Phone: +31 53 4315155 E-mail: paul.d...@dgbes.com Internet: dgbes.com

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