velocity volume loading problem

[Brian Brennan]

Hello Open Detectives.

I'm trying to load a velocity cube in depth and having some trouble. Here is the  deal:

I've loaded an amplitude depth volume and can view it just fine. I used it to create the seismic  grid when I imported it. When I try to load the associated velocity volume, the defaults don't properly pick up the sample rate, so I have to tell it that the volume starts at 0 and has a sample rate of 10 feet. Should work fine. But it doesn't. When I load the volume and try to look at it in the viewer I get nothing, and next to the volume in the item list is a yellow triangle with an exclamation point in it. I don't see anywhere to decode what that means.

Thanks for any help you can give me. 

And a follow up: Once I get the velocity volume in (it is an interval velocity volume), can I use it to convert horizons from time to depth or depth to time?

Thanks for that, too.

--

Brian Brennan

Geophysicist

337-258-3641

email: BrianDB...@gmail.com

website: Third Wave Geophysics.com

[Uwe Strecker]

Hi Brian,

Please find inserted below portions of a reply sent to a user with a similar question last year. The second part of our reply contains some info relevant to your question. I’m out of the office until Monday. Please let me know, if the sub selection range portion (you can set the sample size there) worked for you.

Cheers,

Uwe

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.

 Uwe Strecker, Ph.D.

Geoscience Manager

--------------------------------------------------------------------------------------------------------------------   dGB Earth Sciences
Phone: +31 53 4315155 E-mail: uwe.st...@dgbes.com Internet: dgbes.com

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[Brian Brennan]

Thanks much for the response Uwe.

I'm not sure why it didn't work the first time, and I had to try a couple different things this morning, but I've got the velocity volume loaded. Next step is the model creation and domain conversion.

Thanks again.

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[Uwe Strecker]

You’re welcome, Brian. 

Have a nice weekend. 

Cheers, Uwe

 Uwe Strecker, Ph.D.

Geoscience Manager

--------------------------------------------------------------------------------------------------------------------   dGB Earth Sciences
Phone: +31 53 4315155 E-mail: uwe.st...@dgbes.com Internet: dgbes.com

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