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Author Topic: High Density Velocity Analysis in Claritas 8910 Views

High Density Velocity Analysis in Claritas Link to this post

High Density Velocity Analysis can be performed using the NMOPICK module to automatically pick velocities on a given dataset. The density of these autopicks can be controlled spatially by defining the grid to pick either on input within the DISCREAD module or by using an IF(IFNOT)/ENDIF to define the grid of CDP's to pick on. The density of picks can also be controlled temporally using the WLEN parameter of the NMOPICK module.

However the approach that we have taken when using this module is to use it's capabilities to pick every CDP and every sample on the input dataset.

To obtain optimal results from what we called the VELSECT approach it is necessary to precondition the input data prior to NMOPICK.This would generally consist of Bandpass Filtering to minimise effect of cycle skipping, Radon filtering to remove multiples and diffractions.

The nmo velocity field output from NMOPICK is then conditioned by removing/deleting high-low RMS and Interval velocities in an iterative approach, jitter in the resulting field is then removed by applying a spatial lowpass filter. Finally we can tidy up the resulting field by replacing velocities above a picked waterbottom horizon with a suitable constant velocity, and similarly below a picked base horizon.

Attached are two example powerpoint presentations showing the benefits that can be achieved from applying this approach along with and example NMOPICK job flow.

Re: High Density Velocity Analysis in Claritas Link to this post

In terms of creating a layer based velocity model, you would need a third party software such as Downunder Geosolutions INSIGHTS package to create this model. The model would then be exported from INSIGHTS in a suitable format ie SEGY and read in by the Claritas PreSDM module. Velocity updates would be again performed in INSIGHTS by importing the output Common Image Gathers from PreSDM.

Re: High Density Velocity Analysis in Claritas Link to this post

Dharmender,

If you would want to use the Horizon based velocity model approach for depth migrations then you would need to talk to Downunder yourselves about licensing there Insights software or other similar software providers. DUGeo can be contacted here :- http://www.dugsw.com/

We however have had very good results with pre stack depth migration using the ADMIRE approach which we believe is a robust and effective method of deriving a depth migration velocity model.

Re: High Density Velocity Analysis in Claritas Link to this post

What is the ADMIRE approach? what is the procedure to apply it?

Re: High Density Velocity Analysis in Claritas Link to this post

The ADMIRE approach is effectively iteratively updating the velocity field for depth migration by taking the final PreSTM velocity field smoothing and converting to depth, migrate with the PRESDM module, convert the output gathers back to time run the NMOPICK processing flow to generate a new velocity field, smooth as required and migrate again iterate until you feel you have generated your optimal depth migration model/result.

Re: High Density Velocity Analysis in Claritas Link to this post

Thanks

Could you please explain it in more detail with examples.
i have read the help of PRESDM module, which type of velocity file is suitable for this module. how we make this velocity file. please explain with examples.

Re: High Density Velocity Analysis in Claritas Link to this post

The GRIDS_LIST parameter in the PRESDM module should contain the name of an an ASCII file with the path names to the NetCDF grids or velocity *.segy files you are using. The number of files in this list should match the value in the DEGREE parameter. The velocities in these files must be specified as depth converted interval velocities. You can convert the velocity files from a time to a depth domain using the TDCONV1 module (see the ...convert between time and depth domains? topic in the How do I ... section) or (an easier method) using the V(z) button in the isovels application then use Re-pick to save the converted velocities. If you use the isovels application you will need to convert the created *.nmo file to a segy file using the NMOTRACE module. This module takes an NMO file and overwrites the seismic data in a stacked CDP file creating a file of velocity traces. If needed the isovels application can also be used to create interval velocities by using the Intervals button, before converting to the depth domain.

Re: High Density Velocity Analysis in Claritas Link to this post

Thanks

Can you attached the example of Grids list file and mention the associated Degree value of this file for any 2D line.

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