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obh velocity analysis Link to this post

Hello,

I am new using Claritas and relatively new at processing seismic data, and I would appreciate if someone could give some support in dealing with some obh data. I have received an "old" (2000, 14 years ago!, written in a SparcSun v5 pc, so segy standard, big-endian, ibm floating point) obh (ocean bottom hydrophone) data set acquired at about 1800 m water depth, and I was told it may be possible to perform a detailed velocity analysis of an area with both a BSR and a possible paleo-BSR. Both reflectors are related to the presence of marine gas hydrates in the first 500 m of the sediment column.

I had the idea to perform this analysis with Claritas but so far I only have been able to both read and display the segy with a pretty simple job file. I also used discwrite to convert the segy to hdf5 format. Having that ready, I tried to use CVA and Semblance picks in the Velocities tab to perform an initial velocity analysis on this obh data, but I found that for the former I need a stack file, and for the latter neither the hdf5 or segy file can be read.

This obh file has a quite simple geometry. It is basically a common-receiver gather, where there are in total 1800 shots (about 900 shots per side, ie, positive and negative sx locations, where the BSR/paleo-BSR seem to be just beneath this obh, see image here http://s29.postimg.org/jguqi0exj/image.png) and only one channel (this case labeled as 81). Before applying some filtering (eg. in tau-p domain to attenuate lineal noise), I want to perform an initial velocity analysis with the unfiltered, raw data, but so far I had no success.

I can share the file in case you would like to give it a try, I would appreciate that so you could let me know if it is possible to get the detailed velocity analysis. The file is abut 50 MB, so I can upload it somewhere for you.

Thanks for any pointers and support on this,

Cheers,

Gery

Re: obh velocity analysis Link to this post

As you mention, you probably won't be able to use CVA to carry out your velocity analysis - as you effectively only have one reciever gather and CVA requires a stack. You could try the VELSPEC module - this module can be used to output velocity spectra. The traces are output in ñvelocity gathersî with which may be displayed in SV with a primary key set to CDP, and the secondary key WEATH_VEL. You should then be able to pick your velocities in SV. The OBH data would need the offset header defined if it doesn't already exist.

Re: obh velocity analysis Link to this post

thanks Keleigh, I tried velspec but I am not sure how to proceed after adding it to a job. If I use sv with my hdf5 (the one I get after discwrite) it says it cannot read the header, they are ok afaik. I attached the segy here, it'd be great if you can give me a hand on this, thanks in advance.

Re: obh velocity analysis Link to this post

Thanks for uploading your data. I've attached a simple jobflow that you should be able to modify to create velocity spectra for your data. The READSEGY module reads in your segy file and SEISWRITE writes this out as an hdf5 file. The VELSPEC module creates the 'velocity gathers' that can be displayed in XVIEW/SV with a primary key set to CDP and a secondary key of WEATH_VEL. You can pick your velocities on the output created by VELSPEC by using the Semblance picking tolbar options in SV.

Re: obh velocity analysis Link to this post

cool Keleigh!! thanks for that, now it works, really nice.

You guys rock, keep the great work up!!

Cheers,

Gery

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