Velocity Picking

Bryce Hutchinson Sumit Verma

Objectives : 1. Be able to understand semblance, supergather, CVS 2. Recognize under corrected, over corrected, and properly corrected

reflectors for NMO. 3. Velocity picking on the velocity analysis panel.

Sort the raw data 1. You should have a sort file that you created in a previous lab. Click the to access the flow you created to for this sort.

Note: This is just a simple way to access a previous flow. You can create this flow the same way we did in previous labs if you want.

2. Change flow input to the raw data

3. Be sure the Sort order in the input command is set to CMP_NO

4. Change the name of the output file. Click GO to run the flow

Run semblance flow raw sort1. Open up the semblance flow you have previously run by clicking the in front of semblance in the dataset.

2. Start by right clicking the VelZone icon and Data/Header SELECTIONS. This is where we will define what CMPs are input into semblance.

Important

Before we defined a small CMP range. 1. Delete the selection by clicking the red minus sign and then the aqua colored globe.

2. Now we will define the CMP selection to include every bin Here we were not careful enough, but

you can make it as 20, so that you get a inline numbers like 40, 50, 60.

Run semblance flow with raw sort

1. Now Change the input of Velzone to be the Raw CMP Sort we just created

2. Now Under the Super-Gather Options tab in the Velzone input, select 3-D Analysis and input 5 in the In-Line and X-Line options

3. Change the name of each of the outputs

Click GO to run the flow. It will take a while to run.

Velocity Analysis

1. Under Velocity, select Interactive Velocity Analysis

2. Choose the inputs to be the Raw semblance, cvs, and common offset stack we created

2. Choose the inputs to be the Raw semblance, cvs, and common offset stack we created

Not enough traces to pick velocity !

Press + a few times to see the semblance

You need to zoom on time axis. (Click and drag)

Drag it to reach to this location.

We would like to start our Velocity picking on the bin which has enough number of traces.

Semblance panel

Super gather or Offset gather Constant velocity stacks

Velocity semblance panel shows what velocity has the highest semblance. That means if I choose that velocity to NMO correct the reflector, it will be flat at that time.

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Press + to increase the amplitudes. We will apply a mute in the next slide to get ride of some of this nastiness.

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Just check are reflectors flat ?Play with the velocity by clicking different

velocities in the semblance panel!Check how slower velocity makes it over

corrected or higher velocity makes it over corrected ???

It looks better now ??So stretch mute works !!

Click on the pick again to

delete the pick !

This is over corrected !!What makes it ..

Slow velocity or fast ?

This is under corrected !!

Read if you like !

Reference : http://www.bairdpetro.com/pdf_files/p58-62.pdf

Lets see the CVS : constant velocity stack.

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This square box represents the velocity pick too…

Mark : a processor says that you should pick velocity on the semblance panel first. Then look at the CVS and place the picks correctly.

We moved the boxes, where we think is a reflector.

Here, pick the for time 4000ms too. This will make the velocity look smooth.

We should pick the same initial velocity(say from time 0 to time 100ms) for all the gathers. This makes the velocity model smooth.

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Save your velocity to a file.

Save it often, as the program may crash and you could lose your picked velocity.

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Pick the velocity on next cmp bin!!

Repeat this process for all the bins!!!!

It seems very time taking process butBetter velocity pick may make your processing results valuable . So, guys start picking !!!

Isn’t it fun to pick !

Get your friends involved!