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\title{Computer Assignment 4\\Due 3 June 1994} \begin{center} Your name \end{center} \ABS{ This computer assignment explores how smoothing the velocity field affects the migrated image. In this exercise, you are going to use Promax to migrate a stacked section with an increasingly smoother velocity field. I want you to discuss the differences between the migrated images. }

I will guide you through this exercise. You should be able to answer and check off the questions as you go. Please, be aware that the important questions are the ones in the Match the sections'' paragraph.

# INSTRUCTIONS

1. Login to your student account and open a window on robson''.
2. Start up Promax (command: prompt% promax ).
3. Click on the area class''.
4. Make your own copy of the realdata'' line.
5. Bring up the flow panel of your line.
The Promax processing flow that you will find includes a series of steps. You will use it by commenting in and out certain Promax processes. The flow is complete; you do not need to add or change processes to successfully finish this assignment. However, feel free to change the processing if you like to. Please, document carefully any changes.

## Printing

You will be asked to make print outs of your sections. I suggest you use the grab'' command in your Cakefile''. After typing cake grab'' the mouse cursor turns into an angle; you can crop the area you want to print by dragging the mouse across the screen while keeping the left button pressed. Whenever asked to print out a section, please crop all traces.

To make a print out of a detail on your section, you use the Promax zoom tool (fourth icon from the left in the Promax screen display window) to zoom into the desired detail; then you grab the screen as described above.

## Velocity fields

In this section, you will have a look at different velocity sections. The first section displays RMS-velocity. These RMS-velocities originated by stacking corresponding CDP gathers. I generated the interval velocities by converting the RMS-velocities. The other velocity sections display interval velocities versus depth and are used for the depth migration. Geophysicists routinely derive their migration velocity section in the same fashion.

• Comment in the first velocity viewer and execute.
• Print the entire velocity section.
• Comment out the first velocity viewer.
• This is the RMS-velocity section. Can you see at which CDP's I specified the velocity?
• Comment in the second velocity viewer and execute.
• Print the entire velocity section.
• Comment out the second velocity viewer.
• This is the first interval velocity section. How does it compare to the RMS-velocity section?
• Comment in the third velocity viewer and execute.
• What is the difference between the second and third velocity section?
• Print the third velocity section.
• Comment out the third velocity viewer.
• This velocity section contains more traces than the previous one. Why?
• Comment in the last velocity viewer and execute.
• Print the fourth velocity section.
• Comment out the last velocity viewer.

## Migrated sections

Now you are going to use each of the viewed velocity sections to migrate a small post-stack data set. To prepare the different migration flows, you comment in the Disk Data Input process and the Screen Display process.

• Execute the flow.
• Print out the stacked section.
• Print out two section details, e.g. the shallow faulting in the section center.
• Comment in the first Kirchhoff Depth Mig and execute.
• Print the entire migrated section.
• Print out the same section details as above.
• Comment out the first Kirchhoff Depth Mig.
• What do you think are the steep dipping events at the ends of your sections?
• Draw some colored lines on your section to pinpoint some prominent geological features? What are they? (but do not spend more than 5 minutes interpreting).
• Comment in the second Kirchhoff Depth Mig and execute.
• Print the entire migrated section.
• Comment out the second Kirchhoff Depth Mig.
• Comment in the third Kirchhoff Depth Mig and execute.
• Print the entire migrated section.
• Comment out the third Kirchhoff Depth Mig.

## Match the sections

• Look into the parameter fields for each Kirchhoff Depth Mig process and read the name of the velocity section it used.
• Look into each Velocity Viewer process and read what velocity section you printed out.
• Match your printed velocity sections with the corresponding migrated sections.
Please, clean up: Delete your copy of the realdata'' line in the line panel. If you have any questions, please see Ray. Good luck.