CRP Gather Ray Tracing
CRP Gather Ray Tracing traces rays associated with CRP gathers for use in MVA tomography.
Theory
Please refer to Using MVA for more information about the prestack migration velocity analysis process.
Raypaths are used to relate portions of the velocity field to the residual moveout on CRP gathers. There is presently no way to display these ray paths.
A trace in a CRP gather is produced by migration of an offset section. The ray path of the energy of the reflector on the CRP gather is plotted below. The ray path actually consists of two
rays: that of the energy down to the reflector and back up to the surface. The two ray paths must obey Snell’s law at the reflector and must have the proper offset when they reach the surface. The two rays are not symmetric because of the dip of the reflector.
fixed offset
Rays must obey Snell’s law
The plot of all the rays that correspond to one reflector on the traces of a CRP gather produces a ray fan as shown below.
This ray fan example consists of 4 ray pairs.
A plot of all the ray fans of several reflectors from several CRP gathers would look like this. Normally, the CRP gathers would be more closely spaced producing a more dense overlapping ray coverage.
Parameters
Select interval vs. depth velocity file
Select a velocity file from the interval velocity in depth menu.
This is typically the migration velocity from the last migration ran.
CDP spacing of the velocity field
Enter the CDP spacing of your seismic data. This converts the velocity field’s CDP coordinates to a horizontal position.
You cannot enter 0.0.
Horizontal cell size
Enter the horizontal dimension of a cell, in ft. or m to use for ray tracing. This will also be the cell size used in the
tomographic inversion that uses the ray paths output from this process. Choose this parameter carefully. Choosing too small of a cell size can significantly increase CPU and disk usage, but won’t harm the inversion. We recommend a value of 3-5 times greater that your cdp interval.
Vertical cell size
Enter the vertical dimension, in ft. or m. As with the horizontal cell size, choose this parameter carefully.
Choosing too small of a cell size can significantly increase CPU and disk usage, but won’t harm the inversion. We recommend a value of 3-8 times the cdp interval (Typically 50-150m).
Horizontal smoothing of velocity field
Enter the horizontal smoothing value in ft. or m. Smoothing of the velocity field helps stabilize and speed ray tracing. We recommend a value at least as large as the horizontal cell size.
Vertical smoothing of velocity field
Enter the vertical smoothing value in ft. or m. Smoothing of the velocity field helps stabilize and speed ray tracing. We recommend a value at least as large as the vertical cell size.
CDP increment for ray tracing
Enter the CDP increment for ray tracing. Enter 2.0 to trace rays from every other CDP location. This value should generally be 2.0-8.0.
Minimum offset to trace rays for
Enter the minimum offset for ray tracing. This should be the absolute value of the smallest offset on your prestack depth migrated data.
Maximum offset to trace rays for
Enter the maximum offset for ray tracing. This should be the absolute value of the largest offset on your prestack depth migrated data.
Select a mute table
Select a mute table from the menu. A Top mute should be selected that removes noise and migration artifacts from your prestack depth migrated CRPs. This will decrease run time by letting the ray tracer know where there is no data (the area above the mute) and not shoot rays in those areas.
Number of rays to shoot in the offset range.
Enter the number of rays for sampling your ray fan. More rays increase expense. You generally want a ray for each 100- 400 m of offset, depending on the smoothing you will use in the tomographic inversion and the resolution you desire. This value should be about 8-16.
Robustness factor
Enter a robustness factor value. A value greater than 1.0 will reduce program error, but increase computation time. A value less than 1.0 will do the reverse. If you notice much failure in the ray shooting from a previous run, you may want to increase this value to 2.0 or 3.0. The more smoothing you perform on the velocity field, the smaller this value.
Trace rays to topography or datum?
Select to traces rays from:
• Datum: Source and receiver are at the final datum.
• Topography: Source and receiver are at the surface.
Your choice should be consistent with your prestack migration.
Method of Horizon input
Select how you wish to input your horizons.
• Horizon group: If you created a horizon group contain- ing all of thehorizons you wish to ray trace, select this option.
• Individual horizons: If you did not create a horizon group, select this option and select the horizons one at a time.
Select Horizon Group
This appears if Horizon group is selected as the Method of Horizon input. Select the Horizon Group file you wish to use for ray tracing.
Type of horizons to use
This appears if Individual horizons is selected as the Method of Horizon input.
Select the type of horizon to use from the following choices:
• Interpreted horizon in depth: Edit a parameter table of type IHD. This is the standard default for horizons picked in MVA Xsection Window.
• Other horizons: Input multiple other horizon files.
Number of horizons to use
This appears if Individual horizons to Method of Horizon input. Enter the number of horizons to use for raytracing.
The limit is 30.
Select horizon #1 (-#30)
This appears for each horizon in Number of horizons to use. Select a horizon table from the Parameter table menu.
Select the autostatics horizon file
This appears if Autostatics Horizons to Type of horizons to use. Select an autostatics horizon file from the parameter table menu.
Move output ray files to secondary storage?
Select Yes to move output ray path files to secondary disk storage. These files can be large and may fill up primary disk storage.
Note: You will not be able to remove the raypath files on secondary storage from a ProMAX® menu. (Removing an
entire line will remove these files.) You will have to manually remove them with UNIX commands because this program creates a link from primary to secondary storage, and this file appears to be on primary storage to the rest of the ProMAX®
processing system. Removing the file from the menu just removes the link, not the file.
