13 - 2D AVO Interpreted as a 3D data Set
Please note that these data do not belong to the rest of SeisX data set. The exploration play on these data are focused on shallow Miocene Sands. Examine some inlines and cross lines.
Before we can do these analyses we must have the 2D data properly conditioned at the processing shop. Request that your processing shop cut a tape for the velocity and shot point corrected gathers. Each gather will be a different inline. The final stack should loaded as the first cross line. Be sure the processing house pads out the number traces for each inline (gather) to the maximum fold on the line.
Load the data into SeisX with the cdp's
corresponding as inlines and cross lines running from 1 to the maximum fold.
Cross line one is the final stack, crossline two the near offset gather and
the maximum cross line corresponds to the farthest offset.
Use SeisX to attach this data volume, Project, Attach SeisXed Files,
Create Update SeisX Base Map and label every
100 in lines.
Import the seismic horizon file avo.asc and examine the Miocene horizon. Set
the Seismic Display Attributes to VD/VA Wiggle, Time scale of 7.5 inches per
second, Wiggle Overlay Decimation of 1 and Excursion of 1. Update the System
Display Attributes. Here is Line 623. Notice how the amplitudes increase to
the mid range offset.
Extract Amplitudes
Before we can examine the amplitude distribution we must extract the amplitude
values, Select Horizon, Extract Amplitudes.
Set the map display attributes to 
and press
apply. Zoom into the map to examine the amplitudes.
Examine the first cross line trace (stack section).
Update (correct) the interpretation of this trough event. We will examine this
trough to determine if there are any variations of amplitude will offset. The
offset display will give us a check on how well the velocity has been applied
and if any residual normal moveout is left in the data.
Transfer First Cross lines to all Cross lines
In order to update our interpretation volume we need to export the two
interpreted horizons. Select Horizon, Horizon Export. Select the two horizons
and the one 3D.
I have written a simple awk to take the interpretation from the first cross line and replicate it onto all cross lines. Run this awk:
awk -f near2far.ak < avo.asc > avo3d.ascThe near2far awk takes a 3D horizon interpretation and copies the value from the first trace to all traces on the line. If moveout and statics have been properly applied the interpretated horizon should be perfectly flat.
It's not flat so we need to snap and extract the amplitude values
Change
the Search gate to 
Press the Horizon redraw
button 
button to update the new horizon
interpretation. Display line 635. Did the snap horizon work?
Do you think there might be a positive AVO anomaly ?
Check out what is happening at Line 820. It appears that there is still
residual normal moveout left in the data and the snapped event picked high
instead of low!
We should go back to the seismic data processor to have the velocity decreased
so the events are flat on this section!
Time Slice Analyses
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