Kirchhoff PSTM 2D
A CPSeis process has been written to perform 2D pre-stack time migration by the
straight-ray Kirchhoff method. This is the simplest pre-stack migration and it
is only suitable for certain situations. Notably, the velocities can not vary
much horizontally. The lines must be shot in the dip direction so
that it represents what is under that line.
The plots below compare the migrated stack with a standard NMO stack. They will open
in a new window or tab. It can be seen that the migration has collapsed the
diffractions on the faults.
The data is from the UL9905 survey line 34, which has a mostly flat water bottom, so velocities
varied little. The stack fold is 24. The brute stack flow is simply:
remove gun delay
median despike on shots
FX decon on common offsets
NMO with doppler mute
velocities were picked at 1.25 km intervals
The PSTM uses an aperture of 44 degrees or 3 km (whichever is less).
The following plots are from part of the line with a depression in the water bottom.
This distorts the raypaths, and for the NMO stack, one must pick much higher VRMS
at the depression in order to get a reasonable stack. However, for the PSTM, ignoring
the higher picks and using a smooth velocity across this feature will result in
continuous events. Although if one looks at the gathers, they are not flat in that
zone. Using the raw stacking velocities will for migration will also not yield flat
gathers there, and produces artefacts in migrated stack. The sea floor drooping 24 ms
over 20 CDPs is enough to upset time migration.
The process can be added to CPSeis by downloading the following another.tar
the makefile must be copied to etc subdirectory
kpstm2d.xml must be copied to opencfe/xml
the others to the src subdirectory
then in src subdirectory, touch kpstm2d* touch super*
finally in platform directory, make icps cfe
Note that you need a sourceforge revision of at least 215.