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.
NMO stack
PSTM stack

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
gain T**1.5

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.
high velocities
smooth velocities

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.