PROCESS FKFILT

Document Date:  Jan 1993

FKFILT calculates and applies a filter in the frequency-wavenumber (FK)
domain.

Currently the only type of filter that is implemented is a fan filter or
pie slice filter.  This type of filter is useful for removing or 
retaining signals travelling across the seismic line at certain phase 
velocities.  The filter is defined in terms of a series of lines from 
the origin which delimit pass and cut slices of the filter.  In between
a cut and pass region the filter response is tapered according to a 
chosen window function.

To define a fan filter, the filter lines may be given either in terms of
velocity or in terms of dip. The cut and pass lines may be input in any
order and will be sorted and checked for consistency.   For velocity the
filter region runs 
                    v :  0- -> -inf / +inf -> 0+
While for dips it runs
                  dip :  -inf -> 0 -> +inf.
(Remember horizontal events have 0 dip and infinite velocity.  Steeply 
dipping events have small velocity.)

It is a mistake to define a filter that, when sorted, consists of 3 or 
more lines of the same type within the body of the filter or 2 or more
lines of the same type at either end.

Refer to two articles in the January 1983 "First Break" for more details
on both the FK domain and FK filtering.  The SIOSEIS document fk.forum
contains some discussions about the fk domain and has FKFILT examples.

Prestack FKFILT may be done by using the process TX2FK parameter PRESTK.


The Parameter Dictionary:
--- --------- ----------

DipCut - The dip of the lines defining the FK region(s) to be removed. 
         Dip is measured in ms per trace.

DipPas - The dip of the lines defining the FK region(s) to be retained.
         Dip is measured in ms per trace.
         e.g  DipPas -1 1 DipCut -2 2 retains events with small dip,
              removing dips greater than 2mils trace to trace.

VelCut - The velocity of the lines defining the FK region(s) to be 
         removed. The units for velocity must be consistent with those 
         used for Deltax.

VelPas - The velocity of the lines defining the FK region(s) to be retained.

        e.g.  VelCut -100 -900 900 100
              VelPas - 250 -500 500 250 will retain only arrivals with 
        apparent velocities between +/- 900 & 500.

Deltax - If the filter is defined using VelPas/VelCut the deltax must 
         be given.

Window - The type of window to use when tapering.
       = HAMM, Hamming
       = HANN, Hanning
       = BART, Bartlett (triangular)
       = RECT, Rectangular (box car - no window).
       = BLAC, Blackman
       = EBLA, Exact Blackman
       = BLHA, Blackman-Harris
         Preset = HANN, e.g. window rect

WinOpt - The windowing may be done as a function of angle, wavenumber
         or frequency.  However a window that spans infinite v cannot be
         tapered as a function of frequency.
       = ByA - As a function of angle
       = ByK - As a function of wavenumber
       = ByW - As a function of frequency
         Preset = ByA

End    - Terminates each parameter list.