NAME

     fk  - forward/reverse 2-d fk transform for irregularly  sam-
     pled data.

SYNOPSIS

     fk [ -Nntap ] [ -Ootap ] [ -sist ] [ -eiend ] [ -rsspread  ]
     [ -R ] [ -S ] [ -I ] [ -V ] [ -? ]

DESCRIPTION

     fk in the (default) forward mode transforms an  input  (x,t)
     space worktape into a (k,omega) space worktape. A fast power
     of 2 Fourier transform is applied in the  regularly  sampled
     time domain, while a more general (slower) Fourier transform
     is applied in the irregularly sampled  space  domain.  Trace
     distances and dead traceflags are honored.

     If ntfft is the length of the  fft  that  just  exceeds  the
     seismic  analysis  window,  the output worktape contains the
     seismic amplitudes in sample locations (1:ntfft/2)  and  the
     seismic phase in sample locations (ntfft/2+1:ntfft). The f-k
     (omega,k) spectra are stored such  that  when  plotted  with
     routine  xsd they appear as in usp routine specal , that is,
     with the omega axis positive in the up direction,  and  with
     the  k=0  wavenumber  mapped  to  the  center of the seismic
     (k,omega) gather. Each output trace coresponds  to  fixed  k
     wavenumber.   On  the  forward  transform the string 'fk' is
     placed in the header word DgTrkS. On the  inverse  transform
     this  header  word  is  changed  to 'xt'. All other line and
     trace headers remain unchanged.

     By packing the data in this manner, one is able to  interac-
     tively  pick  data  mutes  in  f-k  space  using usp routine
     polymute. polymute reads the line header word for DgTrkS for
     the  value  'fk'  and  if  found, leaves the phase component
     unchanged.

     fk in the (-R  option)  reverse  mode  transforms  an  input
     (k,omega)  space  worktape  back  to  a (x,t) space worktape
     placing the data back into its original windowed position.

     fk gets both its data and its parameters from  command  line
     arguments.   These  arguments specify the input, output, the
     start and end traces, and verbose printout, if desired.

  Command line arguments
     -N ntap
          Enter the input data set name or file immediately after
          typing -N unless the input is from a pipe in which case
          the -N entry must be omitted.  This input  file  should
          include the complete path name if the file resides in a
          different directory.  Example -N/b/vsp/dummy tells  the
          program to look for file 'dummy' in directory '/b/vsp'.

     -O otap
          Enter the output data  set  name  or  file  immediately
          after typing -O.  This output file is not required when
          piping the output to another process.  The output  data
          set also requires the full path name (see above).

     -s ist
          Enter the start time of the analysis  window  in  msec.
          Default is the beginning of the trace.

     -e iend
          Enter the end time of  the  analysis  window  in  msec.
          Default is the end of the trace.

     -spread spread
          Enter the maximum spread distance (m/ft) to be  encoun-
          tered  along  the seismic line. This distance will form
          the basis  of  the  Fourier  transform  in  x  for  all
          records,  with  or without dead traces. (No default for
          forward transform, read from line header  value  NmSpMi
          for reverse transform).)

     -R   Enter the command line argument '-R' to do the  reverse
          transform

     -S   Enter the command line argument '-S'  to  smooth  early
          mutes on reverse transform.

     -I   Enter the command line argument '-I' if the dead traces
          are  to  be  interpolated during the reverse transform.
          (see NOTE 1 below).

     -V   Enter the command line argument '-V' to get  additional
          printout.

     -?   Enter the command line  argument  '-?'  to  get  online
          help.   The program terminates after the help screen is
          printed.

NOTE 1

     Dead traces containing valid trace distances can be interpo-
     lated by the flow  fk|polymute|fk -I -R where polymute mutes
     out the high wavenumber components that express the  spatial
     discontinuity of a dead trace.

NOTE 2

     The piped flow   fk|polymute|fk  -R  allows  the  user  much
     greater flexibility (and danger!) than the more conventional
     temporal (sctvf) and fan filtering techniques (dipf).

NOTE 3

     The spread length spread is stored in  the  lineheader  word
     NmSpMi. Care should be taken not to destroy this word during
     subsequent processing.

SEE ALSO:

     fft2d, fft2da, specal, polymute, disort, sctvf, dipf

AUTHOR

     Kurt J. Marfurt, APR in response to ugly geometries  encoun-
     tered  in  Canada  and  Oman  as  well as for USA Coal degas
     seismic acquisition.

COPYRIGHT

     copyright 2001, Amoco Production Company
               All Rights Reserved
          an affiliate of BP America Inc.