NAME

     fftxy  - forward/reverse 2-d (X-Y)  fast  fourier  transform
     (amplitude/phase version) for time slice data

SYNOPSIS

     fftxy [ -Nntap ] [ -Ootap ] [ -rsnrst ] [ -renred ] [ -R ] [
     -V ] [ -? ]

DESCRIPTION

     fftxy in the (default)  forward  mode  transforms  an  input
     (x,y)  space  worktape  (each record is a time slice) into a
     (kx,ky) space worktape (differs from fft2da which is specif-
     ically for x-t data).
      The program now uses mixed radix  FFTs  which  can  greatly
     reduce  the  memory  required.  The  output  of  the forward
     transform will no longer necessarily be a power of 2 but the
     same  rules concerning where the amplitude and phase spectra
     are located apply.

     The ky,kx spectra are stored such  that  when  plotted  with
     routine  xsd they appear as in usp routine specal , that is,
     with the ky axis positive in the up direction  (from  sample
     nffy/4  ->  1),  and  with the kx=0 wavenumber mapped to the
     center of the seismic (kx,ky) gather (trace nxpad/2).   Each
     output  trace coresponds to fixed kx wavenumber. nffx is the
     length of the fft that just exceeds the seismic record width
     and  nffy  is  the  length  of the fft that just exceeds the
     seismic trace length.

     By packing the data in this manner, one is able to  interac-
     tively  pick  data  mutes  in  kx,ky space using usp routine
     polymute.  polymute  reads  the   line   header   word   for
     DgTrkS (set by the forward transform) for the value 'kk' and
     if found, leaves the phase component unchanged.

     Upon running the reverse transform the  original  number  of
     traces  per  record  and  the original number of samples per
     trace are restored.  The user might wish to  be  aware  that
     line  header  entries  OrNTRC  and  OrNSMP  are  required to
     preserve these quantities.

     fftxy in the (-R option) reverse mode  transforms  an  input
     (kx,ky)  space worktape back to a (x,t) space worktape plac-
     ing the data back into its original windowed position.

     fftxy 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).

     -rs nrst
          Enter start record number.  Default value is the  first
          record.

     -re nred
          Enter end record number.  Default value is last record.

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

     -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

     Data are assumed to be in regularly  sampled  and  in  their
     correct  relative  position,  with  embedded  dead traces if
     appropriate (yes, the dead traces can all be at the  end  of
     the records).

NOTE 2

     The piped flow  fftxy | polymute ... | fftxy -R  allows  the
     user  flexibility  in enhancing or deleting features in time
     slices.  In addition new polymute zones  can  be  picked  at
     different  times (polymute will interpolate smoothly between
     control points) and so a time varying 2-dimensional  spatial
     filter can be effected.

NOTE 3

     The piped flow  fftxy | dipfk ... | fftxy -R  can  sometimes
     be  a  more  effective velocity filter than traditional dipf
     having  a  narrower  and  deeper  reject  zone  with   fewer
     artifacts.

SEE ALSO:

     fk, fft2d, specal, polymute, disort, filt, dipf, dipfk

AUTHOR

     Paul Gutowski, APR.

COPYRIGHT

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