NAME

     fft2da   -  forward/reverse  2-d  fast   fourier   transform
     (amplitude/phase version)

SYNOPSIS

     fft2da [ -Nntap ] [ -Ootap ] [ -sist ] [ -eiend ] [  -nsnstr
     ] [ -nenetr ] [ -rsnrst ] [ -renred ] [ -R ] [ -S ] [ -V ] [
     -? ]

DESCRIPTION

     fft2da in the (default) forward  mode  transforms  an  input
     (x,t)  space  worktape  into a (kx,omega) space worktape. 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,kx) 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  kx=0  wavenumber  mapped  to  the center of the seismic
     (kx,omega) gather. Each output trace coresponds to fixed  kx
     wavenumber.  The  minimum  kx value corresponds to -nxfft/2,
     while the maximum kx value corresponds to +nxfft-1.

     nxfft is the length of the fft that just exceeds the seismic
     analysis window. 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.

     fft2da in the (-R option) reverse mode transforms  an  input
     (kx,omega)  space  worktape  back  to a (x,t) space worktape
     placing the data back into its original windowed position.

     fft2da 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.

     -ns nstr
          Enter the start trace number.  The default is the first
          trace of the record.

     -ne netr
          Enter the end trace number. The  default  is  the  last
          trace of the record.

     -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

     -S   Enter the command line argument '-S'  to  smooth  early
          mutes on 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. User should consider running more general  rou-
     tine  fk  if which uses a generalized Fourier Transform in x
     if this is not the case.

NOTE 2

     The piped flow  fft2da|polymute|fft2da -R  allows  the  user
     much   greater  flexibility  (and  danger!)  than  the  more
     conventional temporal (sctvf) and fan  filtering  techniques
     (dipf).

BUGS

     Values of StaCor set equal to 30001 will be treated as  dead
     traces  on  the  inverse  transform...this  is bad for cases
     where StaCor is being used for horizon storage etc.

SEE ALSO:

     fk, fft2d, specal, polymute, disort, sctvf, dipf

AUTHOR

     Kurt J. Marfurt and Paul Singer, APR,  modified  from  fft2d
     written by Paul Gutowski.

COPYRIGHT

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