NAME

     fft3da  - module to provide 3D Fourier Transform   amplitude
     and phase volume indexed in Kx, Ky and Omega

SYNOPSIS

     fft3da [ -Nntap ] [ -Ootap ] [ -dtdt ] [ -dxdx ] [ -dydy ] [
     -dzdz ] [ -R ] [ -V ] [ -? ] [ -h ]

DESCRIPTION

     fft3da performs a forward / reverse  2D  FFT  of  the  input
     dataset.   The  input  data  is  expected  to contain 1D fft
     information in the special packed format output by   fftpack
     sorted to frequency slice orientation, i.e.:

          fftpack -Ndatain |
          ttds3d -ndtxy -odxyt -M8 -Tjunk |
          fft3da [parameters] -Odataout


      fft3da  gets both its data and its parameters from  command
     line  arguments.  These arguments specify the input, output,
     the temporal or depth sample interval of the post  stack  3D
     dataset  being used, the in-line and cross-line spacing, the
     forward / reverse transform flag, 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).

     -dt dt
          Enter the temporal sample interval [ms] of the 3D  post
          stack dataset being filtered. The default is 4 ms.

     -dz dz
          Enter the depth sample interval  [m]  of  the  3D  post
          stack  dataset  being filtered. If present, all parame-
          ters input to this run are assumed to  be  measured  in
          terms  of  a  depth  section input.  If not present the
          input is assumed to be in time.

     -dx dx
          Enter the in -  line  spacing  of  the  3D  post  stack
          dataset  being  filtered.  This is the distance between
          traces within a record on the dataset input to  fftpack
          . The default is 25.0 [units].

     -dy dy
          Enter the cross - line trace spacing  of  the  3D  post
          stack  dataset  being  filtered.   This is the distance
          between records on the dataset input to  fftpack .  The
          default is 25.0 [units].

     -R   Enter the command  line  argument  '-R  to  perform  an
          inverse transform on the input data.  The default is to
          do a forward transform.

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

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

DISCUSSION

     This routine uses a full complex fft to  supply  the  output
     [Kx,  Ky,  Omega]  volume separated into amplitude and phase
     components similar to the 2D routine  fft2da .  It  is  then
     possible,  using  the  routines   ttds3d   and  polymute  to
     pass/reject any portion of the volume desired.  It is a good
     idea  to keep a 1 sample volume of the input 3D [x,y,t] data
     in which to store trace header information.  After  all  the
     slicing  and  dicing required to do the filtering the output
     trace headers will no longer be adequate  for  further  pro-
     cessing of the data.

BUGS

     Unknown.

SEE ALSO

     fftpack(1) qdslice(1) vfilt3d(1) vfilt3da(1)

AUTHOR

     P.G.A. Garossino [APR:3932]

COPYRIGHT

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