NAME

     kfilt - resample, filter and phase shift data  for  prestack
     Kirchhoff depth migration.

SYNOPSIS

     kfilt [ -Nfile_sg ] [ -Ofile_filt ] [ -f1f1 ] [  -f2f2  ]  [
     -f3f3 ] [ -f4f4 ] [ -V ] [ -? ]

DESCRIPTION

     kfilt is designed to read in muted common shot  gathers  and
     apply  a  band  pass filter, 45 degree phase shift, and sinc
     based interpolator to prepare  the  data  for  the  prestack
     Kirchhoff depth migration algorithm sammig.

     kfilt allows the user to apply  the  phase  shift  and  data
     resampling  normally  performed  internally to the Kirchhoff
     prestack depth migration algorithm sammig only once, for all
     velocity  model  iterations.  By precomputing this filtering
     operation using kfilt and invoking routine sammig -F one can
     cut  calculation  times  by  25%  to  50% in the workstation
     environment for small to moderate sized data sets. The  out-
     put data file is unfortunately 4 times larger than the input
     data file, negatively impacting disc space and possibly net-
     work  transfer times. Little performance improvement will be
     experienced on the Cray  Supercomputer  environment,  as  it
     performs FFTs so efficiently.

     kfilt gets both its data and  its  parameters  from  command
     line  arguments.   These  arguments specify the inputs, out-
     puts, and certain floating  point  values  to  override  the
     current restrictions to trace header integer values.

  Command line arguments
     -N file_sg
          Enter the input COMMON SHOT GATHER  file  name  immedi-
          ately after typing -N. This may be a pipe (stdin). Data
          are assumed to have undergone muting  or  filtering  of
          direct  arrivals and headwaves, using for instance rou-
          tine bdmute.  This input file also  requires  the  full
          path name (see above).

     -O file_filt
          Enter  the  output  file   name   that   contains   the
          filtered/phase shifted/resampled data.  The output data
          set also requires the full path name (see above) or can
          be   piped  for  further  processing.  The  piped  flow
          'prepmig;kfilt;vsm|vsamp|kray;sammig -F|zstretch'  per-
          forms   a   prestack   depth  migration  equivalent  to
          smallmig.

     -f1 f1
          Enter the first point in a four point Tukey  filter  in
          temporal frequency. Default= 3. Hz.

     -f2 f2
          Enter the second point in a four point Tukey filter  in
          temporal frequency. Default= 8. Hz.

     -f3 f3
          Enter the third point in a four point Tukey  filter  in
          temporal frequency. Default= 50. Hz.

     -f4 f4
          Enter the forth point in a four point Tukey  filter  in
          temporal frequency. Default= 60. Hz.

     -V   Enter the command line argument '-V' to obtain  verbose
          output.

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

BUGS

     No bugs known at present.

SEE ALSO

     vsm, vsamp, kray, sammig, zstretch, tstretch

AUTHOR

     K. J. Marfurt (APR)  extracted from S. H. Gray's   algorithm
     smallmig / smigA

COPYRIGHT

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