NAME

     tvfilt  - do time varying filtering

SYNOPSIS

     tvfilt [ -Nntap ] [ -Ootap ] [ -BBC ] [ -lwlslide ] [  -nor-
     nor  ] [ -nsns ] [ -nsns ] [ -nene ] [ -rsirs ] [ -reire ] [
     -ormsby ] [ -M ] [ -C ] [ -? ]

DESCRIPTION

     tvfilt applies either a time varying  Butterworth  or  Omsby
     (trapezoidal) bandpass filter to input seismic data.

     (1) Butterworth Option: The filter frequencies are  imported
     from a flat file containing time - frequency (lo-cut) - fre-
     quency (hi-cut) triplets. The edges of the  bandpass  define
     the  6db down points with a default rolloff of 48 db/octave.
     The Butterworth filter technology has some advantages   over
     Ormsby  because of its speed (it's about 10x faster) and its
     excellent rejection characteristics. Also a minimum delay or
     causal time varying filter can be specified.

     (2) Ormsby Option: the trapezoidal filter must be  specified
     by  four  corner frequencies (f1, f2, f3, f4) in two pairs -
     one pair for the low side of the trapezoid ramp,  the  other
     pair  for  the  high  side.  Between f2 and f3 the trapezoid
     response is one; outside f1 and f3 it is zero. The responses
     between  f1  and f2 and f3 and f4 vary linearly.  The filter
     frequencies are imported from a flat file containing time f1
     f2 f3 f4 values separated by whitespace.

     tvfilt gets both its data and its  parameters  from  command
     line  arguments.  These arguments specify the input, output,
     the file of time-frequencies, the sliding update window, the
     start  and  end traces and records, 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).


     -B HC
          Enter the name of the flat file containing time varying
          bandpass  time (ms) and frequency (Hz) values.  For the
          default Butterworth these occur in the order time (ms),
          low  cut  frequency  (Hz), high cut frequency (Hz), one
          triple per line.  Whitespace is used to  separate  each
          number on a line.  The frequency refers to the 6db down
          point of the amplitude spectrum and the default rolloff
          is  48db  per octave. There must be at least 2 lines in
          this file. This file must be supplied. For  the  Ormsby
          (trapezoidal)  option  these occur in the order time f1
          f2 f3 f4 (time  in  ms  and  frequency  in  Hz)  values
          separated by whitespace.

     -lw lslide
          Enter the length  in  ms  of  the  update  window.  The
          filters  will  be  constant  within  a  length 1/2 this
          length and will be linearly interpolated from one  half
          window length to the next. Default = 500ms.

     -nor norder
          For Butterworth mode enter the order of the Butterworth
          filters.  The  higher this value the greater the rejec-
          tion but the longer the run time. A maximum order of  8
          is  allowed  which  gives  a  rejection of 96 db/octave
          using a minimum delay filter. Default is 2 which  gives
          a rejection of 48 db/octave for a zero phase filter.

     -rs irs
          (integer) First record to process (default = 1)

     -re ire
          (integer) Last record to process (default = all)

     -ns ns
          First trace in record to process (default = 1)

     -ne ne
          Last trace in record to process (default = all)

     -ormsby
          Enter the command line argument '-ormsby' to use Ormsby
          (trapezoidal)  filters  instead  of the default Butter-
          worth filters.

     -M   Enter the command line argument '-M' to  turn  off  the
          restore mute action.

     -C   Enter the command line argument '-C' to invoke  minimum
          delay  or  causal  filtering,  otherwise  a  zero phase
          filter will be used. Butterworth option only.

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

DISCUSSION

     For the Butterworth mode the time  varying  bandpass  filter
     file will have the form.

     time lo-freq hi-freq

     time lo-freq hi-freq

      ... ... ...

     where the time is in ms and the  frequencies  (low  cut  and
     high cut) are in Hz.

     For the Ormsby mode the time varying  bandpass  filter  file
     will have the form.

     time f1 f2 f3 f4

     time f1 f2 f3 f4

      ... ... ...

     where the time is in ms and the frequencies are in Hz.

BUGS

     The time frequency file must contain the correct  number  of
     entries per line depending on the type of filter to be used,
     i.e. the program  isn't  smart  enough  to  sense  from  the
     entries  in  the  file what type of filter you want. For the
     Butterworth filter you must have time-fl-fh; for the  Ormsby
     mode you must have time-f1-f2-f3-f4

SEE ALSO

     filt

AUTHOR

     Paul   Gutowski,   EPT,   Tulsa,    x4146,    TRCVM(ZPRG03),
     zprg03@trc.amoco.com

COPYRIGHT

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