NAME

     sctvf  - filter traces using Ormsby (trapezoidal) filters

SYNOPSIS

     sctvf [ -Nntap ] [ -Ootap ] [ -dbdb ] [ -rband ] [ -vvtime ]
      -f1f1 ] [ -f2f2 ] [ -f3f3 ] [ -f4f4 ] [ -didec ] [ -sidelay
     ] [ -lo ] [ -M ] [ -T ] [ -I ] [ -R ] [ -V ] [ -? ]

DESCRIPTION

     sctvf Apply an Ormsby filter to trace data  in  either  time
     invariant  or  time  variant manner.  The corner frequencies
     are (going from low to high) f1, f2, f3, f4, with a  rolloff
     of  "db"  decibels  within "band" Hz.  There is an option to
     decimate the data after filtering.

     sctvf gets processing controls from the command  line.  Rea-
     sonable defaults are set up.


  Command line arguments
     -N ntap
          Enter the full path of the file containing the data set
          or nothing for stdin pipe.


     -O otap
          Enter the output file name for the filtered  traces  or
          nothing for stdout pipe.

     -db db
          Rolloff in decibels from the inside 2  corner  frequen-
          cies (def=40 db).

     -r band
          Interval in Hz within which rolloff occurs (def=10 Hz).

     -v vtime
          Name of file containing  time-frequency  pairs.   These
          frequencies  specify the 6db reject points and refer to
          either the high cut time varying  filter  (default)  or
          the  low cut filter.  The pairs are entered into a file
          in free format (values separated by  1  or  more  blank
          spaces)  but  must  be entered in the order time (ms) -
          frequency (Hz).  The user no longer  must  specify  the
          number  of  entries  in this file.  To implement a time
          varyning bandpass filter you will need to  pipe  a  low
          cut  sctvf  into  a high cut sctvf (Note: you will then
          need 2 files, one for the low cut side and one for  the
          high  cut side). It is a good idea to also specify a db
          rolloff of 60 or more and also a band of  5Hz.  Do  not
          exceed 1/2 Nyquist

     -f1,2,3,4f1,2,3,4
          Corner frequencies ordered lowest to highest.  Defaults
          are f1=0, f2=0, f3= 0.9*Nyquist-band, f4= 0.9*Nyquist

     -didec
          Enter decimation factor  if  desired.   -d2  will  take
          every  other  data  point  after  filtering.   The line
          header will reflect these changes.  You will have to be
          careful  about  the f4 entry - the program will stop if
          f4 is not less than .9 nyquist frequency (if -d  option
          is used).          Default = 1

     -sidelay
          Enter filter delay (ms).  The program should  calculate
          its  own delay correctly but the user may wish to alter
          this.   The  "auto"  delay  is  approximately  1/2  the
          predominant wavelet length of the filter.

     -lo  If present do low cut filter for time  variant  option;
          if  present  do  band  reject filter for time invariant
          option (band is defined by f1 f2 f3 f4).

     -M   If present compute and apply the minimum delay  version
          of  the standard Ormsby filter.  Note: the roll off for
          this option is about 25db per band interval

     -T   Time variant option: time-freq pairs  are  input  in  a
          flat  file.  The default assumes a time varying hi-cut;
          putting the -lo flag on the command line allows  lo-cut
          time  varying  filtering.  The  frequencies in the file
          define the f3 point of the Ormsby filter, the f4  point
          being set by the band parameter above.

     -I   For time  variant  option:  if  present  resample  data
          internally  to  the  next lowest sample interval before
          filtering and resample back after filtering.  This  can
          be  useful when hi-cut time variant filtering since the
          default only allows high cut filter  frequencies  below
          1/2  Nyquist  (i.e.  none  of  the  time-freq pairs can
          exceed this value).

     -R   If present restore early on mute zone

     -?   Query mode.  With this flag, sctvf will give a descrip-
          tion of the command line arguments and stop the program
          (use '-?' if you are in c-shell).

DISCUSSION

     EXAMPLE: time variant high cut:

     In a flat file called ffile we insert then lines

     0 50

     500 45

     1000 40

     2000 30

     4000 25

     Then we run the command line:

     sctvf -Nindata -vffile -T -Oodata

     to do the filtering. For a low cut we would add -lo  on  the
     command line.

     Minimum Delay Option

     To check the stability of the minimum delay filter one  will
     need a set of spikes.  The following command line shows how:

     spike -Ostikogram -t050 -X0100 -gi300 -ntr10  -v5000  -L1000
     -Rf

     will generate a 10 trace record of 1000ms with  one  refrac-
     tion  event  of  5000  '/s.  The stability of the filter can
     then be checked by filtering  the  spikes  and  looking  for
     polarity  flips  which  indicate  that the filter has become
     unstable.  At that point the prewhitening should be  changed
     until  stability  is  regained.   Results  can be plotted by
     merging the input and outputs (see mergn with -m  flag)  and
     plotting the results with xmplot

     mergn -N0stikogram -N1sctvf.out -m -Omergn.out

     xmplot

BUGS

     unknown

AUTHORS

     Ken Peacock (and Paul Gutowski - Cray 2 version)

COPYRIGHT

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