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                              PROCESS FILTER
Parameters, alphabetically:
addwb     dbdrop    fillen    fno       ftype
interp    levs      lno       minpha    pass
sets      window    winlen

Document Date: 22 April 2005

Process FILTER applies a frequency filter to every trace.  Filters 
available are: (see parameter FTYPE)
     Time domain (convolutional) zero phase time varying bandpass
     Frequency domain zero phase bandpass
     Frequency domain minimum phase
     Low pass Butterworth
     Frequency domain notch
Time varying filtering is performed by applying different time domain
filters to different parts of the trace.  The different parts of the 
trace are called windows.  The portion of the trace between windows 
are merged by ramping (linear).  The merge zone thus contains data that
has been filtered by different filters and then added together after 
being ramped.  The weights of the windows can be different, however then
the merge zone will contain more of one type of filter than the other.
e.g.          F1            F2            F3
                        ..........     ..........
                       .          .   .
          ..........  .            . .
                    ..              .
                   .  .            . .
                  .    .          .   .

Up to 5 windows may be given, each with a different window level, and
may be spatially varied by shot or rp or by hanging the windows on the
water bottom.  Time varying filters are available only on time domain

All parameters that remain constant for a set of shots (rps) may be
described in a parameter set FNO to LNO.  Windows between two parameter
sets are calculated by linearly interpolating between LNO of one set
and FNO of the next set.  Only the time windows (sets) are spatially
varied.  The filter (pass) remains constant even though the application
window (sets) vary.  Parameter INTERP may be used to turn spatial
interpolation off.

Each parameter list must be terminated with the word end.  The entire
set of filter parameters must be terminated by the word END.

--- --------- ----------

FNO    - The first shot (or rp) to apply the filter(s) to.  Shot (rp)
         numbers must increase monotonically.

LNO    - The last shot (rp) number to apply the filter(s) to.  LNO must
         be larger than FNO in each list and must increase list to list.

SETS   - Start-end time pairs defining the windows of a time varying filter.
         Times are in seconds and may be negative when hanging the
         windows from the water bottom.   A maximum of 5 windows may be 
         given.  Only available on time domain filters.
         Preset= delay to last time. e.g. sets 0 3.0 3.3 6.0

PASS   - For time domain (convolution filtering or FTYPE 99): A list of
         passbands. A passband is a set of two frequencies between which
         the data will be passed.  Frequencies outside the passband will
         be cut.  Pass is an approximate number but very sharp sloped
         filters can be obtained by increasing the filter length.  Up to
         5 passbands may be given.
       - For frequency domain bandpass filters (ftype 0 and 20)
         The two corner frequencies of the passband.  The slopes of the
         filter are given via DBDROP.
       - For low pass or high pass filters (ftype 1 and 2 ):
         The cutoff frequency.
       - For notch filters (ftype 3 and 23):
         The two corner frequencies between which the frequencies
         will be cut out (filtered out).
         Required. e.g. pass 10 70

FILLEN - The length of each time domain filter.  The number of points
         to use in the convolution.  Up to 5 filter lengths may be
         given.  An odd number of points should be used since the
         filters are symmetrical.  Short filters (25 points) may run
         fast, but the filter shape becomes poor.  Short filters do not
         filter low frequencies well.
         Preset = 25 25 25 25 25  e.g.  fillen 39

LEVS   - The amplitude level of each window described by sets. Each
         window may have a different level.  Each level must be >0.
         Up to 5 levels may be given.
         PRESET= 1. 1. 1. 1.

ADDWB  - When given a value of yes, the windows given via sets will be
         added to the water bottom time of the trace.  (Water bottom
         times may be entered via process wbt).

FTYPE  - Filter type.  Time varying filter is permitted with time domain
         filtering only.  Add parameter MINPHA for minimum phase filters.
       = 0, John Shay's frequency domain zero phase bandpass.
       = 1, John Shay's frequency domain low pass. The corner
            frequency is the first value of parameter PASS.
       = 2, John Shay's frequency domain high pass. The corner
            frequency is the first value of parameter PASS.
       = 3, John Shay's frequency domain notch.
       = 10, Low pass 3 pole Butterworth filter.
       = 20, Warren Wood's frequency domain zero phase bandpass.
       = 23, Warren Wood's frequency domain notch.
       = 99, Time domain (convolutional) zero phase time varying.  These
             are VERY fast filters.
         Preset = 99    e.g.   ftype 0

DBDROP - Decibel drop per octave.  The slope of the filter response in
         db/octave.  Valid with FTYPE 0, 1, and 3
         Preset = 48.   e.g. dbdrop 6

WINDOW - The type of window to apply before computing the fft.
       = hamm, Hamming
       = hann, Hanning
       = bart, Bartlett (triangular)
       = rect, rectangular (box car - no window)
       = blac, Blackman
       = ebla, exact Blackman
       = blha, Blackman-Harris
        Preset=hann  e.g. window rect

WINLEN - The window length, in seconds.  A window length of zero causes
         the entire time domain gate to be windowed.  A non zero length
         indicates that winlen data will be modified at both ends of each
         data gate.
         Preset = 0.  e.g.  winlen .2

MINPHA - A yes/no switch indicating that the filter should be a minimum
         phase filter rather than zero phase.  Valid for ftype 0, 1, 2.
         The switch is set by any no-zero value.
         Preset = no     e.g. minpha yes

INTERP - A yes/no switch indicating whether spatial interpolation should
         be done or not.  Normally all shots/rps and traces are filtered,
         but this can be overridden with INTERP NO.  Traces not specified
         in an fno/lno list will not be filtered with INTERP OFF.
         Preset = yes    e.g.   interp no

END    - Terminates each parameter list.

Copyright (C) 1980 The Regents of the University of California
Written by Paul Henkart, May 1980 
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