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                   PROCESS AGC (AUTOMATIC GAIN CONTROL)

Parameters, alphabetically:
center   fno      lno      pctagc   winlen

Document Date: 4 May 2004

PROCESS AGC applies automatic gain control to every trace. AGC is a type
of amplitude normalization (modification) that results in the amplitudes
being more uniform, especially when the window length decreases. AGC
starts by finding the first non-zero sample and then calculates the 
average absolute value of the window.  Successive windows are calculated
by shifting the window down one sample.  Each average absolute value is
then turned into a multiplier by dividing the average by an output level.

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

A null set of AGC parameters must be given even if all the parameters
are the presets.  e.g.  agc end end

AGC honors the mute time in the SEG-Y header by starting the first
AGC window at the mute time rather than the first sample.

THE PARAMETER DICTIONARY
--- --------- ----------
  
FNO    - The first shot (or rp) to apply the agc to.  Shot (rp) 
         numbers must increase monotonically.
         preset=1

LNO    - The last shot (rp) number to apply the agc to.  LNO must
         be larger than FNO in each list and must increase list 
         to list.
         default=fno

WINLEN - The agc window length in seconds.
         preset=.100

PCTAGC - Percent AGC.  The percentage of the computed multiplier 
         to use in each AGC window. e.g. PCTAGC < 100."softens" 
         the effect the AGC.
         Preset = 100.        e.g.    pctagc 50

CENTER - The center point, in seconds, of the AGC window that 
         receives the multiplier of the window.
         Preset = winlen / 2

END    - Terminates each parameter list.

Written by:  Paul Henkart, Scripps Institution of Oceanography, January 1981
Copyright (C) The Regents of The University of California
All Rights Reserved.
Go to the list of seismic processes.      Go to SIOSEIS introduction.