PROCESS GAINS, GAINS2, GAINS3
                  ------- -----  ------  ------

Parameters, alphabetically:
addwb     alpha     etime     fno       lno       rscale
subwb     tadd      tgp       tmult     type      v
winlen


Document Date: 20 June 2011
Updates:  
      Add TYPE 10 (20log(t*v) - ver 2011.2.17 (20 June 2011)
      Add parameter v - ver 2011.2.17 (20 June 2011)
      Allow GAINS2 and GAINS3, January 2007
      Complex modulus changes the sample interval. March 2005
      Have TGP honor SUBWB, March 2004
      Add parameter WINLEN (amplitude running average), June 2003
      Add TADD and TMULT (time add and multiply), Oct 2000
      Add TGP (time gain pair), July 2000
      Add ADDWB, July 2000

Process GAINS applies a gain function.  Chapter 4 of Claerbout's 
"Imaging the Earth's Interior" mentions several of the gain functions 
implemented.
                                               
At least one parameter list must be given, even if no parameters are 
specified, in order that the parameter presets be set.  e.g.
     gains
          end
     end

PARAMETER DICTIONARY
--------- ----------

TYPE   - The type of gain to apply.
       = 1,  a(i) = a(i) * (t*1000.)**alpha    ( USGS gain ) where a(i)
         is the trace and t is the time of the trace sample in seconds.
         Restrictions:  All traces must have the same start time.
         Preset = 1
       = 2,  a(i) = a(i) * (ABS(range)/SIGN(rscale,range)) ** alpha when
             ABS(range) .GE. rscale;  where range is the range in the 
             SEG-Y header, rscale and alpha are given by the user, SIGN
             is the Fortran SIGN function which means that ABS(rscale)
             is used when range is positive and -ABS(rscale) is used 
             when range is negative.
       = 3,  a(i) = a(i) * t ** alpha
       = 4,  a(i) = a(i) ** alpha
       = 5,  a(i) = a(i) * e ** (alpha * t)
       = 6,  a(i) = SIGN(a(i)) * ABS(a(i)) ** alpha
       = 7,  a(i) = SQRT(a(i*2-1)**2+a(i*2)**2) (modulus of complex trace)
             The only SEG-Y header value modified is the number of samples
             (since there are half as many samples after doing the modulus).
             The SEG-Y header value for the number of samples is divided 
             by 2, as is the sample inetrval.
             The deep water delay is NOT modified.
       = 8,  a(i) = a(i) * ABS(range/rscale) ** alpha when
             ABS(range) .GE. rscale;  where range is the range in the
             SEG-Y header, rscale and alpha are given by the user.
       = 9,  Time-Gain-Pairs.  Automatically set to type 9 when 
             parameter TGP (time-gain-pairs) is given.
       = 10, 20log(t*v).  Spherical spreading in water decreases amplitudes
             by 20log(R), where R = t * v and t is the two-way travel time.
             Parameter ETIME is honored.
       Preset = none     e.g. type 4 alpha 2

Additional Parameters:
---------- ----------

ETIME  - The end time of the gain function types 1 and 10.  Data after the end
         time will receive the gain of the end time.  Types 1 and 10 ONLY.
         Preset = the last time of the first trace.   e.g.  etime 4.

ALPHA  - The exponent used in TYPEs 1 - 6 gain.
         Preset = 1.   e.g.  type 3 alpha 1.5

RSCALE - The range scalar used in TYPE 2 gain.
         Preset = 1.

SUBWB  - Subtract water bottom time switch.  Type 3, 5 and 9 ONLY.
       = YES, The water bottom time is subtracted from the data time in
         the gain types that use time as a variable.  e.g.
               a(i) = a(i) * t ** alpha     becomes
               a(i) = a(i) * (t-wbt) ** alpha  for t >= wbt 
         Preset = NO

TGP    - Time-Gain-Pairs.  A list of gains or multipliers to apply
         to each data trace.  Time/gain not specified in TGP are
         obtained through extrapolation and interpolation of TGP.
         The gain for data times between TGP times will be linearly
         interpolated using the adjacent time and gain pairs.
         Times prior to the first time of TGP will use the first gain.
         Times after the last time of TGP will use the last gain.
         Preset = none     e.g. tgp 0 1 1 10 2 100

ADDWB  - When given a value of YES, the times given via TGP
         will be added to the water bottom time of the trace.
         (Water bottom times may be entered via PROCESS WBT).
         Valid with type 9 or TGP gains only.
         Preset=no             e.g.   addwb yes

TMULT  - Time multiplier used in gains type 3 and 5.  Preset = 1.
TADD   - Time additive used in gains type 3 and 5.  Preset = 0.
         When using a(i) = a(i) * t ** alpha, first t is:
         t = delay; if subwb == yes then t = delay - water_bottom;
         t = t * tmult + tadd 
         (note that arithmetic is done left to right)

WINLEN - The window length, in seconds, of an amplitude running average.
         The averaging is independent of gains 1-9 and is done AFTER
         the other gains.  e.g. alpha 2 type 4 winlen .02 is also
         called the trace ENVELOPE.  Every amplitude is squared and
         then a running average over 20 mils. is done.  The window
         average is placed at the center of the window.
         Preset = none.

V      - The velocity in converting time to depth for spherical spreading
         in TYPE 10.  TL = 20log(R), where R = t * v.
         Preset 1500.

FNO    - The first shot/rp number the parameter list applies to.  Data 
         (shots/rps) before FNO WILL NOT HAVE GAINS APPLIED.
         Preset = the first shot/rp received.    e.g.   FNO 101

LNO    - The last shot/rp number the parameter list applies to.  Data 
         (shots/rps) AFTER LNO WILL NOT HAVE GAINS APPLIED.
         Preset = the last shot/rp received.     e.g.   LNO 101

Copyright (C) 1990 Seismic Reflection Processors, Solana Beach, CA.
Written by Paul Henkart.  ALL RIGHTS RESERVED.