PROCESS STK
                          ------- ---

Document Date: 13 March 2003

PROCESS STK contains several alternative algorithms for stacking
(summing) traces.  These stacking algorithms could not be options
of PROCESS STACK because they require the entire "gather" before
the summation can start.  Recall that a "gather" is a set of traces
where the last trace of the "gather" has the special SIOSEIS
end-of-gather flag (header word 51 set to -1).  The end-of-gather
flag may be set by PROCESS GATHER, processes INPUT or DISKIN
parameter NTRGAT, or PROCESS HEADER.

A TRIM stack is a MEDIAN stack where a percentage of the trace values
relative to the median value are summed.  For each time sample, all
traces values within the gather are examined and the median value
is determined by sorting the values.  The closest TRIM values to
the median value are then used in the summation for that time. 

A RANGE varying stack sums traces according to the range of each
trace for each time sample.  This is similar to performing inner and
outer mutes (or tail and front mutes) via processes MUTE and SMUTE.

A stack PANEL is a progressive stack; each trace within the gather
is added to the stack and the partial stack is output.  There will
be as many stacked traces as there are input traces.  e.g. Output
trace 1 is just one trace, trace 2 is the sum of trace 1 + 2,trace 3
is the sum of trace 1 + 2 + 3, etc
    
PARAMETER DICTIONARY
--------- ----------

TRIM   - The TRIM percent of trace values closest to the median value
         are included in the stack.  e.g. trim 30 means a total of 60% 
         of the traces are included in the stack.
         PRESET = 0.

TRIMIN - The TRIMIN percent of trace values farthest from the median
         value are included in the stack.  TRIMIN and TRIM are
         mutually exclusive.
         PRESET = 0.

XXT    - Range-range-time triplets defining the range varying stack.
         Ranges and times not specified are determined through
         interpolation and extrapolation.  For each trace sample time
         a trace is included in the stack if it's range is between
         the two ranges for that time.  The absolute value of all 
         ranges is used, i.e. the sign is ignored.
         Preset 0 0 0

PANEL  - When set non-zero, a progressive stack panel is produced.
         Preset = 0.        e.g.    panel 1

END    - Terminates the parameter list.

Copyright (C) 1999 The Regents of the University of California
ALL RIGHTS RESERVED.