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                              PROCESS GATHER

Parameters, alphabetically:

Document Date: May 2010
7 May 2010 - Change disk addressing to unsigned words - allows 16GB files.
25 May 2006 - Change parameter FRP
24 May 2012 - Change MAXTRS documentation.

A gather is a collection or rearrangement of traces according to some 
criteria.  Process GATHER collects or sorts the input traces according
to the reflection point (rp) number calculated by process GEOM.  The
GEOM parameters may be manipulated by the user to gather the input
traces according to any criteria by fudging the GEOM parameters.
A constant offset gather of a uniform marine line may be made by
omitting traces via process input.

Process GATHER sorts each gather by the absolute value of the shot-
receiver distance (SEG-Y bytes 37-40), so that the shortest range trace
is first within the gather.  Each gather is is terminated by setting a 
special flag in the trace header.  A gather record is the collection of
all these traces.

See PROCESS GEOM for the method of calculating rp numbers.

GATHER creates a temporary disk file to store the partial gathers while
the data are being read.  GATHER assumes that the geometry of the data
does not skip around very much. i.e. the geometry doesn't go backwards
nor does it skip more than a cable length forward.  The temporary disk
file can hold MAXRPS rps (preset to 20 plus the number of traces per shot
from the SEG-Y binary header), with each rp able to hold a maximum of
MAXTRS traces (also preset to the number of traces per shot in the SEG-Y
binary header), with each trace having a maximum of NWRDS samples, with each
sample being 4 bytes long.  The temporary disk file size will be:
   maxtrs * maxrps * (nwrds+60) * 4
The temporary file must be less than 16GB.

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

An input process such as input, diskin, or segdin, must precede the
gather parameters if gather parameter presets are used since the
presets use the number of input traces per shot.

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

MAXRPS - The maximum number of bins (or rp's) that are needed on the 
         disc at any one time.  In marine work the number of traces per
         shot plus a few (+10) will usually suffice since no two
         ungathered traces with the same rp number are more than a cable
         length away.
         Preset=the number of traces from the segy binary header plus 20

MAXTRS - The maximum number of traces any one gather can have.   In rp 
         gathers this is the maximum cdp allowed.  An example of 
         estimating MAXTRS is:  A 468 channel streamer with group spacing
         of 12.5m (6km streamer) and a shot spacing of 37.5m would have
         78 traces in each rp gather. (468/6, 6 because each shot 
         advances 6 rps).  The largest value allowed is 200 - 
         i.e. The maximum number of traces in a gather is 200.
         Preset = 100

NWRDS  - The largest number of samples per trace in the job.  This 
         should be trace length plus the trace header length.  All
         traces output from GATHER will contain NWRDS-60 samples.
         Traces input to gather that are longer than NWRDS-60 samples
         will be truncated.  Traces input to gather that have less
         than NWRDS-60 samples will have unpredicable contents as fill.
         Preset = from first input trace.

MINTRS - The minimum number of traces each gather can have.  If mintrs=0
         and no input traces contribute to a given gather, that gather
         will not be output.
         Preset = 1  e.g. mintrs 24

FRP    - The first rp number to gather.  Traces with rp numbers less 
         than frp are not gathered.  Rp numbers are calculated by 
         PROCESS GEOM.
         Preset = rp number of the first trace.
       - FRP is needed if the first trace read into process GATHER is
         not the first trace in the subsurface (the preset).
******   FRP is needed on marine geometry when trace 1 is closest to
******   to the ship - a reverse streamer.  Some UKOOA processed files
******   may have this.  The subsurface point associated with trace 1
******   is NOT the first subsurface point.  The subsurface point
******   associated with the furtherest from the ship is first.
         For instance, say the RP numbers of the first traces are: 
         5, 4, 3, 2, 1, 6, 7, 8.  The preset value of FRP is 5, which
         would cause RPs 4, 3, 2, and 1 to be omitted.  FRP 1 makes the
         the RP 1 be the first RP.

RPINC  - The increment of rp numbers between the rps to gather.  The
         only traces gathered will have bin numbers frp, frp+rpinc,
         frp+2*rpinc, frp+3*rpinc, . . . . Etc.
         Preset = 1.

Written and copyrighted by:
Paul Henkart, Scripps Institution of Oceanography, March 1980
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