PROCESS XCORR
                           ------- -----

  Document date: 7 April 1999

       PROCESS XCORR performs a cross-correlation between the seismic
traces and a "pilot" trace.  The first sample output is always lag zero.
       When the pilot trace is in a different file from the data, use
parameters PPATH, PSNO, and PTR.  The pilot trace is read only once
when using parameter PPATH; parameters PPATH, PSNO, PTR may not be
given more than once in a SIOSEIS job.
       When the pilot is in the same file as the data to be correlated,
the pilot trace should be the first trace of each shot since XCORR
does not read the file; XCORR saves and uses the same pilot trace
until it receives a trace numbered PILOT.  e.g. PILOT 24 means that
data traces 1-23 do not have a pilot associated with them.  Therefore,
the first trace must be the pilot trace (PILOT 1) when using parameter
PILOT.

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

NLAGS  - The number of correlation lags to perform and output.  This
         is the same as the number of samples to ouput. Normally the
         pilot and uncorrelated traces are very long and NLAGS is
         much smaller.
         REQUIRED.      e.g. nlags 500

SETP   - Start and End Time of the Pilot trace to use in the 
         correlation.  The pilot trace sent out is usually smaller
         than the recorded length, so SETP should be used to eliminate
         the data that is not part of the pilot.  Sometimes the pilot
         may not start at the first sample either, requiring the start
         time to be non-zero.
         Preset = delay end-of-data.     e.g. setp 0 1.

SETD   - Start and End Time of the Data trace.
         Preset = delay end-of-data.    e.g.  setd 0 1.5

PPATH  - Pilot PATHname.  The filename of the file that contains the
         pilot trace when the pilot is not trace 1 of each shot.
         Used with PSNO and PTR to describe the location of the pilot
         trace.
         Preset = none    e.g.  ppath /data/vol3/henkart/cats/pilots

PSNO   - Pilot Shot Number.  Used with PPATH and PTR to describe the
         location of the pilot trace.
         Preset = none.    e.g. psno 101

PTR    - Pilot TRace number.  Used with PPATH and PSNO to describe the
         location of the pilot trace.  The trace number within PSNO.
         Preset = none     e.g.  ptr 3

PILOT  - The trace number of the pilot trace to use in the 
         cross-correlation.  A number other than 1 may result in
         traces before the pilot trace being correlated with an
         incorrect pilot.
         PRESET = 1     e.g. Don't use anything but 1

DOUBLE - WHen given as YES, then the correlation is done in double
         precision,  Long correlations may need double precision
         in order that arithmetic overflow does not occur.
         Preset = NO     e.g. double yes

CATS   - When given YES, the previous uncorrelated trace is used as
         the pilot.  e.g. trace 1 is used to correlate trace 1 and 2,
         trace 2 is used to correlate trace 3, &c.
         Preset = NO   e.g.  cats yes

END    - Terminates each parameter list.

EXAMPLE
-------
     A 1 second sweep frequency vibrator was recorded on a Geometrics
Strataview seismic recorder.  The data were recorded for 1.5 seconds.  
The sample interval was .25 mils (.00025 seconds), so the longest 
output possible is .5 seconds or NLAGS 2000 (.5/.00025).
The Strataview records the pilot in trace 1, so the only XCORR
parameters needed were  NLAGS 2000 SETP 0 1.


Written by: Paul Henkart, Scripps Institution of Oceanography, October 1998
Copyright (C) The Regents of The University of California
All Rights Reserved.