PROCESS SWELL
                                ------- -----

Parameters, alphabetically:
hdr      index     lprint    n        weights

Document Date: 6 November, 2009
Modifications:
     6 Nov. 09 - Add parameter INDEX


     Process SWELL removes or dampens the effects ship heave due to
heavy seas or swells.  Sea swells can cause a periodic time shift in
seismic data especially when the seismic equipment is attached to the
hull of the ship.  The ship motion causes the two-way travel-time to 
change.
     Process SWELL computes the average water bottom time of a group of 
traces, then shifts the middle trace of the group by the difference of
the middle trace water bottom time and the average water bottom time.
For example, if the swell period is 15 traces, the time associated with
the seventh trace is subtracted from the average time of all 15 traces.
The time may be in any SEG-Y header location, but it must be in
seconds since the shift is converted to number of samples using the
SEG-Y sample interval.
     The water bottom time may be picked using process WBT,  The SEG-Y
water bottom depth (long integer word 16) may be converted to time using
process WBT parameter VEL, or with process HEADER.


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

N      - The number of traces in the period of the swell to be removed.
         Either N or WEIGHTS is required.

HDR    - The index of the 32 bit floating point SEG-Y trace header word
         containing the water bottom time.  (The input value to average)
         e.g. hdr 60
         Default = 50 (the sioseis water bottom time)

INDEX  - The index of the 32 bit floating point SEG-Y trace header word
         containing the computed shift.  (How much the trace was shifted.)
         e.g. index 60
         Default = none

WEIGHTS - A list of weights to use.  The total number of weights given
         indicates the number of traces in the swell period.
         Parameter N is ignored when WEIGHTS is given.
         Default = none

END    - Terminates each parameter list.



Written and copyrighted by:
Paul Henkart, Scripps Institution of Oceanography, 15 July 2009
ALL RIGHTS RESERVED