1999 Eel River chirp profiler processing example

      Two sets of notes from processing the 1999 Eel River chirp data

NOTES FROM PROCESSING DONE IN 2003
 The first step was to convert to SEG-Y:
sioseis << eof
procs diskin xstar diskoa end
diskin
   format edgetech ipath yr1999day217-2244z.xstar end end
xstar
  type 2 end end
diskoa
  opath yr1999day217-2244z.segy end end
end
eof


Plot of the raw data.
Plot after mix filter at "low gain".
Plot after mix filter at "high gain".
Plot Trace 1 only with mix filter.
Plot Trace 2 only with mix filter.
Plot xstar mix filter wbt mute plot
(script of mix filter wbt mute plot)



NOTES FROM PROCESSING DONE IN 2002

Kurt Schwehr wanted a small piece of chirp data processed
for his coring project.  In particular:


 23:8:51 8/5/1999 to 23:38:45 8/5/1999
  East end                west end

I also wanted to compare some chirp data (section) without datum 
correction to data (section) with datum correction.

The processing steps were:
1)  Locate the appropriate XSTAR file, convert it to SEG-Y and insert the
    water bottom time into the SEG-Y using an xyz file created from a
    SeaBeam cruise. script.
2)  lsd yr1999day217-2244z.segy 100       listed every 100th ping.  Ping
    86738 is close to 2308z.   Instead of using diskin parameters
    fno 86738 lno 89938 allno no
    I could use fday 217 fgmt 2308 lgmt 2338
3)  Plot the data without a datum correction with script.
 a) Had I not known the best trace amplitude to use for picking the water
    bottom, I would have used process prout to dump the trace amplitudes
    of the first ping.
 b) Finding a best gain function takes a few interations of plots and 
    parameter adjustment.
 c) I used the lsd output to see what the smallest deep water delay was
    and then made the plot stime parameter just before the smallest delay.
4)  Plot the data with  a datum correction with script.
 a) Diskin parameter set .1 .8 was used to pad the input data with zeroes
    so that the ping is big enough for a big datum shift.  e.g. Ping 86738
    has a delay of 289 (mils) and 1988 samples (with a sample interval of ~83
    mils) so the trace goes from .289 sec to ~.450.  The water bottom time
    is ~.520, so the trace needs to go from .289 to .681 (.450 + (.520-.289)).
    Account for possible dip!
 b) While testing various parameters I used:
    fno 86738 lno 87938 allno no noinc 2.    Remember ! starts a comment
 c) Process xstar put the water bottom time (based on depth and 1500 m/s)
    into SEG-Y header real word 50.  Process wbt puts the picked water
    bottom time into SEG-Y header real word 60.  Process header stores
    the datum shift into SEG-Y header real word 59 (segy word 50 - 
    word 60).  Process Mix then averages the shifts over the previous
    20 pings.
 d) Remember to change plot parameter stime since the water after the
    datum correction is different.

    Datum correction using an XYZ file has several sources of errors:

1)  The lat/long associated with the chirp fish is actually the position
    of the boat and not the fish.  The fish can be 100 meters behind
    the GPS antenna.
2)  Similarly, there might be an offset in the XYZ data.
3)  The resolution of the XYZ survey may not be sufficient.  Parameter
    deltad in process xstar was invented to print a warning message
    when there is no XYZ within deltad of the fish.  This dataset had
    over 300 pings further than 1m from an XYZ location.  There were
    only a few pings 10-12m from an XYZ position.
4)  The accuracy of each GPS survey should be examined.  Typical GPS
    positions have 25m RMS accuracy.  Each survey has 25m RMS.
5)  Bathymetric and seismic systems have different focus areas or
    beam widths.  Multibeam systems have very narrow beams while
    seismic systems are very broad.  Seismic systems will misplace
    side echos and inline dipping features.

Return to SIOSEIS examples.