LARSE '93 DATA PROCESSING
The initial uncompressed data set was recorded on
approximately 150 Exabyte and Data tapes, and comprised 87 Gbytes
total with some station data recorded in Reftek format and others in
SEGY format. Using the PASSCAL software program ref2segy
the Reftek DAS format was converted to IRIS SEGY format.
The software processing comprised several phases:
clockcorr - changing the start time in the SEGY trace header
based on the information from the logfile.
refrate - removing the clock drifts that appeared in many of the
stations. The DC mean was also removed from the SEGY files.
FINAL DATA PRODUCT
The final format of the dataset is SEGY format with DC mean removed
and clock drift timing corrections applied. This final dataset has been
archived on the Metrum mass storage system or other media for use by
other institutions in scientific analysis. At UCLA the final dataset was
merged, and then cut into segments containing the teleseism and local events
that were collected during the experiment. This process was applied to the
SEGY format data and transferred to UCLA via ftp. Logfiles were recorded
at almost every station for each week. These are included in the final data
product in storage.
In order to make the appropriate timing offset and drift corrections, the log
files had to be edited in order to be used properly by the PASCALL program
refrate. Because of the older Reftek CPU versions, the required
timing corrections were written incorrectly to the log files, which
refrate uses to make a record of the time corrections it
applies to the data. Refrate reads the log files and pairs
certain statements regarding timing, such as "unlocked-phase error', 'phase
error-DSP CLOCK SET', etc. These pairs create intervals for which timing
corrections are calculated and used by clockcorr. Clockcorr reads the output files of refrate and searches for the
intervals which span the time at which events begin. Most corrections are
simple and straightforward. However, there are some problematic cases, as
All of the log files are in the form I.** where ** is the DAS number of the
Reftek at the sites. The log file lines containing phase error or DSP set
reports contain the essential timing correction. Normally, the corrections
would list a seconds and milliseconds portion of time offset. However, for
almost all of these cases, the milliseconds portion was not recorded in the
log file. Moreover, if the offset was positive, a whole second was added to the
correct milliseconds portion of the offset.
In summary, in all cases of a positive phase error, a DSP line of
(phase error -1) was added. The only case where this was not needed was when
there were no DSP lines after the time jerk. This meant that the total
correction was the same as the phase error and the time jerk completed the
correction. Refrate uses the value of the phase error
to calculate drift.
There still remain some problems with the absolute timing in the seismograms.
Most of the known one-second (leap second) errors were removed during the
data processing from the stations with omega timing receivers. However,
a few stations may still contain these errors if they occurred in the middle
of the week, or if station timing was pulsed with an omega signal only once.
These errors should be relatively obvious to spot in the data set. In
addition, 10-second errors were also removed from some stations with omega
timing receivers. However, as in the one-second case, stations for which the
error occurred in the middle of the week were not corrected. These timing
errors are also easy to spot in the data set.
Kolher, Monica D., Davis, P.M., etc, 1995, Local and teleseismic earthquake
recording during the passive phase of the Los Angeles Region Seismic Experiment
(LARSE) in Southern California , USGS OFR 95-xxx.