LARSE '93 ARRAY INSTRUMENTATION
The array instruments consisted of one-component and
three-component sensors, and the data recording units were
Reftek DAS 72A-06 (16-bit digitizer) and 72A-07 (32-bit), 72A-08
(16-bit), and 72A-02(16-bit). These included Reftek CPU Versions
2.45, 2.53, 2.53H, and 2.53J. Thus, the combination of a one-component
sensor with any digitizer recorded data at 50 samples/sec, three-component
with 16-bit digitization recorded 50 samples/sec, and three component with
32-bit digitization recorded 25 samples/sec. The sensor types included
L4C3D 1Hz (3-component), L4C1D 2Hz (1-component), L4C3D 2Hz
and L22 2Hz (3-component) and the data were collected by exchanging
recorders with internal disks or by copying to tape. Each instrument
recorded data in record lengths of 30 minutes and each 30-minute interval
was automatically written to the hard disk. The sensors were borrowed
from the collections of UCLA, Los Alamos National Lab, the U.S.G.S., PASSCAL
at Lamont-Doherty Geophysical Observatory, the Southern California Earthquake
Center (SCEC), and USC.
The DAS 72A-06 and 72A-07 recorders had 230-Mb internal hard drives. The
DAS 72A-02 and 72A-08 recorders had no internal drives. Either the external
hard disks from these were swapped or the data was copied to tapes in the
field using field Exabyte and DAT drives. All were powered by batteries
which had to be changed each week. The sensors were interspersed so that
no segment of the array would have only one type. However, all stations near
the San Andreas fault were three-component.
Every station was visited approximately once a week. Operators recorded
information related to timing receivers, power, temperature, and data
acquisition in the log files for each DAS.
The timing of each instrument was controlled by GPS receivers (version 2.2),
Omega receivers, or by the Reftek clock alone. GPS receivers were connected
during installation and removal of the stations with no permanent external clocks
in order to correct the internal Reftek clock. Those stations
that did not receive external time pulses should be considered unreliable
in timing. Some Omega stations were pulsed the first week with GPS receivers.
Each station with a log file that indicates permanent time locks contains no
time drift and, therefore, does not require a timing correction. Note that
all of the Omega station data display a time error of exactly one second
(due to incorrect leap second input) and some display, in addition, a 10
second error due to incorrect time pulses.
Most of the San Gabriel Mountain
section stations were calibrated with GPS time receivers; the Mojave stations
were calibrated with either GPS and/or Omega, or had no external timing
receivers. Most of the stations with no permanent external receivers were
located at the northern end of the array. All of these stations were supposed
to be pulsed with GPS receivers at the beginning of the experiment; however,
some were not. Corrections made for timing errors will be described in
the section on data processing . The timing receivers were borrowed from
PASSCAL at Lamont.