principles and applications of microearthquake networks

2.2. Cenrral Cdijbmicr Micwearthyuuke Netu-ork 31
(1977) showed that the frequency spacing between the timing and compensation
subcarriers must be greater than that for the seismic data subcarriers.
The combined signals are recorded on Bell and Howell Model 3700B
tape recorders, in direct-record mode. Fourteen tracks are recorded on
the ]-in.- (25.4-mm) wide tape. The tape recorder speed of 15/16 in./sec
(23.8 mdsec) requires that the standard 7200-ft- (2195-m) length tape be
changed daily. Because each multiplexed signal can have eight seismic
signals within it, each 14-track tape recorder can accommodate I12 seismic
signals. At present four such recorders are used for the USGS Central
California Microearthquake Network.
2.2.4. Timing Systems
Radio signal WWVB of the National Bureau of Standards (broadcasting
at 60 kHz) is taken as the primary time base. The time signal is a serial
binary code at 1 pulseisec. The binary code is synchronized to the 60 kHz
carrier and is referenced to UTC-universal time coordinated (Blair,
1974). Because radio reception of WWVB is of variable quality, a Time
Code Generator made by Datum Products Co. is maintained at the central
recording site as a secondary time base. It is driven by a rubidium frequency
standard, with a rated precision of one part in 10" per second. In
addition, it generates a 30-bit parallel time code and two serial time codes,
in IRIG-C and IRIG-E formats.
All three serial time codes (WWVB, IRIG-C. and IRIG-E) provide time
of day in units of Julian day, hour, minute, and seconds. Because WWVB
time code consists of I-pulse!sec markers, time resolution less than 1 sec
must be obtained by interpolation. Similarly, IRIG-C and IRIG-E consist
of 2-pulse/sec and 10-pulseisec markers, respectively, and time resolution
less than 0.5 or 0.1 sec requires interpolation. The 30-bit parallel time code
generated by the time code generator has a resolution of 1 sec. Serial time
codes are sent to the linear recording devices (Fig. 7): Helicorder, Develocorder,
and magnetic tape recorders. The parallel time code is sent to
digital devices: the on-line computer and other digital data acquisition
devices (not shown in Fig. 7).
The time code generator is kept within 20.005 sec of universal time by
daily comparison of the IRIG-C serial time code with the WWVB radio
signal. Corrections are not made for radio propagation delay. Power failure
or fluctuation at the central recording site does not affect the time code
generator, as it is supplied by a float-charged battery system.
In addition to WWV and WWVB time services (Howe, 19761, which
can suffer from limited range or variable propagation quality. there are