principles and applications of microearthquake networks

24 2. Iris trumentrt tion Systems
FIELD PACKAGE
CENTRAL RECORDING SITE
SEIS. L-PAD AMPL. VCO DISCRIMINATORS OUTPUT MEDIA
+2 0 vpp
ner
1
- MULTIPLEXED SEISMIC SIGNAL
DEMULTIPLEXED SEISMIC SIGNAL
TIME CODE
I
1 %
SERIAL TIME CODE
1 SUMMING AMPLIFIER ONE PER TRACK 1
r
I
4 cm/V
OEVELOCORDER
Zcm/V
at 5 Hr
m
4 cmlV
Fig. 7. Major elements of seismic instrumentation of the USGS Central California Microearthquake
Network.
phone circuit with no special conditioning is used. If the field package
output cannot be connected to a drop, it is transmitted via line-of-sight vhf
radio to a convenient drop and transmitted from there using the telephone
system.
As many as eight seismic signals are carried along one voice-grade
circuit. This is accomplished by frequency division multiplexing (FDM),
as follows. Voice-grade lines transmit audio signals in the frequency range
of 300-3300 Hz. In our system of FDM the usable bandwidth is divided
into eight FM subcarriers, each with a uniquely specified center frequency.
A voltage-controlled oscillator in the field package converts the
analog signal from the seismometer into a frequency-modulated signal for
transmission. Figure 8 shows the eight subcarrier center frequencies used.
The frequencies denoted as T1, T2, and C are not transmitted, and are
discussed later. The maximum deviation allowed from each center frequency
is ? 125 Hz, which corresponds to k3.375 V peak output from the
amplifier. A spectrum analyzer monitoring a well-adjusted telemetry line