principles and applications of microearthquake networks

30 2. Instrumentation Systems
The output impedance and signal level for the single component field
package is compatible with the requirements of the telephone system. If
more than one field package is used at one site, then seismic signals at
different center frequencies are combined by a summing amplifier. For
example, this happens when a two- or three-component system is used or
when radiotelemetry signals are brought together from remote points.
Except for some components of the automatic daily calibration unit, the
entire field package is powered by two 3.65-V lithium cells. The lithium
cells are rated at 30 A . hr each and are the size of standard D cells. The
field package continuously draws about 600 FA from these cells. To provide
power for the automatic daily calibration, additional battery cells are
used. Four alkaline-type AA cells provide a total of 6 V to close the relays
for the 40-sec duration of the daily calibration test. A I .3S-V mercury cell
is used as the voltage standard for the seismometer release test and the
amplifier step test. Battery life under field operation conditions is 2 years.
2.2.3. Central Recording Site
Although the field package usually provides a well-conditioned signal to
the telephone system for transmission, the amplitude of the multiplexed
seismic signals received at the central recording site may have large variations.
These variations pose a serious problem for the analog tape recorders.
To achieve adequate signal-to-noise ratio in playbacks of earthquakes
from the analog tapes, the multiplexed seismic signals must be
recorded within narrow amplitude limits. An automatic gain control
(AGC) unit was designed and installed by Jensen (1976b) to accomplish
this. The AGC unit is used only on multiplexed signals that are sent to the
tape recorders. The multiplexed signals sent to the discriminators do not
go through any signal conditioning circuits (Fig. 7).
Before the multiplexed seismic signals are recorded on analog tapes,
each has three additional signals combined with it (Jensen, 1976a). One of
these is a reference frequency for tape-speed compensation, set at 4687.5
Hz. It is used in a subtractive compensation mode on playback. The other
two signals are serial time codes, set at center frequencies of 3500 and
3950 Hz, with deviation of is0 Hz. These signals are recorded on each
track in order to eliminate some sources of timing error and to increase
signal-to-noise ratio upon playback of the tape. It is possible to combine
these three signals with the eight from the telemetry system because their
frequency range of 3450-4687.5 Hz is well above the highest frequency
(3185 Hz) associated with the telemetry system, and yet is still below the
SO00 Hz limit of the tape recorders running at 15/16 in./sec (Fig. 8). To
obtain the desired rise times and noise reduction, Eaton and Van Schaack