Guralp Systems CMG-3T - Manual (Page 10)

CMG-3T
power will be supplied through the digitizer, which can also activate
the sensor control lines.
The breakout box also provides a CONTROL output, which can be
connected to the Hand-held Control Unit. This device lets you monitor
output signals from the instrument, and perform on-site calibration.
For more information, see Section 6, page 47.
2.2 Installation notes
The goal of any seismic installation is to ensure that wave-trains
arriving at the instrument accurately reflect the internal motion of
subsurface rock formations. To achieve this, the seismometer and its
emplacement need to be considered as a mechanical system, which
will have its own vibrational modes and resonances. These
frequencies should be raised as high as possible so that they do not
interfere with true ground motion: ideally, beyond the range of the
instrument.
In particular, the sensor needs to be protected against environmental
factors such as
•
fluctuations in temperature,
•
turbulent air flow around walls or trees, or around sharp
corners or edges in the immediate vicinity of the sensor;
•
vibration caused by equipment in or near the installation,
particularly computer equipment; and
•
vibration caused by heavy machinery (even at a distance), or by
overhead power lines.
In seismic vaults, instruments are often installed on piers. It is
important to ensure that the interface between the pier and the floor
does not introduce noise, and that the pier itself does not have
resonant frequencies within the passband. Ideally, a seismic pier will
be significantly wider than it is high (to minimize flexing) and will
form a single piece with the floor, e.g. by moulding a poured concrete
floor within a wooden frame.
Many situations do not allow for the construction of a seismic vault.
For example, you may need to deploy quickly to monitor the activity of
a volcano showing signs of rejuvenation, or to study the aftershocks of
a major earthquake. In other cases the site itself may be too remote to
ship in construction equipment.
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