Guralp Systems CMG-6TD User Manual
Page 20
Installing the 6TD
4.3
Installation notes
For the best possible results, a seismometer should be installed on a seismic
pier in a specially-built vault, where conditions are near perfect. Here,
wave-trains arriving at the instrument reflect very well the internal motion of
subsurface rock formations. However, this is not always feasible. For
example,
•
instruments may need to be deployed rapidly, perhaps to monitor the
activity of a volcano showing signs of rejuvenation, or to study the
aftershocks of a major earthquake;
•
installations may be required in remote locations, or otherwise in
circumstances where it is infeasible to build a vault.
In these situations, 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. This is done by
•
standing the sensor on bedrock where possible, or at least deep in
well-compacted subsoil;
•
clearing the floor of the hole of all loose material; and
•
using as little extra mass as possible in preparing the chamber.
In temporary installations, environmental factors are also important. The
sensor needs to be well protected against
•
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 heavy machinery (even at a distance), or by
overhead power lines.
This can be done by selecting a suitable site, and placing the instrument in a
protective enclosure. An open-sided box of 5 cm expanded polystyrene slabs,
placed over the instrument and taped down to exclude draughts, makes an
excellent thermal shield.
After installation, the instrument case and mounting surface will slowly
return to the local temperature, and settle in their positions. This will take
around four hours from the time installation is completed.
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Issue F - February 2014