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have an apparent ground-motion spectrum that is peaked at long periods.
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Movement of anything touching the seismometer, including the digitizer cable
and insulation materials, can cause forces to develop that change with
temperature. Stick-slip effects typically transform these forces into sudden step
changes in tilt. The apparent ground-motion power spectral density is, therefore,
inversely proportional to the square of frequency.
For seismometers that are well temperature-compensated, such as the Trillium Compact Vault,
but are improperly installed, thermally induced tilt on the horizontal channels will be more
significant than direct thermal sensitivity on the vertical channel. Furthermore, due to the
natural convection of air, thermally induced tilt is even observable in sealed underground
vaults where the temperature is very stable.
Therefore, the objectives of a good installation are to:
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Insulate the seismometer from temperature changes.
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Prevent the movement of air on the surface of the seismometer.
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Insulate the installation pier from temperature changes.
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Prevent the movement of air on the surface of the pier, including the sides and
underside of piers that consist of a slab raised above the vault floor.
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Prevent anything from touching and thereby applying a mechanical force to the
seismometer.
To meet these objectives and achieve the best possible performance, observe the following
practices:
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The vault (the space or room where the seismometer is installed) must provide a stable
thermal environment. This environment is typically achieved through careful site
selection and by installing the seismometer below ground.
The site must have good drainage to prevent flooding of an underground vault. Passive
drainage is better than pumps which cause vibration. Roof and wall surfaces that are
directly exposed to the sun and wind may require up to 30 cm (1 foot) of insulation to
completely stabilize the inside temperature. The door and any other openings must be
fully sealed to prevent air drafts.
Chapter 3 - Installing a Trillium Compact Vault Seismometer
16889R10 • 2020-11-12 Page 41