Nanometrics TC120-PH1, User Manual, Page: 29 / 90

Chapter 3
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Performing a Surface Vault Installation
Trillium Compact User Guide
16889R5
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2014-03-04
20
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When you are ready to remove the Trillium Compact seismometer from the box, gently
place it on the installation surface in an approximate north-south alignment.
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Use the adjustable feet, as required, and the levelling bubble on the cover to level the
seismometer. Centre the bubble as precisely as possible inside the black ring to ensure
that the Z output is measuring true vertical motion. To level the Trillium Compact using
the levelling feet:
a) Extend the levelling feet as little as possible to achieve a level seismometer. Try to
keep one of the feet fully retracted into the seismometer base for greatest stability.
b) When the Trillium Compact is level, lock the feet by rotating each locking nut tightly
against the seismometer base, while preventing the foot from turning. Insert an
Allen key into one of the holes in the locking nut to act as a lever and help tighten
the locking nut. A foot that is properly locked will not turn easily when touched.
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Precisely align the Trillium Compact to north-south by:
a) Using the north-south vertically scribed marks: Align the vertical north-south scribe
lines on the base of the seismometer (see Figure 3-1 “Example of seismometer
alignment using vertically scribed marks” on page 18) with the line drawn on the
installation surface.
b) Using the north-south guide: Align the case-top north-south guide to the staked line,
laser level line, or straight edge (see Figure 3-2 “Example of seismometer alignment
using the case-top guide” on page 19).
Care is required when aligning the seismometer to avoid sighting at an angle and
introducing a parallax error.
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After aligning the seismometer, verify that it is still level. It may need to be adjusted due
to unevenness of the installation surface.
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If you relevelled the Trillium Compact and you are levelling it with the adjustable feet,
ensure the feet are locked when finished.
3.6 Theory and Practice of Insulation
Seismometer installations must be thermally insulated to achieve optimal performance,
particularly at long periods. There are two broad categories of thermal effects that can
cause unwanted noise:
a) Direct thermal sensitivity.
The Trillium Compact is designed to minimize temperature sensitivity; however, like all
seismometers, it has some residual thermal response. There are several components in
a seismometer that are temperature sensitive, such as the springs that suspend the
inertial masses. The effect of direct thermal sensitivity typically shows up as very long
period noise on the vertical channel, in particular, a periodic diurnal variation in
response to the day-to-night temperature cycle.
If you are using the optional levelling cradle, you will not be levelling the seismometer using
the levelling feet as described in the next bullet. Instead, see Section C.1 “Using the Levelling
Cradle” on page 74, or if you are using the levelling cradle with its optional spike kit, see
Section C.2 “Using the Levelling Cradle and Spike Kit” on page 75.