Guralp Systems CMG-3T - Manual (Page 17)

Operator's guide
Some installations introduce a layer of ceramic tiles between a rock or
concrete plinth and the seismometer (left):
However, noise tests show that this method of installation is
significantly inferior to the same concrete plinth with the tiles
removed (right). Horizontal sensors show shifting due to moisture
trapped between the concrete and tiling, whilst the vertical sensors
show "pings" as the tile settles.
Other installations have been attempted with the instrument encased
in plaster of Paris, or some other hard-setting compound (left):
Again, this method produces inferior bonding to the instrument, and
moisture becomes trapped between the hard surfaces. We recommend
the use of fine dry sand (right) contained in a box if necessary, which
can also insulate the instrument against convection currents and
temperature changes. Sand has the further advantage of being very
easy to install, requiring no preparation.
Finally, many pit installations have a large space around the
seismometer, covered with a wooden roof. Large air-filled cavities are
susceptible to currents which produce lower-frequency vibrations, and
sharp edges and corners can give rise to turbulence. We recommend
that a wooden box is placed around the sensor to protect it from these
currents. Once in the box, the emplacement may be backfilled with
fresh turf to insulate it from vibrations at the surface, or simply roofed
as before.
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