Guralp Systems CMG-3T - Manual (Page 16)

CMG-3T
prepare a concrete mix with sand and fine grit, and pour it into
the hole. Agitate (“puddle”) it whilst still liquid, to allow it to
flow out and form a level surface, then leave to set. Follow on
from step 7.
Puddled concrete produces a fine-textured, level floor for
situating the seismometer. However, once set hard, the
concrete does not have the best possible coupling to the subsoil
or bedrock, which has some leeway to shift or settle beneath it.
6. Alternatively, for the most rapid installation, place loose soil
over the bottom of the pit, and compact it with a flat stone.
Place the seismometer on top of this stone. This method
emulates that in step 3, but can be performed on-site with no
additional equipment.
7. Set up the instrument as for a vault installation (Section 2.3,
page 11, steps 4 to 9.)
8. The instrument must now be shielded from air currents and
temperature fluctuations. This is best done by covering it with
a thermal shield.
An open-sided box of 5 cm expanded polystyrene slabs is
recommended. If using a seismic plinth on sand (from steps 3–
4 or 5), ensure that the box is firmly placed in the sand,
without touching the plinth at any point. In other installations,
tape the box down to the surface to exclude draughts.
9. Alternatively, if a box is not available, cover the instrument
with fine sand up to the top.
The sand insulates the instrument and protects it from thermal
fluctuations, as well as minimizing unwanted vibration.
10. Ensure that the sensor cable is loose and that it exits the
seismometer enclosure at the base of the instrument. This will
prevent vibrations from being inadvertently transmitted along
the cable.
11. Cover the pit with a wooden lid, and back-fill with fresh turf.
Other installation methods
The recommended installation methods have been extensively tested
in a wide range of situations. However, past practice in seismometer
installation has varied widely.
16 Issue N