Guralp Systems CMG-6TD - Manual (Page 25)

CMG-6TD Installing the 6TD
(“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 emplacing
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 described in section 4.4 on page 21 (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 five centimetre thick 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.
4.6 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.
MAN-T60-0002 25 Issue F - February 2014