Guralp Systems CMG-3T - Manual (Page 15)

Operator's guide
2. On granite or other hard bedrock, use an angle grinder to plane
off the bedrock at the pit bottom so that it is flat and level.
Stand the instrument directly on the bedrock, and go to step 7.
3. On soft bedrock or subsoil, you should install a pier as depicted
below.
4. Pour a layer of loose, fine sand into the pit to cover the base.
The type of sand used for children's sand-pits is ideal, since the
grains are clean, dry and within a small size range. On top of
the sand, place a smooth, flat granite plinth around 20 cm
across, and shift it to compact the sand and provide a near-
level surface.
Placing a granite plinth on a sand layer increases the contact
between the ground and the plinth and improves the
performance of the instrument. There is also no need to mix
concrete or to wait for it to set, as in step 5.
5. Alternatively, if time allows and granite is not available,
July 2009 15