Guralp Systems CMG-3ESPCD User Manual

Page 15

Installing the 3ESPCD

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 4.

5. Alternatively, if time allows and granite is not available, 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 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

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