Installing the 3ESP
possible coupling to the subsoil or bedrock, which has some leeway to shift or
settle beneath it.
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.
5.
Set up the instrument as described in the previous Section.
6.
The instrument must now be shielded from air currents and temperature
fluctuations. This is best done by covering it with a thermal shield.
7.
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.
Alternatively,
if a box is not available, cover the instrument with fine sand up to
the top.
8.
The sand insulates the instrument and protects it from thermal fluctuations, as
well as minimizing unwanted vibration.
9.
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.
10.
Cover the pit with a wooden lid, and back-fill with fresh turf.
3.4.1 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.
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
16
Issue G - February 2016
