Installing the 40T
4.2
Installing in vaults
The 40T is a sensitive instrument designed to measure extremely small movements
of the ground. These movements are the sum of all the vibrations arriving at the
instrument: as well as distant earthquakes and nearby tremors, the ground responds
to surf on nearby beaches, quarry blasts, heavy machinery, traffic, and even people
moving around the building. Temperature changes and air currents in the same
room as the sensor can also affect its output.
4.2.1
Choosing a location
When studying natural earth movements, any other effects introduce unwanted
noise into the system. It is therefore important to choose an appropriate site for the
instrument, ideally in an underground vault with the sensor installed on a concrete
pier that is in direct contact with the bedrock.
This set-up has a number of advantages:
•
It is installed below ground. Most man-made noise tends to travel along the
surface, and natural microseisms (tiny natural flexings of the Earth's crust)
also occur near the surface.
•
Good contact with bedrock means that the signals accurately reflect earth
motions; seismic waves do not have to travel through layers of soft soil and
sediment.
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Issue F - June 2019
