Guralp Systems CMG-40T - Manual (Page 16)

CMG-40T
3 Installing the 40T
3.1 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.
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 setup 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.
• If the vault is inside a larger structure, its foundations are separated from
16 Issue A