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Seismometer installation

We shortly describe the installation of a portable broadband seismometer inside a building, vault, or cave. The first act is to mark the orientation of the sensor on the floor. This is best done with a geodetic gyroscope but a magnetic compass will do in most cases. The magnetic declination must be taken into account. Since a compass may be deflected inside a building, the direction should be taken outside and transferred to the site of installation. A laser pointer may be useful for this purpose. When the magnetic declination is unknown or unpredictable (such as in high latitudes or volcanic areas), the orientation should be determined with a sun compass.

To isolate the seismometer from stray currents, small glass or perspex plates are cemented to the ground under its feet. Then the seismometer is installed, tested, and wrapped with a thick layer of thermally insulating material. The type of material seems not to matter very much; alternate layers of fibrous material and heat-reflecting blankets are probably most effective. The edges of the blankets should be taped to the floor around the seismometer.

Electronic seismometers produce heat and may induce convection in any open space inside the insulation; it is therefore important that the insulation has no gap and fits the seismometer tightly. Another method of insulation is to surround the seismometer with a large box which is then filled with fine styrofoam seeds. For a permanent installation under unfavourable environmental conditions, the seismometer should be enclosed in a hermetic container. A problem with such containers (as with all seismometer housings) is however that they cause tilt noise when they are deformed by the barometric pressure. Essentially three precautions are possible: either the baseplate is carefully cemented to the floor, or it is made so massive that its deformation is negligible (Fig. 19), or a "warp-free" design is used as desribed by [Holcomb & Hutt 1992] for the STS1 seismometers. Some fresh desiccant (silicagel) should be placed inside the container, even into the vacuum bell of STS1 seismometers. Fig. 19 illustrates the shielding of the STS2 seismometers in the German Regional Seismic Network (GRSN).

Guidelines for installing broadband seismic instrumentation are offered at the web site of the Seismological Lab at Berkeley [UCB 1997]. Detailed instructions for the design of seismic vaults are given by [Trnkoczy 1998].


  
Figure 19: The STS2 seismometer of the GRSN inside its shields
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next up previous contents
Next: Magnetic shielding Up: Seismic noise. Site selection. Previous: Site selection
Erhard Wielandt
2002-11-08