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The homogeneous triaxial arrangement

In order to observe ground motion in all directions, a triple set of seismometers oriented towards East, North and upward (Z) has been the standard for a century. However, horizontal and vertical seismometers differ in their construction, and it costs some effort to make their responses equal. An alternative way of manufacturing a three-component set is to use three sensors of identical construction whose sensitive axes are inclined against the vertical like the edges of a cube standing on its corner (Fig. 13), by an angle of $ \arctan \sqrt{2} $, or 54.7 degrees.


  
Figure 13: The `homogeneous triaxial' geometry of the STS2 seismometer
\includegraphics[width=0.7\textwidth]{Fig/sts2c.eps}

At the time of writing, only one commercial seismometer, the Streckeisen STS2, makes use of this geometry, although it was not the first one to do so [Gal'perin 1955,Knothe 1963,Melton & Kirkpatrick 1970,Gal'perin 1977]. Since most seismologists want finally to see the conventional E, N and Z components, the oblique components U, V, W of the STS2 are electrically recombined according to


 \begin{displaymath}\left( \begin{array}{c} X \\ Y \\ Z \end{array} \right) =
\fr...
...right)
\left( \begin{array}{c} U \\ V \\ W \end{array} \right)
\end{displaymath} (35)

The X axis of the STS2 seismometer is normally oriented towards East; the Y axis then points North. Noise originating in one of the sensors of a triaxial seismometer will appear on all three outputs (except for Y being independent of U). Its origin can be traced by transforming the X, Y and Z signals back to U, V and W with the inverse (transposed) matrix. Disturbances affecting only the horizontal outputs are unlikely to originate in the seismometer, and are in general due to tilt.


next up previous contents
Next: Electromagnetic velocity sensing and Up: Design of seismic sensors Previous: Effects of temperature
Erhard Wielandt
2002-11-08