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

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

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