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Calibration on a shake table

Using a shake table is the most direct way of obtaining an absolute calibration. In practice, however, precision is usually poor outside a frequency band roughly from 0.5 to 5 Hz. At higher frequencies, a shake table loaded with a broadband seismometer may develop parasitic resonances, and inertial forces may cause undesired motions of the table. At low frequencies, the maximum displacement and thus the signal-to-noise ratio may be insufficient, and the motion may be nonuniform due to friction or roughness in the bearings. Still worse, most shake tables do not produce a purely translational motion but also some tilt. This has two undesired side-effects: the angular acceleration may be sensed by the seismometer, and gravity may be coupled into the seismic signal (see 3.3). Tilt can be catastrophic for the horizontal components at long periods since the error increases with the square of the signal period. One might think that a tilt of 10 $\mu$rad per mm of linear motion should not matter; however such a tilt will, at a period of 30 s, induce seismic signals twice as large as those originating from the linear motion. At a period of 1 s, the effect of the same tilt would be negligible. Long-period measurements on a shake table, if possible at all, require extreme care.

Although all calibration methods mentioned in the previous section are applicable on a shake table, the preferred method would be to record both the motion of the table (as measured with a displacement transducer) and the output signal of the seismometer, and to analyze these signals with a routine like CALEX. Depending on the definition of active and passive parameters, one might determine only the absolute gain (responsivity, generator constant) or any number of additional parameters of the frequency response.


next up previous contents
Next: Calibration by stepwise motion Up: Procedures for the mechanical Previous: Procedures for the mechanical
Erhard Wielandt
2002-11-08