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If this term is null, we have just the equation for a free swinging damped oscillator. It is thus the ground acceleration that is the single cause for the forced motion of the mass, regardless of the response of the sensor, which can be flat to ground displacement, velocity or acceleration in a given frequency band. To illustrate this, let us show the response to a transient ground displacement. 9a represents a ramp lasting about two seconds. , a simplified model of a near-field observation of an earthquake).

If gravity had been smaller, the natural frequency would have been smaller and the same is true if k had been smaller. The solution is to use astatic suspensions, where the restoring force is very small and, theoretically, any natural frequency can be obtained. 3 Garden-Gate The simplest astatic suspension for horizontal seismometers is the “garden-gate” pendulum (Fig. 13). The mass moves in a nearly horizontal plane around a nearly vertical axis. The restoring force is now g sin(α) where α is the angle between the vertical and the rotation axis, so the natural frequency becomes ω0 ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi g sin ðαÞ=L ð2:48Þ where L is the vertical distance from the mass to the point where the rotation axis intersects the vertical above the mass (see Fig.

The FBA has a flat response down to DC, while, as discussed above, there is an upper frequency limit. g. a permanent tilt will give a permanent output. This can be used to calibrate the FBA, see Chap. 10. For more details of the theory, see Sect. 14. 12 Velocity Broadband Sensors Traditional networks would have weak motion sensors for LP and SP to cover the spectrum of interest. The introduction of the velocity broadband sensor (hereafter just referred to as broadband sensor (BB)) covers the whole frequency band (and more) with the same sensitivity as the individual SP and LP sensors and therefore eliminates the need for two sensors.