On the Use of Mechanical Filters to Attenuate the Transmission of Tilt Motion to Inertial Sensors

Inertial sensors such as seismometers, geophones, and accelerometers cannot distinguish horizontal motion from tilt motion. Rotation measurements can be used to subtract the tilt component from horizontal measurements, but the noise in the tilt sensor is often a limiting factor. No mechanism can change the dual sensitivity of inertial sensors to tilt and translation, but the transmission of ground motion can be mechanically filtered in a frequency-dependent way. This article discusses the use of mechanical filters to reduce the transmission of tilt motion from the ground to inertial instruments, which can be applied to existing sensors, or considered for integration in the design of new horizontal sensors. The limitations of this approach are related to (1) geometrical couplings due to the separation between the reference point and the input point of the mechanical filter, (2) residual tilt transmission through the joints stiffness, (3) effects of the mechanical filtering on the signal-to-noise ratio of the horizontal motion measurement, and (4) practical difficulties with the implementation of such concepts, including thermal noise in the flexures. This study analyzes and quantifies the benefits and limitations of the mechanical filtering approach for seismic studies and for seismic isolation applications.