An angular accelerometer for reducing the effects of tilt in seismic records

Seismometers for measuring horizontal vibrations respond not only to the required ground motion but also to unwanted tilts, caused by atmospheric or local effects. Because of the equivalence of inertial and gravitational accelerations, it is not possible to separate these effects in a single instrument. However, absolute rotation is detectable and an angular accelerometer has been developed, responding only to tilt and not to either horizontal or vertical ground motion. The design of the instrument is very similar to that of a modern horizontal-component seismometer, comprising a double balanced pendulum pivoted at its centre of gravity and operated in a force feedback configuration. Details of the design, testing and cabibration are presented. The instrument was operated in a seismic vault for several weeks, together with standard borehole and surface-mounted seismometers. The outputs were digitized and recorded, and the results analysed off-line on a pc using a data analysis package. Recordings obtained under different conditions of wind and seismic activity are presented and attempts made to reduce the effects of tilt on the surface recordings by subtracting the signal from the angular accelerometer after suitable processing. The tilt signal proved to be rather low, except on very windy days, so that complete correction for tilt was not possible. Suggestions are made for improving the instrument to make it useful in seismic vaults. Key words: angular accelerometers, seismometers, tilt removal.