Tunable mechanical monolithic horizontal accelerometer for low frequency seismic noise measurement

This paper describes a mechanical horizontal monolithic sensor for geophysical applications developed at the University of Salerno. The instrument is basically a monolithic tunable folded pendulum, shaped with precision machining and electric-discharge-machining, that can be used both as seismometer and, in a force-feedback configuration, as accelerometer. It is a very compact instrument, very sensitive in the low-frequency seismic noise band, with a very good immunity to environmental noises, whose main characteristics are the tunability of the resonance frequency and the integrated laser optical readout, consisting of an optical lever and an interferometer. The theoretical sensitivity curves are in a very good agreement with the measurements. A direct comparison of its performances with the STS-2 ones shows that better performances have been reached with the interferometric readout (≈ 10−12 m/√Hz in the band 10−1 ÷ 10Hz as seismometer. Finally, the first results as accelerometer (force feed-back configuration) are also presented and discussed.