A Michelson interferometer for seismic wave measurement: theoretical analysis and system performances

This papers describes a new low-frequency seismic sensor for geophysical applications. The instrument is basically a monolithic tunable folded pendulum with an interferometric readout system, that can be configured as seismometer or as accelerometer. The monolithic mechanical design and the introduction of a laser interferometric technique for the readout implementation make it a very sensitive and compact instrument with a very good immunity to environmental noises. Preliminary tests on the mechanical performances of the monolithic structure and on the optical reaodut have been performed. Interesting result is the measured resonant frequency of the instrument of ≈ 150 mHz obtained with a rough tuning, demonstrating the feasibility of a resonant frequency of the order of 5 mHz with a more refined tuning. The mechanics of the seismic sensor, the optical scheme of the readout system, the theoretical predictions and the preliminary experimental performances as seismometer are discussed in detail, together with the foreseen further improvements.