Mechanical Monolithic Sensor for low frequency seismic noise measurement

摘要

Dipartimento di Scienze Fisiche, Universita degli Studi di Napoli Federico Ⅱ, Complesso Universitario di Monte S.Angelo, Via Cintia, 1-80126 Napoli, ItaliarnINFN Sezione di Napoli, Complesso Universitario di Monte S. Angelo Via Cintia, 1-84084 Napoli, ItaliarnThis paper 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. The theoretical sensitivity curve is calculated considering the brownian noise, the readout noise and the data acquisition noise. Preliminary tests on the mechanical performances of the monolithic structure and on the optical readout 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 transfer function of the folded pendulum in open loop configuration is calculated measuring the resonant frequency and the quality factor for several step responses. Then a PID controller is added to implement the closed loop configuration. The mechanics of the seismic sensor, the optical scheme of the readout system, the theoretical predictions and the preliminary experimental performances as accelerometer are discussed in detail, together with the foreseen further improvements.