GNGTS 2017 - 36° Convegno Nazionale

GNGTS 2017 S essione 1.1 25 thought to be too small for the existing instrumentation. However, in the last decades large frame ring laser gyroscopes (RLG) have demonstrated the capability of sensing the rotational ground motion with a very high resolution. This has open new perspectives in seismology, since the simultaneous observations of ground rotations and translations can improve the inter- pretation of the seismic data, by the correction of the tilting systematic errors of the traditional seismic sensors and by allowing tomographic reconstruction of the seismic velocity profile beneath the observation site (Bernauer et al., 2009). A RLG exploits the Sagnac effect: if a ring laser, operating in a single mode, rotates with respect to an inertial reference system, the radiation beam traveling in the rotation direction sees a round-trip path longer than the counter-propagating one. As a consequence, the stationary frequencies of the radiation emitted in the two directions differ by a quantity, called Sagnac frequency , that can be directly measured by beating the two radiation beams on a photodiode. The Sagnac frequency ƒ Sagnac is related to the rotational speed Ω by a simple geometric factor: 4 A Ω f Sagnac = ǀ f CW – f CCW ǀ = –––––– sin ϑ λ p (1) where p is the length of the perimeter of the cavity, A is the area included in it, λ is the laser wavelength, and ϑ is the angle between the rotational axis and the normal to the RLG plane. The fact that the proportionality factor is purely geometrical is of paramount importance, since the geometry of the apparatus can be controlled. Ring lasers have high resolution, excellent stability, high duty cycle and a wide dynamic range. Furthermore, no movable mechanical parts are required, so these sensors can be manu- factured in a very robust way and with a very high rejection of linear kinematic or gravitational accelerations. The best RLG operating at the moment is the Großring G, which is located in the Bayern geodetic observatory in Wettzell (Schreiber and Wells, 2013). It is a square ring laser with 4 m side, mounted on a monolithic structure made of Zerodur (a glass with practically null expansion coefficient). G has achieved a resolution near to the quantum noise limit, in about 10 4 s of measurement time, reaching an optimal sensitivity better than 10 –13 rad s −1 . Thanks to its sensitivity, it allowed the observation of terrestrial tides, of polar motion, of Earth free oscillation modes, of rotational microseismic noise, and the evaluation of the length-of-the-day competitive with VLBI. At longer times, G is limited by random walk noise, but its stability is anyway good enough to carefully estimate the long-term fluctuation of the orientation of the rotation axis of the Earth (the Chandler wobble). Our group in Italy is active in this field since almost 10 years. Our main purpose is to push the sensitivity of such apparatus in order to test the Lense-Thirring effect foreseen by General Relativity at the level of 1% with and Earth based experiment. Since 2010 a RLG apparatus, called G-Pisa, with a square optical cavity of side 1.35 m was installed near VIRGO gravitatio- nal antenna. During its operation, it detected many earthquake signals, among which the 2011 M w =9.0 Sendai-Honshu earthquake (Belfi et al. , 2011, 2012). Gingerino is an evolution of G- Pisa, with a side of 3.60 m that we have installed in the INFN underground GranSasso Labora- tories (Belfi et al. , 2017). This apparatus cannot compete with G as resolution, since it is based on a simple and cheap heterolithic structure done in steel and not in Zerodur, but it has however demonstrated a shot-noise limited sensitivity in the range of 10 -10 rad/s at 100 s. The noise power spectrum of the apparatus is shown in Fig.1. Mirror quality limits at present the performances of the instrument. This RLG is presently flanked by a long-period seismometer installed by INGV that is inserted in the national seismic network. It is operative since the beginning of 2016, and in this period it has observed many seismic events, included many regional earthquakes of the recent Umbria seismic swarm. Adetailed analysis of the data related to these events is the object of another communication in this GNGTS meeting, by A. Simonelli. At the present days, Gin- gerino is operative completely unattended from the beginning of May 2017. The underground location provides a rather high natural thermal stability, and reduced impact of weather condi-