Published on October 9, 2007
Environmental studies in Virgo: Environmental studies in Virgo Isidoro Ferrante For the Virgo collaboration Talk summary: Talk summary This talk is a review of the status of environmental data analysis in Virgo. It will concentrate on seismic, acoustic and magnetic noise, and to their influence on dark fringe signal. To this effect, data taken during the C5 commissioning run have been used: 5 hours at the beginning of the run (GPS time 786075317) are used for the study of low frequency contribution from seismic noise The interferometer was in science mode in recombined configuration. The first hour alone has been used for the studies about magnetic and acoustic noise. Others more recent data have been used whether possible. The Virgo Enviromental monitoring system: The Virgo Enviromental monitoring system Slow monitoring: 160 temperature probes inside and outside the vacuum chamber 11 humidity probes 11 pressure probes 1 weather station 1 lightning detector Fast monitoring: 13 vertical accelerometers 5 triaxial low frequency accelerometers 1 velocimeter 9 magnetometers 6 microphones Seismic noise: Seismic noise Seismic noise: EpiSensors and accelerometers: Seismic noise: EpiSensors and accelerometers Episensors: Triaxial force-balance accelerometers Frequency range from 0.2 Hz to 200 Hz Accelerometers: vertical piezo accelerometers. Useful frequency range from 1 Hz to 4 kHz Episensors are much more sensitive in the low frequency region, (0.2-10 Hz) were seismic noise contributes to the mirrors motion In addition, vertical and horizontal accelerometers are placed on the top of the inverted pendulum, with a sensitivity band starting at about 10mHz One Guralp velocimeter is taking data since some month. Seismic acceleration spectra: Seismic acceleration spectra Episensor spectra: order of magnitude is the same, but the shape is different. 10÷200 Hz: Electronic equipment 0.1÷1 Hz: Sea microseism 1÷4 Hz: Traffic 4÷10 Hz: Human activity Coherence among seismometers: Coherence among seismometers The very low frequency region is likely due to sea microseism The region around 2 Hz is due to car traffic in the nearby highway Correlation of seismic activity with dark fringe noise: Correlation of seismic activity with dark fringe noise Coherence between seismic acceleration and interferometer signal is significative in the region from 0.2 to about 1 Hz. At lower frequency, the episensor noise masks the coherence However, there is also a significant correlation also between the Central building accelerometers and the dark fringe, in the region around 50 and 100 Hz, in very narrow lines. This region will be better studied using acoustic data. Seismic contribution to dark fringe: Seismic contribution to dark fringe Using coherences, the seismic contribution to dark fringe noise at low frequency can be calculated. Horizontal movements, taking into account all three mirrors, dominate up to 0.5 Hz Vertical movements dominate from 0.5 to about 1.2 Hz. The inertial control will be extended also to vertical direction to further reduce the seismic noise. Acoustic noise: Acoustic noise Acoustic noise: Acoustic noise Acoustic noise is correlated with seismic vibrations in the acoustic band. Microphones placed in the North and West end buildings show no correlation with the dark fringe signal. However, correlation can be seen between the microphone placed in Central building and the dark fringe, in the same frequency region were the coherence with episensors is high. The correlation is bigger with the microphones placed in the laser injection lab. Several lines, due to air conditioning and other electronics have been successfully identified. Coherence and noise contribution: Coherence and noise contribution Acoustic noise contribution estimated using the laser lab microphone Acoustic tests: Acoustic tests To understand how noise enters the interferometer, a dedicated test has been performed during the C5 run placing a loudspeaker inside the laser lab driven with white noise or a frequency sweep. The white noise test did not give conclusive results: however, pushing up the volume level we were able to de-lock the interferometer. The frequency sweep was much more interesting, since showed non-linearities in the noise propagation mechanism Frequency sweep - up to 2kHz: Spectrograms during frequency sweep. In the dark fringe one can see harmonics which are not present in the microphone. Frequency sweep - up to 2kHz Zoom Other signals - zoom to 250 Hz: Other signals - zoom to 250 Hz Top: angular errors Middle: transmitted power from Mode Cleaner and Reference Cavity Bottom: z error common mode Non-linearities seems to affect mainly transmitted power, as expected Test result: Test result The most likely mechanism which could couple acoustic noise to the dark fringe is through laser angular jitter. Structure vibrations in the laser system produce an angular deviation of the beam axis. This modulates the intensity and the phase of the power at the mode cleaner output through a quadratic term. This mechanism could explain the harmonics observed in the dark fringe Power stabilisation, operative only after run C5, should reduce greatly this effect. We hope to solve this problem with the new injection bench. The weather station: The weather station A weather station has been installed on top of the Virgo central building. It provides atmospheric temperature, humidity, pressure and rain rate It measures also wind velocity and direction. The effect of wind on mirrors motion has been studied taking two different periods of no wind and high wind in April. Wind as seen from noise sensors: Central building Mode Cleaner North End West End black: no wind violet: strong wind Wind as seen from noise sensors The wind produces a wide-band (DC-100Hz) seismic and acoustic noise It is measured by seismometers and microphones in all buildings Low frequency velocimeter (Guralp) installed in the Central building measuring down to 10mHz The largest noise increase (below 1 Hz) occurs in the MC building Wind effects on mirrors movements: IP motion Mirror motion by LC Some noise reinjection around 200mHz Wind effects on mirrors movements Magnetic fields: Magnetic fields Magnetic Fields: Magnetic Fields Three MFS-06 magnetometers from Metronix are installed in each building, in an orthogonal configuration Working range from 0.00025Hz to 10kHz Sensor noise: 1*10-2 nT/ÖHz @0.01 Hz 1*10-4 nT/ÖHz @1 Hz 5*10-7 nT/ÖHz @1000 Hz Magnetic spectra show mainly the presence of 50 Hz harmonics up to very high frequencies. Correlation with interferometer signal: Correlation with interferometer signal Correlation with dark fringe signal is very high up to about 20 Hz This is due to the magnetic field produced by the marionetta coils! We can estimate the magnetic field attenuation due to the vacuum tube. From outside to inside: From outside to inside To study the effect of external magnetic fields on the dark fringe signal a big coil has been placed near the wall of the west end tower The field expected in absence of modification from esternal structure is a line of 100 nT at 61 Hz. The test is still in progress. The lightning detector: The lightning detector Boltek LD-250 record lightning strokes with about 1s precision. Provides azimuth and distance of lightning, up to about 450 km Lightinings on magnetometers: Lightinings on magnetometers Near lightning can also be seen on magnetometers. No evidence, so far, of any effect on dark fringe. Conclusion: Conclusion The main environmental disturbances which can interfere in Virgo operation are under control. Work is in progress to understand how the noise enters the interferometer, and to reduce the effects already known. Thanks to the works of Rosario De Rosa, Francesco Fidecaro, Irene Fiori, Lara Giordano, Federico Paoletti and many others I’m forgetting right now....