Modelling of Episodic-Transient Signals in Measurements of Large Ring Lasers
Zusammenfassung der Projektergebnisse
The application of ring lasers for the measurement of the global rotation rate of the Earth provides a direct reference to the instantaneous Earth rotation axis with high temporal resolution. Therefore this independent measurement concept is complementary to the VLBI technique and periodic geophysical signals with distinct frequencies with respect to Earth rotation and local tilts are readily obtained from the measurements. On top of that there are non-periodic transient signatures contained in the time-series of ring laser data, which are difficult to associate to geophysical mechanisms. The data analysis can be separated into models for the ring laser itself, the orientation of the sensor as a function of time and the variable Earth rotation. Finally there is the necessity for additional corrections from local perturbations such as meteorological influences. The objective of this study was to identify and characterize as many of the non-periodic signal contributions, causing a deviation from a measured continuous rotation rate as possible. Over the course of the study more and more potentially contributing geophysical effects or mechanisms were tested against the available measurements. It turned out that many of the observed transient signals are of subtle instrumental origin and that wind loading effects are too small and too incoherent to be detected by the ring laser at the current level of performance. However in the high frequency range a strong correlation with local wind induced deformation is found. The project was aimed at identifying and correcting perturbations in the ring laser record to utilize the full sensitivity and to improve the sensor stability by carrying out the following task: precise stabilization of the optical frequency in the ring laser cavity in real time (thus keeping the phase of the backscatter coupling constant); investigation of a suspected influence of small variations of the magnetic field of the Earth on the ring laser measurements; and the expansion of the environmental model for the improved correction of local perturbations. The precise determination of the optical frequency in the laser cavity has provided a control over scale factor variations by stabilizing the perimeter of the interferometer to keep the optical frequency in the ring resonator constant to within 60 kHz. The observed time series is mostly free from scale factor variations caused by ambient temperature and atmospheric pressure changes. Under a closed loop control of the effective length of the cavity by steering an atmospheric pressure stabilizing vessel enclosing the interferometer, it was possible to detect the annual and the Chandler Wobble by a ring laser for the first time. The improvements in sensor stability also helped to exploit and understand the observed rotational signals from seismic sources, namely free oscillations of the Earth excited by strong earthquakes and direction estimates from micro-seismic and seismic activities based on a point measurement with a ring laser co-located with a seismometer. With respect to variations in the Earth magnetic field no specific influences on the reciprocity of the two laser beams in the cavity were found. Local environmental effects caused by pressure loading and wind friction were studied in a first order analysis of the available measurements based on a finite element analysis over a wide spectral range. While very local wind gusts cause small rotations in the band of 1 second to 1 minute period, these perturbations average out over longer integration times. Regional effects could not be identified. At distances longer than 600 m from the ring laser laboratory, the perturbations from wind increasingly become incoherent as a result of a highly structured driving wind field and the inhomogeneous topographic composition. Therefore wind mostly adds broadband noise to the observations of rotation rates.
Projektbezogene Publikationen (Auswahl)
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: A laser gyroscope system to detect the gravito-magnetic effect on Earth; International Journal of Modern Physics D, 19 (14), pp 2331-2343, World Scientific Publishing Co.
Di Virgilio, A.; Schreiber, K. U.; Gebauer, A.; Wells, J.-P. R.; Tartaglia, A.; Belfi, J.; Beverini, N.; Ortolan, A.
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Can we estimate local Love wave dispersion properties from collocated amplitude measurements of translations and rotations?; Geophysical Research Letters, 37, American Geophysical Union
Kurrle, D.; Igel, H.; Ferreira, A. M. G.; Wassermann, J.; Schreiber, U.
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How to detect the Chandler and the annual wobble of the Earth with a large ring laser gyroscope; Physical Review Letters, 107 (17), American Physical Society
Schreiber, K. U.; Klügel, T.; Wells, J.-P. R.; Hurst, R. B.; Gebauer, A.
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Measuring gravitomagnetic effects by a multi-ring-laser gyroscope; Physical Review D, 84, pp 23, American Physical Society
Bosi, F.; Cella, G.; Di Virgilio, A.; Ortolan, A.; Portio, A.; Solimento, S.; Cerdonio, M.; Zendri, J.P.; Allegrini, M.; Belfi, J.; Schreiber, U.; Gebauer, A., Wells, J.-P. R.
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Observations of Earth's toroidal free oscillations with a rotation sensor: The 2011 magnitude 9.0 Tohoku-Oki earthquake; Geophysical Research Letters, 38, American Geophysical Union, ISSN 0094-8276
Igel, H.; Nader, M.-F.; Kurrle, D.; Ferreira, A. M. G.; Wassermann, J.; Schreiber, K. U.
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Combining VLBI and ring laser observations for determination of high frequency Earth rotation variation; Journal of Geodynamics
Nilsson, T.; Böhm, J.; Schuh, H.; Schreiber, U.; Gebauer, A.; Klügel, T.
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Examining ambient noise using collocated measurements of rotational and translational motion; Journal of Seismology
Hadziioannou, C., Gäbler, P., Schreiber, U., Wassermann, J., Igel, H.
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High-frequency noise caused by wind in large ring laser gyroscope data; Journal of Seismology
Gebauer, A.; Schreiber, K. U.; Klügel, T.; Schön, N.; Ulbrich, U.
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Long-term frequency stabilization of a 16 m² ring laser gyroscope; Optics Letters, 37, pp 1925-1927
Schreiber, K.U.; Gebauer, A.; Wells, J.-P.R.
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Self-locked operation of large He–Ne ring laser gyroscopes; Metrologia, 49 (3), pp 209- 212, IOP Publishing, ISSN 0026-1394
Holdaway, J.; Hurst, R. B.; Graham, R.; Rabeendran, N.; Schreiber, K. U.; Wells, J.-P. R.
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Invited Review Article: Large ring lasers for rotation sensing, Review of Scientific Instruments, vol. 84, no. 4, pp. 041101–041101–26, Apr. 2013
Schreiber, K. U. and Wells, J.-P. R.
