Ring laser gyroscopes as rotation sensors for seismic wave studies

Abstract

Although their importance was understood (Aki and Richards 2002), rotations have long been neglected in seismic studies because no suitable sensors existed. The technology for seismometers exploiting the inertia of a test mass, on the other hand, is well established and such sensors are nowadays sensitive, reliable and reasonably cheap. In general, there is a variety of different concepts for rotation sensors available, such as a pendulum, micromechanic tuning fork gyros, fiber optic gyros and ring lasers. The latter, when scaled up, have the advantage of an extremely high sensitivity. Over the last 40 years, ring laser gyroscopes have become one of the most important instruments in the field of inertial navigation and precise rotation measurements. They have a high resolution, good stability and a wide dynamic range. Furthermore, no spinning mechanical parts are required, so these sensors can be manufactured in a very robust way. These properties made them very suitable for aircraft navigation. For more than 10 years, very large perimeter ring laser gyroscopes have been specifically developed for applications in geodesy and geophysics (Schreiber et al. 2001). By increasing the effective ring laser area by up to a factor of 24,000 over the size of an aircraft gyro, the sensitivity of these ring lasers to rotation has improved by at least 5 orders of magnitude, while the drift rate of the instruments has been reduced substantially. With several of these highly stable large ring lasers, very small periodic signals coming from polar motion, solid Earth tides and ocean loading have been successfully measured (Schreiber et al. 2003, 2004a). However, rotational signal signatures caused by remote earthquakes are stronger than these small perturbations of Earth rotation (McLeod et al. 1998, Pancha et al. 2000). The range of angular velocities to be covered is very wide: 1014 rad/s ≤ ωs ≤ 1 rad/s, with the required frequency bandwidth for the seismic waves in the range of 3 mHz ≤ fs ≤ 10 Hz (Schreiber et al. 2004b). Currently, the large ring lasers are the only available rotation sensors which fulfil these demands. Three such devices mounted in orthogonal orientations may eventually provide the quantitative detection of rotations from shear, Love and Rayleigh waves. These properties inspired the development of a highly sensitive ring laser gyro dedicated to seismological applications. It is important to note that ring laser gyroscopes are sensitive only to rotations around their area normal vector. From that point of view, they provide additional information. The goal of the GEOsensor project was the construction and evaluation of a field-deployable demonstrator unit, which will eventually provide access to all 6 degrees of freedom of motion. The recording of the (complete) earthquake-induced rotational motion is expected to be particularly useful for: (1) further constraining earthquake source processes when observed close to the active faults (Takeo and Ito 1997); (2) estimating permanent displacement from seismic recordings (Trifunac and Todorovska 2001); (3) estimating local (horizontal) phase velocities from collocated observations of translations and rotations (Igel et al. 2005). Because of the relatively short duration of an earthquake, such ring lasers do not need a long term stability over weeks or months, which is difficult and expensive to obtain. An instrumental stability of approximately one hour during a seismic event is sufficient. Therefore it is possible to use a steel structure attached to a solid concrete platform as the main components of the Sagnac interferometer. As indicated above, ring lasers for seismic studies require a high data rate of at least 20 Hz, because of the wide bandwidth of seismic frequencies near an earthquake source. While large ring lasers for geodetic applications are usually optimized for measuring variations in the rotation rate of the Earth in a frequency band below 1 mHz, autoregressive algorithms can be used to determine the Sagnac frequency with a resolution below the Nyquist limit. While this method can still be employed for the strongly bandwidth limited teleseismic signals (McLeod et al. 2001), an entirely different detection scheme is needed for the data evaluation of regional or local seismic events. © Springer-Verlag Berlin Heidelberg 2006.