Rotational Seismology Research Articles

A MEMS Electrochemical Angular Accelerometer with Silicon-Based Four-Electrode Structure.

This paper presents a MEMS electrochemical angular accelerometer with a silicon-based four-electrode structure, which was made of thousands of interconnected microchannels for electrolyte flow, anodes uniformly coated on structure surfaces and cathodes located on the sidewalls of flow holes. From the perspective of device fabrication, in this study, the previously reported multi-piece assembly was simplified into single-piece integrative manufacturing, effectively addressing the problems of complex assembly and manual alignment. From the perspective of the sensitive structure, in this study, the silicon-based four-electrode structure featuring with complete insulation layers between anodes and cathodes can enable fast electrochemical reactions with improved sensitivities. Numerical simulations were conducted to optimize the geometrical parameters of the silicon-based four-electrode structure, where increases in fluid resistance and cathode area were found to expand working bandwidths and improve device sensitivity, respectively. Then, the silicon-based four-electrode structure was fabricated by conventional MEMS processes, mainly composed of wafer-level bonding and wafer-level etching. As to device characterization, the MEMS electrochemical angular accelerometer with the silicon-based four-electrode structure exhibited a maximum sensitivity of 1458 V/(rad/s2) at 0.01 Hz and a minimum noise level of -164 dB at 1 Hz. Compared with previously reported electrochemical angular accelerometers, the angular accelerometer developed in this study offered higher sensitivities and lower noise levels, indicating strong potential for applications in the field of rotational seismology.

Cascaded weak-value amplification for optic-fiber-based Sagnac interferometers

In this paper, we propose a scheme for angular rotation Ω measurement in an optic-fiber-based Sagnac interferometer based on cascaded weak-value amplification (CWVA). CWVA is a modified standard weak-value amplification (SWVA) technique for further enhancing temporal shifts based on the principle of the Vernier effect. By choosing the appropriate CWVA parameters and the repetition time intervals of the Vernier scale, the temporal shifts in SWVA can be further amplified by measuring the envelope shifts in CWVA. Our comprehensive simulations demonstrate that CWVA enables the detection of minuscule rotations in the range of 1.0×10−9rad/s≤Ω≤10×10−9rad/s, with significantly improved sensitivity and superior signal-to-noise ratios compared to SWVA. The enhancement with a larger detection band may have great influence on physics and related sciences, such as rotational seismology and gravitational sensing.

Dual-polarization interferometric fiber optic gyroscope with Shupe effect compensation

Dual-polarization (DP) interferometric fiber optic gyroscope (IFOG) has been studied for over a decade, which has important applications in inertial navigation and rotational seismology. Currently, the Shupe effect is an important factor degrading the accuracy of the DP IFOG. Here, a dual-quadrupolar winding pattern is proposed to compensate for the Shupe effect. The finite-element simulation experiments verify that the phase shifts induced by the Shupe effect in two polarizations are almost reversed and well compensated. Static laboratory test results show that, in this configuration, the angle random walk is reduced 10-fold to 7.1×10−5 °/h, while the bias instability is reduced ninefold to 1.7×10−4 °/h. Moreover, a 3-h temperature variation experiment is carried out. The results show that, compared with the original output, the net rotation rate errors originating from the Shupe effect are effectively compensated and the temperature drift is reduced by 13 times. To summarize, the dual-quadrupolar-wound method is a valid approach to compensate for the Shupe effect, and the temperature adaptability of this DP configuration is greatly enhanced.

Angular accelerometer based on a dual-polarization fiber-optic Sagnac interferometer.

High-performance angular accelerometers are essential for precise dynamics control of aircraft, satellites, etc. Here, we propose, for the first time to the best of our knowledge, an angular accelerometer based on a dual-polarization fiber-optic Sagnac interferometer, which exhibits relatively high sensitivity and a broad bandwidth. The experimental results show that the angular accelerometer achieves a flat frequency response in the bandwidth range of 0.01-100 Hz. The sensitivity reaches 6.6 × 10-8 rad/s2/Hz. In addition, the proposed fiber-optic angular accelerometer does not rely on any mechanical structure and has strong environmental adaptability. This research provides a feasible solution for the design and implementation of new high-performance angular accelerometers, which contributes to their development in the fields of inertial navigation and rotational seismology.

Development of Three-Axis Fibre-Optic Seismograph for Direct and Autonomous Monitoring of Rotational Events with Perspective of Historical Review.

The paper presents historical perspective of fibre-optic seismographs designed and constructed at the Institute of Applied Physics at Military University of Technology, Poland based on the so-called minimum configuration of fibre-optic gyroscope. The briefly presented history, which originated in the 1998 by the system named GS-13P, laid solid foundations for the construction of a three-axis fibre-optic seismograph. The presented system meets all technical requirements of rotational seismology in terms of measurement parameters (measuring range from 10-8 rad/s to several rad/s and frequency from 0.01 Hz to 100 Hz) and utility features (mobility, autonomy, power independence, environmental stability). The presented device provides universal application both for research in engineering applications (high buildings, chimneys, wind towers) as well as in seismological research.

High sensitivity tool for geophysical applications: a geometrically locked ring laser gyroscope.

We demonstrate that a middle sized ring laser gyroscope (RLG) can be a very sensitive and robust instrument for rotational seismology, even if it operates in a quite noisy environment. The RLG has a square cavity, 1.60×1.60m 2, and it lies in a plane orthogonal to the Earth's rotational axis. The Fabry-Perot optical cavities along the diagonals of the square were accessed, and their lengths were locked to a reference laser. Through a quite simple locking circuit, we were able to keep the sensor fully operative for 14days. We verified that the prototype properties are compatible with the seismic requirements. The obtained long term stability is of the order of 3nanorad/s, and the short term sensitivity is close to 2n a n o r a d/s⋅H z -1/2. These results are limited only by the noisy environment; our laboratory is located in a building downtown.

Rotational Seismology with a Quartz Rotation Sensor

Abstract We describe the construction and performance of a new high-precision ground- or platform-rotation sensor called the Quartz Rotation Sensor (QRS). The QRS is a mechanical angular accelerometer that senses rotational torque with an inherently digital, load-sensitive resonant quartz crystal. The noise floor is measured to be ∼45 pico-radians/root (Hz) near 1 Hz, and the resonant period of the sensor is about 10 s, making it a broadband sensor. Among similarly sized broadband rotation sensors, this represents more than two orders of magnitude improvement in noise floor near 0.1 Hz. We present measurements of rotational components of teleseismic waves recorded with the sensor at a vault. The QRS is useful for rotational seismology and for improving low-frequency seismic isolation in demanding applications such as the Laser Interferometer Gravitational-Wave Observatories.

Microtremor surveys based on rotational seismology: theoretical analysis with focus on separation of Rayleigh and Love waves in general wavefield of microtremors

SUMMARYThe applicability of rotational seismology to the general wavefield of microtremors is theoretically demonstrated based on a random process model of a 2-D wavefield. We show the effectiveness of taking the rotations (i.e. spatial differentiation) of microtremor waveforms in separating the Rayleigh and Love waves in a wavefield where waves are simultaneously arriving from various directions with different intensities. This means that a method based on rotational seismology (a rotational method) is capable of separating Rayleigh and Love waves without adopting a specific array geometry or imposing a specific assumption on the microtremor wavefield. This is an important feature of a rotational method because the spatial autocorrelation (SPAC) method, a conventional approach for determining phase velocities in microtremor array surveys, requires either the use of a circular array or the assumption of an isotropic wavefield (i.e. azimuthal averaging of correlations is required). Derivatives of the SPAC method additionally require the assumption that Rayleigh and Love waves are uncorrelated. We also show that it is possible to apply a rotational method to determine the characteristics of Love waves based on a simple three-point microtremor array that consists of translational (i.e. ordinary) three-component sensors. In later sections, we assume realistic data processing for microtremor arrays with translational sensors to construct a theoretical model to evaluate the effects of approximating spatial differentiation via finite differencing (i.e. array-derived rotation, ADR) and the effects of incoherent noise on analysis results. Using this model, it is shown that in a short-wavelength range compared to the distance for finite differencing (e.g. $\lambda < 3h$, where $\lambda $ and $h$ are the wavelength and distance for finite differencing, respectively), the leakage of unwanted wave components can determine the analysis limit. It is also shown that in a long-wavelength range (e.g. $\lambda > 3h$), the signal intensity gradually decreases, and thus the effects of incoherent noise increase (i.e. the signal-to-noise ratio, SNR decreases) and determine the analysis limit. We derive the relation between the SNR and wavelength. Although the analysis results quantitatively depend on the array geometry used for finite differencing, the qualitative understanding supported by mathematical expressions with a physically clear meaning can serve as a guideline for the treatment of data obtained from ADR.

Rotational Seismology: Review of Achievements and Outlooks

Rotational Seismology: Review of Achievements and Outlooks

Generation of 6-C synthetic seismograms in stratified vertically transversely isotropic media using a generalized reflection and transmission coefficient method

SUMMARYWe develop a generalized reflection and transmission coefficient method (GRTM) for generating six-component (6-C) synthetic seismograms in horizontally layered vertically transversely isotropic (VTI) media. Compared with the traditional seismic modelling approaches that only consider translational motion, our method can simultaneously produce three-component translational and three-component rotational data excited by a point vector force or a moment tensor source in a layered half-space. Horizontally layered models are widely used in near surface applications as the properties of near surface formations generally show small lateral variations and change mainly along the depth direction. The use of the VTI constitutive relation can make our method applicable to more general situations because it takes into account the characteristics of sedimentary formations. We compare our method with a finite-difference method (FDM) for a variety of velocity models and acquisition geometries. The numerical results demonstrate that accurate 6-C synthetic seismograms can be calculated using our method. The computational efficiency of our method for 6-C seismic modelling is much higher than the finite-difference method, because it can reduce a 3-D modelling problem to 2.5-D by eliminating the azimuthal dimension. Also, our method does not require to perform additional spatial interpolations to obtain the rotational components. These advantages make our method suitable to serve as a forward modelling tool for rotational seismology.

Sensitivity enhancement through RIN suppression in dual-polarization fiber optic gyroscopes for rotational seismology.

Portable sensors with a sufficiently high sensitivity in detecting small rotational motions have attracted significant attention in the field of rotational seismology. In this study, we propose and demonstrate a dual-polarization fiber optic gyroscope (IFOG) with a portable-sized fiber coil. Excess relative intensity noise (RIN) is effectively compensated for owing to the opposite parities and strong correlation of the two orthogonal polarized light, whereas other noises including coherent phase noise and thermal phase noise have also been handled well. In a test on detecting the rotation rate of the Earth, an enhanced sensitivity of 20nrad/s/Hz over a frequency range of 0.01 Hz to 30 Hz was demonstrated using the proposed design with an enclosed area of only 68 m2.

Length measurement and stabilization of the diagonals of a square area laser gyroscope

Large frame ring laser gyroscopes are top sensitivity inertial sensors able to measure absolute angular rotation rate below prad s−1 in few seconds. The GINGER project is aiming at directly measuring the Lense–Thirring effect with an 1% precision on an Earth based experiment. GINGER is based on an array of large frame ring laser gyroscopes. The mechanical design of this apparatus requires a micrometric precision in the construction and the geometry must be stabilized in order to keep constant the scale factor of the instrument. The proposed control is based on square cavities, and relies on the length stabilization of the two diagonals, which must be equal at micrometric level. GP2 is the prototype devoted to the scale factor control test. As a first step, the lengths of the diagonals of the ring cavity have been measured through an interferometric technique with a statistical accuracy of some tens of nanometers, and they have been locked to the wavelength of a reference optical standard. Continuous operation has been obtained over more than 12 h, without loss of sensitivity. GP2 is located in a laboratory with standard temperature stabilization, with residual fluctuations of the order of 1 °C. Besides the demonstration of the control effectiveness, the analysis of the Sagnac frequency demonstrates that relative small and low-cost ring lasers (around one meter of side) can also achieve a sensitivity of the order of nrad s−1 in the range 0.01–10 Hz in a standard environment, which is the target sensitivity in many different applications, such as rotational seismology and next generation gravitational waves detectors.

Two Correlated Interferometric Optical Fiber Systems Applied to the Mining Activity Recordings

In order to meet new directions of research in the field of rotational seismology, a registration of rotation phenomena from mining seismic shocks was carried out by two interferometric optical systems. Applied instruments are based on the Sagnac effect and measure the rotation rate in a direct way. Systems have a wide measuring range both in the case of signal amplitude as well as frequency range. Allan variance analysis has indicated that they have proper parameters for the rotational seismology investigation (angle random walk equals 3.20 · 10−8 rad/√s, as well as bias instability equals 2.41 · 10−8 rad/s). Moreover, regarding applied special electronic processing, they provide rotation rate as an output instead of a change of an angle like in commercially available fiber optic gyroscopes. The optical head of the constructed systems uses depolarized light in order to avoid polarization effect. Systems are remotely controlled by the Internet and adapted to an independent power supply which makes them fully autonomous. Correlation between data gathered by two systems applied in a geophysical observatory in Ksiąz, Poland, have been calculated according to the person's correlation coefficient.

A Review of the Capacitive MEMS for Seismology.

MEMS (Micro Electro-Mechanical Systems) sensors enable a vast range of applications: among others, the use of MEMS accelerometers for seismology related applications has been emerging considerably in the last decade. In this paper, we provide a comprehensive review of the capacitive MEMS accelerometers: from the physical functioning principles, to the details of the technical precautions, and to the manufacturing procedures. We introduce the applications within seismology and earth sciences related disciplines, namely: earthquake observation and seismological studies, seismic surveying and imaging, structural health monitoring of buildings. Moreover, we describe how the use of the miniaturized technologies is revolutionizing these fields and we present some cutting edge applications that, in the very last years, are taking advantage from the use of MEMS sensors, such as rotational seismology and gravity measurements. In a ten-year outlook, the capability of MEMS sensors will certainly improve through the optimization of existing technologies, the development of new materials, and the implementation of innovative production processes. In particular, the next generation of MEMS seismometers could be capable of reaching a noise floor under the lower seismic noise (few tenths of ng/) and expanding the bandwidth towards lower frequencies (∼0.01 Hz).

The Fiber-Optic Rotational Seismograph-Laboratory Tests and Field Application.

The paper presents construction and laboratory tests, as well as the first field application of a new fiber-optic rotational seismograph. The system is based on a fiber-optic gyroscope (FOG), with determined Angle Random Walk of the order of 10−8 rad/Sqrt(s) and a few rad/s maximum detectable amplitude of rotation in the frequency range from direct current (DC) to 328.12 Hz. It has been designed for the rotational seismology area of interest. This work also presents exemplary relevant measurements, which were conducted using a set of two devices installed in the geophysical observatory in Książ, Poland.

High-rate multi-GNSS attitude determination: experiments, comparisons with inertial measurement units and applications of GNSS rotational seismology to the 2011 Tohoku Mw9.0 earthquake

High-rate GNSS positioning has been widely investigated and applied in science and engineering. We extend it to high-rate attitude determination under a multi-GNSS constellation. A series of experiments of high-rate GNSS attitude determination has been conducted on a platform with three 50 Hz geodetic receivers and two high-grade inertial measurement units (IMU). The high-rate attitude solutions are computed for each of the multi-GNSS systems and the combined constellation by either using short baselines with correct ambiguity resolution or precise point positioning (PPP) and compared with the IMU measurements. In the case of a single GNSS system, the experimental results have shown that GPS is of the best accuracy, followed by GLONASS. The results with Beidou are the noisiest. The combined multi-GNSS constellation can significantly improve the high-rate attitude solutions from any single GNSS system alone, which is, in particular, most suitable for applications to any platform in slow or quasi-static motion. However, the improvement rate could depend on proper weightings of measurements from different GNSS systems in the dynamical experiments. The accuracy of baseline-based high-rate GNSS attitude solutions remains stable over time, while that of PPP-based solutions substantially degrades with time, as theoretically expected. Within a short period of time, the PPP-based high-rate yaw solutions with the combined multi-GNSS constellation are comparable in accuracy with those computed from baselines with correct ambiguity resolution in the dynamical experiments. The attitude results from either static or dynamical experiments have shown that high-rate GNSS attitude determination is sufficiently precise to measure rotatory motions. GNSS rotational seismology is applied to the 2011 Tohoku Mw9.0 earthquake, illustrating the potential of multi-GNSS to precisely detect seismic rotatory motions.

A Numerical Model of the SEIS Leveling System Transfer Matrix and Resonances: Application to SEIS Rotational Seismology and Dynamic Ground Interaction

Abstract Both sensors of the SEIS instrument (VBBs and SPs) are mounted on the mechanical leveling system (LVL), which has to ensure a level placement on the Martian ground under currently unknown local conditions, and provide the mechanical coupling of the seismometers to the ground. We developed a simplified analytical model of the LVL structure in order to reproduce its mechanical behavior by predicting its resonances and transfer function. This model is implemented numerically and allows to estimate the effects of the LVL on the data recorded by the VBBs and SPs on Mars. The model is validated through comparison with the horizontal resonances (between 35 and 50 Hz) observed in laboratory measurements. These modes prove to be highly dependent of the ground horizontal stiffness and torque. For this reason, an inversion study is performed and the results are compared with some experimental measurements of the LVL feet’s penetration in a martian regolith analog. This comparison shows that the analytical model can be used to estimate the elastic ground properties of the InSight landing site. Another application consists in modeling the 6 sensors on the LVL at their real positions, also considering their sensitivity axes, to study the performances of the global SEIS instrument in translation and rotation. It is found that the high frequency ground rotation can be measured by SEIS and, when compared to the ground acceleration, can provide ways to estimate the phase velocity of the seismic surface waves at shallow depths. Finally, synthetic data from the active seismic experiment made during the HP3 penetration and SEIS rotation noise are compared and used for an inversion of the Rayleigh phase velocity. This confirms the perspectives for rotational seismology with SEIS which will be developed with the SEIS data acquired during the commissioning phase after landing.

Recording active-seismic ground rotations using induction-coil magnetometers

Most of the current rotational sensing technology is not geared toward the recording of seismic rotations’ amplitudes and frequencies. There are few instruments that are designed for rotational seismology, and the technology for building them is currently being developed. There are no mass industrial producers of seismic rotation sensors as there are for geophones, and only one current sensor model can be deployed on the ocean bottom. We reviewed some current rotational-seismic acquisition technologies, and developed a new method of recording rotations using an existing, robust and field-deployable technology that had seen extensive use in large exploration surveys: induction-coil magnetometers. We conducted an active seismic experiment, in which we found that magnetometers could be used to record seismic rotations. We converted the magnetometer data to rotation-rate data, and validated them by comparing the waveforms and amplitudes with rotation rates recorded by electrokinetic rotation sensors.

Fibre-Optic Sagnac Interferometer in a FOG Minimum Configuration as Instrumental Challenge for Rotational Seismology

Rotational seismology caused high interest in the investigation on rotational movements generated by earthquakes, mines, and existing in engineering structures. The most oppressive aspects of the research in this field are technical requirements for sensors. They have to be extremely sensitive, as well as possess an extremely wide dynamic range of operation from 0.01 to 100 Hz. This paper points out that solutions basing on the optical technology are the most promising devices in comparison to existing mechanical and electromechanical ones. Especially, instruments basing on the von Laue–Sagnac effect seem to be the most appropriate to investigate rotational phenomena due to the fact that they are entirely insensitive to a translational motion, operate without inertia mass, and are able to measure rotation rate in wide frequency and amplitude band. The paper presents a new device named FOSREM that, based on the FOG minimum configuration, possesses special solutions that makes it perfect, in our knowledge, for any rotation sensing. It enables to measure only a rotational component in a wide range of signal amplitude of 2·10–8–10 rad/s, as well as in a wide frequency band from DC to 328.12 Hz. The presented performance characteristics, as well as a laboratory investigation, confirm FOSREM's opportunity to measure rotation both in seismological observatories and engineering constructions.

The GINGER Project

GINGER (Gyroscopes IN General Relativity) is a project aiming at measuring the Lense-Thirring effect, at 1% level, with an experiment on earth. It is based on an array of ring-lasers, which are the most sensitive inertial sensors to measure the rotation rate of the Earth. The GINGER project is still under discussion; the experiment G-GranSasso is an R&D experiment financed by INFN Group II, it is studying the key points of GINGER and at the same time developing prototypes. In the following the signal coming out of a ring-laser and the present sensitivity are described.The prototypes GP2 and GINGERino and the preliminary results are reported. This project is inter-disciplinary since ring-lasers provide informations for the fast variation of the earth rotation rate, they are used for the rotational seismology and for top sensitivity angle metrology.