Rotational Seismometer Research Articles

Towards uniformity of rotational events recording – initial data from common test engaging more than 40 sensors including a wide number of fiber-optic rotational seismometers

Towards uniformity of rotational events recording – initial data from common test engaging more than 40 sensors including a wide number of fiber-optic rotational seismometers

Development of a High-Performance Rotational Seismometer Based on the Dual-Channel Fiber Optic Gyroscope

Development of a High-Performance Rotational Seismometer Based on the Dual-Channel Fiber Optic Gyroscope

Analyze the Difference Between Rotational and Translational Motions Produced by High-speed Train

The development of fiber optic gyroscope technology makes it possible to directly measure the rotational ground-motion. The joint observation of the three-component rotational seismometer and the traditional three-component translational seismometer is a trend of future. In this paper, the translational and rotational signals generated during the high-speed trainway are studied, and the three-component translational and rotational seismometers are fixed together to realize the joint recording of the high-speed trainway signal. Comparing the translational and rotational three-component data, we find that only one component waveforms and the spectrum have certain consistency. However, the difference between them indicates that the rotation and translation signals generated by the shallow surface wave signal have different frequency bands.

Six‐Component Earthquake Synchronous Observations Across Taiwan Strait: Phase Velocity and Source Location

AbstractWith the development of rotational seismometers, especially the high‐sensitivity optical seismometers, rotational motions have shown increasing benefits for calculation of dispersion curves, estimation of back azimuth and many other applications. Here we present the six‐component observations of 2019 Hualien mww6.1 earthquake simultaneously on both sides of Taiwan Strait. Seismograms and time‐frequency spectrums show that the attenuations for rotations are more significant than translations. Calculations of the surface wave phase velocity are performed for both stations through single‐station records, indicating a distinct difference owning to the instrument installation conditions and site effect. Comparison of theoretical dispersion curves is consistently in trends with estimations, and reveals the influence of higher‐mode surface waves. For the first time, we locate the source of a earthquake through intersection of two stations' back azimuths estimated through rotational methods. Results of comparing to translational polarization verify that rotational observation can distinctly reduce the deviation of source location.

A Rotational Seismometer Based on Dual-polarization Fiber Optic Gyroscopes

A rotational seismometer based on dual-polarization interferometric fiber optic gyroscopes is designed and demonstrated, it can measure three rotation components at the same time. The angle random walk is about 1.67×10−4∘/h . The self-noise ranging from 0.1Hz to 125 Hz is about 7.0×10−8rad/s/Hz . It can be applied to seismic monitoring, seismic exploration, engineering monitoring and other fields.

A New Paradigm for Structural Characterization, including Rotational Measurements at a Single Site

ABSTRACT In this article, we demonstrate that a single station can be used to measure the dynamic properties of a structure. The station includes a collocated accelerometer and rotational sensor, hence, can record both three-component translation and three-component rotation and is referred to as the 6C-station within this study. The key advantage of this approach is to provide a fast and simple path to a comprehensive structural health monitoring characterization that is comparable to the use of a traditional approach using a horizontal array of three-component accelerometers. The deployment of newly developed high-quality rotational sensors allows the direct measurement of structural rotations, facilitating the extraction of structural mode shapes. In this work, we show how an established system identification tool, stochastic subspace identification, can be applied to the 6C-station data and characterize modal properties and structural response. Our results are verified and contrasted against standard horizontal and vertical array configurations. The Prime Tower, a high-rise structure in Zürich, serves as a case study. A structural characterization of this building is presented for the first time. We show that a 6C-station is capable of defining the frequencies of this stiff high-rise building with a fidelity that is on par with a five-sensor horizontal array. The mode shapes of the roof can be precisely determined with a confidence margin that is comparable to conventional sensing array solutions. However, the effectiveness of using only a 6C-station is determined by the noise level of the sensors—in particular, the rotational seismometer needs to be of high quality. The results indicate that, owing to the collocation measurement of translation and rotation, a 6C-station can deliver a comprehensive structural monitoring solution with minimum time, effort, and footprint.

Dynamic Characteristics of TAIPEI 101 Skyscraper from Rotational and Translation Seismometers

ABSTRACTContinuous data streams from translational and rotational seismometers installed in TAIPEI 101 enable monitoring of the natural frequencies on different time scales. Using the 2014 seismic data recorded on the 90th floor of this high-rise building and the meteorological data from a weather station located just 1 km away, we explored the characteristics and controlling factors of the ambient vibrations in TAIPEI 101. Using the random decrement technique, the three modal frequencies in translation were identified as 0.15 Hz (F1), 0.43 Hz (F2), and 0.78 Hz (F3). For rotation around the vertical axis, the modal frequencies were 0.23 Hz (R1), 0.59 Hz (R2), and 0.93 Hz (R3). In translation, TAIPEI 101 exhibits a trend of increasing modal frequency with increasing temperature but decreasing modal frequency with increasing wind velocity. The trend with temperature is reversed in rotation motion. The different frequency versus temperature relationship seen for rotational and translational motion demonstrates the importance of introducing rotational motion analysis into structural health monitoring. The change in modal frequencies were also found to decrease with growing amplitude. It is intriguing that F1 exhibits a weaker dependency with amplitude with respect to the higher modes, which may be associated with the suppression of F1 vibration caused by the damper installed in TAIPEI 101. Other than long-term (seasonable) variation, we also highlight the hourly variation of the first-mode amplitude throughout a day by comparing with weather and mobility data. Other than the atmospheric conditions that strongly influence the modal frequencies in long-term behavior, we found that human activities may play an important role in the short-term vibration characteristics of the building.

Rotational Components of Normal Modes Measured at a Natural Sandstone Tower (Kane Springs Canyon, Utah, U.S.A.)

AbstractModal analysis of freestanding rock formations is crucial for evaluating their vibrational response to external stimuli, aiding accurate assessment of associated geohazards. Whereas conventional seismometers can be used to measure the translational components of normal modes, recent advances in rotational seismometer technology now allow direct measurement of the rotational components. We deployed a portable, three-component rotational seismometer for a short-duration experiment on a 36 m high sandstone tower located near Moab, Utah, in addition to conducting modal analysis using conventional seismic data and numerical modeling. Spectral analysis of rotation rate data resolved the first three natural frequencies of the tower (2.1, 3.1, and 5.9 Hz), and polarization analysis revealed the orientations of the rotation axes. Modal rotations were the strongest for the first two eigenmodes, which are mutually perpendicular, full-height bending modes with horizontal axes of rotation. The third mode is torsional with rotation about a subvertical axis. Measured natural frequencies and the orientations of displacements and rotation axes match our numerical models closely for these first three modes. In situ measurements of modal rotations are valuable at remote field sites with limited access, and contribute to an improved understanding of modal deformation, material properties, and landform response to vibration stimuli.

Connection between the cross correlation and the Green's function: strain and rotation of surface waves

SUMMARY Recent developments of distributed acoustic sensing (DAS) techniques and rotational seismometers enable us to record strain and rotational seismograms. This prompts us to adapt the methods used for traditional translational seismograms to strain and rotational seismograms. Recently, we extended the formulation of the spatial autocorrelation (SPAC) method to strain, rotation and tilt records. According to seismic interferometry for translation seismograms, cross correlations have clear connections to Green's functions under the isotropy and equipartition of noise wavefields. In this study, we clarify similar connections for strain and rotation seismograms. Because we extend the formulation in the frequency domain, we actually study the connection between the cross-spectral matrix of strain and rotation at two receivers and the Green's tensor. First, we provide a proof under a general framework by simply extending the proof for translation to that for strain and rotation. The proof shows the following results: (1) The cross-spectral matrix of (i, j)-component strains at two receivers is found to be proportional to the strain Green's tensor at one receiver for the sum of (i, j)- and (j, i)-components of the moment tensor source at the other receiver. (2) The cross-spectral matrix of (i, j)-component rotations at two receivers is found to be proportional to the rotation Green's tensor at one receiver for the difference of (i, j)- and (j, i)-components of the moment tensor source at the other receiver. Necessary assumptions for the proof are the isotropy and equipartition of the wavefield. We then check this proof with specific calculations for surface waves observed on the free surface. The proof is confirmed for the isotropic incidence of random-phase surface waves with a specific ratio between Rayleigh and Love wave energies. Seismic interferometry for strain and rotation was already studied. However, the connection of cross correlations to the Green's function for strain and rotation seismograms is discovered by this study for the first time. A specific proof for body waves that is assured by the general proof will be our next study.

Study of Rotational Motions Caused by Multiple Mining Blasts Recorded by Different Types of Rotational Seismometers.

Digging two vertical shafts with the multiple blasts technique gave the opportunity to measure the induced angular motions in a horizontal plane with well-defined positions of sources. Three kinds of rotation rate sensors, sharing an underground location, were used. Two of them—a Fiber-Optic System for Rotational Events & phenomena Monitoring (FOSREM) and a prototypical seismometer housing the liquid-filled torus—sensed the rotation, while a microarray of two double-pendulum seismometers sensed both the rotation and symmetric strain. The FOSREM was sampled at 656.168 Hz, while all the others were only sampled at 100 Hz. There were considerable differences within the results gathered from the mining blasts, which should be attributed to two causes. The first one is the difference in principles of the operation and sampling rates of the devices used, while the other is the complex and spatially variable character of the studied wave fields. Additionally, we established that the liquid-filled sensor, due to its relatively low sensitivity, proved to be viable only during a registration of strong ground motions. Overall, a comparative study of three different rotational seismometers was performed during mining-induced strong ground motions with well-localized sources.

Extending the formulation of the spatial autocorrelation (SPAC) method to strain, rotation and tilt

SUMMARYThe spatial autocorrelation (SPAC) method has been applied to ambient seismic noise measured by arrays of translational seismometers for inverting phase-velocity dispersion curves of Rayleigh or Love waves for shallow S-wave velocity structure. Recently, it is becoming possible to observe wave spatial gradients such as strain, rotation and tilt owing to the development of dense seismic networks and improving measurement technologies. Therefore, it is desirable to extend the formulation of the SPAC method to strain, rotation and tilt. This study presents analytical expressions of cross-spectra and coherence of the strain, rotation and tilt components that are measured on the free surface. According to the results, both Rayleigh and Love waves contribute to most components of strains. The exceptions are the areal strain and the vertical axial strain (ezz) on the free surface that are affected by only Rayleigh waves. Only Rayleigh waves contribute to the tilts and rotations around the horizontal axes on the free surface, too. On the other hand, only Love waves contribute to the rotation around the vertical axis. Therefore, different kinds of wave spatial gradients are helpful to separate Rayleigh and Love waves correctly. For practical applications, the analytical expression for the radial axial strain (err) component will be applied directly to distributed acoustic sensing data measured with straight sections of a fibre-optic cable. On the other hand, dense observations of rotation and tilt may still be difficult to carry out at present. However, an application of analytical formulations in this study to arrays of at least several three-component rotational seismometers is attractive because it enables us to separately estimate the phase velocities of Rayleigh and Love waves.

The Development of a New IFOG-Based 3C Rotational Seismometer.

For many years, seismological research mainly focuses on translational ground motions due to the lack of appropriate sensors. However, because of the development of devices based on Sagnac effect, measuring rotational waves directly comes available. In this work, a portable three-component broadband rotational seismometer named RotSensor3C based on open loop interferometric fiber optic gyroscope (IFOG) is designed and demonstrated. Laboratory tests and results are illustrated in detail. The self-noise ranging from 0.005 Hz to 125 Hz is about , and with the harmonics compensation the scale factor variation over is lower than 10 ppm (parts per million). The misalignment matrix method is adopted to revise the output rotation rate. In a special near field experiment using the explosive source, the back-azimuths and phase velocity are estimated by the recorded acceleration and rotation rate. All the results prove the practicability of this new rotational sensor.

Regional Moment Tensor Inversion Using Rotational Observations

AbstractAbstractThere are benefits from the addition of rotational motions to translational displacements for moment tensor (MT) inversions. The rotational radiation pattern is orthogonal to the shear radiation pattern, thus incorporating rotations is equivalent to gaining another observation point on the focal sphere. We demonstrated this by simulating curl and displacement wavefields for a regional distance station. Thus, one 6‐C station (3‐Component translational + 3‐Component rotational) gathers the same information on radiation pattern as two 3‐C stations at 90° azimuth from one another along the focal plane axis, which is sometimes difficult to obtain when restricted to surface sensors. We added rotational Green's functions to a regional MT inversion scheme (long‐period, time‐domain, and linear inversion) by computing spatial gradients from f‐κ reflectivity synthetics. For rotational data, we used Array Derived Rotations (ADR) from Piñon Flats Observatory Array and Golay array deployed during IRIS Community Wavefield Demonstration Experiment. The hope is to ultimately use compact and field deployable broadband rotational seismometers instead of ADRs. Rotational motions were predicted from well‐constrained deviatoric MT solutions of nine earthquakes recorded by the arrays and other seismic networks. We formed three station sparse datasets with (3‐C and 6‐C) and without rotational ground motions (3‐C only) for eight earthquakes testing the benefits of including rotational waveforms in MT inversions when they are equally weighted with translational waveforms. Adding rotational motions improved the double‐couple components and reduced compensated‐linear‐vector‐dipole and isotropic components in full‐MT solutions using sparse datasets with poor coverage.

Rotation, Strain, and Translation Sensors Performance Tests with Active Seismic Sources.

Interest in measuring displacement gradients, such as rotation and strain, is growing in many areas of geophysical research. This results in an urgent demand for reliable and field-deployable instruments measuring these quantities. In order to further establish a high-quality standard for rotation and strain measurements in seismology, we organized a comparative sensor test experiment that took place in November 2019 at the Geophysical Observatory of the Ludwig-Maximilians University Munich in Fürstenfeldbruck, Germany. More than 24 different sensors, including three-component and single-component broadband rotational seismometers, six-component strong-motion sensors and Rotaphone systems, as well as the large ring laser gyroscopes ROMY and a Distributed Acoustic Sensing system, were involved in addition to 14 classical broadband seismometers and a 160 channel, 4.5 Hz geophone chain. The experiment consisted of two parts: during the first part, the sensors were co-located in a huddle test recording self-noise and signals from small, nearby explosions. In a second part, the sensors were distributed into the field in various array configurations recording seismic signals that were generated by small amounts of explosive and a Vibroseis truck. This paper presents details on the experimental setup and a first sensor performance comparison focusing on sensor self-noise, signal-to-noise ratios, and waveform similarities for the rotation rate sensors. Most of the sensors show a high level of coherency and waveform similarity within a narrow frequency range between 10 Hz and 20 Hz for recordings from a nearby explosion signal. Sensor as well as experiment design are critically accessed revealing the great need for reliable reference sensors.

Near-Field Measurement of Six Degrees of Freedom Mining-Induced Tremors in Lower Silesian Copper Basin.

The impact of seismicity on structures is one of the key problems of civil engineering. According to recent knowledge, the reliable analysis should be based on both rotational and translational components of the seismic wave. To determine the six degrees of freedom (6-DoF) characteristic of mining-induced seismicity, two sets of seismic posts were installed in the Lower Silesian Copper Basin, Poland. Long-term continuous 6-DoF measurements were conducted with the use of the R-1 rotational seismometer and EP-300 translational seismometer. In result data collection, the waveforms generated by 39 high-energy seismic events were recorded. The characteristic of the rotational component of the seismic waves were described in terms of their amplitude and frequency characteristics and were compared with translational measurements. The analysis indicated that the characteristic of the rotational component of the seismic wave differs significantly in comparison to translational ones, both in terms of their amplitude and frequency distribution. Also, attenuation of rotational and translational components was qualitatively compared. Finally, the empirical formulas for seismic rotation prediction in the Lower Silesian Copper Basin were developed and validated.

Symmetric Triaxial Rotational Piezoelectric Seismometers

The possibilities of constructive implementation of rotational seismometers using fundamentally new two-and four-element piezoelectric rotation sensors are considered. Based on a detailed review of existing piezoelectric accelerometers for strong earthquakes, a design for triaxial rotational piezoelectric accelerometers symmetric with respect to the vertical location component is proposed. Calculation for an instrument suitable for downhole observations is presented. The prospects of using the new modern piezoelectric materials are noted.

Developments in New Fluid Rotational Seismometers: Instrument Performance and Future Directions

In this article we describe prototype designs and tests for low‐cost rotational medium‐ and strong‐motion seismometers using three types of proof mass (two liquid and one solid) and a number of transducer configurations. This article describes the third set of designs and tests in our development program. The details of our results for most of these are in the Ⓔ electronic supplement to this article, whereas here we concentrate on three of the most promising and representative design combinations. Most of our results pertain to sensors with water or silicon oil as the proof mass, though we also tested a torsion‐bar design with a solid proof mass. We find that most mass–transducer combinations lead to output proportional to rotational acceleration, with varying degrees of fidelity. Most combinations we tested can be dismissed from further development for reasons of performance or inconvenience during analysis of acceleration response (compare with Ⓔ electronic supplement). In this article, we describe three of the more promising combinations, one each for the three types of response functions we measured. Of these three, one mass–transducer combination in particular (a hinged sensing element and capacitive transduction) has output voltage closely proportional to rotational displacement (angle) over a wide frequency range; such displacement proportionality obviates two of the integration steps normally required to solve for continuum single‐point motions or correct for tilt‐induced errors in horizontal translational sensors. Thus, although we illustrate two other designs of some promise, we propose a new design that follows this displacement‐proportional path while increasing the device’s sensitivity to on‐axis rotations, improving its manufacturing ease and lowering its sensitivity to translational motions. Online Material: Results and related material for mass–transceiver combinations, drawings and photographs of liquid proof masses, amplitude and phase transfer functions, clip and linearity tests, and noise plots.

Second generation of a rotational electrochemical seismometer using magnetohydrodynamic technology

Rotational seismometers have many applications. Some require a low self noise with a lower clip specification. Others require many different bandpass specifications, from very low to higher frequencies. The principles of the eentec second-generation R-2 electrochemical triaxial rotational seismometer can achieve many features for various applications. Combining the use of the sophisticated magnetohydrodynamic (MHD) technology increases the current and future features. Principles of the MHD technology used and the many advantages it has in a rotational seismometers are described.

Comparison of point and array-computed rotations for the TAIGER explosions of 4 March 2008

Two large explosions were recorded by a dense array of strong-motion accelerometers and rotational seismometers in northeastern Taiwan associated with a Taiwan Integrated Geodynamics Research long-range refraction experiment. The objective of this experiment was to test the response of the experimental eentec rotational seismometers against calculated array rotations. Computed array rotation rates are seen to have little variation across the array, but point rotation rate measurements obtained from individual rotational seismometers show significant deviations with each other and with the array rotation rates in the ranges of 3–5 Hz and, especially, 3–50 Hz. A cross-correlation method was used to compare array-computed rotation rates and point rotation rate measurements in the frequency band of 3–5 Hz with the result that the absolute value of the normalized maximum correlation coefficient for each station set varied from 0.45 to 0.97 with an average of 0.84. Amplitude differences of the point rotation rate measurements are seen to be factors of 0.2 to 1.8 times the array rotations as well. It is not likely that the differences seen in the point and array-computed rotation rates are due to nonlinear or heterogeneous site conditions under each array element since these effects should also be seen in the acceleration data used to determine rotation rate. A rigorous method for accurately calibrating rotation rate instruments is needed to understand their response in the field.

Characteristics and Response of a Rotational Seismometer to Seismic Signals

A rotational seismometer capable of in situ measurements of rotational oscillations and tilts has been developed and calibrated. Its sensitivity to rotations and tilts at frequencies above 10 mHz is ![Graphic][1] , and this meets the recommendations of the International Working Group on Rotational Seismology (IWGoRS) for instrumental sensitivities for rotational seismology in the near field and for engineering applications. The design and calibration of this instrument and its frequency‐response function are presented, along with the signals initiated by an earthquake at a distance of ∼120 km. Finally, the steps we are taking to build a field instrument based on the experience gained with this engineering model are briefly discussed. [1]: /embed/inline-graphic-1.gif