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.

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.

Measurements of Rotational Events Generated by Artificial Explosions and External Excitations Using the Optical Fiber Sensors Network.

Measurements of artificial events can substantially confirm the data validity of constructed rotational sensors, as well as provide methods for simplifying the measurement process. The above task, especially with international cooperation, can provide full-field measurement results of the target object, which can deliver more significant data and sensor properties. The paper presents vertical rotational velocity recordings gathered during an international experiment that took place at the Geophysical Observatory of the Ludwig Maximilian University of Munich in Fürstenfeldbruck, Germany. Data were obtained during artificial explosions, as well as external excitations induced by a VibroSeis truck. The authors present data recorded by two prototypes of optical fiber rotational sensors. They have been specially designed for rotational seismology needs and are characterized by a theoretical sensitivity equal to 2 × 10-8 rad/s/√Hz and a wide measuring range both in amplitude even up to 10 rad/s, and a frequency from DC to 1000 Hz. Their self-noise investigation during the aforementioned experiment showed that both sensors have precision no worse than 2 × 10-6 rad/s/sqrt (Hz) in all desired frequency range from 0.01 to 100 Hz. A down-sampling and a spectral analysis of the recorded signals are also presented. The recorded data and their analysis confirmed the performance and reliability of the applied optical fiber rotational sensors. Moreover, the presented international experiment underlines a special necessity for specifying the sensors' performance test methodologies in the rotational seismology.

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 Ksiaz, Poland.

Review of the Usefulness of Various Rotational Seismometers with Laboratory Results of Fibre-Optic Ones Tested for Engineering Applications.

Starting with descriptions of rotational seismology, areas of interest and historical field measurements, the fundamental requirements for rotational seismometers for seismological and engineering application are formulated. On the above basis, a review of all existing rotational seismometers is presented with a description of the principles of their operation as well as possibilities to fulfill formulated requirements. This review includes mechanical, acoustical, electrochemical and optical devices and shows that the last of these types are the most promising. It is shown that optical rotational seismometer based on the ring-laser gyroscope concept is the best for seismological applications, whereas systems based on fiber-optic gyroscopes demonstrate parameters which are also required for engineering applications. Laboratory results of the Fibre-Optic System for Rotational Events & Phenomena Monitoring using a small 1-D shaking table modified to generate rotational excitations are presented. The harmonic and time-history tests demonstrate its usefulness for recording rotational motions with rates up to 0.25 rad/s.

Fibre optic system for monitoring rotational seismic phenomena.

We outline the development and the application in a field test of the Autonomous Fibre-Optic Rotational Seismograph (AFORS), which utilizes the Sagnac effect for a direct measurement of the seismic-origin rotations of the ground. The main advantage of AFORS is its complete insensitivity to linear motions, as well as a direct measurement of rotational components emitted during seismic events. The presented system contains a special autonomous signal processing unit which optimizes its operation for the measurement of rotation motions, whereas the applied telemetric system based on the Internet allows for an AFORS remote control. The laboratory investigation of such two devices indicated that they keep an accuracy of no less than 5.1 × 10(-9) to 5.5 × 10(-8) rad/s in the detection frequency band from 0.83~106.15 Hz and protect linear changes of sensitivity in the above bandpass. Some experimental results of an AFORS-1 application for a continuous monitoring of the rotational events in the Ksiaz (Poland) seismological observatory are also presented.