ALBERT

Description: 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.

Description: Recent progress in rotational sensor technology has made it possible to directly measure rotational ground-motion induced by seismic waves. When combined with conventional inertial seismometer recordings, the new sensors allow one to locally observe six degrees of freedom (6DOF) of ground-motion, composed of three orthogonal components of translational motion and three orthogonal components of rotational motion. The applications of such 6DOF measurements are manifold—ranging from wavefield characterization, separation, and reconstruction to the reduction of non-uniqueness in seismic inverse problems—and have the potential to revolutionize the way seismic data are acquired and processed. However, the seismological community has yet to embrace rotational ground-motion as a new observable. The aim of this paper is to give a high-level introduction into the field of 6DOF seismology using illustrative examples and to summarize recent progress made in this relatively young field. It is intended for readers with a general background in seismology. In order to illustrate the seismological value of rotational ground-motion data, we provide the first-ever 6DOF processing example of a teleseismic earthquake recorded on a multicomponent ring laser observatory and demonstrate how wave parameters (phase velocity, propagation direction, and ellipticity angle) and wave types of multiple phases can be automatically estimated using single-station 6DOF processing tools. Python codes to reproduce this processing example are provided in an accompanying Jupyter notebook.

Description: Measurements of rotations are unique because of their inherent property making them absolute and without an external frame of reference [...]

Description: We report the first ground rotation observations on the seafloor from an experiment we carried out in the North Sea close to the island of Helgoland. A slightly modified commercial fiber optic gyroscope was mounted on an ocean‐bottom seismometer (OBS) platform together with an intermediate‐period seismometer. The system was lowered to the seafloor for 4 days. To investigate a potential tilt contamination of horizontal translational recordings, we calculate the coherence between the corresponding motion components (rotations around x axis and translations along y axis, and vice versa). We find very high correlations in the 5–13 s period interval, in which the correlation coefficient reaches 0.94 over 8.5 hrs. This clearly indicates that horizontal translational components are severely contaminated by rotations. We find that these rotational motions are caused by seafloor currents or deformation of the seafloor rather than by seismic waves. The ground rotation observations allow correcting for the cross‐coupling effect, thereby decreasing the power spectral density up to 11 dB at 10 s period on horizontal OBS components. We discuss general requirements for broadband rotation sensors for OBS applications as well as for possible further applications.