Scientific publications

Rotation, Strain, and Translation Sensors Performance Tests with Active Seismic Sources
Felix Bernauer, Kathrin Behnen, Joachim Wassermann, Sven Egdorf, Heiner Igel, Stefanie Donner, Klaus Stammler, Mathias Hoffmann, Pascal Edme, David Sollberger, Cédric Schmelzbach, Johan Robertsson, Patrick Paitz, Jonas Igel, Krystyna Smolinski, Andreas Fichtner, Yara Rossi, Gizem Izgi, Daniel Vollmer, Eva P. S. Eibl, Stefan Buske, Christian Veress, Frederic Guattari, Theo Laudat, Laurent Mattio, Olivie Sèbe, Serge Olivier, Charlie Lallemand, Basil Brunner, Anna T. Kurzych, Michał Dudek, Leszek R. Jaroszewicz, Jerzy K. Kowalski, Piotr A. Bońkowski, Piotr Bobra, Zbigniew Zembaty, Jiří Vackář, Jiří Málek, and Johana Brokesova
Sensors 21(1) 264 (2021)
https://doi.org/10.3390/s21010264

Abstract: 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.
This article is open access and can be found here.

Seismological Processing of Six Degree-of-Freedom Ground-Motion Data
David Sollberger, Heiner Igel,Cedric Schmelzbach, Pascal Edme, Dirk-Jan van Manen, Felix Bernauer, Shihao Yuan, Joachim Wassermann, Ulrich Schreiber, and Johan O. A. Robertsson
Sensors 20(23) 6904 (2020)
https://doi.org/10.3390/s20236904

Abstract: 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.
This article is open access and can be found here.

Exploring planets and asteroids with 6DoF sensors: Utopia and realism
Felix Bernauer, Raphael F. Garcia, Naomi Murdoch, Veronique Dehant, David Sollberger, Cedric Schmelzbach, Simon Stähler, Joachim Wassermann, Heiner Igel, Alexandre Cadu, David Mimoun, Birgit Ritter, Valerio Filice, Özgür Karatekin, Luigi Ferraioli, Johan O. A. Robertsson, Domenico Giardini, Guillaume Lecamp, Frederic Guattari, Jean‑Jacques Bonnefois and Sebastien de Raucourt
Earth, Planets and Space 72, Article number: 191 (2020)
https://doi.org10.1186/s40623-020-01333-9

Abstract: A 6 degrees‑of‑freedom (6DoF) sensor, measuring three components of translational acceleration and three compo‑nents of rotation rate, provides the full history of motion it is exposed to. In Earth sciences 6DoF sensors have shown great potential in exploring the interior of our planet and its seismic sources. In space sciences, apart from naviga‑tion, 6DoF sensors are, up to now, only rarely used to answer scientific questions. As a first step of establishing 6DoF motion sensing deeper into space sciences, this article describes novel scientific approaches based on 6DoF motion sensing with substantial potential for constraining the interior structure of planetary objects and asteroids. Therefore we estimate 6DoF‑signal levels that originate from lander–surface interactions during landing and touchdown, from a body’s rotational dynamics as well as from seismic ground motions. We discuss these signals for an exemplary set of target bodies including Dimorphos, Phobos, Europa, the Earth’s Moon and Mars and compare those to self‑noise levels of state‑of‑the‑art sensors.
This article is open access and can be found here.

Dynamic Tilt Correction Using Direct Rotational Motion Measurements
Felix Bernauer, Joachim Wassermann, and Heiner Igel
Seismological Research Letters (2020) 91 (5): 2872–2880
https://doi.org/10.1785/0220200132

Abstract: Inertial sensors like seismometers or accelerometers are sensitive to tilt motions. In general, from pure acceleration measurements, it is not possible to separate the tilt acceleration from the translational ground acceleration. This can lead to severe misinterpretation of seismograms. Here, we present three different methods that can help solving this problem by correcting translational records for dynamic tilt induced by ground deformation with direct measurements of rotational motions: (1) a simple time-domain method, (2) a frequency-domain method proposed by Crawford and Webb (2000) using a coherence-weighted transfer function between rotation and acceleration, and (3) an adapted frequency-domain method that corrects only those parts of the spectrum with coherence between translational acceleration and rotation angle higher than 0.5. These three methods are discussed in three different experimental settings: (1) a reproducible and precisely known laboratory test using a high-precision tilt table, (2) a synthetic test with a simulated volcanic very-long-period event, and (3) a real data set recorded during the 2018 Mt. Kīlauea caldera collapse. All the three test cases show severe influence of tilt motion on the acceleration measurements. The time-domain method and the adapted frequency-domain method show very similar performance in all three test cases. Those two methods are able to remove the tilt component reliably from the acceleration record.
This article features a video abstract, accessible here.