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.
Highlights
Rotation and strain, which are the antisymmetric and symmetric part of the wavefield displacement gradient tensor, respectively, are moving increasingly into the focus of geophysical research
We analyze the instrument self-noise levels, characterize waveform similarity from recordings from the huddle test explosion “expl2” that happened at 15:16:43 UTC on 19 November 2019, study signal-to-noise ratios within the frequency range of highest coherency, and compare selected recordings to a median reference waveform
Different reference clocks and time synchronisation methods that were used in the various recording units, as well as causal low-pass filtering prior to digital sampling decimation and data archiving, give reasons for these time shifts. blueSeis-3A sensors retrieve the absolute time from Global Navigation Satellite System (GNSS) and continuously synchronize the recorder time to a highly accurate pulse per second (PPS) signal
Summary
Rotation and strain, which are the antisymmetric and symmetric part of the wavefield displacement gradient tensor, respectively, are moving increasingly into the focus of geophysical research. For the relative calibration of rotation sensors in a huddle test, a reliable and well known reference sensor is missing Another possibility for accessing the reliability of ground rotation measurements is the comparison with finite differences being obtained from classical seismometer arrays [43,44,45], which is only valid for a limited frequency range. In order to characterize the instrument response of fiber-optic cables used for distributed acoustic sensing, Wang et al [46] and Lindsey et al [47] co-located fiber-optic DAS-arrays with conventional seismometers and compared the direct strain measurement to the strain that was obtained by finite differencing of two seismometer waveforms In these studies, the observations match well for signal periods from 10 s to 120 s. We access waveform similarity for co-located recordings of an explosion by means of time-frequency analysis
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
