On DAS Recorded Strain Amplitude
Description:
Distributed Acoustic Sensing (DAS) has become popular for recording seismic waves in recent years. The ability to record waveforms with useful phase information has been demonstrated in numerous studies. Validation of the sensitivity factor with respect to strain is commonly done by application of fiber stretching-devices in laboratory environment. The lack of in-situ calibration with respect to actual rock strain leaves room for unknown amplitude loss caused by imperfect coupling of the cable jacket to the rock and the loss through several layers of coating of the fiber in the cable. We use DAS-recordings together with recordings of the colocated invar-wire strainmeter array at BFO and an STS-2 broad-band seismometer to investigate the coupling of DAS fibers in situ. Two types of optical fiber cables are installed in parallel. One is a conventional telecommunication cable, where fibers are embedded in a gel-type substance to avoid damage potentially caused by straining the cable jacket. The other is a dedicated tight-buffered DAS cable. We compare recordings from a section where the cables are simply unreeled with recordings from a section, where sandbags are densely put on the. We use signals from the Mw 7.7 and 7.6 East Anatolian Fault earthquakes on February 6th 2023. A comparison of signals from all three instruments is possible in the frequency-band between 50 mHz and 0.2 Hz. At lower frequencies the DAS signal-to-noise ratio is insufficient. At higher frequencies the invar-wire strainmeters show a parasitic response to vertical ground motion. We find that the DAS fibers do not pick up the full amplitude of rock-strain. Amplitude loss depends on the type of cable. The tight-buffered and the telecommunication cable pick up about 1/2 and 1/4 of the amplitude, respectively. Surprisingly the application of sandbags has little effect. Both sections show similar discrepancy in amplitude. The signal loss apparently is not due to the imperfect coupling of the cable jacket to the rock, but due to reduced internal coupling of the fiber to the cable.
Session: Advancing Seismology with Distributed Fiber Optic Sensing - III
Type: Oral
Date: 5/3/2024
Presentation Time: 02:15 PM (local time)
Presenting Author: Andreas
Student Presenter: No
Invited Presentation:
Authors
Thomas Forbriger thomas.forbriger@kit.edu Karlsruhe Institute of Technology |
Nasim Karamzadeh nasim.karamzadeh@uni-muenster.de University of Munster |
Jerome Azzola jerome.azzola@kit.edu Karlsruhe Institute of Technology |
Rudolf Widmer-Schnidrig widmer@gis.uni-stuttgart.de University of Stuttgart |
Emmanuel Gaucher emmanuel.gaucher@kit.edu Karlsruhe Institute of Technology |
Andreas Rietbrock Presenting Author Corresponding Author rietbrock@kit.edu Karlsruhe Institute of Technology |
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On DAS Recorded Strain Amplitude
Category
Advancing Seismology with Distributed Fiber Optic Sensing