Modeling Subsurface Explosions Recorded on a Distributed Fiber Optic Sensor. Issue 12 (28th November 2021)

• Record Type: Journal Article
• Authors: Mellors, Robert J.; Abbott, Robert; Steedman, David; Podrasky, David; Pitarka, Arben
• Is Part Of: Journal of geophysical research. Volume 126:Issue 12(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-11-28
• Subjects: distributed acoustic sensing -- explosion monitoring -- strong motion; Geomagnetism -- Periodicals; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 551.1
• DOI: 10.1029/2021JB022690
• Languages: English
• ISSNs: 2169-9313

Abstract

Abstract: Fiber optic distributed acoustic sensors (DAS) are becoming a widely used tool for seismic sensing. Here we examine recordings of two subsurface chemical explosions, DAG‐1 and DAG‐3, each of which was about one metric ton (TNT equivalent), that were recorded from a helical fiber installed in two boreholes 80 m away from the source location. Several clear phases including the initial P wave, a weak S wave, and a surface reflected P wave are observed on the helical DAS data. We estimate a velocity model using arrival times measured from the fiber. The DAS waveform data were compared with colocated accelerometers at specific depths in both frequency and time domains. The spectra of the DAS data matched spectra estimated from the accelerometer records. Comparisons of observed waveform shape between the accelerometer records and the fiber measurements (strain‐rate) show reasonable agreement except for the data near the event depth. The DAS data and the accelerometer agreed in relative amplitudes but we had difficulties in matching absolute amplitudes, possibly due to errors in metadata. Synthetic strain‐rate waveforms were calculated using a 2D wavenumber algorithm and matched the waveform shape and relative amplitudes. In general, DAS is effective at recording strong ground motions at high spatial density. Comparison of the synthetic seismograms with observed data indicate that the waveforms are not consistent with a pure isotropic explosion source and that the observed S waves originate from very near the source region. Plain Language Summary: Recently, fiber optic seismometers have become a revolutionary development in measuring seismic waves. Normally, seismic waves are measured using seismometers, which are sensitive instruments that must be carefully installed. Using a new technique named distributed acoustic sensing, the fiber itself is used to measure seismic waves traveling in the ground, making thousands of measurements simply by sending laser pulses from one end of the cable and carefully analyzing reflections from tiny imperfections embedded in the optical fiber. Due to the size and robustness, the fiber optic sensors are well suited for boreholes. In this paper, we study seismic waves from two subsurface chemical explosions as recorded by fiber seismic sensors in two nearby boreholes. The data from the optical sensors are comparable to the data from the standard seismic sensors that were installed in the same borehole even with the high‐amplitude seismic waves produced by the nearby chemical explosion. We see both P and S waves using the new sensors, which provides information about the source. Key Points: Distributed fiber optics sensors are effective at recording strong ground motions from explosions Strain‐rate waveforms can be modeled using standard seismic modeling codes S waves are observed in the fiber sensors data from two subsurface explosions and appear to originate from the source region