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Modeling Source Physics Explosions Recorded on a Distributed Fiber Optic Sensor

Session: Explosion Seismology Applications and Advances II

Type: Oral

Date: 4/19/2021

Presentation Time: 03:00 PM Pacific

Description: 

Fiber optic distributed acoustic sensors (DAS) are becoming a widely used tool for seismic sensing. We model waveforms of two subsurface chemical explosions at depths of 385 and 150 m, each of which was about one metric ton TNT equivalent, that were recorded from a helical fiber (30°) installed in two boreholes 80 m away from the source location. The boreholes were 450 and 390 m in depth and were equipped with both a vertical fiber cable and accelerometers at multiple levels. The fiber sensors remain on scale and several phases including the initial P wave, a weak S wave, and a downgoing reflected P wave are observed. The fiber waveforms are compared using cross-correlation with the downhole accelerometer records. The best match was achieved between the vertical accelerometer component and the DAS strainrate data after converting from strainrate to acceleration. We had difficulty matching the exact amplitudes between the fiber and the accelerometers using the nominal transfer function for the fiber data and used an empirical factor. We estimate a velocity model using arrival times measured from the fiber and then use this velocity model to generate synthetic strain-rate seismograms using both wavenumber and finite difference algorithms. A shallow low-velocity layer which may be a transient feature caused by spall was necessary to match the timing of the downgoing reflected wave. The strainrate synthetics matched the observed DAS data waveform in shape. The helical fiber appears to be sensitive to both P and S waves. We find that the DAS data can be modelled successfully to provide insight into the wavefield generation and propagation. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525. LLNL publication LLNL-ABS-818077. LANL publication LA-UR-21-20078.

Presenting Author: Robert Mellors

Student Presenter: No

Authors