Detection of explosive events by monitoring acoustically-induced geomagnetic perturbations

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The Black Thunder Coal Mine (BTCM) near Gillette, Wyoming was used as a test bed to determine the feasibility of detecting explosion-induced geomagnetic disturbances with ground-based induction magnetometers. Two magnetic observatories were fielded at distances of 50 km and 64 km geomagnetically north from the northernmost edge of BTCM. Each observatory consisted of three separate but mutually orthogonal magnetometers, Global Positioning System (GPS) timing, battery and solar power, a data acquisition and storage system, and a three-axis seismometer. Explosions with yields of 1 to 3 kT of TNT equivalent occur approximately every three weeks at BTCM. We hypothesize that explosion-induced ... continued below

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Lewis, J P; Rock, D R; Shaeffer, D L & Warshaw, S I October 7, 1999.

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The Black Thunder Coal Mine (BTCM) near Gillette, Wyoming was used as a test bed to determine the feasibility of detecting explosion-induced geomagnetic disturbances with ground-based induction magnetometers. Two magnetic observatories were fielded at distances of 50 km and 64 km geomagnetically north from the northernmost edge of BTCM. Each observatory consisted of three separate but mutually orthogonal magnetometers, Global Positioning System (GPS) timing, battery and solar power, a data acquisition and storage system, and a three-axis seismometer. Explosions with yields of 1 to 3 kT of TNT equivalent occur approximately every three weeks at BTCM. We hypothesize that explosion-induced acoustic waves propagate upward and interact collisionally with the ionosphere to produce ionospheric electron density (and concomitant current density) perturbations which act as sources for geomagnetic disturbances. These disturbances propagate through an ionospheric Alfven waveguide that we postulate to be leaky (due to the imperfectly conducting lower ionospheric boundary). Consequently, wave energy may be observed on the ground. We observed transient pulses, known as Q-bursts, with pulse widths about 0.5 s and with spectral energy dominated by the Schumann resonances. These resonances appear to be excited in the earth-ionosphere cavity by Alfven solitons that may have been generated by the explosion-induced acoustic waves reaching the ionospheric E and F regions and that subsequently propagate down through the ionosphere to the atmosphere. In addition, we observe late time (> 800 s) ultra low frequency (ULF) geomagnetic perturbations that appear to originate in the upper F region ({approximately}300 km) and appear to be caused by the explosion-induced acoustic wave interacting with that part of the ionosphere. We suggest that explosion-induced Q-bursts may be discriminated from naturally occurring Q-bursts by association of the former with the late time explosion-induced ULF perturbations. We also present evidence for an acoustically-induced magnetic signal at both magnetic observatories, indicating that magnetometers act as highly sensitive detectors of acoustically-induced ground motion. Further experimental and theoretical work are required to improve confidence in these conclusions.

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20254 Kilobytes pages

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  • Other Information: PBD: 7 Oct 1999

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  • Report No.: UCRL-ID-133240
  • Report No.: DP0101017
  • Report No.: 97-ERD-003
  • Grant Number: W-7405-ENG-48
  • DOI: 10.2172/12544 | External Link
  • Office of Scientific & Technical Information Report Number: 12544
  • Archival Resource Key: ark:/67531/metadc621360

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  • October 7, 1999

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  • June 16, 2015, 7:43 a.m.

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  • May 6, 2016, 1:12 p.m.

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Lewis, J P; Rock, D R; Shaeffer, D L & Warshaw, S I. Detection of explosive events by monitoring acoustically-induced geomagnetic perturbations, report, October 7, 1999; California. (digital.library.unt.edu/ark:/67531/metadc621360/: accessed September 23, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.