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Evaluating Macro and Microscopic Rock Damage from Explosions and the Effects on Shear Wave Generation

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Final rept. 19 Apr 2010-23 Jun 2014

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In July 2008, five small chemical explosions were detonated in low fracture density, homogeneous, yet transversely isotropic, granite in Barre, VT. The physical properties of the granite were measured before and after the explosions on cores recovered in the vicinity of each shot. The changes in the post-shot properties were used to quantify damage at the micro, meso, and macro scales. Near the emplacement, the rock was highly pulverized and granulated. Outward and above the emplacement level, the granite was characterized by high angle fractures parallel to the rift. This zone transitioned to a mostly intact matrix with few randomly oriented fractures out to the elastic region. At the microscale, damage was quantified by measuring the differences in ultrasonic wave speeds between the pre- and post-shot values on specimens of intact matrix rock. Near the working point, the P wave velocity decreased up to 12. Damage at all scales was greatest above the emplacement level of the charge and parallel to the trend of the rift. The shapes of the zones of equal damage were non-spherical. Weston Geophysical Corp. and New England Research, Inc. conducted the field phase of the follow-up New England Damage Experiment NEDE2 during June 2013 in the same granite quarry. A primary objective remained to identify possible sources of shear wave generation resulting from rock damage. We found that an increase in velocity of detonation results in increased seismic corner frequency and high frequency amplitudes and a slight decrease in the low-frequency amplitudes, although host rock water saturation affects this as well. The observed S wave radiation pattern is consistent with radiation due to opening of tensile fractures approximately parallel to the host rock rift direction. The presence of surface fractures for black powder and Composition B shots correlates with an increase in SP amplitudes for these shots.

Subject Categories:

  • Ammunition and Explosives
  • Acoustics
  • Mechanics
  • Radiofrequency Wave Propagation

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