Non-Linear Fracture Mechanics Model for Spallation and Coupling of NuclearExplosions Detonated in Hard Rock

摘要

Fracture heterogeneity is known to cause non-linear effects near the source ofunderground explosions. Preexisting cracks nucleate new fractures which produce significant deviations from numerical computer simulations which assume a simply connected elastic continuum. Such effects were found to he important when computer models of nuclear explosions in granite failed to simulated the 'pulse broadening' observed in the seismic signatures. In order to include the effects of crack growth in computer models, we have developed a micromechanical damage mechanics and incorporated it into two source models: the effective medium source model recently developed by Lane Johnson and the more traditional finite difference model used by the S-Cubed group at Maxwell Labs in San Diego. These models make testable predictions of how the waveform of radiated seismic energy depends on the depth of burial, the presence of ground water, and the size and density of preexisting fractures. They also predict the extend of damage which can be verified using extensive data sets on the density and morphology of the near-source fracture sets generated by several large chemical and nuclear explosions detonated in crystalline rock in the Soviet union. An unexpected result of these source models is the generation of significant secondary high-frequency P and S radiation by the damage could effect source detection and discrimination algorithms.