NUMERICAL SIMULATIONS OF UNDERGROUND EXPLOSIONS: EFFECT OF JOINTS NEAR THE SOURCE ON ENERGY COUPLING AND SHEAR MOTIONS GENERATION

We have performed 3D high resolution simulations of underground explosions conducted recently in jointed rock outcrop as part of the Source Physics Experiment (SPE). The main goal is to understand the nature of the shear motions recorded in the near field at depth. Several hypotheses have been proposed to explain the genesis of shear motions: 1) sliding on the joints, 2) wave conversion at the material boundaries and 3) non spherisity of the source. We suggest yet another mechanism to be responsible for some shear wave generation when the cracks or joints are present in the rock mass containing the source and the explosive products find their way into the cracks. In order to investigate this mechanism, we have conducted several high resolution simulations of the source region using an Eulerian hydrodynamic code GEODYN. We explored the effect of joint orientations, number of family of fracture, energy deposition, joint aperture size, and joint spacing on the overall development of the source itself, sustained damage around the source and shear wave polarization and motions in the vicinity of the source. We have observed that waves interact with the joints and refraction and diffraction of the wave intensify the complexity of the wave field. It is worth noting that the fracture network topology has also dramatically been affected. It is expected that after the pressure has been released and the energy has been dissipated that source cavity may shrink to a different size but will sustain considerable irreversible damage which affect subsequent shots if they were to be conducted in the vicinity or at the same depth. Fracture network connectivity has drastically changed which will affect wave motions and flow of gases. We will present recent 2D and 3D simulations of typical settings for SPE experiments and discuss the most likely scenario that took place during the SPE campaign.

This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.