Uncertainty Propagation and Stochastic Interpretation of Shear Motion Generation due to Underground Chemical Explosions in Jointed Rock

We have performed 3D simulations of underground chemical explosions conducted recently in granitic outcrop as part of the Source Physics Experiment (SPE) campaign. The main goal of these simulations is to understand the nature of the shear motions recorded in the near field considering uncertainties in a) the geological characterization of the joints, such as density, orientation and persistency and b) the geomechanical material properties, such as, friction angle, bulk sonic speed, poroelasticity etc. The approach is probabilistic; joints are depicted using a Boolean stochastic representation of inclusions conditional to observations and their probability density functions inferred from borehole data. Then, using a novel continuum approach, joints and faults are painted into the continuum host material, granite. To ensure the fidelity of the painted joints we have conducted a sensitivity study of continuum vs. discrete representation of joints. Simulating wave propagation in heterogeneous discontinuous rock mass is a highly non-linear problem and uncertainty propagation via intrusive methods is practically forbidden. Therefore, using a series of nested Monte Carlo simulations, we have explored and propagated both the geological and the geomechanical uncertainty parameters. We have probabilistically shown that significant shear motions can be generated by sliding on the joints caused by spherical wave propagation. Polarity of the shear motion may change during unloading when the stress state may favor joint sliding on a different joint set. Although this study focuses on understanding shear wave generation in the near field, the overall goal of our investigation is to understand the far field seismic signatures associated with shear waves generated in the immediate vicinity of an underground explosion. Therefore, we have abstracted the near field behavior into a probabilistic source-zone model which is used in the far field wave propagation.

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