Simulation of Underground Chemical Explosions in Anisotropic Dry Alluvium Geology using Hydrodynamic Generated Source Coupled to Far-Field Linear Anisotropic Wave propagation

Abstract:

The Source Physics Experiment (SPE) is an ongoing effort to improve explosion monitoring by conducting a controlled series of chemical explosions at the Nevada National Security Site (NNSS) and using the resulting observations to improve and validate physics-based simulations of explosion phenomena. Phase I of SPE was conducted on the Climax Stock granite which contains a network of well-characterized joints. It has been shown through hydrodynamic source modeling that sliding on these pre-existing joints may be responsible for a large amount of the tangential motion observed during SPE. Near-field motions generated with hydrodynamic non-linear source models have been coupled to elastic wave propagation codes to propagate these resulting motions into the far-field domain which is assumed to be elastic. However, one simplification that has been made is that the far-field elastic media is isotropic. This is likely not the case as the network of pre-existing joints also continues outside of the inelastic source region of the SPE and can be shown to result in an anisotropic stiffness for the granite.

In the present study we use a hybrid modeling approach with one-way anisotropic hydrodynamic-to-elastic coupling. Near source fully anisotropic hydrodynamic motions are computed using GEODYN-L while anisotropic elastic wave propagation is modeled using SW4, a fourth order finite difference code. Motions are coupled between the two codes by introducing hydrodynamic motions from GEODYN-L as a two-shell internal boundary source to SW4. The anisotropic material model employed in the SW4 domain is derived from the properties of an observed fracture network with relatively well-constrained joint orientations, spacing, and stiffnesses. We show that consideration of anisotropic material in the elastic regime has an important effect on the propagation of tangential motion. Propagation of motions generated in an anisotropic source region into an isotropic far-field domain will introduce some biases. The current GEODYN-L/SW4 coupling is being applied to model Phase II of SPE, conducted in a dry alluvium geology (DAG).