EFFECT OF GEOLOGIC SETTING ON LOCAL RESPONSE TO CHEMICAL EXPLOSIVE

Phase I of the Source Physics Experiment (SPE) is a series of buried chemical explosions in granite. The experiment includes a comprehensive set of measurements including near-source accelerometers, unmanned aerial system (UAS)-borne photogrammetry, and detailed pre- and post-test surface crack mapping. Data analysis is supported by numerical modeling with the goal of better understanding the response of the local geologic setting to explosive loading and identifying potential sources of shear wave generation.

Inspection of near-source velocity traces reveals that non-radial components are initially quiescent as expected for a near-spherical source. However, immediately following the peak radial motion, the non-radial components display a sudden surge to significant amplitude, providing a candidate for generation of shear energy. We propose that this response reflects a build-up of strain on the pre-existing fractures during initial loading. After peak stress, unloading results in an extensional state, allowing release of the stored shear strain. We support this hypothesis with high-fidelity, explicitly-jointed finite element simulations.

We provide evidence to indicate that the geologic setting controls the direction of this release. First, the horizontal slip direction is generally subparallel with the direction of minimum regional horizontal stress. Second, we would expect explosion-induced surface cracking to occur in a radial pattern about the source hole. Instead, post-test surveys indicate that new cracks form in a direction preferentially normal to the slip direction described above suggesting tensile failure related to a general mass movement in the fracture slip direction. Finally, differencing of pre- and post-test photogrammetry data show that horizontal surface displacement aligns with the direction implied by the velocity data. LA-UR-17-24982