We present a mechanism for shear wave generation from buried explosions as part of the National Nuclear Security Administration (NNSA) Source Physics Experiment (SPE) series. SPE fielded various sized cylinder-shaped sensitized heavy ammonium nitrate/fuel oil (SHANFO) sources grouted into a borehole in the jointed Climax stock granite and detonated. A highfidelity site model was included in a numerical simulation that mimics the near field velocity environment measured by an array of in-ground accelerometers. This approach was accommodated through a coupled Euler-Lagrange (CEL) code that allows simultaneous solving of an Eulerian domain to model the high-deformation source region and a Lagrange domain that includes the complex geology with full contact. Specific laboratory-measured geomechanical properties for the rock and the joint sets were included in the model. The simulations compare favorably to the data and provide a possible physical mechanism for unexpected shear motion through the release of stored shear strain on the joints. This research will advance our understanding of explosion generated shear wave energy from low yield nuclear tests. This paper has been approved for unlimited release under LA-UR-15- 21405.
One of the most important phenomena that remains to be understood in the nuclear verification community is how buried explosions generate shear waves. Possible sources for horizontal shear energy generated by explosive sources have been postulated [1, 2], and widely accepted underground nuclear explosion source models currently exist [e.g., 3, 4]. These models are based on a spherical source that couples at some radius to the elastic earth. In more recent models shear is accommodated by invoking a damage mechanism [2, 5]. A shortcoming is that the exact phenomenological mechanism for a specific test site is not included in these models. The analysis of the near-field Source Physics Experiment (SPE) data described herein proposes a mechanism for shear waves in a jointed rock mass.
It has long been observed that near source effects from underground nuclear explosions generate shear waves [1, 2, 6] and that the magnitude and spectra of these shear waves can be used to help discriminate nuclear explosions from earthquakes [7, 8]. Recent experiments in granite have been conducted to investigate the near source dynamics of buried explosions – including detonation velocity, fracture, and spall – and attempt to account for shear content in the received waveforms [9, 10].