Where do Underground Explosion Wavefields Fully Transition to Purely Elastic?

Most source models describing explosions consider two regimes: a near-source regime where shock waves and high-strain rate processes play a major role, and a far-field regime which is governed by pure elastic theory. The representation theorem is often used to connect the two regimes. Consider a surface surrounding an explosive source where the time-history of the wavefield is recorded. This theory states that the recorded information can be used to fully determine the evolution of the wavefield in the remaining full space, allowing one to replace the source volume with a pure elastic representation of the signal. Most seismic source models of explosions project or regress the wavefield at that surface onto a specific mathematical functional basis. For example, the model of Denny & Johnson (1992) uses the Reduced Displacement Potential (RDP) functions. In order for the representation theorem to be valid, the surface used to define the source must be in the elastic domain.

The definition of the elastic radius is the distance from the source at which the shockwave has entirely transitioned to an elastic wave. However, in numerical modeling, different criteria are used to assess where this transition occurs. We perform modeling of SPE-4Prime, an underground chemical explosion of the Source Physics Experiment (SPE, Snelson et al. 2013), which was designed to be as close as possible to an ideal explosion. We used the Hybrid Optimization Software Suite (HOSS), a combined finite-discrete element method code developed at LANL, to model the near-source and elastic regimes. We assess the elastic radius as the distance at which the RDP functions stabilize and compare it to commonly used plastic strain thresholds. HOSS can model the formation of the cavity and allow us to explore the scaling between cavity radius and elastic radius for several scenarios of underground explosions. The representation theorem is also fundamental to coupling between numerical methods where seismic codes take-over hydrodynamic modeling to remote monitoring distances. We couple HOSS to SPECFEM3D using a series of sensors at a certain distance from the source. We vary the distance at which the coupling is performed to propose an independent assessment of the elastic distance.

This is LANL release number LA-UR-22-27704.