Monitoring Underground Nuclear Explosions PDF Download

This paper describes new research being performed to improve understanding of seismic waves generated by underground nuclear explosions (UNE) by using full waveform simulation, high-performance computing and three-dimensional (3D) earth models. The goal of this effort is to develop an end-to-end modeling capability to cover the range of wave propagation required for nuclear explosion monitoring (NEM) from the buried nuclear device to the seismic sensor. The goal of this work is to improve understanding of the physical basis and prediction capabilities of seismic observables for NEM including source and path-propagation effects. We are pursuing research along three main thrusts. Firstly, we are modeling the non-linear hydrodynamic response of geologic materials to underground explosions in order to better understand how source emplacement conditions impact the seismic waves that emerge from the source region and are ultimately observed hundreds or thousands of kilometers away. Empirical evidence shows that the amplitudes and frequency content of seismic waves at all distances are strongly impacted by the physical properties of the source region (e.g. density, strength, porosity). To model the near-source shock-wave motions of an UNE, we use GEODYN, an Eulerian Godunov (finite volume) code incorporating thermodynamically consistent non-linear constitutive relations, including cavity formation, yielding, porous compaction, tensile failure, bulking and damage. In order to propagate motions to seismic distances we are developing a one-way coupling method to pass motions to WPP (a Cartesian anelastic finite difference code). Preliminary investigations of UNE's in canonical materials (granite, tuff and alluvium) confirm that emplacement conditions have a strong effect on seismic amplitudes and the generation of shear waves. Specifically, we find that motions from an explosion in high-strength, low-porosity granite have high compressional wave amplitudes and weak shear waves, while an explosion in low strength, high-porosity alluvium results in much weaker compressional waves and low-frequency compressional and shear waves of nearly equal amplitude. Further work will attempt to model available near-field seismic data from explosions conducted at NTS, where we have accurate characterization of the sub-surface from the wealth of geological and geophysical data from the former nuclear test program. Secondly, we are modeling seismic wave propagation with free-surface topography in WPP. We have model the October 9, 2006 and May 25, 2009 North Korean nuclear tests to investigate the impact of rugged topography on seismic waves. Preliminary results indicate that the topographic relief causes complexity in the direct P-waves that leads to azimuthally dependent behavior and the topographic gradient to the northeast, east and southeast of the presumed test locations generate stronger shear-waves, although each test gives a different pattern. Thirdly, we are modeling intermediate period motions (10-50 seconds) from earthquakes and explosions at regional distances. For these simulations we run SPECFEM3D{_}GLOBE (a spherical geometry spectral element code). We modeled broadband waveforms from well-characterized and well-observed events in the Middle East and central Asia, as well as the North Korean nuclear tests. For the recent North Korean test we found that the one-dimensional iasp91 model predicts the observed waveforms quite well in the band 20-50 seconds, while waveform fits for available 3D earth models are generally poor, with some exceptions. Interestingly 3D models can predict energy on the transverse component for an isotropic source presumably due to surface wave mode conversion and/or multipathing.