Project Description:(Click to expand)
Seismology has historically been a key method for verifying nuclear test detonations. A nuclear detonation's characteristic seismic "signature" is distinct from that of an earthquake; based upon seismic data from numerous instances of either event, a dividing line has been empirically established. This line demarcates earthquakes, which usually generate strong shearing S-waves, from detonations, whose energy is mostly converted into heat and compressional P-waves. However, recent nuclear test detonations executed by the Democratic People's Republic of Korea (DPRK) are poorly characterized by this model, leading to concern over "false negatives" and highlighting the need for a more physics-based event differentiation model. The National Nuclear Security Administration (NNSA) is seeking to create such a model using data from their Source Physics Experiment (SPE). SPE consists of a series of chemical detonations of varying yields emplaced in different media at various depths. SPE shots are executed at the Nevada National Security Site (NNSS, formerly Nevada Test Site), an area where over 900 nuclear detonations occurred between 1951 and 1992. The data from SPE will be used to build a more reliable and physics-based model that more thoroughly accounts for source and medium properties. Recently, project scientists determined that imaging the subsurface of the explosion area prior to the next series of SPE shots would better inform the physical models applied to the SPE data. In 2015, members of a team from Sandia National Laboratories, under project THOR, acquired exquisite active-source seismic data for Yucca Flat, Nevada, the chosen location for the next phase of SPE shots. The source was a 13,000-kilogram modified industrial pile driver called the Seismic Hammer™. This summer I will be performing surface-wave group velocity tomography on THOR data to construct a shear-wave velocity model of the subsurface of Yucca Flat. Surface-wave phase velocity tomography was previously employed in an attempt to obtain a shear-wave velocity model; the results of this approach were significantly distorted in areas where seismic ray paths were proximate to former nuclear detonation sites, resulting in an incomplete velocity model. We hope to obtain higher-quality results using group velocities, leveraging the assumption that - compared to individual phases - the propagation of wave packets is less impacted by the fractured, disrupted media surrounding former detonation areas. The resulting shear-wave velocity model will be integrated with other tomographic results to create a complete model of the subsurface structure of Yucca Flat that can be utilized in the physical modeling of SPE data.