Dense Array Seismic Study of a Legacy Underground Nuclear Test at the Nevada National Security Site

The 1964 nuclear test HADDOCK is characterized by a post-detonation collapse that did not reach the surface. Underground nuclear explosions generally form a vertical rubble-filled chambers known as a chimneys. Compared to explosions where the chimney forms all the way to the surface, nuclear explosions that have little or no surface manifestation are significantly harder to study intact. Understanding the subsurface structure of partially collapsed explosions has significant national security implications as well as applications in mining and construction. Here, we present P-wave tomography and seismic reflection processing results from an active source survey in Yucca Flat, on the Nevada National Security Site. To constrain chimney architecture of the HADDOCK test, the team acquired 6 km of 2-D and 3-D seismic reflection data, recording over 270 survey shots from the Seismic Hammer TM (a 13,000-kg accelerated weight drop) and 65 survey shots from a smaller accelerated weight drop. The hybrid 2-D and 3-D survey geometry consisted of two 2 km-long orthogonal lines, each with 401 receivers at 5 m spacing and a dense grid of fourteen 140 m long lines with an inline and crossline spacing of 10 m. The instruments were cable-less 3C 2 Hz nodes deployed for 1 month. Over 250,000 first break picks were used for P-wave tomography. The initial inversion results suggest a low-velocity anomaly at the depth of burial ( 364 m), a low-velocity anomaly at 200 m depth, which is likely the residual cavity, and a high-velocity anomaly at 50 m depth which is likely due to spall-related compaction. High resolution seismic images from ongoing reflection seismic processing will be used to confirm these results and to further constrain chimney architecture. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-NA0003525.