VELOCITY STRUCTURE AT THE SOURCE PHYSICS EXPERIMENT SITE REVEALED BY LARGE-N ARRAYS

The Source Physics Experiment (SPE) is a multi-institutional, multi-disciplinary project that consists of a series of chemical explosions conducted at the Nevada National Security Site (NNSS). The goal of SPE is to understand the complicated effect of geological structures on seismic wave propagation and source energy partitioning, develop and validate physics-based modeling, and ultimately better monitor nuclear explosions. The SPE program is multi-phase with the completion of Phase I located in hard rock (granite) in 2017 and the initiation of Phase II (DAG – Dry Alluvium Geology) located in weak rock (alluvium) in 2018. A large-N seismic array was deployed at the SPE Phase II site to image the full 3-D wavefields from planned explosions. The large-N seismic array consists of 500 three-component geophones and covers an area of approximately 2 x 2 km. This array recorded multiple chemical explosions (DAG-2, DAG-3, planned DAG-4) with different yields and depths. For each explosion experiment, the large-N array also recorded weeks of continuous data before and after the explosion. The continuous records include ambient noise, aftershocks, and earthquakes. In this work, we use the recorded data by the DAG large-N array to study the velocity structure at the SPE phase II site. Our body-wave analyses reveal distinct geological layers of alluvium, volcanic tuff and Paleozoic sedimentary rocks. Variations of geological structure in 3D are observed and will be incorporated into the large modeling efforts to improve our understanding of seismic characteristics from explosions.