The Source Physics Experiments Large N Array

To improve the understanding of S-wave generation from an explosion, a temporary deployment of 996 geophones, including both one-component (Z) and three-component sensors (3C), was installed from 15 April to 23 May 2016 at the Nevada National Security Site (NNSS). Sensor spacing varied from 25 to 100 m and consisted of 500 Z and 496 3C 5-Hz geophones. Data were continuously recorded during the deployment at low gain (0 dB) from 15 April to 28 April and high gain (36 dB) from 29 April to 23 May. A buried (76.5 m depth) 5035 kg trinitrotoluene (TNT) equivalent chemical explosion (Source Physics Experiments [SPE]-5) was recorded on 26 April. It was situated in a weathered granite body surrounded by volcanic tuffs, Paleozoic carbonates, and alluvium. The array was deployed similar to 400-3000 m from the explosion. A set of large weight drop shots (13,000 kg source) at 53 locations both inside and outside the geophone array were also recorded, as were local, regional, and teleseismic earthquakes. Data recovery was good, with 95% of data recovered from the chemical explosion and up to 99% in the following weeks, including both the weight drop shots campaign and the continuous data. Important initial results from the deployment include estimates of the spatial correlation length of velocity heterogeneities and a higher resolution velocity model. Observations of the data and synthetics indicate that some far-field (elastic) S-wave energy is generated by scattering and conversion outside the near-field (inelastic) region. Interferometric processing was conducted on a Hadoop big-data cluster.