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In preparation for the next phase of the Source Physics Experiments, we acquired an active-source seismic dataset along two transects totaling more than 30 km in length at Yucca Flat, Nevada, on the Nevada National Security Site. Yucca Flat is a sedimentary basin which has hosted more than 650 underground nuclear tests (UGTs). The survey source was a novel 13,000 kg modified industrial pile driver. This weight drop source proved to be broadband and repeatable, richer in low frequencies (1-3 Hz) than traditional vibrator sources and capable of producing peak particle velocities similar to those produced by a 50 kg explosive charge. In this study, we performed a joint inversion of P-wave refraction travel times and Rayleigh-wave phase-velocity dispersion curves for the P- and S-wave velocity structure of Yucca Flat. Phase-velocity surface-wave dispersion measurements were obtained via the refraction microtremor method on 1 km arrays, with 80% overlap. Our P-wave velocity models verify and expand the current understanding of Yucca Flat's subsurface geometry and bulk properties such as depth to Paleozoic basement and shallow alluvium velocity. Areas of disagreement between this study and the current geologic model of Yucca Flat (derived from borehole studies) generally correlate with areas of widely spaced borehole control points. This provides an opportunity to update the existing model, which is used for modeling groundwater flow and radionuclide transport. Scattering caused by UGT-related high-contrast velocity anomalies substantially reduced the number and frequency bandwidth of usable dispersion picks. The S-wave velocity models presented in this study agree with existing basin-wide studies of Yucca Flat, but are compromised by diminished surface-wave coherence as a product of this scattering. As nuclear nonproliferation monitoring moves from teleseismic to regional or even local distances, such high-frequency (>5Hz) scattering could prove challenging when attempting to discriminate events in areas of previous testing. [ABSTRACT FROM AUTHOR]

The Sevilleta Array was a dense seismic array deployed for 12 days in February, 2015 by collaborators from the University of New Mexico, New Mexico Tech, and Colorado State University. Approximately 800 vertical component, 10-Hz geophones were deployed across the Sevilleta National wildlife refuge in northern Socorro County, New Mexico. The array was designed to investigate the northern half of the Socorro magma body, an actively inflating mid-crustal sill. However, the data has provided an opportunity to examine shallow reflectors in the southern portion of the Albuquerque basin in addition to the magma body. Teleseismic virtual source reflection profiling (TVR) utilizes the free surface reflection of a teleseismic P-wave as a "ghost" source in dense arrays. During the deployment, the Sevilleta Array recorded 62 teleseismic events greater than M5. Applying TVR to the data collected by the Sevilleta Array, we present 2-D profiles created from the four events with the highest signal to noise ratio. Shallow reflectors of interest dominate profiles taken from the northwestern-most quadrant of the array (near the Sierra Ladron uplift) and western edge of the Rio Grande rift. These reflectors may represent normal faulting from the region's past and present tectonic activity. [ABSTRACT FROM AUTHOR]