Joint Body‐ and Surface‐Wave Tomography of Yucca Flat, Nevada, Using a Novel Seismic Source

Abstract

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 (>5 Hz) scattering could prove challenging when attempting to discriminate events in areas of previous testing.

Modeling Subsurface Explosions Recorded on a Distributed Fiber Optic Sensor

“…(Bottom left) Theoretical velocities (gray) compared with P well log (blue) and S estimate from Toney et al. (2019) (green). (bottom right).…”

“…A WKBJ algorithm used to calculate travel times from the source to the fiber channel locations (Stockwell, 2008). The parametrization assumes that the velocity increases with depth, which is reasonable given the Yucca Flat velocity structure (Toney et al., 2019; Wagoner & Prothro, 2020). The advantage of a grid search is that it provides a robust indication of the error.…”

“…The primary scientific objective of SPE experiments is to understand the generation of S waves from a subsurface explosion. The DAG chemical explosions were recorded by an extensive suite of subsurface and surface seismic sensors (Larotonda & Townsend, 2021) in a location with extremely well‐characterized subsurface geology (Wagoner & Prothro, 2020) and seismic velocity structure (Pitarka & Mellors, 2021; Pitarka et al., 2015; Toney et al., 2019). The DAG events were the first subsurface explosions recorded by a nearby borehole fiber, although earlier explosions at the NNSS were measured using surface fiber at greater distances.…”

Comparison of Surface Phenomena Created by Underground Chemical Explosions in Dry Alluvium and Granite Geology From Fully Polarimetric VideoSAR Data

“…Borehole data and gravity studies have revealed much about the geologic structure such as its depth, which at its deepest point in the southern part of the basin is over 2 km, and shape, with the long axis of the basin being oriented north-south. The basin fill is composed of Quaternary alluvium overlying Tertiary-aged tuffs that sit above Paleozoic basement, mostly comprised of sedimentary units [2], [3], [4].…”

The Estimation of Depth to Basement Under Sedimentary Basins from Gravity Data: Review of Approaches and the ITRESC Method, with an Application to the Yucca Flat Basin (Nevada)

“…Other gravity data interpretation approaches to modelling the basement morphology use the Tikhonov regularized inversion method (Tikhonov and Arsenin 1977) to obtain the basement geometry, assuming a density contrast. With these approaches, the subsurface is generally discretized into a grid of columnar prisms of known horizontal dimensions and known density, and the parameters to be estimated in the inversion model consist of the thicknesses of the columns.…”