Assessing the Accuracy of a Borehole-Controlled P-Wave Velocity Model of Yucca Flat, Nevada Using Large-N Seismic Data

Abstract

With geologic data from over 950 boreholes, Yucca Flat basin, residing on the Nevada National Security Site, has excellent borehole control on stratigraphy. These data were used to create a Geologic Framework Model (GFM) of the basin. Of these boreholes, 188 have corresponding downhole seismic survey data, which were used to determine average P-wave velocities of the geologic units and create a GFM seismostratigraphic model (GFM-SS). With the acquisition of six new active-source large-N datasets in Yucca Flat, we can now quantitatively assess the accuracy of the GFM-SS previously controlled only by borehole data. For each of the six datasets, we subset the GFM to the region of interest and create a forward model of P-wave travel times for the GFM-SS given the large-N source-receiver geometries. We first made trial-and-error adjustments to the unit velocities (while keeping the layer geometry intact) to improve the travel-time residuals. We then implemented a simulated annealing approach to find the optimal velocity model for each dataset. Our results indicate that the borehole-controlled model overestimates alluvium velocities across Yucca Flat. This result persists even when we make smaller GFM-SS models that are local to individual large-N experiments. We hypothesize that this result is a combination of shorter ray paths and the resulting lack of interaction with large-scale features (such as faults), as well as less attenuation of high frequencies in the borehole data. Both the current GFM-SS and the updated model based on median velocities that we present here overgeneralize local unit velocities, which can be quite heterogeneous in Yucca Flat.