Prediction of sub-surface ³⁷Ar concentrations at locations in the Northwestern United States

The Comprehensive Nuclear-Test-Ban Treaty, which is intended to prevent nuclear weapon test explosions and any other nuclear explosions, includes a verification regime, which provides monitoring to identify potential nuclear explosions. The presence of elevated ³⁷Ar is one way to identify subsurface nuclear explosive testing. However, the naturally occurring formation of ³⁷Ar in the subsurface adds a complicating factor. Prediction of the naturally occurring concentration of ³⁷Ar can help to determine if a measured ³⁷Ar concentration is elevated relative to background. The naturally occurring ³⁷Ar background concentration has been shown to vary between less than 1 mBq/m³ to greater than 100 mBq/m³ (Riedmann and Purtschert, 2011). The purpose of this work was to enhance the understanding of the naturally occurring background concentrations of ³⁷Ar, allowing for better interpretation of results. To that end, we present and evaluate a computationally efficient model for predicting the average concentration of ³⁷Ar at any depth under transient barometric pressures. Further, measurements of ³⁷Ar concentrations in samples collected at multiple locations are provided as validation of the concentration prediction model. The model is shown to compare favorably with concentrations of ³⁷Ar measured at multiple locations in the Northwestern United States.