Rock Physics of the Critical Zone: Models, Inversion, and Interpretation

Abstract Rock physics models link geophysical measurements with subsurface petrophysical properties, such as porosity, mineral composition, and fluid saturation. While originally developed for hydrocarbon exploration, these models are increasingly applied in the near surface for quantitative interpretation of geophysical data. This review focuses on their application to the subsurface component of the critical zone, which extends from soil to the base of weathered bedrock and controls key hydrological, geomorphological, and ecological processes. Its structure and heterogeneity remain difficult to characterize due to limited direct subsurface observations. As a result, critical zone studies of the subsurface have relied on indirect geophysical measurements, which are spatially extensive and are used to interpret structural variations, property heterogeneity, and physical processes. However, geophysical measurements alone do not yield petrophysical properties. Rock physics models combined with geophysical inversion provide a tool to translate geophysical data into subsurface petrophysical properties. In this review, we present a synthesis of rock physics models for the prediction of geophysical properties of unsaturated and saturated porous media, focusing on their formulations and assumptions in near‐surface applications involving seismic, electrical, electromagnetic, and nuclear magnetic resonance methods. We then discuss their integration in geophysical inversion studies for the petrophysical characterization of the critical zone. We assess the capabilities, strengths, and limitations of rock physics models and inversion methods in critical zone applications, illustrated with case studies from the Southern Sierra and Laramie Range, and show how the resulting quantitative estimates of petrophysical properties inform hydrogeological studies and reduce uncertainty in model predictions.