The critical zone is a region of the shallow subsurface that ranges from the top of the vegetation canopy to the base of superficial aquifers. It comprises rocks, soils, water, air, and living organisms; contains the vast majority of life-sustaining resources; and regulates the interaction between the atmosphere and aquifers (e.g., Binley et al., 2015; Parsekian et al., 2015). The combined use of geophysical multiscale probing and imaging techniques along with the integration of hydrological, hydrogeological, and geochemical data is widely practiced for the observation of the partially saturated region of the critical zone, that is, the vadose zone (e.g., Parsekian et al., 2015). This approach to geophysical subsurface characterization, referred to as hydrogeophysics (e.g., Hubbard & Linde, 2011; Rubin & Hubbard, 2006), is dominated by electrical and electromagnetic methods due to their strong sensitivity with regard to water content and salinity (e.g., Friedman, 2005). However, given that seismic waves are inherently sensitive to key hydraulic properties of porous media, such as, porosity, permeability,