Weathering front under a granite ridge revealed through full-3D seismic ambient-noise tomography

Seismic tomography, when calibrated with borehole data, can be a highly effective tool for investigating the deep structure of the critical zone. In this study, we have successfully applied seismic adjoint tomography to obtain a fully three-dimensional shear-wave velocity model under a granitic ridge in the Blair-Wallis watershed, southeastern Wyoming. The seismic data used in our tomography are ambient-noise Green's functions obtained from a minimally invasive, “large-N” seismic survey using a square array of 400 autonomous geophones. Lateral variations of our 3D shear-wave velocity model show strong correlations with surface topography. Depth variations of our velocity model may give clues about the depth interval within which chemical weathering plays a significant role. With the calibration of borehole casing depths and saprolite refusal depths, we selected the isosurface of shear-wave velocity 491 m/s as the interface between highly chemically weathered saprolite and fractured bedrocks, which we use to represent the weathering front in this study. Large-scale spatial variations of the weathering front are consistent with groundwater table, which provides observational support to a recently proposed hypothesis that subsurface weathering is mainly driven by bedrock drainage of chemically equilibrated water. Small-scale spatial variations of the weathering front calls for more sophisticated mechanisms that couple the effect of top-down infiltration of reactive meteoric water with the influence of pre-existing fractures.