Strong Anisotropy in the Critical Zone: Evidence that Weathering Enhances Seismic Anisotropy in Saprolite and that Permeability Anisotropy may be Related

In the critical zone, chemical, physical, and biological processes transform bedrock into saprolite and soil. This transformation is hydrologically important because it creates pore spaces and enhances permeability in weathered materials. As porosity and permeability are created in foliated rocks, rock fabric plays an important role in how weathering processes take effect. For example, past studies have shown that foliation can influence infiltration, and subsequently, weathering rates, while others have noted that the inherent seismic anisotropy of foliated rocks can be enhanced by weathering. Here we provide further evidence that weathering can enhance seismic anisotropy and show that seismic and permeability anisotropy in saprolite may be related. We do this by presenting results from circular seismic anisotropy surveys conducted on the vertical walls of 1.5 - 2 m deep trenches dug in the South Carolina Piedmont through the soil and into saprolite. The circular seismic surveys have radii of 0.35 m and provide direct measurements of in situ saprolite velocities in various directions. From these velocities, we estimate magnitudes of anisotropy as great as 62%. Moreover, a series of constant-head infiltration tests show that seismic and permeability anisotropy may be related in saprolite. In half of the infiltration tests, water was allowed to infiltrate saprolite primarily via lateral flow, while in the other half, vertical flow was allowed. From these data, we estimate both the horizontal and vertical permeability of saprolite, where the ratio between the two is on the order of 2 - 5. Furthermore, the fast directions of seismic velocity and groundwater flow are similarly aligned with the orientation of foliation planes in saprolite (which are nearly horizontal). These results imply that remnant rock fabric in saprolite causes both seismic and permeability anisotropy, and that permeability anisotropy in the critical zone may be geophysically inferable.