Quantifying Depth-Dependent Seismic Anisotropy in the Critical Zone Enhanced by Weathering of a Piedmont Schist

Weathering processes weaken and break apart rock, freeing nutrients and enhancing permeability through the subsurface. To better understand these processes, it is useful to constrain physical properties of materials derived from weathering within the critical zone. Foliated rocks exhibit permeability, strength and seismic anisotropy-the former two bear hydrological and geomorphological consequences while the latter is geophysically quantifiable. Each of these types of anisotropy are related to rock fabric (fractures and foliation); thus, characterizing weathering-dependent changes in rock fabric with depth may have a range of implications (e.g., landslide susceptibility, groundwater modeling, and landscape evolution). To better understand how weathering effects rock fabric, we quantify seismic anisotropy in saprolite and weathered bedrock within two catchments underlain by the Precambrian Loch Raven schist, located in Oregon Ridge Park, MD. Using circular geophone arrays and perpendicular seismic refraction profiles, anisotropy versus depth functions are created for material 0-25 m below ground surface (bgs). We find that anisotropy is relatively low (0%-15%) in the deepest material sampled (12-25 m bgs) but becomes more pronounced (29%-33%) at depths corresponding with saprolite and highly weathered bedrock (5-12 m bgs). At shallow soil depths (0-5 m bgs), material is seismically isotropic, indicating that mixing processes have destroyed parent fabric. Therefore, in situ weathering and anisotropy appear to be correlated, suggesting that in-place weathering amplifies the intrinsic anisotropy of bedrock.