资源环境科技发展态势分析平台

Weathering and hydrological processes in Earth's shallow subsurface are influenced by inherited bedrock structures, such as bedding planes, faults, joints, and fractures. However, these structures are difficult to observe in soil-mantled landscapes. Steeply dipping structures with a dominant orientation are detectable by seismic anisotropy, with fast wave speeds along the strike of structures. We measured shallow (similar to 2-4 m) seismic anisotropy using "circle shots," geophones deployed in a circle around a central shot point, in a weathered granite terrain in the Laramie Range of Wyoming. The inferred remnant fracture orientations agree with brittle fracture orientations measured at tens of meters depth in boreholes, demonstrating that bedrock fractures persist through the weathering process into the shallow critical zone. Seismic anisotropy positively correlates with saprolite thickness, suggesting that inherited bedrock fractures may control saprolite thickness by providing preferential pathways for corrosive meteoric waters to access the deep critical zone.

Plain Language Summary Bedrock undergoes physical and chemical changes near the Earth's surface that turns hard rock, such as granite, into softer, "weathered" rock. This soft rock acts as an important "sponge" that stores water used by vegetation and that feeds runoff in streams. Scientists are currently unable to predict how thick this sponge of soft rock will be at various locations. In this paper we test the idea that inherited cracks in the original bedrock provide pathways for water to percolate deeply into the subsurface, driving the weathering that turns hard rock into soft rock. We use a creative new design of circular seismometer arrays to detect these cracks, by measuring sound waves that travel more quickly along the cracks than across the cracks. Our results show that inherited cracks in the bedrock play an important role in the development of the soft-rock sponge that mantles Earth's surface.