Mapping Inherited Fractures in the Critical Zone Using Seismic Anisotropy From Circular Surveys

Abstract 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 detect able by seismic anisotropy, with fast wave speeds along the strike of structures. We measured shallow (~2–4 m) seismic anisotropy using “circle shots,” geophones deployed in a circle around a central shot point, in a weatheredgraniteterrainintheLaramieRangeofWyoming.Theinferredremnantfractureorientationsagree 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.

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