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

Novitsky, C. G.; Holbrook, W. S.; Carr, Bradley J.; Pasquet, Sylvain; Okaya, D. A.; Flinchum, Brady

doi:10.1002/2017gl075976

Cited by 35 publications

(64 citation statements)

References 47 publications

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“…This is consistent with a large depression in the seismic surface (Fig. 8a) and has a strike consistent with the fast direction identified through seismic anisotropy (Novitsky et al, 2018) and the outcrop to the northeast (Fig. 1b).…”

Section: Insights About the Saprolite–fractured Bedrock Boundarysupporting

confidence: 88%

“…Outcrop is limited within our study area, but outcrops ∼1 km northeast of the study show a clear fracture orientation that has a strike of approximately 50° (Fig. 1b; Novitsky et al, 2018). Underneath the ridge at our study site, the water table is between 12 and 13 m below the ground surface (Flinchum et al, 2018a).…”

Section: Site Descriptionmentioning

confidence: 79%

“…One explanation is that a weathered fracture that is filled with saprolite is sitting below the water table. The saprolite at our site is seismically anisotropic, and the most anisotropic regions correspond to regions of thick saprolite, which implies that inherited bedrock fractures may control saprolite development (Novitsky et al, 2018). Therefore, regions of saturated saprolite could be controlled by the underlying fractures and compartmentalize the groundwater.…”

Section: Discussionmentioning

confidence: 91%

“…Our study site has been targeted by many geophysical experiments and the CZ structure is well characterized at the hillslope scale (Hayes, 2016;Novitsky et al, 2018;Flinchum et al, 2018aFlinchum et al, , 2018bWang et al, 2019;Keifer et al, 2019). An important boundary in the CZ is the one that divides saprolite and weathered bedrock.…”

Section: Addressing the Challenges Of Surface Nmr In A Fractured Envimentioning

confidence: 99%

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Characterizing the Critical Zone Using Borehole and Surface Nuclear Magnetic Resonance

Flinchum

Holbrook²,

Parsekian³

et al. 2019

Vadose Zone Journal

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Core Ideas Unsaturated saprolite, saturated saprolite, and fractured rock have unique NMR responses. Surface NMR signals will be dominated by water in saturated saprolite. The heterogeneous nature of fractured rock results in low‐amplitude surface NMR signals. Understanding critical zone (CZ) structure below the first few meters of Earth's surface is challenging and yet important to understand hydrologic and surface processes that influence life on Earth. Nuclear magnetic resonance (NMR) is an emerging geophysical tool that can quantify the volume of groundwater and pore‐scale properties. Nuclear magnetic resonance has potential to aid in CZ studies, but it can be difficult to collect high‐quality NMR data in weathered and fractured rock. We present data from seven surface NMR soundings and six borehole NMR profiles collected on a weathered and fractured granite in the Laramie Range, Wyoming. First, we show that it is possible to collect high‐quality surface NMR data in a fractured rock. Second, we use the NMR data to delineate the weathering profile into three distinct zones—unsaturated saprolite, saturated saprolite, and fractured rock—and show that the surface NMR signal is dominated by saturated saprolite. Third, we show that lateral heterogeneity significantly reduces the surface NMR signal magnitude, which suggests that the boundary dividing saprolite and fractured rock is laterally heterogeneous. The NMR measurements, when combined with previously collected seismic refraction data, provide a unique opportunity to define the lateral heterogeneity of the boundary dividing saprolite and weathered bedrock in an eroding landscape underlain by crystalline rock.

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Section: Insights About the Saprolite–fractured Bedrock Boundarysupporting

confidence: 88%

Section: Site Descriptionmentioning

confidence: 79%

Section: Discussionmentioning

confidence: 91%

Section: Addressing the Challenges Of Surface Nmr In A Fractured Envimentioning

confidence: 99%

See 2 more Smart Citations

Characterizing the Critical Zone Using Borehole and Surface Nuclear Magnetic Resonance

Flinchum

Holbrook²,

Parsekian³

et al. 2019

Vadose Zone Journal

Self Cite

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show abstract

“…We interpret this feature as a remnant fracture of the parent bedrock material contained within the weathered saprolite. Recent work by Novitsky et al () showed that remnant fractures such as these are common within this watershed and are predominantly aligned with the NE–SW fracture direction of Sherman granite outcrops distributed across the area. At relatively shallow depths (~2.5 to 6 m), fractured bedrock is reached and persists well beneath our area of investigation.…”

Section: Discussionmentioning

confidence: 99%

Characterizing hydrological processes in a semiarid rangeland watershed: A hydrogeophysical approach

Carey

Paige

Carr

et al. 2018

Hydrological Processes

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The complex ecohydrological processes of rangelands can be studied through the framework of ecological sites (ESs) or hillslope‐scale soil–vegetation complexes. High‐quality hydrologic field investigations are needed to quantitatively link ES characteristics to hydrologic function. Geophysical tools are useful in this context because they provide valuable information about the subsurface at appropriate spatial scales. We conducted 20 field experiments in which we deployed time‐lapse electrical resistivity tomography (ERT), variable intensity rainfall simulation, ground‐penetrating radar (GPR), and seismic refraction, on hillslope plots at five different ESs within the Upper Crow Creek Watershed in south‐east Wyoming. Surface runoff was measured using a precalibrated flume. Infiltration data from the rainfall simulations, coupled with site‐specific resistivity–water content relationships and ERT datasets, were used to spatially and temporally track the progression of the wetting front. First‐order constraints on subsurface structure were made at each ES using the geophysical methods. Sites ranged from infiltrating 100% of applied rainfall to infiltrating less than 60%. Analysis of covariance results indicated significant differences in the rate of wetting front progression, ranging from 0.346 m min−1/2 for sites with a subsurface dominated by saprolitic material to 0.156 m min−1/2 for sites with a well‐developed soil profile. There was broad agreement in subsurface structure between the geophysical methods with GPR typically providing the most detail. Joint interpretation of the geophysics showed that subsurface features such as soil layer thickness and the location of subsurface obstructions such as granite corestones and material boundaries had a large effect on the rate of infiltration and subsurface flow processes. These features identified through the geophysics varied significantly by ES. By linking surface hydrologic information from the rainfall simulations with subsurface information provided by the geophysics, we can characterize the ES‐specific hydrologic response. Both surface and subsurface flow processes differed among sites and are directly linked to measured characteristics.

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Strong slope‐aspect control of regolith thickness by bedrock foliation

Leone

Holbrook

Riebe

et al. 2020

Earth Surf Processes Landf

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The porous near‐surface layer of the Earth's crust – the critical zone – constitutes a vital reservoir of water for ecosystems, provides baseflow to streams, guides recharge to deep aquifers, filters contaminants from groundwater, and regulates the long‐term evolution of landscapes. Recent work suggests that the controls on regolith thickness include climate, tectonics, lithology, and vegetation. However, the relative paucity of observations of regolith structure and properties at landscape scales means that theoretical models of critical zone structure are incompletely tested. Here we present seismic refraction and electrical resistivity surveys that thoroughly characterize subsurface structure in a small catchment in the Santa Catalina Mountains, Arizona, USA, where slope‐aspect effects on regolith structure are expected based on differences in vegetation. Our results show a stark contrast in physical properties and inferred regolith thickness on opposing slopes, but in the opposite sense of that expected from environmental models and observed vegetation patterns. Although vegetation (as expressed by normalized difference vegetation index [NDVI]) is denser on the north‐facing slope, regolith on the south‐facing slope is four times thicker (as indicated by lower seismic velocities and resistivities). This contrast cannot be explained by variations in topographic stress or conventional hillslope morphology models. Instead, regolith thickness appears to be controlled by metamorphic foliation: regolith is thicker where foliation dips into the topography, and thinner where foliation is nearly parallel to the surface. We hypothesize that, in this catchment, hydraulic conductivity and infiltration capacity control weathering: infiltration is hindered and regolith is thin where foliation is parallel to the surface topography, whereas water infiltrates deeper and regolith is thicker where foliation intersects topography at a substantial angle. These results suggest that bedrock foliation, and perhaps by extension sedimentary layering, can control regolith thickness and must be accounted for in models of critical zone development. © 2020 John Wiley & Sons, Ltd.

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

Cited by 35 publications

References 47 publications

Characterizing the Critical Zone Using Borehole and Surface Nuclear Magnetic Resonance

Characterizing the Critical Zone Using Borehole and Surface Nuclear Magnetic Resonance

Characterizing hydrological processes in a semiarid rangeland watershed: A hydrogeophysical approach

Strong slope‐aspect control of regolith thickness by bedrock foliation

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