Weathering front under a granite ridge revealed through full-3D seismic ambient-noise tomography

Wang, Wei; Chen, Po; Keifer, Ian; Dueker, K. G.; Lee, En‐Jui; Mu, Dawei; Jiao, Junfeng; Zhang, Ye; Carr, Bradley J.

doi:10.1016/j.epsl.2018.12.038

Cited by 18 publications

(31 citation statements)

References 38 publications

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“…8a). This depression is also present in independent seismic investigations (Wang et al, 2019;Keifer et al, 2019). In cross-sectional view, the water table is much closer to the interpreted seismic Fig.…”

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

confidence: 80%

“…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: Addressing the Challenges Of Surface Nmr In A Fractured Envisupporting

confidence: 80%

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

confidence: 99%

Characterizing the Critical Zone Using Borehole and Surface Nuclear Magnetic Resonance

Flinchum

Holbrook²,

Parsekian³

et al. 2019

Vadose Zone Journal

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“…Following Gao and Shen (2014), we represent the relationship between the phase delay, δ t , and the velocity perturbations of compressional ( Δ m α ) and shear ( Δ m β ) waves as

δ normalt = \int (][, Κ_{α} ()(,, {bold-italicm}_{bold-italicr}, x) boldΔ m_{α} bold+ Κ_{β} ()(,, {bold-italicm}_{bold-italicr}, x) boldΔ m_{β}) normald V,

where Κ α ( m r , x ) and Κ β ( m r , x ) are the 3‐D sensitivity kernels of Rayleigh waves to compressional and shear wave velocities, respectively, m r is the velocity model used in wave propagation simulation, and d V is the volume increment for the integral. The use of 3‐D finite‐frequency sensitivity kernels allows for the ability to resolve structures smaller than the dominant frequency, as demonstrated by previous studies (Chen et al, 2018; Gao & Shen, 2014; Tape et al, 2009; Wang et al, 2018, 2019). From Equation 1, the inversion problem becomes solving for m in

bold-italicGm = bold-italicd,

where G is the matrix built based on the sensitivity kernels in Equation 1 for all phase delay measurements, m is the 3‐D velocity perturbations (the unknowns), and d is the phase delay measurements.…”

Section: Methodsmentioning

confidence: 88%

Segmentation of the Aleutian‐Alaska Subduction Zone Revealed by Full‐Wave Ambient Noise Tomography: Implications for the Along‐Strike Variation of Volcanism

Yang

Gao

2020

JGR Solid Earth

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The along-strike variations of the velocity, thickness, and dip of subducting slabs and the volcano distribution have been observed globally. It is, however, unclear what controls the distribution of volcanoes and the associated magma generation. With the presence of nonuniform volcanism, the Aleutian-Alaska subduction zone (AASZ) is an ideal place to investigate subduction segmentation and its relationship with volcanism. Using full-wave ambient noise tomography, we present a high-resolution 3-D shear wave velocity model of the AASZ for the depths of 15-110 km. The velocity model reveals the distinct high-velocity Pacific slab, the thicker, flatter, and more heterogeneous Yakutat slab, and the northeasterly dipping Wrangell slab. We observe low velocities within the uppermost mantle (at depth <60 km) below the Aleutian arc volcanoes, representing partial melt accumulation. The large crustal low-velocity anomaly beneath the Wrangell volcanic field suggests a large magma reservoir, likely responsible for the clustering of volcanoes. The Denali volcanic gap is above an average-velocity crust but an extremely fast mantle wedge, suggesting the lack of subsurface melt. This is in contrast with the lower-velocity back-arc mantle beneath the adjacent Buzzard Creek-Jumbo Dome volcanoes to the east. The back-arc low velocities associated with the Pacific, the eastern Yakutat, and the Wrangell slabs may reflect subduction-driven mantle upwelling. The structural variation of the downgoing slabs and the overriding plate explains the change of volcanic activity along the AASZ. Our findings demonstrate the combined role of the subducting slab and the overriding plate in controlling the characteristics of arc magmatism. Plain Language Summary The subduction of oceanic plates underneath the continent plate is a complicated, three-dimensional process. The geometry of the downgoing plate and the associated volcanic activity vary along the subduction margin. It is not clearly understood what controls the distribution of arc volcanoes. We utilize an advanced seismic imaging technique to construct a detailed seismic velocity model of the Aleutian-Alaska margin, from crust to the uppermost mantle. The velocity model reveals multiple downgoing slabs, with various seismic velocities, thicknesses, and dip angles. The imaged Pacific slab, Yakutat slab, and Wrangell slab correlate spatially with the Aleutian arc volcanoes, the Buzzard Creek-Jumbo Dome volcanoes, and the Wrangell volcanoes, respectively. The mantle wedge, the wedge-shaped space below the overriding crust and above the downgoing slab, is characterized by different seismic velocities below these three volcanic areas. The Denali volcanic gap, a region with the absence of volcanic activity, is located above a seismically fast mantle wedge and an average crust. There is no melt below this volcanic gap based on our observations. Our findings explain the along-strike variation of volcanic activity along the Aleutian-Alaska subduction zone and demonstrate the combined role of the s...

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“…Downhole hydraulic conductivity logging revealed that hydraulic conductivity varies over two orders of magnitude but generally decreases with depth (Ren et al, 2019). Other geophysical studies have shown that the influence of heterogeneous remnant fabric from fractures extends upward into the saprolite (Novitsky et al, 2018) and that the depth to the intact bedrock is also laterally heterogeneous on the 100s−1,000s of m scale, extending as deep as 60 m below the surface (Flinchum et al, 2018b;Keifer et al, 2019;Wang et al, 2019a). In addition, nuclear magnetic resonance (NMR) logging was completed at all of the boreholes in the study area and show low but variable water contents (< 0.15 m 3 /m 3 ) in the fractured rock region of the CZ; the water is focused in visibly fractured regions (Flinchum et al, 2019;Ren et al, 2019).…”

Section: Heterogeneitymentioning

confidence: 98%

What Do P-Wave Velocities Tell Us About the Critical Zone?

2022

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Fractures in Earth's critical zone influence groundwater flow and storage and promote chemical weathering. Fractured materials are difficult to characterize on large spatial scales because they contain fractures that span a range of sizes, have complex spatial distributions, and are often inaccessible. Therefore, geophysical characterizations of the critical zone depend on the scale of measurements and on the response of the medium to impulses at that scale. Using P-wave velocities collected at two scales, we show that seismic velocities in the fractured bedrock layer of the critical zone are scale-dependent. The smaller-scale velocities, derived from sonic logs with a dominant wavelength of ~0.3 m, show substantial vertical and lateral heterogeneity in the fractured rock, with sonic velocities varying by 2,000 m/s over short lateral distances (~20 m), indicating strong spatial variations in fracture density. In contrast, the larger-scale velocities, derived from seismic refraction surveys with a dominant wavelength of ~50 m, are notably slower than the sonic velocities (a difference of ~3,000 m/s) and lack lateral heterogeneity. We show that this discrepancy is a consequence of contrasting measurement scales between the two methods; in other words, the contrast is not an artifact but rather information—the signature of a fractured medium (weathered/fractured bedrock) when probed at vastly different scales. We explore the sample volumes of each measurement and show that surface refraction velocities provide reliable estimates of critical zone thickness but are relatively insensitive to lateral changes in fracture density at scales of a few tens of meters. At depth, converging refraction and sonic velocities likely indicate the top of unweathered bedrock, indicative of material with similar fracture density across scales.

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Weathering front under a granite ridge revealed through full-3D seismic ambient-noise tomography

Cited by 18 publications

References 38 publications

Characterizing the Critical Zone Using Borehole and Surface Nuclear Magnetic Resonance

Characterizing the Critical Zone Using Borehole and Surface Nuclear Magnetic Resonance

Segmentation of the Aleutian‐Alaska Subduction Zone Revealed by Full‐Wave Ambient Noise Tomography: Implications for the Along‐Strike Variation of Volcanism

What Do P-Wave Velocities Tell Us About the Critical Zone?

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