Permeability and seismic velocity anisotropy across a ductile-brittle
fault zone in crystalline rock

Wenning, Quinn; Madonna, Claudio; Haller, Antoine De; Burg, Jean‐Pierre

doi:10.5194/se-2018-15

Cited by 16 publications

(34 citation statements)

References 35 publications

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“…In the host rock, pulse tests completed in intact rock yield transmissivity values in the range of 10 ‐14 to 10 ‐11 m 2 /s. This is consistent with gas permeability measurements on core samples (both parallel and normal to foliation) in the host rock determined by Wenning et al () using the transient step technique (Brace et al, ). Characterizing the transmissivity range of intact rock between the two S3 faults proved to be difficult, owing to the limited space available in this zone to install straddle packers and isolate rock mass without visible fractures.…”

Section: Resultssupporting

confidence: 90%

“…(b) Selected results showing the range of transmissivity of intact rock ( N f =0) and single fractures ( N f =1). The range of laboratory permeability values of cores from one of the installed boreholes, converted to transmissivity (see Table ), is shown by the gray‐colored region (Wenning et al, ).…”

Section: Resultsmentioning

confidence: 99%

“…Overall, the permeability of damage zones varied from 10 ‐19 to 10 ‐13 m 2 . Wenning et al () sampled the same rock and found that the gas permeability of samples from wall damage zones near the S3.1 fault is ∼10 ‐19 m 2 , corresponding to the lower bound of in situ estimates. While scale effects are well known, particularly in granite (Clauser, ; Huenges et al, ; Martinez‐Landa & Carrera, ), we can directly infer the variation from in situ to core‐scale permeability values for the same rock volume.…”

Section: Discussionmentioning

confidence: 99%

“…Hence, we hereafter refer to S1/S3 shear zones as fault zones and describe their internal structure accordingly: S1 fault cores constitute high‐strain zones characterized by systems of narrow (<10‐cm‐wide) anastamosed mylonitic cores indicative of NW‐directed thrusting (Wehrens et al, ). S1 fault damage zones are associated with fractures oriented parallel to the local Alpine foliation and to fractures in the host rock (Figure c). S3 faults nucleated instead around two E‐W striking pre‐Alpine dykes that now show mylonitic to ultramylonitic textures, with porosities of 0% to 1% (Kralik et al, ; Wenning et al, ). These metabasic dykes are 18–170 cm wide, have generally no visible cracks, but are occasionally interspersed by gouge and quartz veins.…”

Section: Field Setting and Subsurface Test Facility Descriptionmentioning

confidence: 99%

“…Unlike the S1 faults, S3 faults accommodated two sets of fractures (Figure c): (i) a NE‐SW fracture set and (ii) a set striking toward the dykes at large angles, identified as the extension fractures commonly seen with dykes at the Grimsel Test Site (Keusen et al, , Figure 4b). The contact with metabasic dykes is clear and discrete, with evidence of dextral shear (Krietsch et al, ), and foliation remains above background up to 0.5 m from fault cores (Wenning et al, ).…”

Section: Field Setting and Subsurface Test Facility Descriptionmentioning

confidence: 99%

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Tracking Fluid Flow in Shallow Crustal Fault Zones: 1. Insights From Single‐Hole Permeability Estimates

et al. 2020

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New in situ measurements to constrain the range, distribution, and spatial (meter-scale) variations of permeability in shallow crustal fault zones are reported based on systematic downhole tests at 0.5-km depth in crystalline rock. Single and cross-hole hydraulic packer tests were performed at a new dedicated test facility hosted in the Grimsel Test Site, in the Swiss Alps, following the technical instrumentation and isolation of discrete fault zones accessed by an array of boreholes. Single-hole test results are presented in this paper, while cross-hole experiments are reported in the companion paper. Our results reveal a sharp spatial falloff in permeability, from 10 -13 to 10 -21 m 2 , with off-fault distances of 1-5 m and characterized by a power-law relation with fracture density. Fractures linking subparallel faults were detected as high-permeability discrete spots several meters away from off-fault damage. Due to the narrow (centimeter-wide) thickness of fault cores, the hydraulic tests presented in this study do not characterize the permeability of fault core materials. The transmissivity of single fractures spans six orders of magnitude (10 -12 to 10 -6 m 2 /s) and is systematically higher in damage zones. In situ stresses appear to have a minor effect on natural, present-day fracture transmissivity at the borehole scale. We suggest that the geometrical and topological properties of fracture systems instead tend to control the permeability of the shallow crustal faults studied.Characterizing the structural properties of exhumed fault is key to understand their geometrical complexity at depth, and field studies have described how brittle damage around faults develops both along-strike and off-fault (i.e., off-plane) as fractures in damage zones organize themselves in response to off-fault stresses. Consequently, their spatial arrangement is not random (Kim et al., 2004;Peacock et al., 2017). Previous

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Section: Resultssupporting

confidence: 90%

Section: Resultsmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

Section: Field Setting and Subsurface Test Facility Descriptionmentioning

confidence: 99%

Section: Field Setting and Subsurface Test Facility Descriptionmentioning

confidence: 99%

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Tracking Fluid Flow in Shallow Crustal Fault Zones: 1. Insights From Single‐Hole Permeability Estimates

et al. 2020

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A multi-method investigation of the permeability structure of brittle fault zones with ductile precursors in crystalline rock

Osten,

Schaber,

Gaus

et al. 2024

Grundwasser - Zeitschrift der Fachsektion Hydrogeologie

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Brittle faults and fault zones are among the most hydraulically active elements in predominantly impermeable crystalline host rock. They pose a significant challenge to underground infrastructure like nuclear waste repositories. Brittle fault zones frequently occur along pre-existing ductile shear zones as they introduce weakness planes in the rock.Four brittle fault zones of ductile origin were analyzed in the Rotondo Granite at the “BedrettoLab” in the Swiss Central Alps. Scanline mapping, rock sampling and permeability measurements using three different methods provide detailed insights into the heterogeneous fault zone architecture and hydrogeology. Average intact rock permeability is in the range of 10−19 to 10−17 m2. Fluid flow is channeled into single open or partially mineralized fractures of, at point-scale, up to 10−14 m2, demonstrated by selective gas probe permeameter measurements and borehole hydraulic testing. Reduced permeabilities have been measured in close proximity to these permeable features, indicating alteration of and around the fracture walls.

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Subsurface Fluid Pressure and Rock Deformation Monitoring Using Seismic Velocity Observations

Doetsch

Gischig

Villiger

et al. 2018

Geophysical Research Letters

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Fluid pressure within the Earth's crust is a key driver for triggering natural and human-induced seismicity. Measuring fluid pressure evolution would be highly beneficial for understanding the underlying driving mechanisms and supporting seismic hazard assessment. Here we show that seismic velocities monitored on the 20-m scale respond directly to changes in fluid pressure. Our data show that volumetric strain resulting from effective stress changes is sensed by seismic velocity, while shear dislocation is not. We are able to calibrate seismic velocity evolution against fluid pressure and strain with in situ measurements during a decameter-scale fluid injection experiment in crystalline rock. Thus, our 4-D seismic tomograms enable tracking of fluid pressure and strain evolution. Our findings demonstrate a strong potential toward monitoring transient fluid pressure variations and stress changes for well-instrumented field sites and could be extended to monitoring hydraulic stimulations in deep reservoirs. Plain Language Summary The pressure of fluids in the subsurface is generally a function of depth as well as the regional geological history. Changes to the subsurface fluid pressure-be it natural or human induced-disturb the stress field and are known to drive volcanic eruptions, as well as to trigger earthquakes. For example, pressure increase by fluid injection for hydraulic stimulation and wastewater disposal has been linked to earthquake activity. Unfortunately, pressure measurements need direct access through boreholes, so that pressure data are only available for few locations. A method for estimating the spatial distribution of fluid pressure remotely would thus be highly beneficial. From measurements in a 20-m-scale experiment in granite, we find that fluid pressure propagation can be predicted from observed seismic velocity variations, based on a strong correlation between observed changes in seismic velocities and fluid pressure measured within the rock. As seismic velocities can be readily measured on the reservoir scale, our results demonstrate a strong potential of seismic velocity monitoring for remotely estimating fluid pressure changes in deep reservoirs, along faults, or in volcanic systems. The estimated pressure and stress changes could be an important input to real-time risk analysis of fault reactivation and volcanic eruptions.

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Permeability and seismic velocity anisotropy across a ductile-brittle fault zone in crystalline rock

Cited by 16 publications

References 35 publications

Tracking Fluid Flow in Shallow Crustal Fault Zones: 1. Insights From Single‐Hole Permeability Estimates

Tracking Fluid Flow in Shallow Crustal Fault Zones: 1. Insights From Single‐Hole Permeability Estimates

A multi-method investigation of the permeability structure of brittle fault zones with ductile precursors in crystalline rock

Subsurface Fluid Pressure and Rock Deformation Monitoring Using Seismic Velocity Observations

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