Structural characteristics, bulk porosity and evolution of an exhumed long-lived hydrothermal system

Egli, Daniel; Baumann, Rahel; Küng, Sulamith; Berger, Alfons; Baron, Ludovic; Herwegh, Marco

doi:10.1016/j.tecto.2018.10.008

Cited by 20 publications

(54 citation statements)

References 57 publications

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“…All picked events are plotted in Fig. 3b, and their values are compared to the fracture dips inferred from the OTV data, which are shown as turquoise dots whose diameter is indicative of the fracture aperture (Egli et al, 2018).…”

Section: Estimation Of Fracture Dipmentioning

confidence: 99%

“…Following Egli et al (2018), fractures with very large apertures are classified as cataclastic zones. Overall, the range of dip angles picked from the imaged BHR reflection data is consistent with those inferred from the OTV data, although it is difficult to match individual reflection events with specific fractures in the OTV images.…”

Section: Estimation Of Fracture Dipmentioning

confidence: 99%

“…To account for borehole breakouts, we compare the FWS data with the caliper and the Neutron-Neutron log data. While the Neutron-Neutron log is also affected by the borehole breakouts, it is primarily sensitive to the water content and thus to the porosity, which in the considered environment is dominated by fracture porosity (Egli et al, 2018).…”

Section: Full-waveform Sonic Data: Identification Of Brittle Deformatmentioning

confidence: 99%

“…In Fig. 4, we compare the aforementioned log data to the OTV-based brittle deformation data of Egli et al (2018). Fig.…”

Section: Full-waveform Sonic Data: Identification Of Brittle Deformatmentioning

confidence: 99%

“…A quantitative analysis of the amplitudes of the hydrophone VSP data in terms of hydraulic conductivity was, however, not possible due to the abundance of tube wave events and the resulting interference of the various parts of the recorded seismic wavefield. Egli et al (2018) performed a structural characterization of the GBF system and its evolution with a specific focus on porosity, permeability, fracture distribution, and fluid flow reconstruction based on the combined analysis of drill cores, optical televiewer (OTV) data, and geological mapping. Building on the results of these previous works, the focus of this study is a detailed characterization of the fracture network in the damage zone of the main fault core from geophysical borehole log data with a particular focus on the network's geometrical and petrophysical properties as well as the links of the latter to brittle deformation.…”

Section: Introductionmentioning

confidence: 99%

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Characteristics of a fracture network surrounding a hydrothermally altered shear zone from geophysical borehole logs

Caspari¹,

Greenwood²,

Baron³

et al. 2019

Preprint

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Abstract. Hydrothermally active and altered fault/shear zones in crystalline rocks are of practical importance because of their potential similarities with petrothermal reservoirs and exploitable natural hydrothermal systems. The petrophysical and hydraulic characterization of such structures is therefore of significant interest. Here, we report the results of corresponding investigations on a prominent shear zone of this type located in the crystalline Aar massif of the central Swiss Alps. A shallow borehole was drilled, which acutely intersects the core of the shear zone and is entirely situated in its surrounding damage zone. The focus of this study is a detailed characterization of this damage zone based on geophysical borehole measurements. For this purpose, a comprehensive suite of borehole logs, comprising passive and active nuclear, full-waveform sonic, resistivity, self-potential, optical televiewer, and borehole radar data, was collected. The migrated images of the borehole radar reflection data together with the optical televiewer data reveal a complicated network of intersecting fractures in the damage zone. Consequently, the associated petrophysical properties, notably the sonic velocities and porosities, are distinctly different from intact granitic formations. Cluster analyses of the borehole logs in combination with the structural interpretations of the optical televiewer data illustrate that the variations in the petrophysical properties are predominantly governed by the intense brittle deformation. The imaged fracture network and the high-porosity zones associated with brittle deformation represent the main flow pathways. This interpretation is consistent with the available geophysical measurements as well as the analyses of the retrieved core material. Furthermore, the interpretation of the self-potential and fluid resistivity log data suggests a compartmentalized hydraulic behavior, as evidenced by inflows of water into the borehole from different sources, which is likely to be governed by the steeply dipping structures.

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Section: Estimation Of Fracture Dipmentioning

confidence: 99%

Section: Estimation Of Fracture Dipmentioning

confidence: 99%

Section: Full-waveform Sonic Data: Identification Of Brittle Deformatmentioning

confidence: 99%

“…In Fig. 4, we compare the aforementioned log data to the OTV-based brittle deformation data of Egli et al (2018). Fig.…”

Section: Full-waveform Sonic Data: Identification Of Brittle Deformatmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Characteristics of a fracture network surrounding a hydrothermally altered shear zone from geophysical borehole logs

Caspari¹,

Greenwood²,

Baron³

et al. 2019

Preprint

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Remote Sensing and Field Data Based Structural 3D Modelling (Haslital, Switzerland) in Combination with Uncertainty Estimation and Verification by Underground Data

Baumberger

Herwegh

Kissling

2022

3D Digital Geological Models

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Permeability and groundwater flow dynamics in deep-reaching orogenic faults estimated from regional-scale hydraulic simulations

Alt‐Epping

Diamond

Wanner

2022

Preprint

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Observations from warm springs discharging through a fault at Grimsel Pass (2,164 m.a.s.l.) in the Swiss Alps are used to constrain a numerical model of the deep water circulation feeding the springs. The springs are known to have been active for at least 3.3 million years and to be due to ascent of meteoric water that penetrated to depths exceeding 10 km, where it acquired temperatures above 250°C. Simulations show that the circulation along the fault connects a meteoric recharge zone at high altitude to the west with a sub-vertical permeable discharge zone at Grimsel Pass. A key unknown is the permeability of the fault. Temperature, discharge rate and chemical composition of the spring water depend on flow conditions at depth and can be used to estimate the fault's bulk permeability. Our study shows that the range of fault permeabilities can be narrowed down to roughly half an order of magnitude: 1e−15 m 2 -5e−15 m 2 . This permeability range is consistent with a currently stable, single-pass flow pattern. Long water-residence times inferred from the isotopic composition of the spring water suggest low recharge rates during the last glaciation and the dominance of a multi-pass flow pattern during that time.

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Structural characteristics, bulk porosity and evolution of an exhumed long-lived hydrothermal system

Cited by 20 publications

References 57 publications

Characteristics of a fracture network surrounding a hydrothermally altered shear zone from geophysical borehole logs

Characteristics of a fracture network surrounding a hydrothermally altered shear zone from geophysical borehole logs

Remote Sensing and Field Data Based Structural 3D Modelling (Haslital, Switzerland) in Combination with Uncertainty Estimation and Verification by Underground Data

Permeability and groundwater flow dynamics in deep-reaching orogenic faults estimated from regional-scale hydraulic simulations

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