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Structural Geology Applied to Fractured Aquifer Characterization

Book Cover for Structural Geology Applied to Fractured Aquifer Characterization

Publication year: 2023
Number of pages: 189
ISBN: 978-1-77470-009-9

Authors:
Amélia João Fernandes – Environmental Research Institute, Brazil
Alain Rouleau – University of Québec at Chicoutimi, Canada
Eurípedes do Amaral Vargas Junior – Catholic University and Federal University of Rio de Janeiro, Brazil

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Updated: 18 April 2023

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Structural Geology Applied to Fractured Aquifer Characterization

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Book Description

The study of fractured (hard rock) aquifers is increasingly pressing because they occupy vast areas of all continents, and the dependence on this type of aquifer for water supply is growing fast. In hard rocks, groundwater flows through void spaces that are present in fractures that form a connected network; its characterization requires knowledge of structural geology and rock mechanics. This book shows how these disciplines allow us to unveil the logic of the organization of a fracture network and help us build more realistic conceptual models.

The permeability of hard rocks is related to fracture connectivity and aperture, and both are constrained by a number of structural geology and rock mechanics factors. In this way, this book brings insight to questions such as: How does the geometry of different fracture types influence connectivity and aperture? How do tectonic regimes (compressive, extensional and strike slip) influence the general configuration of a fracture network? How does the brittle deformation history affect the architecture and connectivity of the fracture system? How does the current in situ stress field affects the aperture of fractures?

Different rock types, such as sedimentary, volcanic, metamorphic, and intrusive rocks have their own typical discontinuities and pre-existent structures. This influences how the in-situ stresses develop different fracture network architectures. Conceptual models of fracture networks, their connectivity, and preferential groundwater flow pathways, in different geological settings, are presented and explained.

Due to the heterogeneous distribution of the fracture porosity, hard rock aquifers pose many scientific and methodological challenges. One way to overcome these difficulties is to conduct detailed fracture surveys, on large rock exposures, when possible, and to apply structural geology and rock mechanics fundamentals to the collected data. Such work gradually builds knowledge on the organizational logic of fracture systems and how it affects groundwater flow.

Contents

1 INTRODUCTION

1.1 What is a Fractured Aquifer?

1.2 Why Should We Study Fractured Aquifers?

1.3 Are Fracture Systems Predictable?

1.4 Groundwater from Hard Rocks has a Major Role for Water Supply

2 WHERE AND HOW ARE FRACTURES FORMED?

2.1 Crustal Level: Brittle Versus Ductile Deformation

2.2 Terminology: Types of Fractures

2.3 Sources of In-Situ Stress and Differential Stress

2.4 Stress Magnitude and Fracture Types

2.4.1 Shear Fracture
2.4.2 Extension Fracture
2.4.3 Hybrid Fracture

2.5 Fluid Pressure and Hydraulic Fractures

2.6 Expected Influence of Fracture Types on Flow

2.7 Highlights on Fracture Types and Groundwater Flow with Opportunities to Exercise Knowledge Gained by Reading Sections 1 and 2

3 TECTONIC REGIMES, FRACTURE PATTERNS AND REACTIVATION

3.1 Stress Orientation, Tectonic Regimes and Geometric Patterns of Faults and Joints

3.2 Influence of the Tectonic Regime on the Connected Fracture Network

3.3 How Are Joints, Hybrid Fractures and Faults Recognized on Rock Exposures?

3.4 Reactivation of Pre-Existing Structures

3.5 Expected Influence of Reactivation on Flow

3.6 Further Theories and Readings

3.7 Highlights on Tectonic Regimes and Groundwater Flow with Opportunities to Exercise Knowledge Gained by Reading Sections 1, 2 and 3

4 CONCEPTUAL FRACTURE NETWORK MODELS

4.1 Geological Settings

4.1.1 Sedimentary Rocks
4.1.2 Continental Flood Basalts
4.1.3 Metamorphic and Intrusive Igneous Rocks
4.1.4 Fault Zones and Fluid Flow

4.2 Past Deformation and Present Stress-Field Effects on Fracture System Properties

4.3 Brittle Deformation History and Current Fracture Network Flow Properties

4.4 Systematic Collection of Data for Fracture Network Modeling

4.5 Highlights on Groundwater Flow Along Fracture Networks with Opportunities to Exercise Knowledge Gained by Reading Sections 1 Through 4

5 INSIGHTS ON CONCEPTUAL MODELS AND GEOLOGICAL SETTINGS FROM CASE STUDIES

5.1 Fracture Network and Conceptual Flow Models in Sedimentary Rocks

5.2 Fracture Network Conceptual Models of Flood Basalts

5.3 Fracture Network Conceptual Models for Metamorphic and Intrusive Igneous Rocks

6 WRAP-UP

7 GLOSSARY

8 EXERCISES

8.1 Exercises on Constraints on Fracture Formation and Implications for Fracture Network Properties

8.2 Exercises on The Interpretation of Fracture Data Collected in the Field and Their Consequences for Groundwater Flow

9 REFERENCES

10 BOXES

Box 1 Effects of Shear Displacement on The Transmissivity of a Rock Fracture

Box 1.1 Fabricating Replicas of a Natural Fracture

Box 1.2 Test Equipment and Procedure

Box 1.3 Selected Results

Box 1.4 Applications to Field Conditions

Box 1.5 Wrap-Up

Box 1.6 References

Box 1.7 Complementary Bibliography on Shear-Flow Coupling Experiments (1990-2019)

Box 2 Lineament Extraction for Hydrogeologic Applications

Box 2.1 Constraints on Lineament Interpretation

Box 2.2 Rectilinear Morphological Features

Box 2.3 Geometric Properties of the Fractures

Box 2.4 Scale of Lineament Extraction

Box 2.5 Subjectivity of Lineament Interpretation

Box 2.6 Rugged Topography

Box 2.7 Lineament Map Representativeness

Box 2.8 Recommendations

Box 2.9 References

11 EXERCISE SOLUTIONS

12 NOTATIONS

13 ABOUT THE AUTHORS

Interview with Author