1 Introduction
Knowledge of rock mechanics and structural geology reveals the logic underlying the organization of a fracture network in both space and time. This knowledge makes it apparent that the network is not random. Conceptual models of fracture networks based on rock mechanics and structural geology are able to predict fracture character and distribution that are closer to reality than networks developed by generating random fracture fields using statistical measurements.
The flow of groundwater, or any fluid, in rocks whose main permeability is due to the voids associated with fractures, depends on the configuration of the connected fracture network, as well as on the opening variable width (aperture) of the fracture voids within the network.
The connected fracture network and apertures are constrained by geological and mechanical factors, which bestow certain geometric characteristics to the network, as shown schematically in Figure 1.
Figure 1 – Geological and mechanical factors (left) constrain geometric fracture network characteristics (right).
This book is divided into multiple sections. Section 1 defines fractured aquifers and presents the reasons why they have to be studied. Sections 2 and 3 summarize the fundamentals of rock mechanics and structural geology in a way that intends to facilitate understanding the relationships between causes (constraining factors) and effects (associated characteristics). Those two sections allow the following questions to be answered:
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Under what conditions are joints and faults generated? Can they be generated at the same time? What are the implications in terms of variation in aperture and the degree of connectivity?
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What characteristics allow the identification of fractures that were generated during the same tectonic event?
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Why are some fractures more continuous (persistent) than others?
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How and why do different lithological types impose different characteristics on fracture networks and, consequently, variations in the degree of connection and in the groundwater flow properties?
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How does the deformation history influence the groundwater flow properties?
In Section 4 models of fracture networks in different geological contexts (sedimentary rocks, volcanic rocks, metamorphic and igneous rocks, and fault zones) are presented, along with the main consequences for the architecture of the connected network and presence of preferential groundwater flow paths. The relevance of understanding the role of the lithological types is that, given a geological map of sufficient detail, important predictions can be made about characteristics of the fracture network in each geological unit. This allows forecasts to be made, which is significant because one of the greatest difficulties in studying fractured aquifers is the inference of the fracture network geometry in places where it is not possible to collect data directly.
Section 5 is intended to highlight hydrogeological characteristics (e.g., hydraulic conductivity, transmissivity, and preferential groundwater flow pathways) of conceptual fracture network models in the geological settings of sedimentary, volcanic, igneous, and metamorphic rocks based on case studies.
We conclude by expressing our desire that this book will inspire and motivate practitioners to carry out structural geology surveys that provide a sound basis for a better hydrogeological characterization of a site or a region.