1.3 Are Fracture Systems Predictable?
While the needs are pressing, the scientific and methodological challenges for characterizing fractured aquifers are substantial. Anyone who has ever read something about fractured aquifers has certainly come across an initial statement such as: fractured media are (extremely) complex due to their inherent heterogeneity and resulting unpredictability. This heterogeneity controls the groundwater flow field. Especially in crystalline rocks such as granites and gneisses, a common first impression is that the fracture network is random, as there are fractures with several different orientations. Given this, it is natural to ask the following questions:
- Are the fractures organized in a logical way?
- Is it possible to unravel this logical organization so it makes physical sense?
- What will be the implications of this organization for the flow of groundwater in fractured aquifers?
In answering the above questions, two types of theoretical fundamentals must be used. They are:
- rock mechanics, which control the initiation and propagation of fractures; and,
- structural geology, which describes conditions in the Earth’s crust where deformation occurs.
A structural geologist investigates the fracture sets based on field data, as well as the spatial and temporal relationships between these sets, that is, the evolution of the deformation over time. Even when the fracture networks are highly heterogeneous, a representative fracture survey quantitatively samples the characteristic orientations (sets), sizes and other fracture properties. However, a structural geology investigation, along with the application of rock mechanics principles, provides a better understanding of how fractures were formed throughout the deformation history; this history has important implications for assessing connectivity and apertures.
A great advantage of structural geology surveys based on data collected from outcrops is their relatively low cost. Thus, it is recommended that research on fractured aquifers should make the most of data collected from outcrops to develop realistic conceptual models. In addition to proposing a fracture network conceptual model, such studies also provide a qualitative, or semi-quantitative, classification of each fracture set with respect to groundwater flow potential based on evidence of flow along fractures observed on the outcrops. Conceptual fracture networks that include the classification of each fracture set, with regard to their potential for carrying groundwater flow, can be called hydro-structural conceptual models. Such models can be used to optimize (and reduce costs of) subsequent expensive research methods such as drilling, core logging and well testing.