Permeability and Elastic Properties of Fractured Rock: Systematic Experimental Investigation and Model Development

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Fractured-rock aquifers are an important water resource. Complex fracture distributions in most rocks make it difficult to characterize and understand groundwater flow in them. Cost-effective methods are needed to ensure sound decisions in groundwater management. Methods using well water levels and cross-hole tests to identify groundwater flow pathways are useful, but the resolution of maps based on these data alone depends on the number of wells, which is commonly inadequate due to financial and time constraints. In these cases, it is necessary to rely on additional data, particularly seismic velocities, to improve the mapping of subsurface flow pathways. This research will be the first systematic experimental study of the flow properties in well-characterized 3D fracture networks that will relate changes in seismic velocity to changes in permeability. This proposal builds on more than a decade of work using ice as a model for rock, which has shown that the basic processes controlling brittle failure of rocks and ice are similar. However, ice has distinct experimental advantages over rock. The proposed experiments and model development offer the opportunity to gain insights into the elastic properties and permeabilities of a broad range of fractured, crystalline natural (i.e., rocks) and synthetic materials. Thus, this research has applicability to many disciplines in materials science, engineering, and the geosciences. Identifying narrow, continuous high conductivity zones in fractured rock is a pressing challenge in hydrogeology. Hydraulic tomography (the inverse modelling of multiple cross-hole well tests) has shown increasing promise for this task, but the resolution of hydraulic tomography is sensitive to the number of boreholes and cross-hole tests, which are commonly too few. In these cases it is necessary to rely on additional data types, particularly geophysical surveys, to improve the resolution of the hydrogeological characterization. Combining hydraulic and geophysical data is most effective when the form of the relationship that links geophysical and hydrogeological parameters, particularly the relationship between seismic velocities and permeability, is known. Both permeability and seismic velocity depend on fracture density. This research examines these relationships to link seismic velocity to permeability. Specifically, using ice as a model material, this will be the first systematic experimental study of the flow properties of well-characterized 3D fracture networks designed to elucidate the relationship between changes in seismic velocity and changes in permeability in fractured geologic media. Despite the importance of fracture-induced changes in elastic properties and permeability, there are few systematic observations of the co-evolution of these properties during brittle deformation. Such data are critical for refining constitutive models linking hydrogeologic and geophysical parameters. The proposed research is a combination of laboratory measurements and mechanistic model development that serves as a necessary precursor to field applications designed to obtain spatially dense information about permeability that will improve the quantitative characterization of groundwater flow and transport.