Fracture Network Characterization using Hydrological and Geophysical Data

Groundwater management decisions must often rely on quantitative aquifer models that capture the heterogeneity of hydraulic properties. Fractured aquifers are one of the most heterogeneous types of aquifers and are very challenging to investigate. Connected millimeter-aperture fractures can act as hydraulic conductors and may provide very localized preferential pathways for fluid flow. The generally poor knowledge about the spatial distribution of hydraulically important fractures limits the ability to derive quantitative flow and transport models in such formations. The distribution of fractures are often poorly constrained by data that are spatially restricted, noisy, and represent indirect measures of physical properties. These data limitations can be partly overcome by combining different data types, such as hydrological and ground penetrating radar (GPR) reflection data. Borehole GPR imaging is a promising tool to image individual fractures up to some tens of meters away from boreholes. In this thesis, I develop approaches to combine GPR and hydrological data to improve the characterization of fractured aquifers. At a crystalline aquifer in Brittany, France, intensive borehole-based hydrologic investigations in three adjacent boreholes were previously performed, but a fracture length scale and the 3-D geometry of the conductive fracture network was poorly known. To overcome these limitations, I use GPR data to improve the fracture network characterization. First, I propose an advanced processing work flow that enables the imaging of fractures using multiple-offset borehole GPR data. The results illustrate that permeable fractures previously identified in borehole logs can be geometrically characterized away from their borehole intersections and that fractures that do not intersect the borehole can be identified. The results from a second study show that GPR reflection monitoring of saline tracer transport is a powerful tool to identify the fractures that are part of the conductive and connected fracture network that dominates local flow and transport processes. This is the first time that tracer transport over tens of meters in individual connected fractures could be imaged. A third study con- firms these findings by repeated experiments and additional tracer tests stimulating different parts of the connected discrete fracture network. Furthermore, the combined interpretation of hydrological and GPR monitoring data acquired during tracer tests provides evidence that temporal changes of GPR amplitudes can provide information about relative changes in tracer concentrations away from the boreholes. Hence, the hydrological and geophysical imaging data are complementary. I then propose a stochastic inversion approach to generate 3-D discrete fracture models that are conditioned to all available data within their uncertainty ranges. The stochastic generation of GPR-conditioned fracture models is able to reproduce the observed hydraulic connections and their contribution to flow. The set of proposed conditioned models provides quantitative estimates of the length scales and the spatial organization of hydraulically important fractures. The different studies described herein give clear evidence that GPR imaging is a useful tool to characterize fractures. Its combination with hydrological data is able to successfully condition networks of hydraulically connected fractures and to provide quantitative fracture models. The presented approaches can be applied in other types of fractured rock formations where the host rock is electrically resistive.