Abstract. In this study, we infer the structural and hydraulic properties of the highly fractured zone at the Grimsel Test Site in Switzerland using a stochastic inversion method. The fractured rock is modeled directly as a discrete fracture network (DFN) within an impermeable rock matrix. Cross-hole transient pressure signals recorded from constant-rate injection tests at different intervals provide the basis for the (herein presented) first field application of the inversion. The experimental setup is realized by a multi-packer system. The geological mapping of the structures intercepted by boreholes as well as data from previous studies that were undertaken as part of the In Situ Stimulation and Circulation (ISC) experiments facilitate the setup of the site-dependent conceptual and forward model. The inversion results show that two preferential flow paths between the two boreholes can be distinguished: one is dominated by fractures with large hydraulic apertures, whereas the other path consists mainly of fractures with a smaller aperture. The probability of fractures linking both flow paths increases the closer we get to the second injection borehole. These results are in accordance with the findings of other studies conducted at the site during the ISC measurement campaign and add new insights into the highly fractured zone at this prominent study site.

Groundwater flow through rocks with a low-permeability matrix is usually dominated by the presence of fractures, associated with pronounced local permeability contrasts. Multiple connected fractures yield preferential flow paths along a fracture network permeating the rock mass. Implemented in a model, the network is mostly represented either by a single or multiple continuum method that translates the hydraulic properties of the fractures into an upscaled effective permeability tensor or explicitly as a discrete fracture network (DFN). Combinations of both methods are also possible, such as realized by the discrete fracture matrix model (Berre et al., 2019). Dense fracture networks with many interconnections are more appropriate for the representation in a continuum model. In contrast, if a few fractures dominate the hydraulic conditions, resolving the fractures explicitly in flow models allows for a more detailed insight into preferential flow and transport paths, specific processes such as flow focusing, spatial fracture connectivity, and quantification of the individual influence of single fracture pa-

<p>This study is aimed at the characterization of discrete fracture networks (DFN) by a transdimensional inversion methodology. It has been demonstrated that the reversible-jump Markov chain Monte Carlo (rjMCMC) is suitable for the inversion of two-dimensional (2D) DFNs. Based on given statistical information and measured data, the algorithm identifies the main characteristics of a DFN correctly.</p><p>For this reason, the method will be extended to the inversion of three-dimensional (3D) DFNs which allows more realistic examples. Two main difficulties arise here. First, further constraints have to be defined to limit the number of unknowns due to the high dimensionality of the inversion problem. Second, the forward modelling is a restricting factor concerning the computational costs and the robustness of the iteration. The assumptions made to simplify the governing fluid equations are to be evaluated and the resulting limitations are presented, e.g. small Reynolds number, smooth fracture walls, impermeable rock matrix. Moreover, the errors caused by the numerical solution of the partial differential equation are estimated to verify the correctness of the implementation.</p>

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Abstract. In this study, we infer the structural and hydraulic properties of the highly fractured zone at the Grimsel test site in Switzerland by a stochastic inversion method. The fractured rock is modeled directly as a discrete fracture network (DFN) within an impermeable rock matrix. Cross-hole transient pressure signals recorded from constant rate injection tests in different intervals provide the basis for the herein presented first field application of the inversion. The experimental setup is realized by a multi-packer system. The geological mapping of the structures intercepted by boreholes and data from previous studies that were undertaken as part of the in-situ stimulation and circulation (ISC) experiments facilitate the setup of the site-dependent conceptual and forward model. The inversion results show that two preferential flow paths between the two boreholes can be distinguished. One is dominated by fractures with large hydraulic apertures while the other path consists mainly of fractures with a smaller aperture. The probability of fractures linking both flow paths increases the closer we are at the second injection borehole. The results accord with the findings from other studies conducted at the site during the ISC measurement campaign and add new insights about the highly fractured zone at the prominent study site.