A massive hydraulic tomography experiment for the high-resolution characterization of the excavation induced fracture network using a travel-time based inversion scheme

The Meuse/Haute-Marne Underground Research Laboratory hosts a large-scale hydraulic tomography experiment designed to characterize fracture networks induced around open and sealed galleries. The objective of this study is to reconstruct their hydraulic properties and fracture geometry to validate a conceptual model describing stress redistribution during tunnel excavation. In the first step, cross-hole responses from gas permeability tests conducted by Solexperts SA were analyzed using an equivalent porous media approach consisting of a 3D travel-time-based tomographic inversion. In the second step, a sequential inversion scheme was applied to reconstruct discrete fracture planes in 3D.

Gas injection tests were carried out across 11 boreholes, each equipped with a movable multi-packer system comprising six intervals for injection or observation. This configuration enabled the recording of 1168 pressure interference signals with a signal-to-noise ratio sufficient for inversion.

The applied 3D travel-time-based tomographic approach relies on transforming the transient groundwater flow equation into the eikonal equation using an asymptotic approximation. This inversion method allowed reconstruction of the 3D gas diffusivity distribution, capturing the key features of the conceptual model related to stress redistribution during excavation. The sequential inversion approach integrates the 3D travel-time inversion with multivariate statistics and basic geological constraints. This method enables significant mesh refinement within the model domain while avoiding a strong ill-posed inversion problem. It successfully reconstructed fracture traces of the induced network parallel to the tunnel surface, including both extension and shear fractures.

Combining results from both approaches enhanced understanding of the spatial geometry of the induced fracture network around galleries: the 3D travel-time tomography provided a comprehensive spatial representation of the conceptual model, while the sequential inversion delivered high-resolution 3D images of fracture traces associated with its main properties.