Poster: A field assessment of the ability of Ground Penetrating Radar to detect fractures in very low permeable crystalline rock

Event: AGU Fall Meeting 2019, San Francisco (USA)
Poster by Justine Molron, Niklas Linde, Ludovic Baron, Jan-Olof Selroos, Caroline Darcel, Philippe Davy

Abstract

The identification of (open) fractures in the subsurface is critical for evaluating potential routes for contaminant transport from deep disposal sites. Ground Penetrating Radar (GPR) is suitable for this task and its detection capacity depends on fracture characteristics (orientation, aperture and size) and on the dielectrical and electrical contrast between the fluid or material filling the fractures and the surrounding bedrock.

A GPR experiment was performed in the Äspö Hard Rock Laboratory (Sweden) in a tunnel located 410 m below the sea level with a length of 20 m long, a width of 4 m and a height of 4.5 m. The geological formations are fractured granite, diorite and granodiorite with negligible matrix permeability and very low transmissive fractures (10E-9 to 10E-10 m2/s for most permeable zones).

We used 160 MHz, 450 MHz and 750 MHz antennas, pulled along the clean and flat tunnel floor along parallel lines separated by 0.10 m for 160 MHz and by 0.05 m for 450 and 750 MHz antennas. This measurement set-up and antenna choices allow for a 3D identification of fractures from GPR diffractions and reflections, with different image resolutions and investigation depths reaching 10m, 8m and 5m for 160, 450 and 750 MHz, respectively.

Based on the data, we identify 15 reflections that could correspond to larger fractures with dimensions of 2 to 5 m. We compare the GPR-inferred fractures with the corelogging of three 9.5 m deep boreholes that were drilled after the GPR campaign. The strong GPR reflections in the borehole area largely correspond to the depth and orientation to the fractures identified by the Optical Televiewer (OPTV) data.

Additionally, pumping and injection tests in each borehole showed that the GPR-inferred fractures are situated in the most permeable regions. The occurrence of GPR fractures was then compared with a statistical description of the fracture network built from the intersection of boreholes and 2D trace maps from tunnel walls. Given the average size of the GPR-inferred fractures, we demonstrate that they are overall consistent with the expected fracture density below the tunnel.

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Presentation: “Identification of 3D fracture distribution and fracture connectivity by combined Ground Penetrating Radar Imagery and Tracer Tests at the Äspö Hard Rock Laboratory, Sweden

Event: CBH Studiedag Hydrogeofysica – Journée d’étude Hydrogéophysique, Rochefort (Belgium), May 2019
Presentation by Justine Molron, Niklas Linde, Ludovic Baron, Peter Andersson, Diane Doolaeghe, Tanguy Le Borgne, Johanna Ragvald, Jan-Olof Selroos, Caroline Darcel, Philippe Davy


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Poster: Identification of 3D fracture distribution and fracture connectivity by combined Ground Penetrating Radar Imagery and tracer Tests at the Äspö Hard Rock Laboratory, Sweden

Event: Journées Scientificques de l’ED GAAL, 2019
Poster by Justine Molron, Niklas Linde, Ludovic Baron, Peter Andersson, Diane Doolaeghe, Tanguy Le Borgne, Johanna Ragvald, Jan-Olof Selroos, Caroline Darcel


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Poster: Reducing the uncertainty of discrete fracture network models by ground penetrating radar imagery: case study at the Äspö Hard Rock Laboratory, Sweden

Event: ENIGMA Summer School, June 2018
Poster by Justine Molron, Niklas Linde, Ludovic Baron, Caroline Darcel, Philippe Davy, Jan-Olof Selroos


Abstract

Predicting the flow and mechanical behaviour of fractured rock masses is a major challenge for a large number of hydrological and geotechnical applications. This is mostly achieved by deriving DFN models from field and core mapping and pumping tests. The present research project is focused on the conditioning of DFN models to geophysical imaging data close to tunnel walls. The objective is to develop a new approach for better assessing the safety of the bedrock barrier around canisters for nuclear waste disposal. We firstly investigate the possibility to use the Ground Penetrating Radar (GPR) geophysical method to identify fractures close to tunnel walls at depth. Secondly, the geophysical data shall be introduced in a broader DFN modelling framework which already involves structuralfractureand hydrological data.

The first GPR campaign was completed at the SKB Äspö tunnel Hard Rock Laboratory (HRL) in Sweden. The tunnel is roughly20 m long and 4 m width and height. Data interpretation is currently ongoing. Several significant fractures can be already identified. This information will be used to design a second experiment involving GPR measurements during pumping and tracer tests.

All the data acquired during the project will be made open sourceto contribute to improving our understandingofflow and transport patterns in fractured media. The methodology will offer a new approach for SKB to assess confinement properties of the bedrock barrier around canisters for nuclear waste disposal.


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