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Solid Earth An interactive open-access journal of the European Geosciences Union
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Short summary
Two time-lapse borehole ground penetrating radar (GPR) surveys were conducted during saline tracer experiments in weakly fractured crystalline rock with sub-mm fractures apertures, targeting electrical conductivity changes. Through the combination of time-lapse reflection and transmission GPR surveys from different boreholes we were able to monitor the tracer flow and reconstruct the flow path and its temporal evolution in 3D and provided a realistic visualization of the hydrological processes.
Preprints
https://doi.org/10.5194/se-2020-215
https://doi.org/10.5194/se-2020-215

  13 Jan 2021

13 Jan 2021

Review status: this preprint is currently under review for the journal SE.

4D Tracer Flow Reconstruction in Fractured Rock through Borehole GPR Monitoring

Peter-Lasse Giertzuch1, Joseph Doetsch1,2, Alexis Shakas1, Mohammadreza Jalali3, Bernard Brixel1, and Hansruedi Maurer1 Peter-Lasse Giertzuch et al.
  • 1Institute of Geophysics, ETH Zurich, Zurich, Switzerland
  • 2Lufthansa Industry Solutions, Raunheim, Germany
  • 3Chair of Engineering Geology and Hydrogeology, RWTH Aachen, Aachen, Germany

Abstract. Two borehole ground penetrating radar (GPR) surveys were conducted during saline tracer injection experiments in fully-saturated crystalline rock at the Grimsel Test Site in Switzerland. The saline tracer is characterized by an increased electrical conductivity in comparison to formation water. It was injected under steady state flow conditions into the rock mass that features sub-mm fracture apertures. The GPR surveys were designed as time-lapse reflection GPR from separate boreholes and a time-lapse transmission survey between the two boreholes. The local increase in conductivity, introduced by the injected tracer, was captured by GPR in terms of reflectivity increase for the reflection surveys, and attenuation increase for the transmission survey. Data processing and difference imaging was used to extract the tracer signal in the reflection surveys, despite the presence of multiple static reflectors that could shadow the tracer reflection. The transmission survey was analyzed by a difference attenuation inversion scheme, targeting conductivity changes in the tomography plane. By combining the time-lapse difference reflection images, it was possible to reconstruct and visualize the tracer propagation in 3D. This was achieved by calculating the potential radially-symmetric tracer reflection locations in each survey and determining their intersections, to delineate the possible tracer locations. Localization ambiguity imposed by the lack of a third borehole for a full triangulation was reduced by including the attenuation tomography results into the analysis. The resulting tracer flow reconstruction was found to be in good agreement with data from conductivity sensors in multiple observation locations in the experiment volume and gave a realistic visualization of the hydrological processes during the tracer experiments. Our methodology proved to be successful for characterizing flow paths related with geothermal reservoirs in crystalline rocks, but it can be transferred in a straightforward manner to other applications, such as radioactive repository monitoring or civil engineering projects.

Peter-Lasse Giertzuch et al.

Status: open (until 24 Feb 2021)

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Peter-Lasse Giertzuch et al.

Peter-Lasse Giertzuch et al.

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Short summary
Two time-lapse borehole ground penetrating radar (GPR) surveys were conducted during saline tracer experiments in weakly fractured crystalline rock with sub-mm fractures apertures, targeting electrical conductivity changes. Through the combination of time-lapse reflection and transmission GPR surveys from different boreholes we were able to monitor the tracer flow and reconstruct the flow path and its temporal evolution in 3D and provided a realistic visualization of the hydrological processes.
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