Abstract
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-millimeter 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 in 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 was demonstrated to be applicable for monitoring tracer flow and transport and characterizing flow paths related to 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.
Highlights
Flow and transport processes in fractured rock have been a key focus of basic hydrogeological research and are relevant for numerous applications and research fields
We have presented results from two borehole ground-penetrating radar (GPR) experiments that were performed to monitor saline tracer flow in weakly fractured crystalline rock with smallaperture fractures
A geometrical reconstruction approach was applied that successfully overcame the radial ambiguity in the data and resulted in a 4D tracer flow reconstruction
Summary
Flow and transport processes in fractured rock have been a key focus of basic hydrogeological research and are relevant for numerous applications and research fields. These include risk assessment of contaminants (e.g., Andricevicand Cvetkovic, 1996), nuclear waste disposal (Cvetkovic et al, 2004), and the exploitation of deep geothermal energy (DGE) (Brown et al, 2012). All fluid transport in granitic crystalline rock is carried by discrete permeable fractures that are connected within a fracture network Field investigations in such complex subsurface environments are extremely challenging, as no complete direct observations of the fracture geometries and hydrological processes can be made.
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