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.

中文翻译：

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通过钻孔GPR监测裂缝岩中的4D示踪剂流动

摘要。在瑞士的格里姆瑟尔试验场，在完全饱和的结晶岩中进行盐水示踪剂注入实验期间，进行了两次井下探地雷达（GPR）调查。与地层水相比，盐水示踪剂的特征在于电导率增加。它在稳态流动条件下被注入具有亚毫米断裂孔的岩体中。GPR测量被设计为来自不同钻孔的延时反射GPR，以及两个钻孔之间的延时透射测量。GPR捕获了由注入的示踪剂引起的电导率的局部增加，这反映了反射测量的反射率增加和透射测量的衰减增加。尽管存在多个可能掩盖示踪剂反射的静态反射器，但在反射测量中仍使用数据处理和差异成像来提取示踪剂信号。通过差异衰减反演方案分析了透射测量，目标是断层扫描平面中的电导率变化。通过组合时差反射图像，可以重建并可视化3D中的示踪剂传播。这是通过在每次测量中计算潜在的径向对称示踪物反射位置并确定它们的交点以描绘可能的示踪物位置来实现的。通过将衰减层析成像结果纳入分析，减少了由于缺少第三个井眼而无法进行完全三角测量而造成的定位模糊性。发现所得的示踪剂流量重建与来自实验体积中多个观测位置的电导率传感器的数据非常吻合，并在示踪剂实验过程中提供了对水文过程的真实可视化显示。我们的方法论已被证明可以成功地表征与晶体岩石中地热储层有关的流动路径，但是它可以以直接的方式转移到其他应用程序中，例如放射性储层监测或土木工程项目。