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.

中文翻译：

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通过钻孔探地雷达（GPR）监测在裂隙岩中四维示踪流重建

在瑞士格里姆塞尔试验场的完全饱和结晶岩中进行盐水示踪剂注入实验期间进行了两次钻孔探地雷达 (GPR) 调查。与地层水相比，盐水示踪剂的特点是电导率增加。它在稳态流动条件下注入具有亚毫米裂缝孔径的岩体中。探地雷达勘测被设计为来自不同钻孔的延时反射探地雷达和两个钻孔之间的延时透射勘测。由注入示踪剂引入的局部电导率增加被 GPR 根据反射测量的反射率增加和透射测量的衰减增加捕获。尽管存在多个可能遮蔽示踪剂反射的静态反射器，但仍使用数据处理和差异成像来提取反射勘测中的示踪剂信号。传输测量通过差分衰减反演方案进行分析，针对断层扫描平面中的电导率变化。通过组合延时差异反射图像，可以在 3D 中重建和可视化示踪剂传播。这是通过计算每次调查中潜在的径向对称示踪剂反射位置并确定它们的交叉点来描绘可能的示踪剂位置来实现的。通过在分析中包含衰减断层扫描结果，减少了由于缺少第三个钻孔用于完整三角测量而造成的定位模糊性。发现所得示踪剂流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一致，并在示踪剂实验期间提供了水文过程的真实可视化。我们的方法被证明适用于监测示踪剂流动和运输以及表征与结晶岩中地热储层相关的流动路径，但它可以以直接的方式转移到其他应用中，例如放射性储存库监测或土木工程项目。