Abstract Identifying fractures in the subsurface is crucial for many geomechanical and hydrogeological applications. Here, we assess the ability of the Ground Penetrating Radar (GPR) method to image open fractures with sub-mm apertures in the context of future deep disposal of radioactive waste. GPR experiments were conducted in a tunnel located 410 m below sea level within the Aspo Hard Rock Laboratory (Sweden) using 3-D surface-based acquisitions (3.4 m × 19 m) with 160 MHz, 450 MHz and 750 MHz antennas. The nature of 17 identified GPR reflections was analyzed by means of three new boreholes (BH1-BH3; 9–9.5 m deep). Out of 21 injection and outflow tests in packed-off 1-m sections, only five provided responses above the detection threshold with the maximum transmissivity reaching 7.0 × 10−10 m2/s. Most GPR reflections are situated in these permeable regions and their characteristics agree well with core and Optical Televiewer data. A 3-D statistical fracture model deduced from fracture traces on neighboring tunnel walls show that the GPR data mainly identify fractures with dips between 0 and 25°. Since the GPR data are mostly sensitive to open fractures, we deduce that the surface GPR method can identify 80% of open sub-horizontal fractures. We also find that the scaling of GPR fractures in the range of 1–10 m2 agrees well with the statistical model distribution indicating that fracture lengths are preserved by the GPR imaging (no measurement bias). Our results suggests that surface-GPR carries the resolution needed to identify the most permeable sub-horizontal fractures even in very low-permeability formations, thereby, suggesting that surface-GPR could play an important role in geotechnical workflows, for instance, for industrial-scale siting of waste canisters below tunnel floors in nuclear waste repositories.

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哪些裂缝是用探地雷达成像的？瑞典 Äspö Hardrock 实验室的实验结果

摘要 识别地下裂缝对于许多地质力学和水文地质应用至关重要。在这里，我们评估了探地雷达 (GPR) 方法在未来深度处置放射性废物的背景下对具有亚毫米孔径的开放裂缝进行成像的能力。GPR 实验在位于 Aspo Hard Rock 实验室（瑞典）海平面以下 410 m 的隧道中进行，使用 3-D 地基采集（3.4 m × 19 m）和 160 MHz、450 MHz 和 750 MHz 天线。通过三个新钻孔（BH1-BH3；9-9.5 m 深）分析了 17 个已识别 GPR 反射的性质。在密闭 1 米段的 21 次注入和流出测试中，只有 5 次提供高于检测阈值的响应，最大透射率达到 7.0 × 10-10 m2/s。大多数 GPR 反射位于这些可渗透区域，它们的特性与核心和光学望远镜数据非常吻合。从相邻隧道壁上的裂缝痕迹推导出的 3-D 统计裂缝模型表明，探地雷达数据主要识别倾角在 0 到 25°之间的裂缝。由于探地雷达数据大多对开放裂缝敏感，我们推论地面探地雷达方法可以识别80%的开放亚水平裂缝。我们还发现 GPR 裂缝在 1-10 m2 范围内的比例与统计模型分布非常吻合，表明 GPR 成像保留了裂缝长度（无测量偏差）。我们的结果表明，即使在渗透率非常低的地层中，地表 GPR 也具有识别最具渗透性的亚水平裂缝所需的分辨率，因此，