Ground-penetrating radar (GPR) and seismic imaging have proven to be important tools for the characterization of rock volumes. Both methods provide information about the physical rock mass properties and geological structures away from boreholes or tunnel walls. Here, we present the results from a geophysical characterization campaign that was conducted as part of a decametre-scale hydraulic stimulation experiment in the crystalline rock volume of the Grimsel Test Site (central Switzerland). For this characterization experiment, we used tunnel-based GPR reflection imaging as well as seismic travel-time tomography to investigate the volumes between several tunnels and boreholes. The interpretation of the GPR data with respect to geological structures is based on the unmigrated and migrated images. For the tomographic analysis of the seismic first-arrival travel-time data, we inverted for an anisotropic velocity model described by the Thomsen parameters v₀, ϵ and δ to account for the rock mass foliation. Subsequently, the GPR and seismic images were interpreted in combination with the geological model of the test volume and the known in situ stress states. We found that the ductile shear zones are clearly imaged by GPR and show an increase in seismic anisotropy due to a stronger foliation, while they are not visible in the p-wave (v₀) velocity model. Regions of decreased seismic p-wave velocity, however, correlate with regions of high fracture density. For geophysical characterization of potential deep geothermal reservoirs, our results imply that wireline-compatible borehole GPR should be considered for shear zone characterization, and that seismic anisotropy and velocity information are desirable to acquire in order to gain information about ductile shear zones and fracture density, respectively.

中文翻译：

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使用探地雷达和地震方法表征十米级花岗岩储层

探地雷达（GPR）和地震成像已被证明是表征岩石体积的重要工具。两种方法都可以提供有关远离钻孔或隧道壁的物理岩体性质和地质结构的信息。在这里，我们介绍了地球物理表征活动的结果，该活动是在格里姆瑟尔试验场（瑞士中部）的结晶岩体积中进行的分米级水力刺激实验的一部分。对于此表征实验，我们使用了基于隧道的GPR反射成像以及地震行进时间层析成像技术来研究多个隧道和井眼之间的体积。关于地质构造的GPR数据的解释基于未迁移和已迁移的图像。v ₀， Iμ和I'占岩体叶理。随后，结合测试体积的地质模型和已知的原地应力状态解释了GPR和地震图像。我们发现，韧性剪切带由GPR清晰地成像，并显示出由于较强的叶面而引起的地震各向异性的增加，而在p波（ v ₀）速度模型中不可见。地震p减小的区域然而，波速与高裂缝密度区域相关。对于潜在的深层地热储层的地球物理特征，我们的结果表明，应考虑与电缆兼容的钻孔GPR来进行剪切带的表征，并希望获得地震各向异性和速度信息，以获取有关延性剪切带和裂缝密度的信息，分别。