Electric resistivity and seismic refraction tomography: a challenging joint underwater survey at Äspö Hard Rock Laboratory
Electric resistivity and seismic refraction tomography: a challenging joint underwater survey at Äspö Hard Rock Laboratory
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DOI：
10.5194/se-8-671-2017
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发表时间：
2016-11
期刊：
影响因子：
3.4
通讯作者：
M. Ronczka;K. Hellman;T. Günther;R. Wisén;T. Dahlin
中科院分区：
文献类型：
--
作者：
M. Ronczka;K. Hellman;T. Günther;R. Wisén;T. Dahlin
抽象的。水下通道施工无论是规划、前期勘察还是施工，都是一项具有挑战性的任务。下伏基岩中的裂隙带导致岩石质量低，从而降低了稳定性。由于自然原因，它们往往更频繁地出现在水道上。在施工阶段之前，提供地下信息的地面调查是必要的，但这在后勤上可能是困难的。地球物理可以通过产生地下图像来帮助缩小局部点信息之间的差距。一种将地震折射层析成像和电阻率层析成像相结合的方法已经在Aspo硬岩实验室(HRL)进行了测试。其目的是从测量的角度，在一个众所周知但在后勤方面具有挑战性的地区发现断裂带。目前的调查覆盖了一条连接地面设施和地下测试实验室的隧道沿线的一条水道。该隧道位于测量线下方约100米，以东约20米，显示了一个大断裂带和几个小断裂带。地质和一般试验场条件，例如来自附近核电站的强电力线噪声，对地球物理测量具有挑战性。在700米剖面的450米水下段，地震和ERT传感器的位置和震源位置一起使用。由于ERT结果中出现了较大的过渡带，加上地震数据的缺失，无法通过分离反演来检测水下通道南段和北段的裂隙带。综合研究表明，ERT模型中存在明显的三维伪影，甚至超过了水下电极的定位误差。模型覆盖率与分辨率密切相关，通过对中、低分辨率的淡出区域引入阈值，可以用来显示模型的不确定性。构造耦合协同反演法能够成功地对北部断裂带进行成像。此外，在异常有据可查的地质环境中发现了以前未知的、厚度相当大的沉积矿床。结果显著改善了一些地质特征的成像，否则这些地质特征可能无法被发现或被误解，并通过聚类分析将图像组合成概念性的地下模型。
Abstract. Tunnelling below water passages is a challenging task in terms of planning, pre-investigation and construction. Fracture zones in the underlying bedrock lead to low rock quality and thus reduced stability. For natural reasons, they tend to be more frequent at water passages. Ground investigations that provide information on the subsurface are necessary prior to the construction phase, but these can be logistically difficult. Geophysics can help close the gaps between local point information by producing subsurface images. An approach that combines seismic refraction tomography and electrical resistivity tomography has been tested at the Aspo Hard Rock Laboratory (HRL). The aim was to detect fracture zones in a well-known but logistically challenging area from a measuring perspective. The presented surveys cover a water passage along part of a tunnel that connects surface facilities with an underground test laboratory. The tunnel is approximately 100 m below and 20 m east of the survey line and gives evidence for one major and several minor fracture zones. The geological and general test site conditions, e.g. with strong power line noise from the nearby nuclear power plant, are challenging for geophysical measurements. Co-located positions for seismic and ERT sensors and source positions are used on the 450 m underwater section of the 700 m profile. Because of a large transition zone that appeared in the ERT result and the missing coverage of the seismic data, fracture zones at the southern and northern parts of the underwater passage cannot be detected by separated inversion. Synthetic studies show that significant three-dimensional (3-D) artefacts occur in the ERT model that even exceed the positioning errors of underwater electrodes. The model coverage is closely connected to the resolution and can be used to display the model uncertainty by introducing thresholds to fade-out regions of medium and low resolution. A structural coupling cooperative inversion approach is able to image the northern fracture zone successfully. In addition, previously unknown sedimentary deposits with a significantly large thickness are detected in the otherwise unusually well-documented geological environment. The results significantly improve the imaging of some geologic features, which would have been undetected or misinterpreted otherwise, and combines the images by means of cluster analysis into a conceptual subsurface model.