Identification of water-conducting fractures is important for the safety assessment of underground projects in crystalline rocks at geological disposal sites. We applied a portable methane gas analyzer by wavelength-scanned cavity ring-down spectroscopy to detect the water-conducting fractures in the underground tunnel of the Mizunami Underground Research Laboratory, central Japan. The tunnel is excavated in granite with CH₄-rich groundwater. Two approaches were taken to obtain the profile of CH₄ concentration along the gallery walls: (1) “Scan by walking” at the speed of 0.5 m/s and (2) monitoring for 30 s at 0.5 or 1 m intervals. In the Scan by walking approach, the peaks of the CH₄ concentration corresponded well with the occurrence of high water flow rate fractures. Thus, this method is useful for rapid identification of major water-conducting fractures. Monitoring at constant intervals takes more time than the Scan by walking approach; however, this method can largely detect occurrences of fractures with low fluid fluxes.

中文翻译：

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使用痕量甲烷气体测量快速识别导水裂缝

导水裂缝识别对于地质处置场结晶岩地下工程安全评价具有重要意义。我们通过波长扫描腔衰荡光谱应用便携式甲烷气体分析仪来检测日本中部水波地下研究实验室地下隧道中的导水裂缝。隧道在花岗岩中开挖，地下水富含CH ₄。采取了两种方法来获得沿廊壁的 CH ₄浓度分布：（1）以 0.5 m/s 的速度“步行扫描”和（2）以 0.5 或 1 m 的间隔监测 30 秒。在步行扫描方法中，CH ₄的峰值浓度与高水流量裂缝的发生很好地对应。因此，该方法可用于快速识别主要导水裂缝。以恒定间隔进行监测比步行扫描方法花费更多时间；然而，这种方法可以在很大程度上检测低流体流量的裂缝的出现。