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Report No.

Rapid identification of water-conducting fractures using a trace methane gas measurement

Niwa, Masakazu   ; Amano, Kenji  ; Takeuchi, Ryuji  ; Shimada, Koji   

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 an underground tunnel excavated in granite with CH$$_{4}$$-rich groundwater. Two approaches were taken to obtain the profile of CH$$_{4}$$ 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 CH$$_{4}$$ 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.



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Category:Water Resources



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