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Report No.

Efficient detection of methane seepage using a vehicle-mounted cavity ring-down spectroscopy analyzer

Niwa, Masakazu   ; Shimo, Michito*; Shimada, Koji   ; Goto, Akira

Methane (CH$$_{4}$$) seepage to the surface in the early stage of hydrocarbon exploration has attracted increasing attention. Also, detection of CH$$_{4}$$-rich fluid emission can be applied to reconnaissance surveys for safety management in geological disposal of radioactive waste because high-temperature fluids that come from slab dehydration along a subduction zone are often rich in CH$$_{4}$$. These fluids likely migrate along faults and fractures. In this study, we employed a portable analyzer that used a wavelength-scanned cavity ring-down spectroscopy (CRDS) with high accuracy (in ppb levels) and short measurement intervals ($$sim$$1 s). An on-vehicle measurement that employed the CRDS system was performed in the Hongu area in southeast Kii Peninsula, Japan, which included CH$$_{4}$$-bearing hot springs (Kawayu and Yunomine). The measurement near the hot springs detected a clear CH$$_{4}$$ anomaly ($$>$$2 ppm) that exceeded the background concentration. The estimation of CH$$_{4}$$ diffusion in air based on the Gaussian plume model corresponded to the result of the on-vehicle measurement. This study confirmed that the method using the vehicle-mounted CRDS analyzer can help in quickly and easily identifying CH$$_{4}$$-bearing fluid emissions at the surface. Additionally, directions for use of the analyzer were summarized in this report.



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