Using SF 6 and Xe to Monitor Gas Migration Through Explosion‐Generated Fracture Networks

We describe a field study where tracer gas was injected into a subsurface cavity created by a small chemical explosion beneath the water table. The main objective of the study is to compare the migration of sulfur hexafluoride (SF 6 ) and xenon (Xe) through an explosion‐generated fracture network and to study the influence of ground water on gas transport. A mixture of tracer gases (50% of SF 6 and 50% of Xe) was injected on 31 October 2018 and gas sampling continued until 8 November 2018. We observe similar trends in SF 6 and Xe concentrations at four ground surface sampling sites. The changes in the SF 6 /Xe ratios with time show that more SF 6 than Xe is observed during the barometric pressure lows when the absolute measured concentrations are highest. Conversely, the ratio SF 6 /Xe is less than 1 during the high‐pressure intervals when absolute measured concentrations are low. The results of the experiment suggest that during barometric pressure lows the tracer is migrating to the surface primarily by advective gas phase transport, whereas during barometric pressure highs, advection is suppressed and near‐surface evaporation of interstitial pore fluid with tracer dissolved in it becomes more important. Thus, the results of the experiment show that the gas concentrations at the surface are controlled by the combined effects of the gas dissolution into pore water and the barometric pressure fluctuations. We injected a mixture of two tracer gases (Xe and SF 6 ) into an underground explosion‐generated cavity and monitored their concentrations at the surface. The arrival times at the surface were similar for both gases. The observed ratios of SF 6 and Xe concentrations typically varied by about a factor of 2 over a million‐fold range of measured concentrations and correlate with the absolute gas concentrations. Our results indicate that the breakthrough times and the concentrations of the tracer gases are affected by fluctuations of the barometric pressure. We conducted a unique field‐scale experiment to compare the migration of two tracer gases, SF 6 and Xe, through fracture networks, generated by chemical explosions The observed arrival times were similar for both gases; SF 6 concentrations usually tracked Xe concentrations within a factor of 2 over concentration variations of 6 orders of magnitude The observed surface concentrations are affected by the fluctuations in the atmospheric pressure and by gas solubilities in pore water