In-soil hydrogen concentration monitoring using MONHyTOR: application for natural hydrogen exploration

Abstract. Soil gas measurements are extensively utilized for seepage detection in natural hydrogen exploration; hydrogen concentration in the soil exhibits significant temporal and spatial variability. We developed an in-soil hydrogen concentration monitoring instrument, named MONHyTOR, capable of up to 1 s sampling rate with up to months of autonomy. Laboratory tests using a closed chamber demonstrate the accuracy of the instrument for diffuse gas flow and a monitoring campaign conducted in the southwest of France yielded several interesting recordings. Each field dataset begins with a hydrogen concentration peak linked to the drilling of the borehole during installation, where parameters from fitting an exponential decay to its decrease indicate the effects of soil type on hydrogen diffusivity and retention in the soil. While the origin of the measured hydrogen is not identified in this paper, our long-term monitoring data suggest that soil gas movements vary with air pressure and temperature values, as well as water circulation in the soil. This paper shows that sampling rate in the order of seconds is appropriate for the commonly observed wavelengths of hydrogen concentration variation in the soil and data processing, and the absence of pump allows for a study on proxies for soil gas diffusivity.