Space and time distribution of subsurface H2concentration in so-called “fairy circles”: Insight from a conceptual 2-D transport model

Andrey Myagkiy,Isabelle Moretti,Fabrice Brunet

doi:10.1051/bsgf/2020010

Abstract

Natural H2emissions from the ground have now been measured in many places worldwide. These emissions can be localized on faults or be more diffuse in some sedimentary basins, usually of Proterozoic age. In such a case, emanation zones are often visible from aerial images or on high-resolution topographic maps since they correspond to slight depressions of circular to elliptic shape. Furthermore, the rounded depressions are covered with a scrubby vegetation which often contrasts with the surrounding vegetation. Although the emission structure displays a very regular shape, the distribution of H2concentration in the first meter of soil in such a structure does show a clear pattern. For example, the maximum concentration is almost never measured in the center of the structure and the few time-resolved data show that the soil H2concentration is variable with time. Here, the time and space evolution of H2concentration is simulated using a 2-D advective-diffusive model of H2transport in porous media. Several parameters have been tested as the depth and periodicity of the H2point source (pulsed), bacterial H2consumption and permeability heterogeneities of the soil. The radius of the structure is linked to the time spent by the H2in the soil that depends on the soil permeability, the depth of the gas leakage point and the pressure of the bubble. To account for field observations, the case of a shaly, less permeable, heterogeneity in the center of the structures has been modeled. It resulted in an increase of the concentration toward the rim of the structure and a close to zero signal in its center. If the deep signal is periodic with a frequency smaller than a few hours, H2concentration within the soil is almost constant; in other cases, the near surface concentration wave reflects the concentration periodicity of the source with a delay (in the range of 12 h for 30 m of soil) and so the near surface H2concentration values will be highly dependent on the time at which the measurement is performed. H2monitoring through a sensor network is thus mandatory to characterize the H2dynamics in the soil of fairy circles.

Highlights

1.1 H2 transfer in subsurfaceSubsurface H2 gas concentrations far above atmospheric level and subsequent H2 flow towards the surface have been reported in an increasing number of publications
H2 emissions are observed in convergence area where the oceanic lithosphere outcrops onshore such as in Oman, Philippines or New Caledonia (Neal and Stanger, 1983; Abrajano et al, 1988; Sano et al, 1993; Monnin et al, 2014; Deville and Prinzhofer, 2016; Vacquand et al, 2018)
After a synthesis of the characteristics of H2 emitting structures in terms of shape and H2 concentrations in the soils, we explore, in the current study, through two-dimensional reaction-transport modeling, the situation where the H2 migration toward the surface ends in a soil cover

Summary

Introduction

1.1 H2 transfer in subsurfaceSubsurface H2 gas concentrations far above atmospheric level and subsequent H2 flow towards the surface have been reported in an increasing number of publications. H2 gas concentrations at around 1 m below the surface have been measured in Russia and USA among others (Larin et al, 2014; Zgonnik et al, 2015) and even monitored in Brazil (Prinzhofer et al, 2019) on a time-resolved basis. The source of this hydrogen is still debated since different H2-forming reactions and processes are likely to produce H2 in the subsurface (Guélard et al, 2017). The same type of process based on the interaction between ultramafic rocks and water may occur at lower temperature and the fluid could be meteoric water (e.g., Mayhew et al, 2013; Miller et al, 2017)

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