Establishing meaningful soil gas measurements for geological hydrogen research and exploration

Unlike traditional hydrocarbon and mineral exploration, where decades of empirical data informed threshold values, natural hydrogen exploration requires establishing new baselines for what constitutes an economically significant anomaly. To use soil gas measurements as an effective tool in the geological hydrogen research and exploration we must understand the limitations of the existing instruments, what are background hydrogen values in soil and what other data are required for the reliable interpretation of the soil gas measurements and monitoring data sets.
 Current technology constraints remain a significant challenge in natural hydrogen soil gas sensing. Field-appropriate commercially available sensors exhibit combinations of limited operating ranges, cross-sensitivity to humidity and other gases, baseline drift over time and exposure, and hysteretic dynamics. CSIRO has developed Seeptracker multi-gas (hydrogen, methane, carbon monoxide and carbon dioxide) monitoring device. In this presentation we want to share findings regarding the commercially available hydrogen sensing components comprising Seeptracker and results of deploying this instrument around the world to collect soil gas data in various geological settings. Seeptracker utilises multiple commercially available sensors to measure hydrogen and other gases and the output is enhanced by an extensive calibration routine to improve gas measurement accuracy. Developing Seeptracker revealed the challenge of balancing sensing quality, deployment compatibility, and cost/effort scaling. To achieve suitable long-term large-scale autonomous field deployment requires a clearly and concisely defined study scope, together with a well-characterised sensor package and robust calibration routine to address the multi-variate challenge. 
 Interpreting multi-gas measurements introduces both opportunities and risks for false positives. Effective interpretation of soil gas data for geological hydrogen research requires integration with multiple complementary datasets. Geological mapping identifying serpentinisation fronts, radiolytic source rocks, or fault systems provides essential structural context. Geophysical surveys, particularly magnetotellurics and gravity, can delineate subsurface fluid pathways and potential trap geometries. Geochemical analysis of associated gases, including methane, helium, nitrogen, carbon and noble gas isotopes, potentially enables source discrimination and migration pathway delineation. 
Our work with Seeptracker deployments across diverse geological settings around the world suggests that sustained hydrogen concentrations in soil gas can be used as an effective tool for natural hydrogen exploration, but it cannot be used in isolation. The detailed follow-up investigation is required, particularly when accompanied by spatial coherence and temporal stability and crucially ensuring that measured natural hydrogen is geological. Our studies demonstrate that continuous monitoring data capturing temporal variability, rather than single-point measurements, enhances interpretation confidence. In this presentation we show the performance of the current hydrogen sensors within the CSIRO multi-gas monitoring system Seeptracker, including limitations, and present soil gas monitoring results from various sites around the world. We also show in greater detail soil gas studies from Australia, and the interpretation of the soil gas monitoring results is constrained by geochemical, geophysical and isotope data sets.