# Sensing Hydrogen Seeps in the Subsurface for Natural Hydrogen Exploration
## 3 Results and Discussion
### Hydrogen Seep Detection  
The sensor calibration and testing revealed that the hydrogen gas readings/results from GA5000 drift by around 9% after 1 week. As a result of the use of multiple reference gases in our studies, the measurements of different mixtures created with the mass flow controllers are only accurate to within about 20%, which did encompass the theoretical concentrations of the mixtures. In addition, we established that the sensor was relatively stable over a long period of time (>1 day) with drift less than 1 ppm per day. To further evaluate the sensor under real field conditions, the GA5000 instrument was deployed on a site north of Coolgardie, which is a town located near Kalgoorlie, Western Australia. Figure 6 presents the soil-natural hydrogen gas values, which were measured at a 50 cm depth along a path cross-cutting a hydrogen seep from the centre of the feature to the edge of the surrounding vegetation ring. The record shows high hydrogen values (>100 ppm) at the centre of the seeps and in the external green vegetation ring, with a maximum hydrogen reading of 447 ppm. A minor increase of the hydrogen values up to 31 ppm is also noticeable at the transition zone, between the centre of the feature without vegetation and the sparse vegetation located in the internal ring. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Methane Concentration** & **Gradient** & **R\({}^{2}\)** \\ \hline
0 ppm & 1.31 \(\pm\) 0.27 & 0.9998 \\
468 \(\pm\) 2 ppm & 1.35 \(\pm\) 0.28 & 0.9999 \\
895 \(\pm\) 3 ppm & 1.34 \(\pm\) 0.27 & 0.999 \\  
As stated in the introduction, elevated levels of hydrogen in the soil have been associated with \"fairy circles\", or circular surface features, particularly in several countries including Australia, Russia, USA, and Brazil. Various theories have been proposed for the origin of hydrogen in these observations. One of these is through the generation of hydrogen from water via water/rock interactions. Several possibilities for this exist, including the process of serpentinization, which occurs when sea water comes into contact with ultramafic rocks along mid ocean ridges and which has been well studied [24; 25]. In this work, we have studied the effects of water/rock interactions on the surface of the soil. Figure 6: Soil-gas hydrogen measurement with GA5000 along a natural seepage profile located north of Coolsardie, WA. essence, this is a redox process where Fe\({}^{2+}\) from certain rocks is oxidized to Fe\({}^{3+}\) and H\({}_{2}\)O is reduced to form hydrogen. This has also been suggested as a source for hydrogen observed in continental settings if Fe\({}^{2+}\) rocks are present, such as the hydrogen seep in Brazil [10]. Another possible source for the hydrogen is by degassing of the Earth's mantle or core, which is left over from planetary formation as the early solar system contained abundant hydrogen [26; 27]. Another possible source is by mechano-radical processes. Here, constant shearing along tectonic faults causes dissociation of Si-O bonds in silicates, producing surface radicals that can react with water to form hydrogen [28]. Other theories include the decomposition of organic matter and biological activity [26]. The present study revealed hydrogen concentration values of up to 447 ppm in the field. While one of the motivations for studies of fairy circles is for potential commercial development, there are still insufficient data on what level of hydrogen in the soil would indicate a potentially economic hydrogen resource. Studies on fairy circles in the Carolina Bays, USA, found soil hydrogen concentrations above 1200 ppm, although no deeper wells were drilled to evaluate the concentration at a greater depth [11]. Studies on fairy circles in the Sao Francisco Basin, Brazil, however, which gave soil hydrogen levels of up to 547 ppm, were associated with hydrogen concentrations of up to 40 mol% from nearby wells that were 1500 m deep. This indicates that surface anomalies can be used as an indicator for significant hydrogen deposits.