Abstract
Fairy circles are subcircular surface depression features that are widely distributed all over the world and they are commonly considered as a potential indicator to the presence of subsurface geologic hydrogen. Understanding the relationship between fairy circles and geological structures could assist in understanding potential hydrogen migration pathways and accumulations. We have carried out an integrated geophysical investigation over a group of such circles in the central U.S. We integrate digital elevation model (DEM), airborne gamma-ray spectrometry (AGRS) data, and ground electrical resistivity data to understand the connection between the surficial expressions and the underlying geology of the fairy circles. The study has shown that correlation between the locations of the fairy circles with either radiometric data or subsurface mafic or ultramafic rocks cannot be established. Meanwhile, the imaged geo-electrical structure within 500 m depth from the surface shows that the rim of the studied circle is directly above a ring of more resistive gap in a conductive layer. The low water saturation or lower salinity imaged by the high resistivity in this ring would allow the subsurface H2 to seep through more easily and, therefore, can explain the elevated H2 concentration from soil gas sampling.
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Introduction
Geologic hydrogen including natural H2 and stimulated H2 has the potential to become a significant component of new energy supply mix1,2,3. Suggested global reserve potential of geologic hydrogen from recent research varies from 500,000 tones/year4) to billions of tones/year5,6,7,8. The evidence for the presence of geologic H2 has been found all over the world, such as Mali9,10,11, Oman12, United States13, Australia14, France15, Brazil16, and Bolivia17. Zgonnik8 presents a global H2 seeping map, which provides a global view of the potential geologic hydrogen distribution.
Currently, two sources of H2 are considered in geologic hydrogen exploration among many that have been identified1. The first is serpentinization, which is primarily a hydration reaction between Fe(ii)-rich minerals and water. The second is radiolysis, in which the radiation from radioactive elements in the geology breaks down water to release hydrogen gas (H2). Some of the hydrogen gas would be consumed through biotic and abiotic reactions, some migrates to the surface and dissipates, and some could accumulate in the geologic formations. The latter has the potential to form economic reservoirs.
To explore for geologic hydrogen, both surface exploration methods [e.g.,18] and subsurface imaging-focused geophysical methods as well as efficient data acquisition19 are needed. Significant physical property changes are present in different components of known geologic hydrogen systems [e.g.,20,21]. These subsurface physical property changes provide the basis for geophysical exploration for geologic hydrogen [e.g.,22,23,24,25,26].
Within the context of integrated exploration for geologic hydrogen, one feature has gained significant attention, namely, fairy circles or subcircular depressions (SCD). Fairy circles are ring-like geomorphological features that are oval or elliptical in shape and measure several kilometers across. These features commonly have a flat interior and an elevated rim. The name fairy circles may have the origin in the ecological studies in Namibia and southern Africa. A more scientific name used currently is subcircular depressions [e.g.,27]. Because of the elevated hydrogen gas concentrations in soil gas sampling that are observed near fairy circles, these features have often been considered as an indicator of the potential subsurface hydrogen gas accumulation. For example, Hand (2023)28 referred to fairy circles in N. Carolina, US in his influential reporting on the potential of geologic hydrogen. Frery et al. (2021)18 investigated the hydrogen seep in and near fairy circles in Western Australia.
Given the prominence of fairy circles in the current research and exploration effort on geologic hydrogen, it is important to underhand the connection between these surficial features and the subsurface structures in the context of potential hydrogen generation or the migration pathways of geologic hydrogen.
Towards this goal, we have investigated an area with a large number of fairy circles in eastern Nebraska, U.S. (Fig. 1). We aim to image the subsurface structures directly beneath the circles, understand the geomorphological setting of fairy circles, investigate the relationship between the locations of fairy circles and commonly available regional geophysical data such as magnetic and radiometric data, and image the 3D spatial relationship between fairy circles and deep geologic formation that may serves as the hydrogen source rocks in a serpentinization-driven system.
Figure 1 shows the geographic location of the study site, and the corresponding slope map derived from the digital elevation model (DEM). The slope parameter was computed using standard surface analysis tools in ArcGIS Pro to highlight subtle variations in surface gradient, which helps enhance the visualization of the fairy circles and their relationship with the surrounding geomorphology.
Geographic location of the study site (indicated by the rectangle) in eastern Nebraska, U.S. The map was created using the USGS 1/3 arc-second DEM dataset29.
For this study, we carried out a field campaign to collect geo-electrical (DC resistivity) data over two perpendicular lines crossing a fairy circle. We have also made use of public-domain digital terrain model (DEM) data, regional airborne gamma-ray spectrometry (AGRS) data (also called radiometric data) and regional magnetic data available from the United States Geological Survey (USGS earth MRI website).
In the following, we first describe the field geo-electrical data acquisition and the acquired data. We then present the DEM data29, and the surface slope data computed from the DEM, radiometric data, and magnetic data. We form an interpretation of the shallow geo-electrical structure of beneath one circle and then investigate the general relationship between the group of circles in the area with the three regional-scale data sets, namely DEM, radiometric, and magnetic data.
Field site and data acquisition
We chose a site with fairy circles in Nebraska shown in Fig. 1. The area has fairy circles ranging in size from sub-km to 3 km across. We selected a larger circle for its typical characteristics and easy access as the target circle for detailed investigation.
We carried out the field work in late April 2024. This was a short time window following the snow melt and before crop seeding by the farmers. Our primary field objective was to collect DC (direct current) resistivity data for imaging the electrical conductivity beneath the circle. For general audience of this journal, we will refer to this data as the geo-electrical data.
Figure 2 shows the electrical data acquisition design, which consists of two lines of electrical geophysical survey crossing the fairy circle in perpendicular directions (NS and EW). The lines run alongside respective road and all station locations are determined by differential GPS.
(a) Map view of electrical survey over the chosen fairy circle, which is shown by the gray-scale DEM data29. (b) Apparent resistivity pseudo-volume.
On the right in Fig. 2 is a pseudo-volume of apparent resistivity. The apparent resistivity is a standard form for presenting geo-electrical data and is equivalent to the resistivity of a hypothetical uniform ground (i.e., a uniform half space). The pseudo-volume shows all measurements taken by plotting each datum below the center of the four electrodes at a depth proportional to the electrode separation. This is a common form for presenting the data and for first-order quality control, but such a pseudo-volume does not lend itself directly to interpretation.
In addition, we have extracted digital elevation model (DEM) data, airborne gamma-ray spectrometry data, and magnetic data over a 1-degree by 1-degree area enclosing the study site. These data are shown in Fig. 3. When displayed at this scale, the DEM data29 primarily show the drainage systems. Larger fairy circles are visible at this scale. As we will see in the later section, the circles are much better imaged in the slope data calculated from DEM. The fairy circle over which we carried out geo-electrical imaging is located toward the central west of the area marked in Fig. 1 (c). The vintage radiometric data (only uranium channel is shown) are of lower resolution and we aim to examine the area-wise relation between fairy circles and radiometric data. The magnetic data are of limited resolution with a nominal grid spacing of 1,000 m. Since the magnetic data are meant to image the deep magnetic structure, the available resolution is deemed sufficient for our study.
Thus, we have a set of fine-scale fit-for-purpose geo-electrical data (Fig. 2) and the high-resolution DEM data29 (Fig. 3a) to study individual fairy circles. Meanwhile, we use the regional-scale radiometric and magnetic data (Fig. 3b & c) to investigate the connection between groups of fairy circles and underlying geology at large depths. The radiometric data have a depth of investigation of about 0.5 m and primarily image the lateral variations of radioelement concentrations in the top soil in this case. The magnetic data are sensitive down to 15 to 20 km in this case and can provide an image of 3D heterogeneity of magnetic minerals. These two data sets provide a geologic backdrop within which we investigate the specific fair circle which is about 3 km across. We present the analyses in the next section.
(a) High-resolution digital elevation model (DEM). (b) Uranium channel (eU) of radiometric data in an area of 1-degree by 1-degree. Radiometric data are sensitive to lateral heterogeneities of the radioelement concentrations near the surface. (c) Magnetic data in the same area as the radiometric data. Magnetic data can sense 3D magnetic heterogeneities down to a depth of 15 km in this case. The fairy circle over which geo-electrical data were collected is located near the central west edge of the area.
Analyses of DEM and geophysical data sets
Geo-electrical imaging and DEM data
We first perform 2D inversions of the geo-electrical data we collected over one of the fairy circles. Since each line crosses the circle close to its center and the data do not indicate strong 3D heterogeneities compared to the scale of sensitivity of the geo-electrical profile data, 2D inversions are warranted in this case. We use the algorithm presented in Oldenburg and Li (1994)30 and its implementation in the open-source package SimPEG31. The inversion uses a finite volume discretization of the surface and also include the terrain data to account for the topographic effect. For each inversion we estimate a depth of investigation index as a function of horizontal location using the method developed by Oldenburg and Li (1999)32. We adopt a commonly used convention of expressing the data as apparent resistivity and subsurface model in electrical conductivity. We have estimated that the average depth of the geo-electrical imaging is 500 m, which is determined by the array configuration used, data noise level, and the actual electrical conductivity at the site. Of the sections of data collected, the southern portion of the NS line has poor data quality and was not inverted. The remain section (entire EW line and northern portion of the NW line) have been inverted and results are displayed below a cut-away image of the DEM data29 in Fig. 4.
Electrical conductivity images below the two survey lines obtained by inverting the electrical data shown in Fig. 2. (a) A partial view of the digital elevation model (DEM) is displayed for reference. Three quarters of the fairy circle are visible in the DEM data29. (b) a half DEM data29 over displayed on a E-W line of electrical inversion result.
We observe that the subsurface volume is characterized by a more conductive layer situated in a more resistive background, which appears to be quite uniform. Interestingly, this conductive layer is broken by a more resistive gap that is located directly below the rim (elevated terrain) of the fairy circle. This resistive gap is observed at all locations where a 2D geo-electrical line crosses the rim. It follows that this resistive gap is a ring situated direction below the rim of the circle. In addition, we observe smaller spatial variation of electrical conductivity inside the ring (i.e., below the interior of the circle) and outside the ring. However, the amplitude of these variations is notably lower than that of the resistive ring.
The conductivity of a rock unit is primarily a function of porosity, water saturation, and water salinity. In this flat lying sedimentary environment we do not expect significant horizontal porosity variation but can expect vertical porosity variations in unique layers or sedimentary units. One possible interpretation for the ring of resistive gap directly below the fairy circle’s rim is that it is drier than the rest of the layer. A simple explanation for this would be that the local topography directs rainwater away from the rim and thus lower infiltration rates leads to a dryer subsurface33. If so, the dryer region beneath the rim could increase the relative permeability experienced by H2 gas.
However, the top of the highly conductive persistent layer in which the ring appears is estimated to be at a depth of about 120 m. Nearby water supply wells34 produce small amounts of water from depths between 20 m and 30 m, suggesting that this layer should be fully saturated. The last possible explanation is thus the variations in salinity. Without complete explanation for the origin of the depression circles, it may be possible that whatever deformation caused the surface expression also caused damage such as fracturing in lower layers and thus increased the absolute permeability in this layer. If that is the case, it is possible that greater quantities of fresh water from the surface are able to infiltrate into this layer directly beneath the rim and cause higher resistivity. A greater absolute permeability would also allow for elevated H2 gas migration.
Thus, we have established a direct correlation between the local elevation variations associated with this fairy circle, i.e., the geomorphology, and the underlying resistive ring by the geo-electrical data. Although multiple explanations are plausible, they all point to the resistive ring serving as a pathway for upward migration of H2 gas. This observation and inference will have a direct implication on the H2 concentrations from surface soil gas sampling, as we will discuss in more details in the next section.
Regional scale radiometric and magnetic data
From the relationship between the geo-electrical structure with DEM shown for a single fairy circle in the preceding subsection, we now move to a larger setting with multiple fairy circles in a block of 10 km by 10 km. In Fig. 5, the panel on the right displays the terrain slope calculated from DEM, which clearly shows a set of fairy circles. All fairy circles show a subcircular (or elliptical) rim with a flat interior. The circles vary in size and appear to be located between local drainage systems. The larger circle located towards the north of this area is the one over which we have carried out geo-electrical imaging. For reference, we display the gray-scale DEM image with the sections of inverted electrical conductivity.
The setting of fairy circles in the context of surface terrain slope data. The area is 10 km by 10 km and encompasses several circles of different sizes. All circles are situated between local drainage systems and in the areas that is more stable and not directly affected by the drainages such as creeks and streams.
We further zoom out to an area of 85-km by 100-km shown in Fig. 6, which corresponds to the 1-degee by 1-degee area in latitude and longitude shown in Fig. 3. The terrain slope data again show many fairy circles in this area, and the circles are concentrated in areas between drainage systems (Fig. 6a). These areas are expetced to have more stable surface and not directly affected by drainages such as streams and creeks. Considering this persistent pattern at 10-km (Fig. 5) and 100-km (Fig. 6) scales, it is reasonable to interpret it as indicating that the fairy circles are primarily a geomorphologic feature rather than a surface expression of geologic structure.
Terrain slope calculated from DEM (left) and Uranium (eU) channel of the radiometric data in the same area. The location of fairy circles extracted from slope data are shown as ellipses in the radiometric data. The circles remain to be located between drainage systems at this scale, and there is no correlation between the areas with multiple fairy circles and the regional-scale radiometric data.
We now examine radiometric data in more detail in relationship to the locations of fairy circles. Radiometric data measure the concentration of radioelements in the soil down to a depth of approximately 0.5 m. We display the uranium (eU) channel of the radiometric data in the 100-km by 85-km area on the right in Fig. 6, while repeating the slope data in the same area on the left. We also overlay fairy circles identified from the terrain slope data on the radiometric data. There is no correlation between the radiometric data and the locations or sizes of the fairy circles at this scale.
We now zoom in to the red square and display the two data sets in Fig. 7. There are still no correlations. We note, however, that the resolution of this radiometric data here is low. Our understanding is that the mechanism of H2 generation in Nebraska is serpentinization instead of radiolysis. Therefore, we don’t expect the radiometric be an indicator to H2 in this case. Correspondingly, we do not expect a correlation. This understanding and expectation are consistent with the data available at the study site.
Zoom-in view of slope data and radiometric data (eU) the 10-km by 10-km area marked by the red square in Fig. 6.
Since there is no correlation between the radiometric data and fairy circles at the site, a logical question to ask is whether there is any correlation between these circles and the magnetic data or the corresponding subsurface magnetic susceptibilities. We zoom out back to the larger area of 85 km by 100 km. We display the magnetic data with the terrain slope data again in Fig. 8. We note that the fairy circle we have focused on with geo-electrical data is located at an area with low magnetic anomaly.
Terrain slope from DEM and magnetic data in a large area of 85 km by 100 km.
We have performed a 3D inversion of the magnetic data35 to recover the distribution of magnetic susceptibility down to a depth of 15 km. The mean depth to the basement in the area is 1.5 km, and the spectral analysis of the magnetic data indicates a negligible contribution from sedimentary basin above 1.5 km. Based on this information, we have constrained the magnetic sources to lie entirely below 1.5 km from the ground surface. Thus, the susceptibility represents the magnetic source rocks in the basement. The susceptibility model in Fig. 9 delineates the concentration of magnetic minerals such as magnetite in the basement rocks. It is therefore reasonable to use the susceptibility value as a proxy of the potential hydrogen source rocks at depth. We note that the presence of relatively high value of susceptibility is a necessary condition for the presence of mafic or ultramafic rocks in this area adjacent to the Midcontinent Rift, so any H2 source rocks would be confined within the high susceptibility volume imaged by the magnetic inversion.
3D magnetic inversion result showing the magnetic susceptibility between the depth range from 1.5 km to 15 km.
Examining spatial relationship between the fairy circles and the magnetic data in Fig. 8 as well as the imaged magnetic susceptibility in Fig. 9, we observe no correlation between the locations of the fairy circles and positive magnetic anomaly nor any correlation in 3D with the imaged susceptibility.
In summary, our analyses have shown a strong connection between the terrain elevation variation associated with the fairy circle and the shallow subsurface geo-electrical structure. Meanwhile, no discernable correlations have been observed between fairy circles and radiometric data. No correlation has been observed between fairy circles and the magnetic data or the inverted magnetic susceptibility. We then integrate the results in the next section in the context of geologic hydrogen exploration.
Integrated interpretation
Based on the data analyses presented in the preceding section, we now proceed to form an integrated interpretation and examine the connection, or lack thereof, between fairy circles on the surface and different subsurface features pertinent to geologic hydrogen exploration.
We first examine the information gained from geo-electrical imaging of a fairy circle together with the DEM data29 in the context of hydrogen exploration. The interpretation is summarized in Fig. 10. The high-conductivity subsurface region has either relatively lower permeability or more saline water. This zone can play the role of H2 migration barrier. The resistive ring within the layer of high conductivity forms a protruding feature that may stem from increased fractures with infiltrated rainwater. This resistive ring corresponds to subsurface rock fractures with high permeability and would allow H2 to seep to the surface, regardless the origin of the H2gas. These features are consistent with the results in the literature [e.g17., that show higher H2 concentrations are usually near the rim of fairy circles as indicated by soil gas sampling results.
Integrated interpretation of the electrical conductivity image below the two survey lines obtained by inverting the electrical data and the digital elevation model (DEM) at the fairy circle.
We assemble the surface DEM data29, the geo-electrical image of shallow subsurface, and the magnetic susceptibility of deeper formation. A spatial relationship between the imaged fairy circle and possible H2 source rocks emerges. If one plans to drill a well in the middle of a fairy circle as some recent explorers have done, they may have assumed that the rocks under the interior of fairy circles act as a barrier to suppress the seeping of H2 and therefore drilling through these rocks may reach the naturally occurring H2 accumulation. However, our geophysical results reveal that the source rocks are located both laterally and vertically far away from the fairy circles. Meanwhile, there are diverse H2 migration paths such as drier rocks or fracture zones, which can lead to H2 accumulations at multiple depth and different lateral locations. As a result, there is a low likelihood that the natural occurring H2 accumulation is directly below the fairy circle. Therefore, a fairy circle is not a good indicator if one wishes to reach the H2 source rocks and target the H2 accumulation. Such is the scenario of the well location markedin the source rock image in Fig. 11.
Spatial relationship between the studied fairy circle, and potential deep source rocks. Drilling at the center of a fairy circle with elevated H2 concentration near the rim would be targeting the deep subsurface region without source rocks.
We also display the spatial relation in map view for better visualization in Fig. 12. The red dot represents the location of the fairy circle over which we have collected geo-electrical data and performed imaging. Once we overlay the multiple fairy circle locations obtained from DEM data29 over the magnetic susceptibility distribution, we can reach an important conclusion: For most fairy circles in this area of 85 km by 100 km, there is no source rocks beneath them, nor in the vicinity of them because of the depth of source rocks.
Composite view of the spatial relationship between fairy circles and the potential source rocks images by magnetic susceptibility. The images related to the fairy circle in Fig. 10 are reproduced on the left. On the right, a set of fairy circles are overlaid on a 3D model of the inverted susceptibility viewed directly from the top. In the study area, there is no relationship between the presence of these surface features with deep source rocks.
A succinct summary of the integrated interpretation is that the geomorphological feature of fairy circles offers one explanation of the spatial variation of H2 concentration seen in surface soil gas sampling, but these circles may not serve as reliable indicators of the locations of subsurface hydrogen generation and possible hydrogen accumulation as they are not related to the presence of H2 source rocks.
Conclusions
We have carried out an integrated investigation of a group of fairy circles in Nebraska, US using a set of geo-electrical data collected over one fairy circle and digital elevation model (DEM), magnetic, and radiometric data in the area with multiple fairy circles. Geo-electrical imaging of the fairy circle using the collected data reveals the near-surface structure within upper 500 m below the circle. The terrain slopes calculated from DEM data29 show that the circles are offset from active surface drainage systems, while there is no discernable correlation between the locations of fairy circles and radiometric data and magnetic data in an area of 85 km by 100 km surrounding the study site. Based on the evidence, we have reached the conclusion that there is no relationship between fairy circles and radioelement concentrations. We also observe no correlation between the horizontal locations of these fairy circles and the possible H2 source rocks imaged by the magnetic data. Thus, the integrated investigation suggests that drilling for geologic H2 by targeting fairy circles may not be a sound approach when exploring for serpentinization-generated H2.
Our study using the geo-electrical data specifically collected for this investigation, however, does provide one possible explanation for the elevated H2 concentration near the rims of fairy circles as often highlighted in the published literature. The ring in the subsurface directly below the fairy circle rim in our study has low electrical conductivity and it indicates either a lower water saturation or rainwater with a lower salinity filling the fractures beneath the rim of the circle, which would allow more H2 to seep to the surface.
Data availability
The geo-electrical data acquired for this research are available upon request by contacting the lead author (M. Zhang). The DEM, radiometric, and magnetic data are available in the public domain from the United States Geologic Survey.
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Acknowledgements
We thank Skye Hart for assistance in the field data acquisition. This research was supported in part by a grant from the Grantham Foundation and the funding from the JIP Potential of Geologic Hydrogen Gas Resources.
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Mengli Zhang: field work, idea, interpretation, writing; Noah Perkovich: field work, DC data; Yaoguo Li: field work, magnetic data, writing; Jessica Weihermann: radiometric data work; R. Nate Crummett: field work.
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Zhang, M., Perkovich, N., Li, Y. et al. A geophysical investigation of the fairy circles in Nebraska for geologic hydrogen exploration. Sci Rep 15, 22344 (2025). https://doi.org/10.1038/s41598-025-07335-5
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DOI: https://doi.org/10.1038/s41598-025-07335-5
