# Natural hydrogen emanations in Namibia: Field acquisition and vegetation indexes from multispectral satellite image analysis

Isabelle Moretti, Ugo Geymond, Gabriel Pasquet, Léo Aimar, Alain Rabaute

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Isabelle Moretti, Ugo Geymond, Gabriel Pasquet, Léo Aimar, Alain Rabaute. Natural hydrogen emanations in Namibia: Field acquisition and vegetation indexes from multispectral satellite image analysis. International Journal of Hydrogen Energy, 2022, 47 (84), pp.35588-35607. 10.1016/j.ijhydene.2022.08.135. hal-03981115

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# Natural hydrogen emanations in Namibia: field acquisition and vegetation indexes from multispectral satellite image analysis

Isabelle Moretti¹&³, Ugo Geymond², Gabriel Pasquet¹, Leo Aimar³, Alain Rabaute³&⁴

1. LFCR-UPPA av de l'université 64012 Pau, France

2. Université Paris Cité, Institut de Physique du Globe de Paris, CNRS, F-75005 Paris, France

3. Istep, Sorbonne Université, Paris, France

4. Geosubsight, Paris area, France

Corresponding author: Isabelle Moretti, Isabelle.moretti@univ-pau.fr

Submitted to: International Journal of Hydrogen, revised version

Date: 10/28/2022

**Abstract:** Natural hydrogen exploration is now active in various places of the world. Onshore, correlation between natural H₂ generation and the presence of iron rich rocks especially from Archean and Neoproterozoic cratons have been observed. Emanations and accumulations of H₂ have already been confirmed in such geological settings in Australia, South Africa and Brazil. The geological similitude and the presence of numerous sub circular depressions that are a good proxy for hydrogen emanations suggest that hydrogen resources may also exist in Namibia. We present here the results of a data acquisition campaign which allowed us to confirm the presence of natural hydrogen in this country in the vicinity of Neoproterozoic Banded Iron Formation. The H₂ content in the soil, as in Brazil, is variable within the depressions in time and space and is particularly time sensitive across the day. Comparison of the H₂ signal versus time within these two regions shows a similar behavior of the soils with an increase of the H₂ flow at the middle of the day. In addition, these new data allow us to better constrain the morphological characteristics of such H₂-emitting depressions. By using satellite images and digital elevation model we propose a new proxy to differentiate potentially H₂-emitting features from other type of depressions such as Salt Pan. The Landsat multispectral images and their processing through NDVI and SAVI indexes, that highlight a ring of healthy vegetation around the sub circular area with scarce vegetation already observed appear able to discriminate between H₂ emitting structures and other soft depressions.

**Key words:**

Namibia, natural hydrogen, Neoproterozoic, Vegetation Index, SAVI and NDVI
# 1. Introduction

Within the frame of the race to reduce the environmental impact of the industry and mobility, hydrogen appeared as a versatile energy vector and an interesting fuel. However, it is also a raw material which is, up to now, still mainly produced from hydrocarbon and coal. Methane pyrolysis will potentially allow us to produce a carbon free $H_2$ but these methods are not widespread (Moretti, 2019). Electrolysis using carbon free electricity is another solution unfortunately much more expensive and which requests about 8 times more energy than pyrolysis or Steam Methane Reforming (SMR). Within the SMR, part of the $H_2$ is coming from the $CH_4$ and part from the $H_2O$; in the electrolysis all the hydrogen is coming from the water and so the water consumption is also larger. Natural hydrogen appears in this landscape as a very promising alternative (Smith et al., 2005; Gaucher, 2020; Lapi et al., 2022), the understanding of the underground $H_2$ generation and evaluation of the resources is quickly progressing. The exploration started in various countries such as Australia (Boreham et al., 2021; Frery et al., 2021) and USA (Zgonnik et al., 2015; Guelard et al., 2017) while some other countries have promising geological settings. In addition, $H_2$ has been measured in various places even if it was not targeted (Zgonnik, 2020). Up to now the presence of hydrogen was suspected in Namibia, due to the geological context, but there was no data to confirm it.

After an overview of the reactions that allow hydrogen generation in subsurface and the synthesis of the current understanding of the hydrogen systems, we will present in this paper new data acquired during a field campaign in this country of south west Africa. They allow us to prove the generation of natural hydrogen in Namibia, to confirm the breathing of the emitting structures already observed in Brazil (Prinzhofer et al., 2019; Moretti et al, 2021a). However, we didn't measure any $H_2$ within some of the initially targeted structures, especially in the Kalahari Desert. The morphological differences and the characteristic of the vegetation cover of these structures, that we realized *a posteriori*, will be discussed and a new *a priori* data processing of Landsat images will be proposed for the next field campaigns. It improves the proxy that was proposed by Moretti et al (2021b) to recognize potential emitting structures from space images.

# 2. Current understanding of the $H_2$ generation in subsurface

## 2-1 Origins of hydrogen

The hydrogen element is the most common element of the universe and an almost negligible part of our atmosphere (0.5 ppm) in the form of $H_2$. It was, up to recently, considered as only present chemically combined with other elements in subsurface ($H_2O$, $CH_4$ ...). Some researchers alerted that it could be present in large quantity in the deep Earth (Larin et al., 2015; Smith et al., 2005) and that $H_2$ emanations were common (Larin et al., 2015; Zgonnik et al., 2015) onshore in intracratonic basins but without receiving too much attention. After the discovery of the $H_2$ accumulation in Mali at Bourakebougou (Prinzhofer et al., 2018), and because $H_2$ is now considered as a fuel as explained above, the exploration is starting and the understanding of the hydrogen system or at least of the hydrogen generation in subsurface is improving.

The various processes that may generate hydrogen in subsurface could be classified depending on the origin of the $H_2$ and on the reactions that result in the gas phase release (Table 1).
The main set of reactions is related to the water reduction and oxidation of the iron contained in ferrous minerals. The serpentinization is the more studied consisting in the alteration of ferromagnesian minerals (olivine and pyroxene) contained in basic and ultrabasic rocks of the oceanic lithosphere (Charlou et al., 2002; McCollom & Bach, 2009, Mével 2003). H₂ generation has been described in the Mid Oceanic Ridges (MOR) next to hydrothermal vents by serpentinization (white smoker) or pyritization (Black smokers, Arrouvel and Prinzhofer, 2021). In addition, H₂ content within fumaroles and steams of the geothermal areas has been reported in Iceland where the Mid-Atlantic Ridge is uplifted by the presence of a hotspot and emerges (Combaudon et al., 2022) as well as above the new oceanic material of the Afar Rift opening zone (Pasquet et al., 2022). However, reduction of water onshore by old oceanic lithosphere is also generating hydrogen as in Oman, New Caledonia or other ophiolitic contexts (Deville and Prinzhofer, 2016). Close to surface mantellic bodies may also be oxidized and serpentinized as proposed for the Pyrenees (Lefeuvre et al., 2021). More recently the oxidation of iron-rich formations such as the Banded Iron Formation (BIF) has been proposed as an additional source of hydrogen (Geymond et al., 2022). These last authors suggested the water reduction due to the direct oxidation of the Fe²⁺ present in these iron rich facies (essentially contained in Fe²⁺-silicates and carbonates like riebeckite or siderite) as source of the H₂. Oxidation of biotite-rich or peralkaline granite, and reduction of water, might also have the potential to generate H₂ in granitic context such as the Rhine Graben in France (Murray et al., 2020), Strange Lake Pluton in Canada (Truche et al., 2021) and in the basement underlying the Stansbury Basin in Australia (Boreham et al., 2021).

Where the crust is naturally enriched in radioactive elements, if water is present, radiolysis could also generate hydrogen in subsurface; in such a case the water molecule is broken by the highly energetic rays emitted by the natural radioactivity of the crust (Sherwood-Lollard et al, 2014; Truche et al., 2018). Since the radioactive decay is accompanied by the emission of He, the recent discoveries of blends including H₂ and He may suggest a common source. However, in some basins such as the Amadeus Basin in central Australia where H₂ and He have been found together, the gas in the reservoir includes also hydrocarbons that indicate another source, a classical organic rich source rock (Leila et al., 2022). At the opposite, in South Africa in the Orange Free State where a blend of CH₄ and He has been also found, Karolyte et al (2022) suggested that all the CH₄ is derived from the H₂ produced by the radiolysis.

The role of sulfur has recently been investigated in H₂ generation through the process of pyritization. In this case, H₂ production is associated with both a reduction of ferric-iron into ferrous iron and an oxidation of sulfur in pyrite. This reaction is supposed to occur in high temperature environments (as for instance the black smokers of MOR). Pyritization and serpentinization can be seen as complementary during H₂ generation and can co-exist through an iron cycle (Arrouvel & Prinzhofer, 2020). Other sources have been proposed and are under investigation such as the mechanoradical one due to the friction along the active faults (Hirose et al., 2011 & 2012). Other researchers consider the primordial hydrogen, trapped in the mantle and the core of the Earth at the beginning of the solar system as the main source (Larin et al., 2015, Zgonnik, 2020; Toulhoat & Zgonnik, 2022).

The last alternative that started to be studied is the late maturation of some organic rich source rocks. Town gases issued to the pyrolysis of coal was indeed at 50% made up of H₂ and natural hydrogen presence has been reported in many coal mines accident (Grossman et al., 1995; Wang et al., 2017). H₂ generation from the late maturation of shales or shaly coal are so also suggested in various basins (Suzuki et al., 2017; Horsfield et al., 2022; Mahlstedt et al., 2022).

Recent synthesis of these various reactions and on the existing data on H₂ presence and H₂ isotopes published by Mikov (2022) concludes that the main reaction is the serpentinization. Our view is that
the serpentinization is the most well documented process but that there is not enough subsurface data to rank its importance.

## 2-2 Emitting structures and how they work

The hydrogen may be found in subsurface without any apparent leakage at the surface above the accumulations. That is the case in Mali (Prinzhofer et al., 2018) or in the Amadeus Basin in Australia (Bache et al., 2018; Leila et al; 2022). In other cases, the flow of hydrogen is almost permanent as through some active fault zones (Pasquet et al., 2022) or in the steam of some geothermal areas (Leila et al., 2021; Combaudon et al., 2022). However, onshore, when a soil is present, the emanations of hydrogen result in many cases in Sub-Circular Depressions (SCD) sometimes also called fairy circles. It is the case in Russia south east of Moscow (Larin et al., 2015), in USA, North Carolina (Zgonnick et al., 2015) in Brazil, Minas Gerais, (Prinzhofer et al., 2019) or in Australia, Perth Basin, (Moretti et al., 2021b; Frery et al., 2021, 2022) among other places.

These SCD are slightly negative structures with a typical depth of few meters and a size ranging in equivalent diameters from few tens of meters to few hundreds. They are rarely very large (over 500 meters of diameter) and never very deep. Statistical approach published for the Russian, Brazilian and Australian cases display a depth/equivalent diameter ratio around 1% (+/- 0.2%) (Moretti et al., 2021b; Geymond et al., 2022). The slopes are always very gentle, with a maximum around 3%, and these two characteristics help to differentiate them from karstic dissolution holes (dolines). The shape is concave but some second order positive reliefs may also exist in the depressed part. In terms of footprint and number, in the area where H₂ emanation within the SCDs have been proven, they are always very numerous. More than 540 have been mapped by Larin et al (2015) SE from Moscow in Russia and more than 670 in Brazil near the monitored structure within the São Francisco Basin (Moretti et al., 2021b).

These structures, even not very deep, are easy to map from space, Google Earth type, images since they correspond to gaps of vegetation. It is still unclear why the vegetation, especially the trees die when the hydrogen leakage starts but it is systematically observed. Larin et al. (2015) as well as Zgonnik et al (2015) showed clearly the disappearance of trees in a few years from the formation of a H₂-emitting SCD, highlighting that H₂-emitting SCD could be living features at human time scale. The presence of permanent water in the SCDs as well as the role of microorganisms that consume the H₂ have been proposed as an explanation for the vegetation death (Sukhanova et al., 2013; Myagkiy et al., 2020a).

The presence of these SCD has been proposed as a proxy to start an exploration process (Moretti et al., 2021b) and happened to be pertinent to quickly find emanations (Frery et al., 2021). However, SCD presence is not mandatory and H₂ anomalies in the soil have been also discovered by doing systematic measurements of the H₂ content within soil without any mark indicating gas leakage (Lefeuvre et al., 2021).

Obviously, all the depressions are not related to H₂ emissions. For instance, Halas et al (2021) monitored for a year a SCD in the south eastern part of France without measuring any H₂ flow although the shape and size of the various structures in the area are very similar to the H₂-emitting ones described previously. The authors concluded that, in that specific case, the depressions could be due to ice lenses (palsa) formed during the Würm glaciation but other cause can't be disregarded.
## 2-3 Sporadic pulse and 24h soil breathing

Since the description of the H₂-emitting SCDs started, authors noted that the H₂ content in the soil is highly variable, in space (horizontally but also with depth) as well as in time (Larin et al., 2015; Zgonnik et al., 2015). The monitoring conducted on two emitting structures in the São Francisco Basin in Brazil allowed us to better understand this apparent inconsistency of the gas content signature (Prinzhofer et al., 2019; Moretti et al., 2021a). One observes a kind of permanent signal, a 24h wavelength with a maximum at the middle of the day and very often 0 by night. In the Brazilian case this periodic signal reaches about few hundreds of ppm. In addition, sporadic pulses of thousands of ppm (the sensors are usually saturated) happen from time to time at different places in the SCD (see Moretti et al., 2021a fig 4 & 5). The authors interpreted the pulses with high H₂ content as the mark of the deep signal, and the 24h periodic signal as the mark of the soil behavior with respect to H₂ release. Numerical models show that it could be due to the pressure variation in the air at the middle of the day (Cathles and Prinzhofer, 2020) and seem to preclude a major influence of the Earth tides (Simon et al., 2020).

The maximum of H₂ is very rarely found in the center of the SCD (Larin et al., 2015, Zgonnik et al., 2015, Prinzhofer et al., 2019, Moretti et al. 2021a). Myagkiy et al. (2020a, 2020b) interpreted these low values in term of soil permeability. Indeed, the gas transport in the soil is influenced by the water content and the permeability, these parameters have a strong influence on the velocity of the H₂ upward transport. In the center of the SCD, the soil is usually more shaly and some permanent water pool may be present, at least in the rainy season. Since the microorganism activity results in a large consumption of H₂ near the surface, a slow gas velocity will result in an increase of the time spent in the soil and so in a major consumption of the H₂ implying a lower near surface H₂ content.

## 2-4 Place of the world where H₂ has been already measured and tentative source rock correlation

Hydrogen onshore has been first described in ophiolitic context such as Oman or New Caledonia (Deville and Prinzhofer, 2016). The presence of carbonate and brucite, chemically formed by the water/oceanic lithosphere interactions, is the usual marker of the reaction. In addition, H₂ emitting SCDs are observed in many places, and H₂ has also been measured in mines and wells. The maps that synthetize these confirmed presences of H₂ onshore are regularly upgraded since the first publications by Sherwood Lollar et al. (2014), Prinzhofer & Deville (2015), Zgonnik (2020). Onshore, in continental crust, it has been noted that the H₂ emanations are usually above Archean or Neoproterozoic cratons (Moretti et al., 2021b) and Geymond et al. (2022) proposed to link these H₂ generation with the presence of iron rich series. Indeed, the BIFs deposited in the anoxic oceans of the Archean-Paleoproterozoic or during the Snowball Earth events of the Neoproterozoic are very rich in ferrous iron and could also be good potential source rocks.

The rather well-known H₂ emanations measured in surface or subsurface and cited in this paper, are synthesized Figure 1 where we highlight the more likely sources.

As explained before, the sources of hydrogen could be multiple and data are missing to discriminate the main one. The single criteria is often the spatial correlation between the H₂ presence and the known subsurface rocks. In Africa, numerous Archean and Neoproterozoic Cratons outcrop, hydrogen has been already found and it is produced in Mali (Prinzhofer et al., 2018). The presence of H₂ dissolved in the underground water of the Precambrian shields has also been measured, the values being much higher than within the younger aquifers (Lin et al., 2005). In the Orange Free State, South Africa, Sherwood Lollar et al. (2015) proposed both radiolysis and hydration, namely
serpentinization, as sources for this H₂. More recently, again in the Free State, Karolyte et al. (2022) studied the gases that currently escape from a ten of wells. These gases are a blend of CH₄, N₂ and He; the methane is abiotic and interpreted by the authors as the mark of the H₂ initial presence, the cogeneration being due to radioelement decay (He and may be N₂) in presence of water, which allows H₂ generation by radiolysis, in the area. These different H₂ potential sources are obviously nonexclusive and, considering the size of South Africa, the main one may differ depending on the area. Namibia borders South Africa to the West and has some common geological features that motivated this study. In addition, SCDs are numerous and clearly visible on space images in this country.

### 3. Namibia

#### 3-1 Geological setting

Namibia is located in the southwestern part of the African continent. The country spans over Paleo-to Meso-Proterozoic Congo and Kalahari Cratons as shown in Figure 2. These cratons are separated by the Damara Orogenic Belt of Neoproterozoic age. Along with the Kaoko Orogenic Belt (and the Gariep Belts out of the border of Figure 2), these belts are gathered under the term "Pan African Orogeny" and mark the colliding of Kalahari, Congo and Rio de la Plata (South-America) cratons during the formation of Gondwana supercontinent at the end of the Neoproterozoic (Frimmel et al., 2009). As such, the Damara Belt records a complex tectonic history that can be summarized in three steps: (i) Rifting period initiated around 1000 My leading to the formation of NE-SW rifting basins; (ii) Subduction of the oceanic crust under the Congo Craton; (iii) Collision between Congo and Kalahari Cratons and exhumation of former oceanic material around 550 My (Cairncross et al., 1997; Nascimento et al., 2016). Major NE-SW oriented faults are inherited from these tectonic events and delineate the different Neoproterozoic tectono-stratigraphic zones:

* In the North of the country, the Otavi Platform corresponds mainly to post-rift units overlying the Meso-Proterozoic basement of the Congo Craton. The stratigraphy is mainly constituted by proximal marine material deposited in a continental platform context, namely the Otavi Group (770-600 Myr). This Group is dominated by carbonates and intercalated ironstones and diamictites. These formations of Cryogenian and Ediacaran ages are likely related to Snowball Earth events. Higher in the stratigraphy, the Mulden Group overlies the Otavi Group and corresponds to molassic sediments concomitant to latter obduction (Borg, 2000).

* Southward, in the center of the country, the Damara Orogenic Fold Belt is separated from the Otavi Platform by the Khorixas-Gaseneirob Thrust, a former normal-fault remobilized during the orogeny. The syn-rift Nosib Group constitutes the basal unit. It corresponds to rift filling by fluvial clastic sediments and evaporites in alternance with magmatic series of 750 My that constrain the age of basement rifting as well as age of first deposition (Borg, 2000; Lehmann et al., 2016). The Nosib Group is overlaid by distal marine deposits of the Swakop Group. Within it, siltstone and quartzite alternate with carbonates originating from marine transgressions that induced the growth of shelf carbonates in much deeper environments
(Borg, 2000; Lechte et al., 2019). The presence of diamictites related ironstones in Chuos and Ghaub formations must also be pointed out in the Swakop Group. These facies are temporally correlated with those of the Otavi Group in the Otavi Platform (Lechte et al., 2019), but are thinner, in line with distal marine environment. In the South of the Damara Fold Belt, mafic rocks and pelagic sediments of the former mid-oceanic lithosphere are exhumed, highlighted by the Matchless Amphibolite Member (Borg, 2000). The Central Zone of the Damara Belt records extensive magmatic activity dated to 560-500 My, consisting in granitic pluton emplacement and considered as syn-orogenic (Meneghini et al., 2014).

* In the South of the country, Paleoproterozoic rocks of the Kalahari constitute the basement and are separated from the Damara Fold Belt by the Areb Mylonite Zone (AMZ). There, the Nama Group overlies the basement and is made up of carbonates and clastic sediments deposited from 560 to 530 Myr, corresponding to the foreland basin of the Damara orogenesis (Borg, 2000).

Finally, all these Precambrian terranes are covered or intruded at various extents by magmatic and sedimentary rocks of much younger ages. Indeed, sedimentary rocks related to the Karoo Supergroup of South Africa span over a wide part of the eastern part of Namibia and are dated to Jurassic. In parallel, Cretaceous granites likely linked to the opening of South Atlantic Ocean are signaled mainly in the western part of the country (Will et al., 2016).

## 3-2 Potential H₂ source rocks in Namibia

By comparison with the other areas where H₂ has been detected and considering the main reactions that may produce H₂ by water/rock interaction, there are at least 3 potential H₂ source rocks in Namibia: (1) the BIF from the Neoproterozoic (Chuos and Ghaub Fms in the Damara Belt) and Paleo to Mesoproterozoic (in the Kalahari and Congo Cratons), (2) the Green Belts (for instance the Marmora oceanic terrane in the Gariep orogenic domain or the Matchless Amphibolite Member in the Damara orogenic domain), and (3) granite intrusions in the north (syn and post-orogenic Damara Granites).

The present work is not focused on producing an exhaustive list of the Namibian H₂ source rock neither on their characterization however the presence of these facies let to be optimistic concerning an H₂ system in the country. For instance, the Neoproterozoic Chuos (740 Myr) and Ghaub (635 Myr) iron-rich formations are a matter of interest since they are likely to constitute the basement southwest of Okakarara where SCDs are numerous (see section 4.1). They are inherited from the two Cryogenian glaciations, respectively Sturtian and Marinoan (Bechstädt et al., 2018), with an interglacial carbonate formation between both (Le Heron et al., 2013):

* The Chuos Formation ironstone facies precede, and are vertically gradational into, complete diamictites. These ironstones contain evidence for tractional processes (large-scale cross bedding) and biogenic growth (stromatolites) in strata rich in magnetite and hematite. The Chuos Formation has a high iron potential. It is formed by alternations between diamictites and ironstones. Diamictites are metric to multi-decimetric detritic units, whereas ironstones are less powerful, a few meters, when outcropping. The ironstones can reach values above 60% FeO (wt%) whereas the diamictites are more variable, between 2 and 65 FeO (wt%) (Lechte et al., 2019). Different types of ironstones are observed: (Fig 3A) the fine laminated ones, paler, showing a plurimilimetric alternance between siltstones, cherts and iron oxides (hematite and
magnetite); (Fig 3B) centimetric laminations, darker, between cherts and microcrystalline magnetites; (Fig 3C) finally centimetric to decimetric beddings between cherts and jasper levels, magnetites are present in layers or around the jasper levels. Mineralogically, the composition of Chuos ironstones has been described as simple, essentially constituted by magnetite with minor amount of hematite and siderite (Lechte et al., 2019). On the contrary to hematite that only contains Fe³⁺, the presence of magnetite as well as siderite that contain Fe²⁺ may correspond to a potential source for H₂ generation.

In the diamictites, massive or foliated, the proportion of clasts is extremely variable and sizes range from 2 mm to 1 cm and up to 1 m (Nascimento et al. 2016). Depending on the lateral position, the source and the lithology of the clasts change (Figs 3D & 3E) between more or less rounded clasts of quartzite, siltstone, dolomite, granite, andesite and basalt from the Ugab subgroup, Nosib group, Ombombo subgroup, Congo crystalline basement (Le Heron et al. 2013; Lechte et al. 2019; Nascimento et al. 2016). The matrix, rich in magnetite or hematite in places, is a sandstone with more clayey levels (Fig. 3F) and traces of detrital dolomite, feldspars, quartz and aluminosilicates (Lechte et al. 2019). Sometimes euhedral magnetites (5 mm) (Fig. 3G) are present in the siliceous matrix of diamictites. These large magnetites are visible at the top of some diamictite levels and could be interpreted as secondary recrystallisation following alteration processes, as these levels are also associated with hematite crusts.

* The Ghaub Formation has a lower iron content. It is characterised by a high lateral thickness variation from 0 to 150 m (Nascimento et al. 2016). We observe a succession of diamictite with a carbonate matrix (dolomite or calcite) more or less rich in clasts (Fig. 3H) mainly carbonated (Bechstädt et al. 2018; Domack & Hoffman, 2011) interbedded with mudstone (Fig. 3H) and coarse conglomerates marked by a ferruginous alteration (Domack & Hoffman, 2011). Depending on the lateral position, the source of the clasts changes from carbonates of the Abenab Subgroup and Auros Formation, to sandstones and quartzites of the Nosib Group and finally polycrystalline rocks (granite, diorite) of the Grootfontein metamorphic complex (Bechstädt et al. 2018). The alteration of this formation has led to the enrichment of the matrix with pyrite depending on the area.

### 3-3 Fairy circles and depressions from the space images:

In Namibia, circular features shaping the landscapes have already been a matter of interest. Some of them exhibit the usual gap of vegetation, are flat in topography and measure only few meters in diameter. Their origin remains a matter of debate but works suggested that these rounded structures originate in soil dryness resulting in vegetal preferential organization (Tschinkel, 2015). The ones we studied which are related to gas escape are much larger and not flat, they are easy to differentiate. In parallel, other rounded features have also been pointed out concomitant with sand dunes, especially in the southeastern part of the country. Their shape is rather ovoid than circular and they are typically of few hundreds of meters in diameter and tens of meters in depth. In literature, these structures called Salt Pan are assumed to originate from wind flows blowing out the ground to transport sand particles further and built up the dunes (Hesp, 2002; Telfer & Thomas, 2007; Barchyn & Hugenholtz, 2013).

As stated before, we study the possibility for Namibian territory to source H₂ emissions since many SCDs can be detected through satellite imagery in addition to this favorable geological setting. We present in this paper data from two areas displaying many SCDs and located into the favorable
geological context, namely old cratonic rocks covered by young Cenozoic sedimentary rocks. These
areas are located (1) in the inner region of the Damara Fold Belt, south of Okakarara City and (2) in
the Kalahari Desert in the East - South East of the country (location Fig. 2).

4. New field data: SCD characteristics and H₂ Emanations

H₂ has been measured in the soil using a BIOGAS5000 after positioning in the soil an about 1 m long tube with some voids in the lower part. The protocol is the same as the ones used by Larin et al (2015) Zgonnik et al. (2015), Prinzhofer et al., (2019) or Frery et al. (2021) among others in their various field acquisitions. Our analyzer is measuring CH₄, O₂, CO₂, in % and CO, H₂S and H₂ in ppm. In case of water presence or of high H₂S content we use filters in order to protect the detector and so the H₂S value is meaningless. The H₂S never exceeds few ppm in Namibia and this filter was thus not necessary.

We tested various ways to drill and we didn't notice any systematic difference in the H₂ content,
neither H₂ emissions, due to drilling in the various soils encountered during this field work in
Namibia. All data acquired are reported in the supplementary Table_S1. In all the structures we
measured few, when not many, zero values that seems the mark of not artificial H₂ generation by our
tools. It could be the case in other soils, Halas et al (2021) observed H₂ generation due to rotative
drilling in a specific layer on the ground in an area (south west of France) apparently without any
deeper H₂ flow. Frery et al. (2022) noted also some slight variations between rotary and percolation
drilling mode but they remained low, and lower that the variability of the H₂ content in the soil
observed during the monitoring (Prinzhofer et al., 2019, Moretti et al., 2021a). In Namibia, we have
also to note that calcareous concretions were often present, very hard to drill and precluding 1 m
deep perforation with the material we had. When the measurements have been done at a significant
shallower depth it is noted in the Table_S1.

4.1 Area of Okakarara

Based on space images the area (Location Fig. 2) shows a high density of SCD which have more or less
the same characteristics than the ones related to H₂ emissions in Brazil, Russia and Australia (Fig. 4).
The average equivalent diameter is around 100 m and the footprint in the studied area around 1 %.
The depth/equivalent diameter ratio is about 1.4% as in Brazilian and Russian H₂-emitting SCDs
already published (Moretti et al., 2021b).

The H₂ content in the soil of various SCDs was measured and all structures show some H₂ content,
knowing that the signal is variable in space and time as described previously, we then focused our
measurements on 2 of them. Figure 5 shows the pictures of these two SCD, named SCD_6 and
SCD_7, as well as their Google Earth images. The SCD_6 is an almost perfect circle with a radius of 88
m, a relative maximum depth of fewer than 3 m and an area of 24800 m². The SCD_7 is less circular,
rather flat (the depth is around 1 m) and smaller, 7300 m², the equivalent radius is around 48 m. In
both cases, the number of trees within the SCD was very low compared to the area around, see on
figure 5 the pictures and the space image, but not null. In March 2022, it was raining almost every
day (for a short period at the end of the afternoon) and the vegetation was not rare. The center of
the SCD_6 was full of rushes and the soil was rather shaly. All around the trees were present and the
soil sandier. We went on three different days at different times in these two structures, Figure 5
shows the map of the H₂ maximum content we measured in the soil at a given coordinate, without
regarding the time. Each day is highlighted by a different form (1, 2 or 8th of March 2022).
The values range from 0 to 400 ppm and are, as usual, highly variable. In the center, values are low and the highest H₂ contents are found in the periphery of the SCDs. These maps of maximum display are rather coherent signal even if, as already noted, there are also many low values.

It can be seen in Figure 5 that some H₂ has also been measured in the soil outside of the area without vegetation, and even at a few tens of meters from the SCD, especially for the SCD_7. A ring of dense vegetation could be also noticed around the area without trees. We will discuss later what it means in terms of the external border of the H₂ emitting zone.

The two structures are located at 6.6 km of distance. Their behaviors are rather similar even if higher H₂ contents are registered on the SCD_7. Since we spent more time and did more measurements in this structure and because the signal is highly variable also in time, it could be just a question of “luck”. Globally we may consider that the SCDs of this area are daily emitting hydrogen.

## 4.2 Kalahari

Other acquisitions have been done in the basin above the Kalahari Craton. Based on the same pre-field trip space image analysis, we noticed many SCDs in the area and so went to the Kalahari zone (location Fig 2). This area is characterized by N150/N155 long dunes perpendicular to the principal wind direction. The distance between the dune crests is around 550 m and there are many SCDs in between. Eastward toward Botswana's border the number of SCD remains very high but the dunes are less pronounced and are N-S oriented. As already stated, these SCDs are usually considered as Salt Pan, i.e. sebkha a feature due to evaporation in this arid environment (Hesp, 2002; Telfer & Thomas, 2007; Barchyn & Hugenholtz, 2013).

The mapping and the morphological statistics are presented figure 6. The structures are in average larger than the ones in the Okakarara zone and deeper. The average equivalent diameter is 421 m, the maximum about 1570 m and the maximum depth reaches 12 m. The ratio depth/equivalent diameter is about 0.75% which is smaller than in the Okakarara zone but on the same range of values. Figure 7 shows some pictures of these SCDs and profile across two of them. Figure 7C illustrates how the dunes visible on Figure 6 look like on the field.

We spent 2 days in this area, and measured 40 points within 7 different structures without measuring any H₂ (the maximum has been 7 ppm) and so we doubt that H₂ emanations exist in this zone. However, to avoid concluding too quickly, we have to note that the soil of all these depressions was very hard to drill and the surface was covered by a hard carbonated deposit that was impossible to drill through and our sampling tool never reached one meter deep. Deeper wells may be required to confirm this lack of H₂, however this calcrete can't be considered as a seal and so gas flow is reasonably possible through this soil.

# 5. Discussions: Gas concentration in the emitting structures

## 5-1 H₂ upward transport versus time

As stated in section 2.3, the data of all the H₂-emitting SCDs are variable in time and in space. As a result, many measures are mandatory in order to know which structures are emitting and to start to have an idea of the emission per day. In fact, weeks of measures should be ideally realized. Permanent monitoring has not yet been installed in Namibia but many measurements have been done on the SCD_6 and SCD_7 in order to quantify the variability of the signal. Figure 8 shows the results presented as function of the hours and a comparison with the data of some days of measurement in the Brazilian monitored structure (Fig. 8A).
We have to note that the two points at the end of the afternoon on the first of March have been measured at shallower depth since we had a drilling issue and so have to be considered as a minimum and with caution. We don't have night data.

The increase of the signal, and so of the H₂ flow toward the surface, in the middle of the day is imaged by this new data and seems to confirm the breathing of the soil initially evidenced by the first monitoring in Brazil - 2 months of measurements with 10 sensors (Prinzhofer et al., 2019) and better defined by the second one - 6 months of measurements with a hundred of sensors (Moretti et al., 2021a). The Namibian data, even partial since the night signal is missing, clearly confirm the phenomena. As already stated, it could be explained in terms of a decrease of pressure in the near soil atmosphere during the hottest part of the day resulting in a pumping effect (Cathles & Prinzhofer, 2021).

In comparison with the work done in Brazil, due to the too short sampling period, we didn't catch stronger pulses of H₂ that happen more sporadically, at least considering the Brazilian case as representative of all this kind of structures behavior. A 24h period for the "regular" signal will be perfectly compatible with this new data in Namibia.

These results have two implications, in terms of the H₂ potential of the Okakarara zone, H₂ is generated in subsurface in this area. One may note that the maximum fund by chance in the SCD_6 is superior to the daily one in Brazil which rarely overpass 200 ppm however the current data set does not allow us to quantify the flow. A permanent monitoring should be scheduled if a quantification of the leaking H₂ by days need to be done. In addition, this near surface H₂ content is strongly affected by the microorganism activity within the soil (Myagkiy et al., 2020a, 2020b) and a characterization of the microorganisms in this specific soil should be also carried out. We consider that we cannot deduce anything regarding the flow of H₂ from the currently available dataset. However, by analogy and since the flow from the structure monitored in Brazil has been evaluated as over 600 kg/day for a structure of 220000 m², the measured values allow us to be optimistic about the quantity of H₂ leaking in this part of Namibia.

At this stage, different hypotheses could be proposed for the H₂ flow origin. It may correspond to emission directly related to a subsurface generation, to a dismigration from a deeper accumulation, or a couple of accumulations. In addition, the transport mode of H₂ remains also an open question, if the hydrogen is in a gas phase, the transport is likely rather vertical, and the source or intermediary accumulation has to be looked for below the SCD. If the hydrogen is dissolved in water, the H₂ may migrate from another area, the solubility of H₂ when the pressure increases potentially allows long distance migration in deep aquifer (Lopez et al., 2019). Acquisition of subsurface data will be necessary to image potential structures and thus potential accumulations below the area where the SCDs are present and also to know at which depth the potential H₂ source rocks of the Chuos and Ghaub formations are buried in the drainage area of this zone.

## 5-2 Minimum sampling requirements

If we are restricted to surface acquisition, the question of data representation is obviously important. In exploration, data collection aims to reduce uncertainties. An accumulation of data that does not reduce doubt about the resource is of no value. It is therefore mandatory to have a sampling strategy and to have a clear view of what could be deduced from each data set.
Concerning the sampling strategy, the variability of the H₂ content in the soil reinforces the need to make numerous measurements at different times of the day and at different locations in the structures. It is wrong to conclude that a structure does not emit H₂ by having just a few points without H₂, especially if the measurements have been done early morning or late afternoon. If a permanent monitoring with many sensors is not possible, building the envelope of the measurements versus time over a couple of hours as presented figure 8 seems us a minimum, as well as mapping the maximum in various locations of the structure at various time, and ideally various days.

We interpret the large number of “zero” within a structure as the mark of the transport mode of the gas, having as analog gas bubbles in a lake. Taking into account the power of the pump of the BIOGAS5000, about 400ml/mn, the volume of soil which is sampled during a measurement is very small, the distance to the tube is no more than few cm. In Brazil it has been computed that the investigated area is of 8 cm² so a distance of less than 2 cm around the tube (Moretti et al. 2021a). This value depends on the soil permeability but the range of value will remain very low, at centimetric scale, in all soil. This point is crucial to interpret the results: the BIOGAS5000 measurements cannot be considered as the average H₂ soil content. We capture a snapshot of a non-permanent signal that has another temporary and spatial logic than our measurements. During the high-density permanent monitoring done in the Brazilian structure, the distance between the permanent sensors was around 80 meters, even with this density the values given by each sensor were poorly correlated and the number of “null”, H₂ content less than 20 ppm, was high every day (Moretti et al., 2021a, see figure 7 in this article). The variability is real and we need to explain it not to hide it by claiming that it is due to our tools, which obviously may also generate temporary perturbation in the soil. The simple model of a single daily pulse (Maygkyi et al., 2020a and 2020b) has the advantage to quantify the impact of the soil and soil bacterial content on the H₂ transport but the initial signal (a 6 hours long gas escape every 24 hours) which was an input in this model is more complex in the reality.

To sum up, regarding estimations of the H₂ emanations, only the multiplication of the measures in both time and space may give us a more or less complete picture. Any interpretation of an isolated value or even of a few values may lead to erroneous conclusions. Since the time available for field acquisition may be limited, it so may be interesting to look for additional data that will allow us to better anticipate the presence of hydrogen through a characteristic sensitive to the mean hydrogen flow and not to individual occasional pulses, a value that average the H₂ signal.

## 6. H₂-emitting and no emitting SCD characteristics: Use of vegetation index

Up to now the SCDs have been imaged relying on a lack of vegetation as the main characteristic. In addition, a proxy based on size, size distribution and slope has been proposed to differentiate these structures potentially related to gas emission to other depressions (Moretti et al., 2021b). These morphological characteristics are far from being discriminatory. We didn't measure any H₂ in the Kalahari, and other researchers also mentioned SCDs with promising morphology but without H₂ when a single point monitoring has been done (Halas et al. 2021). The identification of other characteristics, such for instance features that could be seen on satellite images whose processing can be automated, should accelerate the recognition of these structures. Accordingly, we worked in this way and because the first criterion is the gap of vegetation, we tested the vegetation indexes that could be measured or computed, on satellite data.
The other advantage that can be expected from this approach is to get an average over time. The vegetation doesn't grow or die within a week and so if the H₂-emitting structures display a specific signal on some satellite images it may be more representative of the leakage over months. In addition of the size, depth and slope, we tested the Normalized Difference Vegetation Index (NDVI), Soil Adjusted Vegetation Index (SAVI) and the Normalized Derived Built-up Index (NDBI) in the areas where H₂ leakage has been proven, or not been found, in the soil of the SCDs.

## 6-1 Remote sensing data.

### Digital elevation model

In addition to the easy to access Google Earth Images, a Digital Elevation Model (DEM) has been used for the topography of Namibia. It is the instance GLO-30 of the Copernicus DEM provided by the European Space Agency, based on the radar satellite data acquired during the TanDEM-X Mission between 2010 and 2015. Its resolution is 30 meters. The slope on the border of the SCD has been computed using this DEM under the software QGIS®.

As morphological characteristics three values are so available: the image (in the case of the SCDs the gap of vegetation is the more obvious information), the elevation and the slope. This data has been combined with Landsat images.

### Satellite imagery

The images used for this study are HLS Landsat Operational Land Imager Surface Reflectance v2.0 acquired by LANDSAT 8 with the Operational Land Imager (OLI) sensor in 2016. They all include atmospheric correction, cloud and cloud-shadow masking, spatial co-registration and common gridding, illumination and view angle normalization, and spectral bandpass adjustment. They are homogenized at a spatial resolution of 30 meters. The surface reflectance bands used are as follows:

* Band 1 - Coastal Aerosols - 433-453 nm - 30 m resolution

* Band 2 - Blue - 450-515 nm - 30 m resolution

* Band 3 - Green - 525-600 nm - 30 m resolution

* Band 4 - Red - 630-680 nm - 30 m resolution

* Band 5 - Near Infrared - 845-885 nm - 30 m resolution

* Band 6 - Short Wave Infrared - 1560-1600 nm - 30 m resolution

### Index-derived images

These raw data have been combined to compute indexes. The Normalized Difference Vegetation Index (NDVI) is calculated from the Red and NIR bands and is used to assess the vegetation and its health. Its value ranges between -1 and 1, with -1 to 0 representing dead vegetation, building areas or body of water. At the opposite, 0 to 1, represents more or less healthy (active) and dense vegetation. It is widely used in agriculture to monitor crops health or droughts. It is calculated as follows:

$$NDVI = \frac{NIR - red}{NIR + red} \qquad (1)$$

However, it is sensitive to the reflectance of the soil when there is a low or no vegetative cover. To overcome this limitation, Huete (1988) proposed the SAVI, acting as an image dynamic range
enhancer. The introduction of a correction factor L allows to account for no to moderate vegetation densities, with L = 0, 0.5 and 1, corresponding to high vegetative cover (NDVI equivalent), moderate vegetative cover, or no vegetative cover, respectively. Ray (1994) showed that NDVI can only work effectively in areas with plant cover above 30 percent, while SAVI can work with plant cover as low as 15%. As we estimated the vegetation cover in our Namibian studied area to be around 30%, we used a value of L=0.5 (Xu, 2007). The SAVI index varies between -1 and 1. It is calculated as (Huete, 1988):

$$SAVI = \frac{(NIR - red)(1 + L)}{NIR + red + L} \quad (2)$$

Finally, we also use the NDBI. This index uses the SWIR and NIR band to show the reflectance for certain types of vegetation; the index is higher for dry vegetation and water with high suspended matter concentration (Gao, 1996).

$$NDBI = \frac{(SWIR - NIR)}{(SWIR + NIR)} \quad (3)$$

The coastal aerosol band has also been studied, because it is supposed to show a modification of the reflectance due to dust, smoke and water particles in the air. The human activities, a quarry or a track, are obviously very visible on this type of index but we suppose that the emission of fluids and gases like H₂ could also locally modify the reflectance and we wanted to test it.

## 6.2 Appearance of the H₂ emitting SCD vs the non-emitting structures

Maps of six different characteristics, three morphological and three deduced from the remote sensing indexes, on one H₂-emitting SCD (in the Okakarara area) and a depression without H₂ signal (Kalahari Desert) are presented figure 9 and 10.

In the Okakarara area where the SCDs related to H₂ emissions are numerous we did the analysis around the SCD_6. On all the six images, this H₂-emitting SCD_6 is clearly visible (Fig. 9). The morphological ones correspond to already known characteristics: appearance on Google Earth Image (Fig. 9A), topography on the DEM (Fig. 9B), and depressions through the slopes that border the structure (Fig. 9C). They reach 2.7° when the area around is rather flat and the fact to compute it on 3D DEM allows an automatic extraction of the values. Besides, and it is the new feature of this study, the SCD_6 is also visible on the vegetation indexes (SAVI and NDBI) and on the coastal aerosol band.

The SAVI image (Fig. 9D) shows a general background SAVI of around 0.18, (green colors) and some areas with a lower value of around 0.14 (blue colors). The lowest values on the figure correspond to the road. The SCD_6 shows higher SAVI values, around 0.22. The circular shape of the SCD is preserved in this index and the outer part shows increased vegetative activity. At the center of the SCD, we observe the highest value of around 0.27, which may correspond to a dense cane patch observed on the field. One may note that the areas without vegetation related to river beds eastward and westward have a low SAVI of around 0.1 and are easy to recognize from the H₂ emitting structures although on Google Earth Image they look very similar.

On the NDBI index image (Fig. 9E), we observe a general background of around 0.07 (medium blue colors), with locally higher value areas of about 0.14 (dark blue). The circular shape of the SCD associated with SCD_6 can be seen again, with low values around 0.00, which would be interpreted as a sign of less dry vegetation. The low values (0.02) at the center of the SCD, can be also attributed to the cane patch.
On the coastal aerosol band (Fig. 9F), the background value is around 0.05 (light green), with local patches showing higher value of 0.08 (dark green), corresponding to the lowest values of SAVI and the highest of NDBI. The highest values of 0.15 correspond to the road which is not asphalted. The circular patch corresponding to the SCD_6 is again visible, with low values around 0.04. The center of the SCD shows lower values around 0.03.

One of the interesting points is that the vegetation indexes and the aerosol coastal band show a specific signature on a zone that is larger. As a reminder, the SCD is up to now mapped based on the area without vegetation. The high SAVI values define a ring of healthy vegetation around the zone without tree. This ring has a width of about 35 m, which is close to the resolution limit of our data but the ring is visible in all Figures 9D, 9E and 9F. The ring touches the road whereas the area without vegetation, the one that we mapped originally as the SCD, is separated from the road by about 40 m of trees. The map of the H₂ content (Fig. 5) shows that some of the highest H₂ ratio in the soil have been measured in this ring. The SAVI, NDVI and aerosol show all 3 zones: the center of the structures, a first ring (the area without vegetation) and a second ring. This external ring is the area with an increase of vegetation also visible on the Google Earth Image. Interestingly, little attention has been paid to this ring until now because the absence of vegetation is a more obvious characteristic, especially when we are in the field. A treeless area attracts the attention but the area affected by the gas presence is clearly the whole of the two.

The highest slopes are roughly at the border between the two rings. Since the SCD in the Okakarara area are rather small (see Fig. 4) and the ones we have sampled have a diameter lower than 200m it is impossible to be more accurate on that point for this Namibian zone.

One may note that the other SCD visible in Fig 9, South East from the SCD_6, presents the same characteristics.

The same six maps are presented on figure 10 in an area of the Kalahari Desert (see location Fig. 6) where none of the measures done in the soil found any H₂ content. Various structures are visible on the Google Earth Image (Fig. 10A) and on the DEM (Fig. 10B). The N-NW/S-SE dunes are also clearly imaged on the elevation and the slope values (Fig. 10C). The structure is not very deep (about 1 m) and the slope remains lower than 2° but the border of the SCD is still visible on Figure 10C. At the opposite, none on the other indexes allows us to guess a structure (Figs 10D, 10E, 10F). The vegetation doesn't highlight any difference from the surrounding area and the coastal aerosol is completely homogeneous in the area.

These contrasted values, especially the one on the SAVI, that allow differentiating the area without vegetation not related to H₂ emanation (Figure 10D but also river bed Figure 9D) from the areas with H₂ appear to be an additional proxy potentially relevant for early H₂ exploration.

## 7. Conclusions

This study allows us to draw conclusions on the natural H₂ resources in Namibia but also on H₂ exploration methodologies.

### Namibia:

The presence of cratons, both Neoproterozoic and Archean/Paleo-Mesoproterozoic, and of iron-rich formations coupled with numerous SCDs observable from space images suggested than Namibian have potentially natural H₂ resources. The collected data presented here confirms it. In the Damara Orogenic Belt, especially in the Okakarara area the SCDs are related to H₂ emanations. The
area where H₂ is currently escaping is rather large, more than 300 structures are present in an area of about 180 km². The iron-rich Chuos and Ghaub formations, from the two glaciations of the Neoproterozoic are very likely present in subsurface in the drainage area of this zone and could be the source of the emanations. The petrological characterization of these rocks as well as the other H₂ potential source rocks remain to be done.

Other places in Namibia need to be explored in order to have an exhaustive view of the various promising basins. At this point, we didn't collect any data south or near the green belt in the country, for instance close to the Matchless Amphibolite Member. We measured few points around the Etosha zone (location Fig. 2), the H₂ content were low but our data are not numerous enough to conclude anything. Southward, the Archean craton near the South African border will also deserve additional data acquisition.

On the Kalahari Desert, in the area westward from Leonardville, we didn't find any H₂ emanation although many structures were observed on space images. At that stage, evaporation not related to gas escape is the more likely cause of these Salt Pan, as it has been classically considered (Hesp, 2002; Telfer & Thomas, 2007; Barchyn & Hugenholtz, 2013).

**H₂ Exploration in area with SCD:**

In Namibia, the H₂ content in the soil is variable in time and in space. This H₂ content follows the same trend that already observed in the H₂ related SCD in Brazil: the values are often low in the center of the structures and there is a daily variation of the H₂ flow with a maximum around the middle of the day in the two structures where we have enough data to conclude (respectively more than 70 and 30 measurements, done at various times over 3 days). We suggest always measuring many points and doing profile versus time, in addition to the maps or cross-sections. The signal being variable an envelope of the values is surely more representative of the structure behaviors when permanent monitoring is not possible. We are very circumspect about the interpretation, not to say the over-interpretation of these values of soil content as quantitative. At our current level of understanding of these emanation phenomena, values of 200 or 400 ppm on the BIOGAS5000 analyzer have the same meaning.

The measurements, done by pumping one or two minutes into a soil, characterize only a few cm around the stem. These microseepage values are by no means an average of the percentage of hydrogen in the soil. They only indicate that H₂ is generated and/or leak from a reservoir below the zone of H₂ emitting SCD. Measurements in fault zone connected to a reservoir or a generation zone and that correspond to seepage will more likely present high values of H₂ content, and also more constant values. It is what has been observed in rift zone (Pasquet et al., 2022) or in the thrusts of compressive zones (Lefeuvre et al., 2022). The sketch Figure 11 synthetizes these differences between seepage and microseepage already introduced by previous authors for the hydrocarbon seepages (Schumacher, 1996). The figure has been drawn with a structural trap as the source of the hydrogen but it could be either a H₂ generation zone or a deep aquifer from which the H₂ is degazing. The various characteristics related to the morphology (size, depth, slope) as well as vegetation (weak/dense, SAVI, NDVI) are all impacted by the gas microseepage.

The exact position of the slope break, versus the SAVI and NDVI anomalies can not be precisely defined of the studied area of Okakarara since the SCD are small in comparison of the Landsat image resolution we used. It rises the question of what we have to consider as the H₂-emitting feature, the depressed zone, the one that have been mapped so far in the SCD statistic, or the external ring of positive SAVI anomalies. The maps of the measured values for SCD_6 and SCD_7 suggest different
answers. On SCD_7 the emitting area includes the halo of dense vegetation. It is not the case for SCD_6 but our data set is limited. A few days long monitoring will be also mandatory to conclude.

## New Proxy

Up to now the SCDs have been defined as an area without vegetation, sub-circular and slightly negative in terms of reliefs. The comparison realized in Namibia between the H₂-emitting SCDs and the others SCDs using DEM, slope, vegetation indexes (SAVI and NDVI) and coastal aerosol band shows that these indexes may allow a more accurate definition and/or another *a priori* criteria when starting field acquisition. SAVI and NDVI both highlight rings in case of H₂ emitting structures that is not the case for the non-emitting SCDs of the Kalahari. A halo of healthy vegetation, high SAVI, low NDVI, around the area without tree that are used to define the SCD appears to be systematic in case of H₂ emission (Fig. 11). Presence and morphology of the SCD are not the only characteristics to check before targeting an area. The healthy vegetation ring around the emitting structures appear to be an additional good proxy that is rather easy to test before field acquisition. In the studied SCDs in Namibia, the center of the structure display also high SAVI values that remain to be characterized and understood. The resolution limit of 30m is not universal, in many countries the resolution is much better and should allow a good, and rather automatic, mapping of all the structures.

Knowing that hydrogen is generated in subsurface in Namibia, exploration efforts may continue. Local need of energy for the mines industry and infrastructure development for the projects of green H₂ production from electrolysis would help for a rapid growth of a hydrogen industry in this country.
**Credit Authorship contribution statement:**

Field acquisition in Namibia: UG, GP, IM. Data curation and formal analysis of the GA results: UG.
Satellite image processing: LA (master student at Sorbonne University) under the supervision of AR
(Geosubsight, SU). Conceptualization and methodology: IM, All the authors participated to the
writing of the paper.

**Acknowledgement:** We really thank Laurie Barrier (IPGP) for sharing are expertise on Namibian sand dunes and sedimentology, as well as for providing the DEM. We also grateful to the three reviewers, A Prinzhofer, C Boreham and E Gaucher that allowed the improvements of the manuscript.

**Funding:** G P PHD founded by the Isite E2S and the field trips by the Carnot isifor, UG PhD is founded by the MESRI (Ministère de l'Enseignement Superieur et de la Recherche)
**Table 1:** Synthesis of various potential origins for H₂ emissions onshore with some references

**Figure captions:**

**Figure 1:** Map of well documented H₂ emissions worldwide, depending on their nature. Precambrian cratonic areas are colored in dark gray.

**Figure 2:** Simplified geological map of Namibia. KGT corresponds to Khorixas-Gaseneirob Thrust. AMZ corresponds to Areb Mylonite Zone.

**Figure 3:** Chuos Formation from (A) to (G) and Ghaub Formation with (H) (A) Fine laminated ironstone plurimillimetric layers of chert (dark blue), siltstones (cream) and iron oxides (reddish brown). (B) Centimetric laminations of cherts (dark blue) and microcrystalline magnetites (black). (C) Centimetric beddings between cherts (dark blue) and jasper (purple red). (D) Diamictite ferruginous matrix with pluricentimetric granitic clasts. (E) Sandy diamictite with centimetric clasts of granite, dolomite and siltstone. (F) Diamictite clay matrix with siltstone clasts. (G) 1 to 5 mm euhedral magnetites in sandy diamictite. (H) Ghaub formation, succession between carbonate matrix diamictite (left) and mudstone (right).

**Figure 4:** Global view zone 1 (location Fig. 2)

A- Map of the subcircular depressions base of space images and contours of the area where they have been mapped

B- Statistic on size presented through the equivalent diameter of perfectly circular depression with the same area.

C- Relationship depth/equivalent diameter for this zone.

**Figure 5:** Details on two SCD (left: N°6 and right n°7, location Fig. 4)

A_ Space image appearance of the SCD, note the difference of vegetation within the depressions.

B_ Picture on the field (March 2022), the depressions was not without any vegetation but rather with grass instead of trees.

C_ Map of the maximum hydrogen contents measured on the soil on various locations.

**Figure 6:** Global view of the zone 2 within the Kalahari Desert (location Fig. 2)

A- Map of the subcircular depressions base of space images and contours of the area where they have been mapped. 111 depressions have been mapped on an area of about 1400 km². their average equivalent diameters is 421 m, the maximum around 1500 m and the maximum depth reaches 12 m.

B- Statistic on size presented through the equivalent diameter of perfectly circular depressions with the same areas.

C- Relationship maximum depth of the SCD/equivalent diameter for this zone.

**Figure 7:** Pictures of the Kalahari A: one of the sampled depression, B: topography profile across it, oriented about N140 as visible on the yellow line.

C : Picture of dune between the depressions, they are also visible from the space image Fig 6.
D : another of these depression, rather large about 1.5 km in the larger axis.
**Figure 8:** H₂ emanation versus time, comparison of the breathing of the soil between Brazil and Namibia.

A - Data from the Mina Gerais, Brazil, extracted from the permanent monitoring done by Engie and GEO4U between 2018 and 2002 and published by (Pinzhofer et al., 2019 and Moretti et al., 2021). The height sensors are fix and positioned around the structure, they measure the H₂ content every hour, only two days of data set are represented (square and circle). The H₂ content is measured with PArHys, the permanent sensor developed by Engie. The number (s_1) corresponds to the identity of the sensor.

B- Data from the structure N°7 in Namibia. Measurements have been done during couple of hours the 2 and the 8th of march 2022 but with only one sensor (the BIOGAS5000). We don't have night data. The measures have been done in different positions around and across the SCD, the map view of this data is presented figure 5.

**Figure 9** Structure (SCD_6) with H₂ emanations observed in (A) Satellite imagery from Google Earth ; (B) DEM in meters, from GLO-30 ; (C) Slope in degrees, derived from the DEM ; (D) SAVI index with L=0.5 ; (E) NDBI index ; (F) Coastal Aerosols band. The map of the various H₂ measurements in that structure could be see figure 5.

**Figure 10** Monitored circle observed in (A) Satellite imagery from Google Earth ; (B) DEM in meters from GLO-30 ; (C) Slope in degrees, derived from the DEM; (D) SAVI index with L=0.5 ; (E) NDBI index; (F) Coastal Aerosols band

**Figure 11** Schematic section above a H₂ rich zone in subsurface. (A) and (B) vegetation indexes, (C) apparence of the vegetation on the field, (D) usual H₂ content in the soil, micro-seepages are found above the H₂ rich zone when larger seepages are present where migration pathways reach the surface as here in the fault zones. (E) schematic cross-section without scale.

Negative topography, SAVI and NDVI anomalies help to map these prospective areas of micro-seepage. Small and not permanent H₂ pulses in case of micro-seepage, added to the microbial H₂ consumption in the soils, result in highly variable values in H₂ soil content. Values are usually more constant, and higher do to faster transport velocity, in macro-seepage areas.

**Additional material:**

GA 5000 dataset on zone 1 and 2: sheet of values
Biblio

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<table><caption>Table 1</caption><thead><tr><th rowspan="2">H₂ origin</th><th colspan="3">Water reduction</th><th rowspan="2">Deep Gas</th><th rowspan="2">Late OM maturation</th><th rowspan="2">Others</th></tr><tr><th>Fe oxidation</th><th>radiolysis</th><th>mechanoradical</th></tr></thead><tbody><tr><th>Reaction</th><td>Fe oxidation</td><td>radiolysis</td><td>mechanoradical</td><td></td><td>Coal, shale</td><td>H₂S, NH₄</td></tr><tr><th>Geological context</th><td>Basalt weathering<br>Early<br>Serpentinisation<br>Ophiolitic context<br>Iron rich facies<br>Biotite rich granite</td><td>Lower<br>crust<br>Granite<br>Uranium<br>mines</td><td>Active faults</td><td>Mantle<br>degassing<br>of<br>primary<br>H₂</td><td>Pyrolysis</td><td>Sulfur<br>oxidation<br>Ammonia<br>&gt; H₂ + N₂</td></tr><tr><th>Where</th><td>Mid Oceanic Ridge<br>Oman, Australia<br>Iceland<br>Mali</td><td>Canada<br>South<br>Africa</td><td>Jura ?</td><td>?</td><td>Australia</td><td>Italy<br>(volcanic<br>area)</td></tr><tr><th>Reference</th><td>Klein et al. (2019)<br>Deville & Prinzhofer (2017)<br>Moretti et al. (2021)<br>Truche et al. (2021)<br>Combaudon et al. (2022)<br>Geymond et al (2022)</td><td>Truche et al., 2018<br>Sherwood<br>lollar et al. (2014)<br>Karolyte et al. (2022)</td><td>Hirose et al. (2011)<br>Deronzier et Guise (2020)</td><td>Larin et al. (2015)<br>Zgonnik (2020)</td><td>Suzuki et al. (2017)<br>Mahlstedt et al. (2022)</td><td>Arouvel & Prinzhofer (2021)</td></tr></tbody></table>

<figure><img src="image_2.png" /><figcaption>Fig 1</figcaption></figure>
<figure><img src="image_3.png" /><figcaption>FIG 3</figcaption></figure>

<figure><img src="image_4.png" /></figure>
<figure><img src="image_5.png" /><figcaption>Fig 4 :</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>FIG 5</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>Fig 6</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>Fig 8</figcaption></figure>

<figure><img src="image_9.png" /></figure>
<figure><img src="image_10.png" /><figcaption>Fig 9</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>Fig 10</figcaption></figure>
<figure><img src="image_12.png" /><figcaption>Fig 11</figcaption></figure>