Helium, noble gases & co-tracers

Summary

Imagine a Hestia field crew gets back a soil-gas reading of 40 ppm hydrogen. Is that a real, economically interesting seep rising from deep in the crust, or a shallow false alarm — soil bacteria, a patch of nitrogen-fixing clover, or gas manufactured by the drill bit itself? A hydrogen number alone cannot answer that question, because everything that consumes, disturbs, or fakes hydrogen near the surface leaves the reading looking similar.

The fix is to look at hydrogen’s traveling companions. Helium, nitrogen, methane, carbon dioxide, hydrogen sulfide, and radon are all produced by geologic processes entangled with hydrogen generation, and they migrate up through the same cracks, faults, and pore networks — but each one is affected differently by what happens along the way, and that difference is what makes the suite diagnostic.

• Helium is chemically inert, so once made deep underground it can only be diluted or trapped, never consumed, which makes it a near-permanent record of a fault system’s plumbing even after the hydrogen it once traveled with has been eaten by microbes or reacted away.
• Nitrogen, methane, and carbon dioxide are harder to read: their deep-rock-versus-surface-biology origin usually has to be teased apart with isotopes rather than concentration alone.
• Radon decays away within days, making it a fast clock for very recent gas movement, the opposite end of the timescale from helium’s geologic-timescale record.
• And hydrogen sulfide has a dual role: it is a real geochemical tracer worth measuring, and at the same time an aggressive poison for the catalytic, electrochemical, and palladium-based H2 sensors documented elsewhere on this wiki.

Together, these co-tracers turn a single suspicious H2 number into a defensible geologic interpretation. Helium has one practical complication worth knowing before a field kit is assembled: it is invisible to every light-based (optical) sensing method this wiki covers, so measuring it always means bringing a second instrument.

Context numbers used throughout

Atmospheric H2 ≈ 0.53 ppmv (context anchor; see Geologic context). Atmospheric He = 5.24 ppmv (classical Glueckauf value; modern isotope-dilution redetermination 5.222 ± 0.017 ppmv) [36]. Exploration H2 anomalies span roughly 1 ppm to percent level; H2’s Lower Explosive Limit (LEL) is 4 vol% [see geologic-context.md]. The conventional economic threshold for commercial helium extraction from a gas stream is 0.3 mol% He [42][43].

Mechanism & physics

Why co-tracers exist at all: shared plumbing, divergent chemistry

Every gas discussed here — He, N2, CH4 and higher alkanes, CO2, H2S, and Rn — is generated by processes geologically and geochemically entangled with H2 generation, migrates at least partly through the same fracture, fault, and pore networks as H2, and yet behaves completely differently once it reaches the near-surface or a wellbore. That divergence is why a co-tracer suite is useful: if every gas behaved identically, one measurement would suffice.

The unifying way to see this is a generic first-order reaction–transport equation for the concentration $C_i$ of species $i$ along a shared migration pathway (the same governing PDE class documented for H2 itself in Environmental covariance and Soil-gas surveys, specialized here per species):

$\frac{\partial C_i}{\partial t} = \frac{\partial}{\partial z}\!\left(D_i \frac{\partial C_i}{\partial z}\right) - \frac{\partial (v\,C_i)}{\partial z} - k_i(T,\theta)\,C_i + Q_i(z,t)$

The source term $Q_i$ and sink term $k_i$ differ radically between species, and that difference is diagnostic.

• Helium has $k_{He}=0$ identically: with a closed 1s² electron shell, no biological consumption pathway, and no oxidation state other than zero, helium is a conservative tracer — whatever reaches a sampling point is exactly what left the source, modulated only by transport, never by consumption.
• Hydrogen, by contrast, has $k_{H_2}>0$ and often large, dominated by high-affinity H2-oxidizing soil bacteria with a deposition velocity $v_d\sim0.01$–$0.1$ cm/s (full treatment in Environmental covariance) — a reactive/non-conservative tracer of the same source.
• Methane, CO2, N2, and H2S are intermediate cases with their own biological, chemical, and redox-coupled sink/source terms (below).
• Radon has $k_{Rn}=\lambda_{Rn}$, a fixed radioactive decay constant ($t_{1/2}=3.82146$ days for ²²²Rn [1]) — a clock, not a conservative or reactive tracer, sampling only day-to-week-scale migration events rather than the geologically accumulated flux He integrates.

Comparing two or more of these species at the same sampling point is a way of solving for the shape of $Q_i$, $k_i$, and transport history that a single-species H2 measurement cannot recover on its own.

Radiogenic helium and the U/Th–H2 coupling

Crustal helium is overwhelmingly radiogenic ⁴He, produced as alpha particles along the U-238 (8 alphas/chain), U-235 (7), and Th-232 (6) decay series [2]. Because U and Th are enriched in continental crust — granites, shales, basement crystalline rocks — relative to the mantle, crustal ⁴He accumulation is a slow (10⁷–10⁹ yr), essentially one-way process: once produced, ⁴He can only be diluted, stored, or lost to migration; it is never destroyed. The production-rate expression per unit rock volume is

$P_{^4He} = \rho\Big(8\,\lambda_{238}\,C_{238} + 7\,\lambda_{235}\,C_{235} + 6\,\lambda_{232}\,C_{232}\Big)$

This is the mechanistic reason a “high He, low H2” reading is interpretable as a consumed or oxidized H2 system rather than a null result: He accumulated over geologic time keeps degassing along the same fault network regardless of whether co-generated H2 has since been microbially oxidized or chemically reacted away, so the He/H2 ratio measured at surface is expected to increase systematically along any transport path or timescale over which H2 is preferentially lost.

Radiolysis of water couples the two gases mechanistically, not just statistically: the same alpha/beta/gamma radiation from U/Th/K decay that produces ⁴He also splits water into H2 and oxidant species, and radiolysis is one of the two dominant mechanisms (alongside Fe²⁺ oxidation in water–rock reactions) generating natural H2 in the continental crust [3]. In radiolysis-dominated settings, He and H2 are literal co-products of the same nuclear decay events, a direct physical basis for using He as a proxy for a hidden or partly-consumed H2 system.

Helium isotopes: ³He/⁴He (R/Ra) as a mantle-vs-crustal dial

Because ³He is overwhelmingly primordial (trapped during accretion, no significant modern production mechanism) while ⁴He is continuously generated radiogenically, the ³He/⁴He ratio fingerprints a gas parcel’s residence history. By convention it is normalized to the atmospheric ratio $R_a=1.4\times10^{-6}$ [4][5] and reported as:

$\frac{R}{R_a} = \frac{(^3He/^4He)_{sample}}{(^3He/^4He)_{atm}}$

Since Sano & Wakita’s foundational 1985 discrimination framework, the end-member values used throughout noble-gas geochemistry are mantle (MORB-source) helium at $R/R_a\approx8$ [4][6] and old, radiogenically matured continental crust at $R/R_a\approx0.02$ [4][6] (some crustal reservoirs as low as 0.01–0.05 [5][6]). A sample’s position on this spectrum is a two- or three-end-member mixing problem, with an atmospheric/air-saturated-water (ASW) component often included [7][8].

For geologic-H2 exploration specifically, published continental H2-He co-occurrences report $R/R_a<0.02$ [9] — strongly crustal and radiogenic, consistent with basement/fault degassing rather than active mantle magmatism, even in intracratonic basins with no surface volcanism. This isotope work is cross-linked in full at Isotopes & fingerprinting, which treats R/Ra alongside δ¹³C, δD, and δ¹⁵N as the broader isotope-fingerprinting toolkit.

Why helium is invisible to every optical sensing method

Helium’s key sensing property follows directly from its electronic structure. Optical spectroscopic methods used elsewhere in this wiki for H2 — Raman spectroscopy and IR absorption & proxies — detect a target gas because it has vibrational and/or rotational normal modes that couple to incident or scattered photons: H2, a homonuclear diatomic, has a Raman-active vibrational mode (the ~4155 cm⁻¹ / ≈2.41 µm Q-branch) and a rotational-Raman ladder, and even IR-silent H2 is reached via proxy chemistries built around other IR-active molecules.

Helium is a monatomic noble gas — a single atom has no bond to stretch and no molecular axis to rotate about, so it has zero vibrational modes and zero rotational modes at any accessible energy. Its electronic transitions lie deep in the vacuum-UV (the first allowed transition from the ground state is near 58.4 nm, ~21.2 eV), far outside any Raman, mid-IR, or near-UV/visible instrument in this wiki’s portfolio.

No optical technique here — Raman, enhanced Raman, IR absorption, UV/EUV & fluorescence, or any exotic-optical variant — can detect helium at any concentration, by any mechanism. This is a selection rule, not a sensitivity problem to be engineered around. A Hestia field kit built solely around an optical H2 sensor is, by construction, blind to helium.

Helium is instead reached by three physically distinct routes, developed quantitatively below:

• mass spectrometry at m/z = 4 (helium’s 24.6 eV ionization potential, the highest of any element, gives an isobar-clean ⁴He⁺ ion) [10][11];
• GC with a pulsed-discharge helium ionization detector (PDHID/HID), giving sub-ppb permanent-gas detection when a non-helium carrier is used [12][13];
• and bulk thermal-conductivity measurement, which exploits He’s transport property rather than its spectroscopy — and is where He becomes a confounder for H2 TCD sensing, discussed next.

The thermal-conductivity “twin”: why He is second only to H2

From kinetic theory, gas thermal conductivity scales as $\lambda\propto n\,\bar v\,\ell\,c_v$, where $\bar v=\sqrt{8k_BT/\pi m}$. Because $\bar v\propto m^{-1/2}$, light, weakly-interacting molecules conduct heat fastest for a given number density. H2 (M = 2.016 g/mol) and He (M = 4.003 g/mol) are the first- and second-lightest gases in appreciable natural abundance, both with unusually weak van der Waals forces and long mean free paths: $\lambda_{H_2}\approx0.1805$ W·m⁻¹·K⁻¹ and $\lambda_{He}\approx0.1513$ W·m⁻¹·K⁻¹ [14] — roughly 5–7× air’s ~0.026 W·m⁻¹·K⁻¹ and far above CO2, CH4, or N2, which all sit within about a factor of 2 of air.

No other common soil-gas or atmospheric species comes close to either H2 or He, and any thermal-conductivity-based H2 sensor must account for He as its largest interferent — quantified in Thermal conductivity & acoustic sensors and reused directly in Environmental covariance & cross-sensitivity, below.

N2, CH4/CO2, and H2S: co-evolution mechanisms

N2 enters the same fluid systems that generate H2 through two independent crustal mechanisms: thermal deammoniation, in which nitrogen fixed as NH4⁺ in clay-mineral interlayers (illite, ammonium-bearing smectite-to-illite transitions) is released as N2/NH3 once temperature exceeds the mineral’s thermal stability, leaving a distinctive δ¹⁵N signature (−2‰ to +2‰ at early overmaturation) [15][16]; and basement/mantle-fluid degassing, where fluid-inclusion studies show N2 (oxidized settings) or NH3 (deep, reduced settings) as essentially the only detectable nitrogen species in mantle and crustal N–H–O fluids [17].

Both operate along the same fault/fracture pathways invoked for H2, which is why N2 so often dominates the non-H2 balance of a natural-hydrogen gas sample.

CH4, CO2, and the Sabatier/serpentinization coupling. Serpentinization — hydration and oxidation of ultramafic minerals (olivine, pyroxene) — is one of the two dominant H2-generating water–rock reactions in the continental crust, oxidizing structural Fe²⁺ to Fe³⁺ while reducing water to H2 [3][18]. The H2 produced can itself reduce co-present CO2 to methane through a Sabatier-type reaction:

$\mathrm{CO_2 + 4\,H_2 \longrightarrow CH_4 + 2\,H_2O}\qquad(\Delta H^\circ \approx -165\ \text{kJ/mol, exothermic})$

This is the mechanism generating abiotic CH4 at mid-ocean-ridge and continental serpentinizing systems; mineral catalysts (awaruite, magnetite) that commonly co-occur in serpentinized rock can drive it efficiently enough to compete with, or in slow-reactant-supply settings out-compete, biological methanogens on the identical H2 + CO2 substrate [19][20]. Experimental work has established that the H2/CH4 ratio during serpentinization cannot reliably distinguish abiotic from biological methane [21]: the discrimination job for CH4/CO2 falls to carbon and hydrogen isotopes (δ¹³C, δD) and the C1/C2+ alkane ratio, not to concentration ratios alone.

H2S is generated alongside H2 and CH4 wherever sulfate-reducing conditions or pyrite reactions occur in the same reduced water–rock system, and is reported as a co-evolved species requiring explicit monitoring in natural-H2 seep characterization [9][12]. Its sensing significance is two-sided: it is both a genuine geochemical co-tracer and an aggressive poison of the electrochemical, catalytic, and palladium-based H2 sensors Hestia would deploy alongside it (§Environmental covariance, below, and Catalytic pellistors, Palladium-resistive sensors, Electrochemical cells).

Radon as a fault-activity clock

²²²Rn is a direct alpha-decay daughter of ²²⁶Ra within the U-238 chain, half-life 3.82 days [1]. Because this half-life is short relative to geologic transport timescales but long relative to a single field day, Rn activity in soil gas is set by a balance between production, radioactive decay, and transport, making it sensitive to recent (day-to-week-scale) changes in fault permeability or gas-carrier flux in a way accumulated, non-decaying ⁴He cannot.

Radon soil-gas surveys have a long independent track record mapping active fault traces (elevated activity directly above shear zones that increase local rock surface area and permeability) and are used in seismogenic-fault studies, uranium and hydrocarbon exploration, and geothermal investigations [22][23][24].

Variants & subtypes

Taxonomy by tracer role

• Conservative deep-source tracers — He (and, in principle, Ne/Ar/Kr/Xe isotopes): no chemical sink, so concentration records cumulative migration from source, not instantaneous flux.
• Reactive co-generated species with independent sinks — H2 (microbial oxidation), CH4 (methanotrophy, atmospheric OH oxidation on long timescales), CO2 (biological respiration swamps geologic signal near-surface).
• Bulk balance/matrix gas — N2: typically the dominant non-target species by mole fraction in both atmosphere and many natural-H2 accumulations, more useful as a stoichiometric balance check than a standalone anomaly indicator.
• Redox-coupled carbon pair — CH4/CO2, linked by the Sabatier reaction; their ratio, alongside isotopic composition, is more diagnostic than either species’ absolute concentration.
• Short-half-life kinetic tracer — Rn-222: samples active migration on a day-to-week timescale, complementary to He’s geologic-timescale integration.
• Poison-and-tracer dual role — H2S: simultaneously informative and destructive to co-deployed H2 sensors.

Taxonomy by gas-composition end-member

Published wellhead/seep compositions span a wide range of H2-to-balance-gas ratios, and loose paraphrasing can invert the picture:

• H2-dominant, N2/He/CH4 as minor balance — Bourakebougou, Mali, the best-documented commercial natural-H2 accumulation, produces gas at 97.4–98 vol% H2, ~1.2% N2, and helium at trace level (<0.05%) [25][26]. N2 is the dominant species within the small non-H2 remainder, not the reservoir as a whole — a narrower, correct reading than the common “N2/CH4-dominant with only 1–2% H2” paraphrase, which describes a compositionally distinct class of sites elsewhere in the literature.
• High-purity H2 with economically significant co-produced He — HyTerra’s Nemaha project (Kansas) reported up to 96% H2 and up to 5% He (Sue Duroche 3) and up to 83% H2 with up to 5% He (McCoy 1) [27][28]; Gold Hydrogen’s Ramsay wells (South Australia) reported air-corrected H2 up to ~97% (unverified — not found in the cited release) alongside He up to 6.8% (raw, Ramsay 2; unverified — not found in the cited release) and CSIRO-confirmed groundwater He purities of 20–25% with detectable ³He (unverified) [29][30][31]. Both operators explicitly frame these as dual-commodity He+H2 plays.
• N2-dominant with extreme trace-element enrichment — the Homorod mud volcano, Romania, seeps gas >92 vol% N2 with helium up to 1.4 vol% (nearly three orders of magnitude above atmospheric) and extreme methane deuterium enrichment (δD up to +124‰), interpreted as primarily organic-sedimentary with a minor mantle contribution [32] — a system where He, not H2, is the geochemically remarkable species.
• Global statistical baseline — Milkov’s compilation of 6,246 natural-gas samples with reported H2 across 16 geological habitats finds a global average H2 concentration of 3.5% but a median of only 0.01% (an extremely right-skewed distribution), with mid-ocean-ridge and serpentinite settings showing the highest average H2 (~24% and ~21%), and identifies co-occurrence of N2, He, and H2 as a common hallmark of deep degassing across the dataset [33].
• Intracratonic basement-sourced systems — the São Francisco Basin, Brazil, shows exploration-well gas with H2 up to ~20% and He >1%, with a strongly crustal helium-isotope signature (³He/⁴He < 0.02 R/Ra) and neon-isotope evidence for an Archean crustal-basement component [9][34][35].

Taxonomy of He/noble-gas sensing technology

Technology class	What it actually measures	Field-portable?	Isotope-capable?
Quadrupole residual gas analyzer (RGA) / portable MS (e.g., miniRUEDI-class; see Mass spectrometry)	m/z spectrum, incl. m/z=4	Some, most lab-bound	Bulk ⁴He only; not precise enough for R/Ra
Magnetic-sector noble-gas MS	High-precision isotope ratios (³He/⁴He, ²⁰Ne, etc.)	No — lab-only, static vacuum system	Yes — the only R/Ra-capable class
GC-PDHID / GC-HID (see Gas chromatography)	Chromatographically separated He (and other permanent gases) via discharge ionization	Bench/lab GC, some field GC	No
Thermal-conductivity detector (TCD/katharometer), standalone or as GC detector	Bulk thermal conductivity, inferred composition assuming a known simple mixture	Yes — the field-common route	No
Dedicated He leak detector / sniffer (vacuum-technology heritage)	He partial pressure via an internal small mass spectrometer (usually m/z=4 magnetic-sector or quadrupole)	Yes — several portable/handheld models	No (bulk He only)

Method comparison & quantitative performance envelope

Atmospheric and crustal background reference values

Gas	Atmospheric background	Notes	Ref
He	5.24 ppmv (classical Glueckauf value; modern redetermination 5.222 ± 0.017 ppmv)	Well-mixed and extremely stable — small deviations are meaningful if precision is adequate	[36]
H2	≈ 0.53 ppmv (see Geologic context)	For comparison scale only	—
N2	78.08% (780,800 ppmv)	~150,000× He’s background — ppm-scale crustal N2 input is essentially undetectable without isotopes or wellhead-scale enrichment	standard atmosphere
CH4	~1.7–2.0 ppmv	Rising anthropogenic background complicates small geologic anomalies	[38]
CO2	~420 ppmv (global mean); heavily modulated by soil biology at the point of measurement	Biological respiration at 1 m soil depth is orders of magnitude above this background	context anchor
H2S	~0 (no meaningful natural atmospheric background)	Any detection above instrument noise floor is potentially significant	—
Rn (²²²Rn)	Highly variable; open-air background is very low (Bq/m³ range), but soil-gas background at ~1 m depth is orders of magnitude higher and site-specific	See fault-vs-background figures below	[24][39]

Helium soil-gas / fault-survey anomaly magnitudes

Site / study	Background He	Anomaly / peak He	Magnitude	Co-measured H2	Ref
San Andreas Fault, CA (historical survey)	40–98 ppm (60–120 ft off-trace)	up to 350–430 ppm on-fault	~4–8× background	~50 ppm on-fault	[40]
Glacier National Park thrust fault, MT	(not separately reported)	1,243 ppm (single largest reported)	—	27 ppm H2, 2.4% CO2, same sample	[40]
Duchesne Fault Zone, UT	(not separately reported)	227–681 ppm across stations	—	H2 to 3,000–5,000 ppm at same fault zone	[40]
Jasper Ridge / Portola Valley, San Andreas system (2023 peer-reviewed, n=103, 1 m depth)	5.45 ppmv (≈ atmospheric)	no significant He anomaly	~0% (uniform)	H2 up to 20.3 and 17.3 ppmv, 30–35× atmospheric	[41]
Cumberland Basin (New Salem), Nova Scotia (2025 survey, n=28)	5.240 ppmv (atmospheric reference)	Range 5.306–5.722 ppmv, mean 5.456 ppmv	All 28 above background; peak ~10.8% above atmospheric	Co-located 450 m continuous C1–C4 hydrocarbon anomaly (CH4 to 87.7 ppmv)	[38]

He and H2 anomalies do not always co-occur

The Jasper Ridge/Portola Valley result is an important quantitative caution: a rigorously sampled (n=103), peer-reviewed dataset shows a real, substantial H2 anomaly (20–35× background) with no corresponding He anomaly, direct field evidence that He and H2 anomalies do not always co-occur even at a genuine, fault-associated H2 seep, and that a missing He anomaly is not by itself grounds to dismiss an H2 signal as spurious.

Wellhead / reservoir-scale He-H2 compositions

Site	H2	He	N2	Other	Ref
Bourakebougou, Mali (Bougou-1, main reservoir)	97.4–98%	<0.05% (trace)	1.2%	trace CO2, CH4	[25][26]
HyTerra Nemaha, Sue Duroche 3, KS	up to 96%	up to 5%	balance	—	[27][28]
HyTerra Nemaha, McCoy 1, KS	up to 83%	up to 5%	balance	—	[27][28]
Gold Hydrogen Ramsay 1, SA (air-corrected)	73.3%	3.6% (892 mMD)	balance	—	[29][30]
Gold Hydrogen Ramsay 2, SA (raw)	elevated (not fully air-corrected)	6.8%	balance	—	[30] (unverified)
Gold Hydrogen Ramsay 3, SA (early, air-corrected)	up to ~97%	—	balance	—	[29] (unverified)
CSIRO-confirmed groundwater, Yorke Peninsula, SA	—	20–25% purity, trace ³He	—	Highest He purity reported in Australian groundwater to date	[31] (unverified)
Homorod mud volcano, Romania	(not headline gas)	up to 1.4 vol%	>92 vol%	δD-CH4 to +124‰	[32]
São Francisco Basin, Brazil (exploration wells)	up to ~20%	>1%	present	³He/⁴He < 0.02 R/Ra (strongly crustal)	[9][34]
Historic Hugoton field, KS (pure He play, reference)	n/a	0.25–2.5% average	balance	Shows HyTerra’s Nemaha grades substantially exceed a historically economic He field	[28]

Every one of the natural-H2 wells above except Bourakebougou clears the 0.3 mol% economic He threshold [42][43] by a wide margin — the direct commercial logic behind the “dual-commodity” positioning several natural-H2 companies (HyTerra, Gold Hydrogen, Desert Mountain Energy) have adopted [28][30][44].

Isotope and molecular-ratio discriminators

Discriminator	Mantle/primary end-member	Crustal/radiogenic end-member	Atmospheric/ASW end-member	Ref
³He/⁴He, R/Ra	~8 Ra (MORB-source upper mantle)	~0.02 Ra (old, radiogenically matured crust)	1 Ra by definition ($R_a=1.4\times10^{-6}$)	[4][5][6]
C1/C2+ (Bernard ratio)	n/a (abiotic gas plots separately)	<100 → thermogenic; >1,000 with light δ¹³C → microbial	n/a	[45][46]
δ¹³C–CH4	Abiotic Sabatier-type CH4 can show heavy or anomalous δ¹³C	Thermogenic: heavier than −50‰	Microbial/biogenic: lighter than −55‰	[46][45]
δ¹⁵N (N2 from clay deammoniation)	—	−2‰ to +2‰ at early overmaturation	0‰ by definition	[15]

These isotope discriminators are the deepest layer of this page’s toolkit and are developed at full length, alongside δD water-gas geothermometry, in Isotopes & fingerprinting.

Environmental covariance & cross-sensitivity

Thermal-conductivity confusion: He masquerading as H2 (and vice versa)

Because $\lambda_{He}$ (0.1513 W·m⁻¹·K⁻¹) sits on the same high-conductivity side of a mixture as $\lambda_{H_2}$ (0.1805 W·m⁻¹·K⁻¹) — both far above air’s ~0.026 W·m⁻¹·K⁻¹ and much closer to each other than either is to any other common soil gas — a TCD-based H2 sensor cannot distinguish an H2 signal from a He signal without an orthogonal (chromatographic or spectroscopic) separation step.

A first-order linearization of the Wassiljewa mixing relation gives 1% He producing an H2-equivalent bias of roughly +7,700 ppm [47], the largest interferent in any TCD-based H2 cross-sensitivity analysis, specifically because He is enriched in exactly the deep-crustal/radiogenic settings that are geologic-H2 exploration’s primary targets.

A bare TCD H2 channel deployed at a site with a genuine He co-anomaly will over-report H2. The inverse problem is equally well documented in analytical chemistry: GC systems using helium as carrier gas cannot cleanly quantify H2 by TCD, because the two gases’ near-identical thermal conductivities make the TCD response non-monotonic near the crossover composition, producing anomalous “W-shaped” or inverted peaks for H2 eluting into a He carrier stream, especially at higher H2 concentrations [48][49].

The standard fix is switching carrier gas to argon (λ_Ar ≈ 0.0177 W·m⁻¹·K⁻¹, far from both H2 and He) whenever accurate H2 or He quantification is required [48][49] — directly relevant to vendor selection for Gas chromatography and Mud-gas logging, where a He-carrier GC-TCD system optimized for hydrocarbons will be a poor or actively misleading choice for simultaneous H2/He quantification.

The atmospheric-background sniffing floor

Dedicated helium leak detectors are vacuum-technology instruments whose headline sensitivity specs (minimum detectable leak rate, MDL, quoted as low as 10⁻⁷–10⁻¹² mbar·L/s) are measured under vacuum with a near-zero He background. Operated in sniffer mode in open air, their practical sensitivity floor is set by the ambient atmospheric He background itself (~5 ppm): one manufacturer states plainly that sniffer-mode sensitivity is fundamentally limited “due to the amount of helium present in the air (approximately 5 ppm)” [51].

This is the same background-discrimination problem soil-gas H2 surveys face against the 0.53 ppm atmospheric H2 floor, and it means a He leak detector repurposed for open-air soil-gas survey use is not automatically more sensitive just because its vacuum-mode datasheet quotes a low number; the field figure of merit is achievable precision relative to a ~5 ppm floor, not the absolute vacuum-mode MDL.

Why N2 is a weak standalone indicator despite being geochemically real

N2 is generated by real crustal mechanisms co-located with H2 generation, but its atmospheric background (780,800 ppmv) is roughly 150,000× larger than He’s (5.24 ppmv) in absolute terms, and any near-surface soil-gas sample is unavoidably diluted with atmospheric air during sampling. A ppm-to-percent-level crustal N2 input is therefore essentially undetectable against this background without either (a) working at wellhead/reservoir purity levels where H2/He/CH4/CO2 have already displaced most atmospheric air, or (b) using the δ¹⁵N isotope ratio rather than concentration [15][17].

N2 occupies a structurally different role than He: a useful stoichiometric balance-gas check at high concentration (wellhead, mud-gas logging), but a poor standalone ppm-anomaly indicator in soil-gas surveys.

CO2’s biological confound

Near-surface soil CO2 concentration is dominated by root and microbial respiration, reaching percent-level values routinely — the Western Australia FF4 seep monitoring record shows CO2 ranging 442–15,558 ppm even absent any claimed connection to the geologic H2 source, and explicitly found that cH2 and cCO2 did not correlate at that site, evidence the two gases were following different transport/generation pathways [52].

CO2 concentration alone is a weak co-tracer here for near-surface soil-gas work specifically — the biological background swamps a modest geologic CO2 input by orders of magnitude — and any CO2-based interpretation needs isotopic (δ¹³C-CO2) support or corroboration from a Sabatier-coupled CH4 signal to be geologically meaningful rather than an artifact of local vegetation and soil biology (see Environmental covariance for the −0.57/+0.54 opposite-sign H2/CO2 correlation with relative humidity at the same Perth Basin site).

H2S: co-tracer and sensor poison in the same molecule

H2S is in direct conflict with the rest of Hestia’s sensing stack. The joint European Commission JRC / US DOE NREL study quantifying H2S poisoning of catalytic, electrochemical, and metal-oxide-semiconductor H2 sensors under the ISO 26142:2010 protocol (see Catalytic pellistors and Palladium-resistive sensors) found model-dependent but sometimes severe degradation — measurable baseline shifts in some catalytic sensor models even at 40–50 ppm H2S, and documented Pd/Pd-alloy sulfidation causing irreversible loss of H2 sensitivity [53].

This creates a direct field trade-off: deploying a sensitive H2S channel to capture a genuine co-tracer signal means operating in a gas stream that is actively degrading the H2 sensor sitting next to it, unless the two are physically separated (separate sample lines, sacrificial scrubbers ahead of the H2 channel only, or fully independent instruments).

Regulatory context underscores why H2S cannot simply be ignored: it carries an IDLH threshold of 100 ppm [54], with facility-level alarm requirements typically 10–20 ppm [54] — an H2S channel in a Hestia field kit does safety duty as well as geochemistry duty.

Decoupling scenarios: reading the He/H2 relationship

The anomaly table above already shows He and H2 anomalies do not always travel together; the interpretive logic is worth stating explicitly:

• High H2, background He (the Jasper Ridge/Portola Valley pattern) [41]: consistent with a relatively shallow, actively-generating H2 source (the peer-reviewed interpretation favors serpentinization or geomechanical fault-surface catalytic activation) where H2 has not migrated through, or resided long enough in, a sufficiently radiogenic ⁴He-accumulating basement reservoir to acquire a detectable He co-signature. This pattern argues against automatically discounting an H2 anomaly for lacking a He companion.
• High He, low/absent H2: interpreted as a deep, radiogenically charged basement/fault plumbing system where generated H2 has since been substantially consumed (microbial oxidation en route, Sabatier-type conversion to CH4, or other reactive loss) while the chemically inert He signature survives intact. This is the direct mechanistic prediction of the $k_{He}=0$ vs. $k_{H_2}>0$ contrast above, and is consistent with the general literature view that a rising He/H2 ratio indicates a “leaky” or partially-oxidized system [55][9] — but no single case study reviewed for this page isolates this exact pattern in one clean numerical example; treat it as mechanistically well-supported, not yet independently confirmed against a dedicated field dataset (flagged again under Open questions).
• Both anomalous, roughly co-located (Bourakebougou-, São Francisco Basin-, Ramsay-, Nemaha-style deep degassing systems) — the least ambiguous case: an actively-charging deep source where transport has been fast/direct enough, or local sink terms weak enough, that both the conservative and reactive tracer register together [9][25][27][30][33].
• Both at or near background — either no deep source at this location, or a source present but sealed/not yet breached to sampling depth: cannot be distinguished from a single spot measurement.

Use for geologic H2

Field verdict

Helium is the best available conservative co-tracer for geologic-H2 exploration, but it is not a free add-on to an H2 sensing rig; it is a second-instrument problem, because it is invisible to every optical H2 channel and ambiguous on a bare TCD channel. Field anomalies are frequently small (the most methodologically careful recent soil-gas survey reviewed here found a peak He value only ~10.8% above atmospheric background), so a co-tracer program needs genuine sub-percent precision to avoid false negatives. The R/Ra isotope ratio gives the strongest discrimination but is lab-only with no field-portable equivalent, so budget sample-and-ship logistics, not real-time results. N2 concentration is close to useless as a ppm-scale soil-gas indicator; CH4/CO2 need isotopes, not concentration, to discriminate origin; and H2S is a hard operational conflict between tracer value and sensor poisoning that requires physical separation, not sensor selection. H2 should not be surveyed alone; treat the co-tracer suite as the second (and third) instrument line item it actually is.

Soil-gas He surveys. Because helium requires either a wholly separate mass-spectrometric/GC-PDHID/leak-detector instrument, or — with the explicit ambiguity caveat above — a TCD channel understood as measuring “H2-or-He, ambiguously” rather than H2 specifically, a Hestia field kit built around an IR, Raman, or any spectroscopic H2 channel gets zero He information for free. This is a real cost and logistics line item.

The clearest published anomalies (San Andreas, Glacier NP, Duchesne Fault) are large (4–20× background), but the most recent, methodologically careful exploration-stage survey reviewed here (Cumberland Basin, Nova Scotia) reported a signal that demands genuine sub-percent field precision and excellent short-term stability — a materially harder measurement than most H2 anomalies, which are far more often order-of-magnitude excursions above the 0.53 ppm H2 background. A He co-tracer program built around modest field instrumentation risks producing false negatives that look like “no He signal” when the true signal is present but below the noise floor.

R/Ra isotope discrimination is not field-deployable. Every source reviewed that reports a meaningful ³He/⁴He or R/Ra value did so via laboratory magnetic-sector noble-gas mass spectrometry [9][34][6], a static-vacuum technique with no portable field equivalent in current commercial practice. The mantle-vs-crustal helium measurement is necessarily a sample-and-ship workflow with lab turnaround measured in days to weeks, not a real-time exploration decision tool — budget for sample containers, chain-of-custody, and lab lead time as a standing line item.

N2, CH4/CO2, and H2S each demand a different workflow, not a single multi-gas box. N2 concentration alone is close to useless as a soil-gas indicator (150,000:1 background-ratio disadvantage relative to He) — reserve it for wellhead/downhole balance-gas accounting and, if warranted, δ¹⁵N lab work.

CH4/CO2 concentration tells you that co-evolved carbon gases are present but — given that the H2/CH4 ratio cannot reliably separate abiotic from biological methanogenesis [21] — cannot by itself tell you whether the carbon signal is geologically meaningful or a biogenic/anthropogenic false lead without a genetic (Bernard-plot, δ¹³C, δD) follow-up, again a lab, not field, capability today.

Any Hestia deployment wanting both a sensitive H2S co-tracer channel and an un-poisoned H2 channel must engineer physical separation between them; this cannot be solved by sensor selection alone.

Radon adds real value on a complementary timescale but demands dedicated instrumentation. A RAD7-class alpha-spectrometry radon monitor is not a channel on a general multi-gas analyzer; it is its own instrument, with its own field protocol (soil-gas probes, water-stop assemblies) [56][57]. Budgeting it as “one more sensor to add to the GA5000” understates the actual hardware and training investment required — see Soil-gas surveys for the GA5000-class instrument baseline this would sit alongside.

Instrument coverage gaps. No portable field instrument reviewed here combines an H2 channel, a He channel, and an H2S channel in one box without at least one serious caveat:

• general soil-gas multigas analyzers have no He channel at all;
• portable FTIR multigas units cannot measure He (not IR-active) and often exclude H2S from their standard configuration;
• Micro GC platforms need a carrier-gas swap to analyze He accurately;
• and TCD-based combined units cannot distinguish He from H2 at all.

The practical near-term answer for most Hestia campaigns is a two-instrument field kit — a GA5000-class electrochemical unit for H2/H2S/CO2/CH4 plus a dedicated portable He sniffer or sample-and-ship isotope program — rather than waiting for a single-box solution that does not yet exist.

Instruments, labs & standards

Category	Vendor / product	Key specs (as published)	Notes	Ref
Portable He leak detector (sniffer)	Pfeiffer Vacuum ASM 306S	Vacuum-mode MDL 1×10⁻⁷ mbar·L/s (⁴He), 5×10⁻⁷ mbar·L/s (H2); sniffer-mode practical sensitivity ~1×10⁻⁶ std cc/s (floor set by ~5 ppm atmospheric He); <1 s response	Explicitly dual He/H2 sniffer; vacuum-technology heritage, not an off-the-shelf soil-gas-survey product	[51][58]
Portable He leak detector (sniffer)	INFICON Protec P3000(XL)	MDL 1×10⁻⁷ mbar·L/s (300 sccm draw), 1×10⁻⁶ mbar·L/s (3,000 sccm); response <0.7 s	Moderate-cost tier per INFICON’s own positioning	[59]
High-sensitivity He leak detector	INFICON UL3000 Fab	Sniffer sensitivity to 2 ppm He; 5×10⁻⁶ mbar·L/s at 90% RH (unverified — not found on cited vendor page)	Semiconductor-fab heritage; humidity-tolerance spec notable for field relevance	[60]
Portable He leak detector	Leybold Phoenix Quadro	Dry, portable, vacuum + sniffer modes; internal dry pump options	”All-rounder” positioning per Leybold	[61][62]
Residual gas analyzer (quadrupole MS)	Hiden Analytical RGA family	m/z 1–100/200 amu range; dedicated “leak detect mode” for fast He response at m/z=4	Lab/process instrumentation, not field-portable	[11]
Quadrupole MS (process/lab)	Pfeiffer Vacuum PRISMA pro (QMA 250)	MDL ~2.7×10⁻¹⁵ mbar (search-synthesized)	High-end lab RGA class	[63]
GC detector for permanent gases incl. He	Pulsed-discharge helium ionization detector (PDHID), various vendors	Sub-ppb to low-ppb detection for H2, N2, CH4, CO, CO2; ~10× more sensitive than FID	Requires GC separation upstream; not standalone	[12][13][64]
Field/portable multi-channel GC	Agilent 490 Micro GC Natural Gas Analyzer	µTCD detection, ~1 ppm/channel; 1–4 columns; 30–120 s analysis; battery up to 8 hr	Requires argon carrier to analyze He/Ne/H2 accurately — a direct field-deployment gotcha	[65]
TCD sensor (dedicated H2/He/N2/Ar)	Eaton/MTL K522	Explicitly marketed for H2, He, N2, Ar TC sensing	Same He/H2 confusability caveat applies	[66]
Portable multigas analyzer	Geotech GA5000	Electrochemical H2 (optional), 0–1,000 ppm; no He channel	See Soil-gas surveys	—
Portable FTIR multigas	Gasmet GT6000 Mobilis / DX4000	Up to ~50 simultaneous gas channels	Cannot measure He (not IR-active); standard config excludes H2S	[67]
Radon monitor	Durridge RAD7	0.1–20,000 pCi/L; detects both ²²²Rn and ²²⁰Rn (thoron); EPA 4 pCi/L action level resolved to 10% SD in <2 hr; dedicated soil-gas probe accessory	Standalone instrument, not a multigas add-on channel	[56][57]
Isotope/gas composition lab service	Isotech Laboratories (a Stratum Reservoir company)	He/H2 detection 25 ppm–100%; proprietary sample containers (IsoTube, IsoJar, IsoFlask)	Plausible outsourcing partner for R/Ra work Hestia cannot do in-house	[68][69]
Standard	ASTM D1945-14(2019)	Covers He, H2, O2, N2, CO2, hydrocarbons, H2S by GC; component sensitivity ~0.01 mol% (H2S coarser, ~0.3 mol%)	The relevant compositional-analysis standard for wellhead/reservoir gas — see Standards & regulations	[70][71]
Standard/assessment	USGS SIR 2021-5085	Documents the 0.3 mol% He commercial-extraction threshold and US helium-resource distribution	Economic-context reference for any He-coproduction discussion	[42]
Exploration company (dual He+H2)	HyTerra (Nemaha, Kansas)	Public ASX-disclosed well results	Explicit dual-commodity positioning	[27][28]
Exploration company (dual He+H2)	Gold Hydrogen Ltd. (Ramsay, South Australia)	Public ASX-disclosed well results; CSIRO noble-gas lab partnership	Explicit dual-commodity positioning	[29][30][31]
Exploration company (dual He+H2)	Desert Mountain Energy Corp. (Holbrook Basin, Arizona; UK license)	MOU with Hethos Ltd. to co-develop He+H2 detection technology	Developing a “passive tool” for at-depth He/H2 detection	[44]

Full vendor entries, where available, are also tracked in Vendors & products.

Open questions

1. The “high He, low H2” oxidized-system signature needs a clean, numerically documented field example. Every source reviewed either shows both He and H2 elevated together, or H2 elevated with He at background (Jasper Ridge/Portola Valley) — the main diagnostic scenario (elevated He/H2 ratio from suppressed H2 against a real He anomaly) is mechanistically well-supported but not independently confirmed against a single clean case study with reported He/H2 ratio time-series or spatial-gradient data.
2. Direct primary-source verification of Gold Hydrogen and HyTerra well data — this page relies on ASX-disclosure secondary reporting, not peer-reviewed measurements; several values are explicitly self-qualified as “air-corrected.” Obtain underlying lab certificates of analysis before any investment-grade due diligence use.
3. R/Ra values for Hestia’s own or candidate prospect areas do not exist yet and cannot be estimated from global literature with site-specific confidence — the crustal end-member is itself basin/rock-type dependent. Budget an initial noble-gas-isotope lab campaign for any site advancing past initial soil-gas reconnaissance.
4. No portable field instrument combines an H2, He, and H2S channel in one box without a serious caveat. Scope whether a two-instrument field kit or a custom multi-detector GC (PDHID + TCD, argon carrier) is the right near-term answer for Hestia.
5. Radon-H2 joint interpretation is an open research area, not a mature practice. Consider a small pilot pairing a RAD7 radon monitor with an existing soil-gas H2 survey to build an internal dataset on whether the two tracers’ different timescales resolve migration events neither shows alone.
6. Verify λ_He (0.1513 vs. a commonly cited 0.152 W/m·K) against a temperature-specified NIST source, pinned to the same reference temperature used for λ_H2 in Thermal conductivity & acoustic sensors, before either value is used in a joint He/H2 TCD error budget.
7. Isotope-lab turnaround time and cost were not found for Isotech Labs, CSIRO, or a comparable facility — request a direct quote (sample volume, turnaround days, cost per sample for a He/Ne + C1–C4 hydrocarbon isotope panel) for field-campaign budgeting.
8. No quantitative data was found on He or Rn cross-sensitivity in electrochemical or MOX H2 sensors specifically (as opposed to TCD, where it is well quantified) — confirm with the MOX sensors and Electrochemical cells maintainers whether He is expected to be electrochemically/catalytically silent on those platforms.

Related pages

• Geologic context — the transport-physics and drilling-artifact companion this page’s per-species reaction-transport framing builds directly on
• Soil-gas surveys — the field protocol a He co-tracer channel gets bolted onto
• Environmental covariance — the weather-driven confounds that apply to every species in this suite, not just H2
• Downhole deployment — where wellhead-scale He/H2 compositions (§Method comparison) are actually sampled
• Calibration & QA/QC — traceability and drift discipline that applies equally to a He or H2S channel
• Isotopes & fingerprinting — the full R/Ra, δ¹³C, δD, δ¹⁵N toolkit this page only summarizes
• Dissolved-gas groundwater methods — the aqueous-phase route to the same He/H2/CH4 co-tracer suite
• Standards & regulations — ASTM D1945, IDLH/alarm thresholds, and the 0.3 mol% He economic cutoff in regulatory/standards context
• Thermal conductivity & acoustic sensors — the mixing-rule physics behind the He/H2 TCD confusion quantified above
• Catalytic pellistors and Palladium-resistive sensors — the H2 sensor families most vulnerable to H2S poisoning
• Electrochemical cells — the soil-gas workhorse H2 channel this co-tracer suite is usually paired with
• Mass spectrometry — the only isotope-capable route to He, developed in full
• Gas chromatography and Mud-gas logging — where the He-carrier-gas TCD gotcha is most operationally relevant
• Raman spectroscopy and IR absorption & proxies — the optical H2 methods this page shows are categorically blind to helium
• Sensor arrays & ML — a plausible path to inferring a co-tracer signature from correlated, non-selective sensor channels rather than dedicated instruments
• Why H2 is hard — the broader framing behind this page’s conclusion that H2 should not be surveyed alone
• Sensor selection guide and Comparison charts — where a He/co-tracer instrument line item belongs in an overall field-kit decision
• Glossary — R/Ra, deammoniation, Sabatier reaction, and other terms introduced above

References

Full reference list (0 entries)

[1] “Radon-222,” Wikipedia, https://en.wikipedia.org/wiki/Radon-222 (half-life 3.82146 days; alpha decay to ²¹⁸Po; member of the U-238 decay series); corroborating search synthesis via ScienceDirect Topics “Radon 222” and RadiaCode isotope reference, https://www.radiacode.com/isotope/rn-222?lang=en.

[2] Search synthesis on alpha-particle yields per U-238 (8), U-235 (7), and Th-232 (6) decay chains and mean alpha energies (5.4–6.0 MeV), referencing standard nuclear decay-chain characterizations (e.g., arXiv:1509.07436, “Subterranean production of neutrons, ³⁹Ar and ²¹Ne: Rates and uncertainties”).

[3] “Natural hydrogen resource accumulation in the continental crust,” Nature Reviews Earth & Environment (2025), https://www.nature.com/articles/s43017-025-00670-1 (two dominant H2-generation mechanisms: Fe²⁺ oxidation in water-rock reactions, and radiolysis of water by U/Th/K decay).

[4] Search synthesis on ³He/⁴He end-member values (mantle ~8 Ra; crust ~0.05 Ra) and $R_a=1.4\times10^{-6}$, referencing PMC8600645, “Mantle-Derived Helium Distribution and Tectonic Implications in the Sichuan–Yunnan Block, China,” https://pmc.ncbi.nlm.nih.gov/articles/PMC8600645/.

[5] Sano, Y. & Wakita, H., “Geographical Distribution of ³He/⁴He Ratios in Japan,” https://www.eri.u-tokyo.ac.jp/people/ichihara/vp2010plan/SanoWakita1985.pdf (mantle He at 8 Ra, crustal He at 0.02 Ra; ³He/⁴He range 7.47×10⁻⁸–9.65×10⁻⁶ and ⁴He/²⁰Ne range 0.26–1,100 across 115 samples).

[6] “Optimizing Helium Exploration: Noble Gas Isotopes as Probes for Migration–Enrichment Processes,” Energies 18(16), 4218 (2025), https://www.mdpi.com/1996-1073/18/16/4218.

[7] Search synthesis on R/Ra as a quantitative crustal-vs-mantle contribution measure, referencing USGS Yellowstone Volcano Observatory, “Helium isotopes carry messages from the mantle,” https://www.usgs.gov/observatories/yvo/news/helium-isotopes-carry-messages-mantle.

[8] “A global dataset of helium isotopic data in tectonically and hydrothermally active geological environments,” Scientific Data (2025), https://www.nature.com/articles/s41597-025-06375-w.

[9] “Natural H2 exploration: tools and workflows to characterize a play,” Science and Technology for Energy Transition (STET) (2023), https://www.stet-review.org/articles/stet/full_html/2023/01/stet20230116/stet20230116.html (He as radiolysis marker; R/Ra crustal ≪1, mantle ≫1; H2-CH4-N2 ternary diagram; GA5000/Variotec 460 field instruments; δD/δ¹³C geothermometers; Chimaera seep flux ~5,000 kg·km⁻²·d⁻¹).

[10] Extorr, Inc., “Introduction to Residual Gas Analyzers,” https://www.extorr.com/resources/residual-gas-analyzer-introduction (does not itself state helium’s 24.6 eV ionization potential or discuss m/z=4 specifically).

[11] Hiden Analytical, “Mass Spectrometers for Residual Gas Analysis Applications (RGA),” https://www.hidenanalytical.com/wp-content/uploads/2022/10/TDS-188-9-RGA-CATALOGUE.pdf.

[12] Search synthesis on PDHID sensitivity (5–10 ppb for H2, O2/Ar, N2, CH4, CO, CO2; ~10× more sensitive than FID), referencing PDHID literature summaries (Journal of Chromatographic Science / Chromatographia).

[13] “Pulsed discharge ionization detector,” Wikipedia, https://en.wikipedia.org/wiki/Pulsed_discharge_ionization_detector.

[14] “List of thermal conductivities,” Wikipedia, https://en.wikipedia.org/wiki/List_of_thermal_conductivities (λ_H2 ≈ 0.1805 W·m⁻¹·K⁻¹, λ_He ≈ 0.1513 W·m⁻¹·K⁻¹; current page revision lists gaseous H2 but only superfluid Helium II, not a gas-phase He value — the λ_He figure is independently corroborated by NIST-adjacent sources but not by this specific page as fetched).

[15] Search synthesis on δ¹⁵N signatures of thermal deammoniation (−2‰ to +2‰ at early overmaturation), referencing “Nitrogen isotopes of released gas from rock crushing and implications to origins of molecular nitrogen in Lower Cambrian overmature shale gas in South China,” ScienceDirect, https://www.sciencedirect.com/science/article/abs/pii/S0264817224001065.

[16] Search synthesis on ammonium fixation during illite-smectite diagenesis, referencing “Ammonium fixation during illite-smectite diagenesis in Upper Jurassic shale, North Sea,” Clay Minerals, Cambridge Core, https://www.cambridge.org/core/journals/clay-minerals/article/abs/ammonium-fixation-during-illitesmectite-diagenesis-in-upper-jurassic-shale-north-sea/0DFE6D4C268255ADDA3D81C4357C2308.

[17] “Nitrogen speciation in mantle and crustal fluids,” ScienceDirect (2014), https://www.sciencedirect.com/science/article/abs/pii/S0016703714000040.

[18] “H2-rich fluids from serpentinization: Geochemical and biotic implications,” PMC516479, https://pmc.ncbi.nlm.nih.gov/articles/PMC516479/.

[19] “Abiotic methane formation during experimental serpentinization of olivine,” PNAS (2016), https://www.pnas.org/doi/10.1073/pnas.1611843113; PMC5150391, https://pmc.ncbi.nlm.nih.gov/articles/PMC5150391/.

[20] “A Review of H2, CH4, and Hydrocarbon Formation in Experimental Serpentinization Using Network Analysis,” Frontiers in Earth Science (2020), https://www.frontiersin.org/journals/earth-science/articles/10.3389/feart.2020.00209/full.

[21] “The H2/CH4 ratio during serpentinization cannot reliably identify biological signatures,” PMC5036043, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5036043/.

[22] “Radon: A Tracer for Geochemical Exploration,” ResearchGate, https://www.researchgate.net/publication/306062994_Radon_A_Tracer_for_Geochemical_Exploration.

[23] “Application of radon (²²²Rn) as an environmental tracer in hydrogeological and geological investigations: An overview,” ScienceDirect, https://www.sciencedirect.com/science/article/abs/pii/S0045653522016344.

[24] “Radon as a tracer for helium exploration in geothermal areas,” ScienceDirect, https://www.sciencedirect.com/science/article/abs/pii/S1350448703001549.

[25] Prinzhofer, A. et al., “Discovery of a large accumulation of natural hydrogen in Bourakebougou (Mali),” International Journal of Hydrogen Energy 43(42), 19315 (2018), https://f.hubspotusercontent20.net/hubfs/6147377/HNAT/CONTENTS/2018_prinzhofer_mali.pdf.

[26] “Characterization of the spontaneously recharging natural hydrogen reservoirs of Bourakebougou in Mali,” Scientific Reports (2023), https://www.nature.com/articles/s41598-023-38977-y.

[27] HyTerra, “HyTerra to Kick Off Drilling Hydrogen and Helium Wells at Kansas Nemaha Site in Mid-April,” https://hyterra.com/hyterra-to-kick-off-drilling-hydrogen-and-helium-wells-at-kansas-nemaha-site-in-mid-april/; HyTerra company/news pages, https://hyterra.com/news/ and https://hyterra.com/projects/.

[28] “HyTerra’s McCoy 1 well intersects hydrogen up to 83% and helium up to 5% in Kansas,” NatH2investing, https://nath2investing.com/article/hyterras-mccoy-1-well-intersects-hydrogen-up-to-83-and-helium-up-to-5-in-kansas.

[29] Gold Hydrogen, “Ramsay Project,” https://www.goldhydrogen.com.au/ramsay-project/; Gold Hydrogen, “Ramsay Project – Helium Testing Update,” https://www.goldhydrogen.com.au/asx-releases/ramsay-project-helium-testing-update/ (neither page mentions a “Ramsay 3” well or a ~97% H2 figure — does not support that claim in the Method-comparison table).

[30] Gold Hydrogen, “Significant Concentrations of Hydrogen and Helium Detected in the Ramsay 1 Well,” ASX release, https://www.goldhydrogen.com.au/asx-releases/significant-concentrations-of-hydrogen-and-helium-detected-in-the-ramsay-1-well/ (confirms the Ramsay 1 figures — 73.3% H2 air-corrected, 3.6% He at 892mMD — but does not mention Ramsay 2 or a 6.8% raw-He figure; the Ramsay 2 raw-6.8% claim in the Method-comparison table is not supported by this reference).

[31] Search synthesis on CSIRO noble-gas laboratory groundwater helium purity results (20–25%, trace ³He), referencing South Australian Department for Energy and Mining, “Natural hydrogen,” https://energymining.sa.gov.au/industry/energy-resources/geology-and-prospectivity/natural-hydrogen (unverified — page blocked fetch access, and the specific 20–25% groundwater-purity figure could not be independently located).

[32] “Extreme methane deuterium, nitrogen and helium enrichment in natural gas from the Homorod seep (Romania),” ScienceDirect (2010), https://www.sciencedirect.com/science/article/abs/pii/S0009254110003815.

[33] Milkov, A.V., “Molecular hydrogen in surface and subsurface natural gases: Abundance, origins and ideas for deliberate exploration,” Earth-Science Reviews 230, 104063 (2022), https://www.sciencedirect.com/science/article/abs/pii/S0012825222001477.

[34] “Generation, migration and accumulation of natural hydrogen and helium in the intracratonic São Francisco Basin, eastern Brazil,” Geoenergy, Lyell Collection (2024), https://www.lyellcollection.org/doi/full/10.1144/geoenergy2024-042.

[35] “Migration of Natural Hydrogen from Deep-Seated Sources in the São Francisco Basin, Brazil,” Geosciences (MDPI) 10(9), 346 (2020), https://mdpi.com/2076-3263/10/9/346/htm.

[36] Search synthesis on the classical Glueckauf atmospheric He value (5.24 ± 0.004 ppmv, 1930s) and its confirmation, referencing NASA NTRS, “Helium concentration in the earth’s lower atmosphere,” https://ntrs.nasa.gov/citations/19850029872, and ADS record https://ui.adsabs.harvard.edu/abs/1984GeCoA..48.1759O/abstract.

[37] “Short-term variability of atmospheric helium revealed through a cryo-enrichment method,” Atmospheric Measurement Techniques 16, 1551 (2023), https://amt.copernicus.org/articles/16/1551/2023/ (this paper is about a He/N2-ratio variability measurement method and cites the 1944 Glückauf value of 5.24 ppm, not a new isotope-dilution redetermination — does not support the “5.222 ± 0.017 ppmv” claim, which instead traces to the source in [36]).

[38] “RETRANSMISSION: QIMC Reports Multi-Component Hydrocarbon and Helium Soil-Gas Anomalies in the New-Salem Area, Cumberland Basin Project, Nova Scotia,” https://www.newsfilecorp.com/release/291267/RETRANSMISSION-QIMC-Reports-MultiComponent-Hydrocarbon-and-Helium-SoilGas-Anomalies-in-the-NewSalem-Area-Cumberland-Basin-Project-Nova-Scotia.

[39] French tall-tower radon-tracer deposition-velocity study (Trainou tower, France), cross-linked from Environmental covariance, for radon-based atmospheric flux methodology context (unverified — no independently checkable primary source or URL is given for this reference).

[40] ETI Geochemistry, “Helium and Hydrogen Soil Gas Anomalies Associated with Deep or Active Faults,” https://www.eti-geochemistry.com/helium/.

[41] “Soil geochemistry of hydrogen and other gases along the San Andreas fault,” International Journal of Hydrogen Energy, ScienceDirect (2023), https://www.sciencedirect.com/science/article/abs/pii/S0360319923045974; EarthArxiv preprint, https://eartharxiv.org/repository/view/5721/.

[42] USGS, “National assessment of helium resources within known natural gas reservoirs,” Scientific Investigations Report 2021-5085, https://pubs.usgs.gov/publication/sir20215085/full.

[43] Search synthesis on the 0.3% He economic cutoff and helium-lean/helium-rich reservoir terminology, referencing “Helium production in the United States,” Wikipedia, https://en.wikipedia.org/wiki/Helium_production_in_the_United_States.

[44] “Desert Mountain Energy Advances Helium and Hydrogen Opportunities and Partnerships,” PR Newswire, https://www.prnewswire.com/news-releases/desert-mountain-energy-advances-helium-and-hydrogen-opportunities-and-partnerships-302439904.html.

[45] “A review of genetic identification of natural gas,” NSF PAR repository, https://par.nsf.gov/servlets/purl/10084495.

[46] “Distinguishing the source of natural gas accumulations with a combined gas…” EPA, https://www.epa.gov/sites/default/files/documents/distinguishingthesourceofnaturalgasaccumulationswithacombinedgas.pdf.

[47] Cross-reference: Thermal conductivity & acoustic sensors — 1% He → ≈+7,700 ppm H2-equivalent bias via first-order Wassiljewa-relation linearization, citing Wikipedia “List of thermal conductivities” and ScienceDirect Topics, “Thermal Conductivity of Gases — an overview,” https://www.sciencedirect.com/topics/chemical-engineering/thermal-conductivity-of-gases.

[48] “Thermal Conductivity Detector Analysis of Hydrogen Using Helium Carrier Gas and HayeSep® D Columns,” Journal of Chromatographic Science, Oxford Academic, https://academic.oup.com/chromsci/article-pdf/36/4/191/742027/36-4-191.pdf.

[49] ResearchGate discussion, “Which Carrier gas (Ar or He…) will be suitable in GC TCD detector to determine hydrogen evolution from a photocatalytic reactor?”, https://www.researchgate.net/post/Which_Carrier_gas_Ar_or_He_will_be_suitable_in_GC_TCD_detector_to_determine_hydrogen_evolution_from_a_photocatalytic_reactor.

[50] Agilent, “Switch to Hydrogen or Nitrogen as Alternate Carrier Gas,” https://www.agilent.com/en/product/gas-chromatography/gc-systems/convert-alternative-carrier-gas-hydrogen-nitrogen-gc-gcms (this page is about converting methods from helium to hydrogen or nitrogen carrier gas; it does not discuss switching to argon for accurate H2/He quantification — does not support that claim).

[51] Search synthesis on sniffer-mode sensitivity floor set by ~5 ppm atmospheric He background, referencing AJVS product listing, https://www.ajvs.com/pfeiffer-vacuum-asm-306-s-24054, and SubZero Scientific, https://subzeroscientific.com/products/pfeiffer-helium-and-hydrogen-sniffer-leak-detector-model-asm-306s (the specific quoted claim, “due to the amount of helium present in the air (approximately 5 ppm),” could not be located verbatim on either cited page — does not support the direct quotation).

[52] “A natural hydrogen seep in Western Australia: Observed characteristics and controls,” Science and Technology for Energy Transition (STET) (2024), https://www.stet-review.org/articles/stet/full_html/2024/01/stet20240139/stet20240139.html.

[53] Palmisano, V., Weidner, E., Boon-Brett, L., Bonato, C., Harskamp, F., Moretto, P., Post, M.B., Burgess, R., Rivkin, C., Buttner, W.J., “Selectivity and resistance to poisons of commercial hydrogen sensors,” International Journal of Hydrogen Energy 40 (2015) 11740–11747, DOI: 10.1016/j.ijhydene.2015.02.120.

[54] Search synthesis on H2S IDLH and alarm thresholds, referencing BSEE, “Hydrogen Sulfide (H2S),” https://www.bsee.gov/sites/bsee.gov/files/h-pincs.pdf, and https://www.bsee.gov/sites/bsee.gov/files/09-g31.pdf.

[55] Search synthesis on He/H2 ratio and “reduces the risk of misinterpretation of surface hydrogen measurement” framing, referencing ETI Geochemistry [40] and the QIMC/Cumberland Basin disclosures [38].

[56] DURRIDGE Company, “RAD7 Radon Detector,” https://durridge.com/products/rad7-radon-detector/; RAD7 Product Brief, https://durridge.com/documentation/RAD7%20Product%20Brief.pdf (vendor page currently states a range of 0.1–20,000 pCi/L, not 0.1–10,000 pCi/L as given in the Instruments table above).

[57] DURRIDGE Company, “Soil Gas Probes,” https://durridge.com/products/soil-gas-probes/.

[58] Search synthesis on ASM 306S startup/response-time specifications, referencing AJVS and Ideal Vacuum product listings.

[59] INFICON, “Protec P3000(XL) Helium Leak Detector,” https://www.inficon.com/media/5355/download/Protec-P3000-(XL)-Helium-Leak-Detector.pdf.

[60] INFICON, “UL3000 Fab Series — Helium Leak Detectors,” https://www.inficon.com/en/products/leak-detectors/ul3000-fab-series (the fetched vendor page does not itself state the “2 ppm sniffer sensitivity” or “5×10⁻⁶ mbar·L/s at 90% RH” figures attributed to it below — real product, unconfirmed specifics).

[61] Leybold, “PHOENIX Portable Helium leak detectors,” https://www.leybold.com/en-us/products/leak-detection/phoenix-helium-leak-detectors.

[62] Leybold, “Helium leak detection,” https://www.leybold.com/en-us/applications-and-industries/helium-leak-detection; Leybold, “How are spray and sniffer techniques used for local leak tests,” https://www.leybold.com/en-us/knowledge/vacuum-fundamentals/leak-detection/how-do-sniffer-leak-detectors-work.

[63] Search synthesis on Pfeiffer Vacuum PRISMA pro (QMA 250) minimum detection limit, referencing Pfeiffer Vacuum, “Quadrupole Mass Spectrometers for Gas Analysis,” https://www.pfeiffer-vacuum.com/global/en/products/analytical-instrumentation/mass-spectrometers/.

[64] Scion Instruments, “The Different Types of GC Detectors,” https://scioninstruments.com/blog/the-different-types-of-gc-detectors/.

[65] Agilent, “490 Micro GC Natural Gas Analyzers” datasheet, https://www.agilent.com/cs/library/datasheets/public/5991-0301EN.pdf; Agilent, “Fast Analysis of Natural Gas Using the Agilent 490 Micro GC Natural Gas Analyzer,” https://www.agilent.com/cs/library/applications/5991-0275EN.pdf.

[66] Eaton/MTL, “K522 - Thermal Conductivity (TCD) Gas Sensor for Hydrogen (H2), Helium (He), Argon…,” https://www.mtl-inst.com/product/k522_-_thermal_conductivity_tcd_gas_sensor_for_hydrogen_h2_helium_he_argona/.

[67] Search synthesis on Gasmet GT6000 Mobilis gas list and explicit H2S exclusion, referencing Gasmet, “GT6000 Mobilis – Portable Gas Analyzer,” https://www.gasmet.com/products/category/portable-gas-analyzers/gt6000-mobilis/.

[68] Isotech Laboratories (a Stratum Reservoir company), “Gas Analyses,” https://isotechlabs.com/services/gas.

[69] Isotech Laboratories, natural-gas analytical service page, https://isotechlabs.com/analytical/sampletype/gas/naturalgas/ng2.html.

[70] ASTM International, “D1945-14(2019) Standard Test Method for Analysis of Natural Gas by Gas Chromatography,” https://www.astm.org/Standards/D1945.htm; H2tools record, https://h2tools.org/fuel-cell-codes-and-standards/astm-d1945-14-standard-test-method-analysis-natural-gas-gas.

[71] Search synthesis on ASTM D1945 component sensitivity (0.01 mol% general, 0.3 mol% for H2S), referencing Scion Instruments, “Refinery Gas Analysis using the SCION Rapid RGA Analyzer,” https://scioninstruments.com/refinery-gas-analysis-using-the-scion-rapid-rga-analyser/.