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To help meet emission standards, hydrogen sulfide (H2S) from geothermal production may be injected back into the subsurface, where basalt offers, in theory, the capacity to mineralize H2S into pyrite. Ensuring the viability of this pollution mitigation technology requires information on how much H2S is mineralized, at what rate and where. To date, monitoring efforts of field‐scale H2S reinjection have mostly occurred via mass balance calculations, typically capturing less than 5% of the injected fluid. While these studies, along with laboratory experiments and geochemical models, conclude effective H2S mineralization, their extrapolation to quantify mineralization and its persistence over time leads to considerable uncertainty. Here, a geophysical methodology, using time‐domain induced polarization (TDIP) logging in two of the injection wells (NN3 and NN4), is developed as a complementary tool to follow the fate of H2S re‐injected at Nesjavellir geothermal site (Iceland). Results show a strong chargeability increase at +40 days, interpreted as precipitation of up to 2 vol.% based on laboratory relationships. A uniform increase is observed along NN4, whereas it is localized below 450 m in NN3. Changes are more pronounced with larger electrode spacing, indicating that pyrite precipitation takes place away from the wells. Furthermore, a chargeability decrease is observed at later monitoring rounds in both wells, suggesting that pyrite is either passivated or re‐dissolved after precipitating. These results highlight that a sequence of overlapping reactive processes (pyrite precipitation, passivation, pore clogging and possibly pyrite re‐dissolution) results from H2S injection and that TDIP monitoring is sensitive to this sequence. Plain Language Summary: High‐temperature geothermal production is responsible for air pollution due to hydrogen sulfide (H2S) present in the magmatic fluid. To help meet emission standards, H2S may be injected back into the subsurface, where basalt offers the capacity to transform H2S into pyrite. Transformation into an immobile mineral prevents further transport into the atmosphere, sea, surface streams, or lakes. However, ensuring the viability of this pollution mitigation technology requires information such as how much H2S is mineralized, at what rate and where, which are highly uncertain due to the heterogeneity and inaccessibility of subsurface processes. Here, a geophysical monitoring methodology is developed and tested during the re‐injection of H2S at Nesjavellir geothermal site in south‐west Iceland. According to laboratory studies, pyrite precipitation is expected to increase the electrical capacitance, "chargeability," of the subsurface. Using the so‐called time‐domain induced‐polarization (TDIP) method embedded in a wireline logging tool, the chargeability of a 2 m‐wide cylinder around the injection wells is measured with high spatial‐resolution before and during H2S injection. A strong chargeability increase at +40 days indicates that up to 2% pyrite is formed. A subsequent decrease at later rounds raises questions on whether pyrite is re‐dissolved or passivated by other secondary minerals. Key Points: H2S reinjection in basalt and mineralization into pyrite was monitored using TDIP logging in two of the injection wellsChargeability increases observed at +40 days (first monitoring round) correspond to 1%–2% pyrite precipitation with great spatial variabilitySubsequent monitoring rounds show a decrease in chargeability, suggesting that pyrite is either passivated or re‐dissolved [ABSTRACT FROM AUTHOR]

Yellowstone National Park (YNP) is home to roughly 500 geysers, making it the most concentrated geyser field in the world. Recent studies exploring the mechanics of geyser eruptions utilizing laboratory models were limited by a lack of knowledge of the subsurface geometry of the geyser features. This study presents results from active hydrogeophysical surveys, including ground penetrating radar, nuclear magnetic resonance (NMR), seismic refraction, electrical resistivity tomography, and transient electromagnetics to image subsurface geyser structure and constrain the geophysical response of the reservoir structure of Spouter Geyser, Yellowstone National Park. Previous geophysical studies on similar geyser systems in Yellowstone characterize the hydrothermal reservoir that supplies the geyser with hydrothermal fluids and vapors as high porosity, low density structures. Whereas our imaging highlights the hydrothermal conduits and reservoir as high resistivity and high velocity structures, interpreted as silica precipitate decreasing the porosity and increasing the bulk modulus (i.e., from unconsolidated media to semi/fully consolidated). Ground penetrating radar identifies sinter thickness around the geyser, NMR provides porosity measurements of the geyser reservoir, and transient electromagnetics maps the depth to the Biscuit Basin Rhyolite bedrock throughout the field site. Electrical resistivity tomography and seismic refraction image the shallow geyser conduit structures to ∼15 m depth and establish the extent of the geyser hydrothermal reservoir structure. Furthermore, a spatial correlation of the electrical resistivity and seismic velocity results establishes the interpreted hydrothermal geyser reservoir as a high resistivity, high velocity structure located to the northeast of Spouter Geyser at depths greater than 15 m. Plain Language Summary: The dynamics of geyser eruptions are uncertain. Much of this uncertainty stems from a lack of knowledge of their subsurface structure. Therefore, to better understand geyser eruption dynamics, we must first establish the subsurface structure of geysers and their associated hydrothermal geyser reservoirs that supply the erupted hydrothermal fluids and vapors. Geophysical imaging provides an opportunity to image these structures, but active geophysical methods have not been extensively explored for this application. In this study, we use various active geophysical methods to (a) constrain the characteristics of the structure and (b) better understand the subsurface geometry of Spouter Geyser (YNP) and its hydrothermal reservoir. We find that Spouter Geyser's structure and its reservoir have high electrical resistivity and seismic velocity responses, indicative of low porosity and highly cohesive characteristics due to silica precipitate from hydrothermal fluids. We also find that Spouter Geyser's subsurface geometry consists of silica‐lined conduits that extend 15 m to a broad, laterally offset geyser hydrothermal reservoir structure. With the knowledge of the characteristics and geometry of Spouter Geyser and its hydrothermal reservoir, an additional study (Geyser Dynamics II) is better positioned to explore the dynamics of geyser eruptions with active geophysical methods. Key Points: Geophysical results reveal a laterally offset, low porosity and highly consolidated geyser reservoir structure at Spouter GeyserDistinct vertical conduit structures extending to the hydrothermal geyser reservoir at 15 m depth are interpreted at Spouter GeyserVelocity and resistivity distribution characterize Spouter Geyser's reservoir structure as high velocity‐high resistivity [ABSTRACT FROM AUTHOR]

Geysers are unique hydrothermal features, requiring specific geometry, fluid, and vapor input, and heat to produce eruptions. In a geyser eruption, the decompression of superheated hydrothermal fluids results in a conversion of thermal to kinetic energy. Recent studies conclude that a laterally offset cavity structure (bubble trap) plays an integral role in the geyser eruption process. In this second study of Spouter Geyser, Yellowstone National Park (YNP), we build on the geyser structure developed in the first publication and explore how structural components, such as the bubble trap, control eruption dynamics. We utilize time‐lapse electrical resistivity tomography (ERT) and transient electromagnetics (TEM) geophysical methods to track changes in the saturation of conductive hydrothermal fluid and resistive vapor through the eruption cycle. Additionally, we use pressure‐temperature transducer data to measure eruption and recharge durations over the past 23 years at Spouter Geyser, identifying long‐period trends in geyser behavior. The geophysics results support the bubble trap model, capturing an increase in resistivity in the bubble trap structure during the recharge phase interpreted as a vapor saturation increase. The TEM also captures a resistivity increase in the 20–30 m depth interval during the eruption phase, interpreted as a vapor‐dominated "flash eruption zone" where the superheated hydrothermal fluids flash into steam and drive fluid out of this zone during the eruption. From the temperature time‐series detailing 23 years of eruption and recharge durations, we find that eruption durations have remained constant at ∼2 hr while recharge durations have decreased linearly at 6.6 min/year. Plain Language Summary: Current geyser eruption models include a bubble trap structure, a laterally offset, subsurface cavity that pressurizes with hydrothermal fluid and vapor before an eruption. Hydrothermal fluid conducts electrical current more efficiently than hydrothermal vapors, and thus the two phases have different electrical resistivity properties, a measurable geophysical parameter. Here, we pioneer the application of active, time‐lapse electrical and electromagnetic geophysical methods, sensitive to the subsurface electrical resistivity properties, to track hydrothermal fluid and vapor through a geyser system at Spouter Geyser, Yellowstone National Park. Through this, we expand on conclusions of Spouter Geyser's structure (Geyser Dynamics I) by investigating the movement of hydrothermal fluids and vapors to contribute to the understanding of geyser eruption dynamics. We find that electrical resistivity at the top of the geyser hydrothermal reservoir increases while the geyser is not erupting, indicative of vapor buildup in a bubble trap structure. We also find that electrical resistivity increases in a section of the geyser hydrothermal reservoir during the eruption, interpreted as hydrothermal vapors flashing to steam and driving fluids to the surface. Finally, we investigate eruption durations over the past two decades at Spouter Geyser, revealing constant average eruption durations but an increase in eruption frequency. Key Points: Time‐lapse electrical and electromagnetic methods track changes in hydrothermal fluid and vapor saturations in Spouter Geyser's structureTime‐lapse geophysics informs a conceptual model of eruption dynamics at Spouter Geyser that supports the bubble trap geyser modelA 6.6 min/year decrease in Spouter Geyser's average recharge duration over the last 23 years offers insight into long‐duration behavior [ABSTRACT FROM AUTHOR]