Hydrothermal Reservoir Research Articles

Optimizing the heat extraction performance in the Qingfeng Karst geothermal reservoir

Faults are typically associated with hydrothermal reservoirs and may control the heat extraction of geothermal reservoirs. However, in Karst geothermal reservoirs, the dissolution of soluble rocks results in pronounced spatial porosity and permeability anisotropy (PPA), potentially weakening the dominant role of faults. The question arises: which plays a dominant role, PPA or faults? Should geothermal exploitation strategies be adjusted under the interaction of both factors? In this study, we proposed a coupled Thermal-Hydraulic-Geothermal Wellbore (THG) numerical simulation model to streamline the integration of non-isothermal flow and heat transfer processes within a 3D wellbore. Utilizing this model, we analyzed the influence of faults compared to PPA and assessed the geothermal well doublet's heat extraction performance, including thermal breakthrough time, temperature losses along the wellbore wall, and recovery factor (Rg). There are specific geothermal reservoir development conditions leading to the latest thermal breakthrough time and the maximum Rg. The results of contribution rates highlight the influence of injection rate in determining Rg. It is crucial to note that thermal breakthrough time does not necessarily increase with injection temperatures. A more suitable definition for thermal breakthrough time in the Karst geothermal reservoir has also been identified. This study provides an essential reference value for the efficient development of similar Karst geothermal reservoirs.

Frictional Properties of Feldspar‐Chlorite Gouges and Implications for Fault Reactivation in Hydrothermal Systems

AbstractAs a particularly common mineral in granites, the presence of feldspar and feldspar‐chlorite gouges at hydrothermal conditions has important implications in fault strength and reactivation. We present laboratory observations of frictional strength and stability of feldspar (K‐feldspar and albite) and feldspar‐chlorite gouges under conditions representative of deep geothermal reservoirs to evaluate the impact on fault stability. Velocity‐stepping experiments are performed at a confining stress of 95 MPa, pore pressures of 35–90 MPa, and temperatures of 120–400°C representative of in situ conditions for such reservoirs. Our experiment results indicate that the feldspar gouge exhibits strong friction (μ ∼ 0.71) at all experimental temperatures (∼120–400°C) but when T > 120°C, the frictional response transitions from velocity‐strengthening to slightly velocity‐weakening. At constant confining pressure and temperature, increasing the pore pressure increases the friction coefficient (∼0.70–0.85) and the gouge remains slightly velocity weakening. The presence of alteration‐sourced chlorite leads to a transition from velocity weakening to velocity strengthening in the mixed gouge at experimental temperatures and pore pressures. As a ubiquitous mineral in reservoir rocks, feldspar is shown to potentially contribute to unstable sliding over ranges in temperature and pressure typical in deep hydrothermal reservoirs. These findings emphasize that feldspar minerals may increase the potential for injection‐induced seismicity on pre‐existing faults if devoid of chloritization.

Gas geothermometry, soil CO2 degassing, and heat release estimation to assess the geothermal potential of the Alpehue Hydrothermal Field (Sollipulli volcano, Southern Chile)

The Alpehue Hydrothermal Field (AHF) near the Sollipulli Volcano in the Southern Volcanic Zone of Chile shows promise as a significant geothermal resource. A comprehensive geothermal exploration survey was conducted, including the evaluation of hydrothermal gases, geothermometer calculations, and CO2 flux measurements, to assess the AHF's geothermal potential. Our results indicate that the hydrothermal gasses at the AHF primarily originate from primitive, mantle-derived sources, with some contribution from crustal sediments. Two different CO2 populations of fluxes were identified. One corresponds to the background emission related to the soil biological activity (mean ∼7.7 g·m−2·d−1), and the other, much more significant, emanates from an endogenous source related to the Alpehue hydrothermal reservoir (mean ∼461 g·m−2·d−1). Reservoir temperatures were calculated using gas geothermometry yielding average temperatures of 249 °C. The calculated heat flow rate of the AHF is approximately 3.3 MW and the heat flux corresponds to 156 thermal MW⋅km−2, which could be considered a medium geothermal potential comparable to other systems worldwide. Although further studies are needed to fully address its exploitability, this study presents favorable characteristics of the AHF that make it a promising avenue for further exploration.

Effect of fracture structure on heat transfer in heat pipes in a submarine hydrothermal reservoir

Effect of fracture structure on heat transfer in heat pipes in a submarine hydrothermal reservoir

Fluid geochemistry of the Cerro Galán geothermal system (Southern Puna, Argentina): Implications for the geothermal potential of one of the youngest giant calderas in the Andes

The exploration of novel geothermal systems, particularly those promising for electrical power generation, plays a fundamental role in incorporating new renewable sources into the energy matrix. Geothermal systems associated with volcanic calderas are considered ideal targets for exploration. This study focuses on the geochemical features of fluids from the Cerro Galán hydrothermal system, which is hosted within a major resurgent caldera with >3.5 Myr of magmatic evolution situated on the Southern Puna (Central Volcanic Zone of the Andes, NW Argentina). The main aim is constructing the first geochemical conceptual model and provide information on the geothermal potential of this interesting resource. The main hydrothermal reservoir consists of a NaCl aquifer with estimated temperatures up to 187 °C at depth. This reservoir is likely hosted within the fractured pre-caldera basement rocks, mainly including Miocene-Pliocene volcanic rocks and Proterozoic-Cambrian igneous and metamorphic rocks. The confinement of the deep reservoir is attributed to the deposits of the Toconquis Group and Cueva Negra Ignimbrite, along with the basal section of the Cerro Galán Ignimbrite, which exhibit low permeability due to hydrothermal alteration. The presence of a phreatic explosion crater near one of the hot spring-rich areas is likely indicating past over-pressurization of the hydrothermal aquifer, resulting from efficient sealing. Furthermore, the absence of anomalous soil CO2 flux values on the top of the reservoir, except where the thermal spring discharges are located, can be explained by an effective cap-rock layer. Deep circulation of meteoric water, enriched with atmospheric gases, receives inputs of magmatic fluids (∼11% of primordial helium), leading to the development of the hydrothermal NaCl aquifer. However, this deep fluid contribution might be underestimated due to significant crustal assimilation (up to 50%) involved in the magma genesis of the Cerro Galán Volcanic Complex, a process which modifies the He isotopic signature of the magmatic endmember. The hot springs, characterized by high flow rate (up to 459 m3/h) are positioned at the intersection between the caldera margins and the NNE-SSW oriented tectonic structures, suggesting favorable permeability conditions. The preliminary geothermal gradient for the Cerro Galán area is estimated at around 98–101 °C/km. Such a high gradient can be attributed to the considerable heat flux generated by the transcrustal plumbing system of the Cerro Galán caldera, which includes the shallow crystal mush reservoir (<4 km depth). The preliminary geothermal potential of this giant caldera was performed using the volumetric method along with Monte Carlo simulations. The results indicate a probable power production capacity of 2.09 MWe and 10.85 MWe at 90 and 50% confidence level, respectively. The results presented in this work constitute a foundational knowledge base to promote a more advanced exploration phase for the geothermal resource. Additionally to the local energy demand, lithium and other metal mining operations, which are operating independently from the National Interconnected System, could potentially be interested in power generation through binary cycles.

The effects of fractures on porous flow and heat transfer in a reservoir of a U-shaped closed geothermal system

In geothermal reservoirs, fractures, serving as preferential channels for heat transfer fluid circulation, can cause complicated migration patterns and heat transfer characteristics of the fluids. However, the crucial issue of whether and how fractures within a hydrothermal reservoir of a closed geothermal system (CGS) affect reservoir heat transfer fluid migration and the system’s heat transfer is still poorly understood. In this study, we construct a thermo-hydraulic coupling finite-element model to investigate the effects of fractures on fluid flow and heat transfer enhanced by natural convection in a U-shaped closed geothermal system (UCGS) with a horizontal well. We systematically analyze the responses of fluid migration and heat extraction in the system to key factors, such as fracture aperture, fracture azimuth, fracture distribution, and reservoir permeability. The results show that, in a high permeability reservoir (such as 2 × 10−13 m2), the fracture with an aperture of a millimeter scale can greatly strengthen and control the natural convection induced by heat transfer around the horizontal well in a UCGS, achieving a heat recovery efficiency of six times higher than conduction alone within the reservoir. The fracture azimuth can control the heat production region by guiding the evolution of natural convection. Intersecting network-distributed fractures can effectively improve heat recovery area and performance (more than 15%) compared to a single fracture. Some specific fracture segments within a fracture network may exhibit synergy or inhibition in different heat extraction stages, while a fracture connecting in the deep reservoir and orienting to the horizontal well may lead to heat breakthrough. In addition, a low-permeability interlayer below the horizontal well can restrict the heat recovery (nearly 20% drop in our models), but a fracture penetrating the layer can compensate for this hindrance. The findings of this study can contribute to understanding the thermodynamic mechanisms of fluid flow and heat transfer in a fractured reservoir, and then utilizing them to maximize heat recovery rather than considering conduction only, thus providing scientific insights into engineering practices of the UCGS.

Three-dimensional resistivity structure in Toya caldera region, Southwest Hokkaido, Japan — Constraints on magmatic and geothermal activities

Southwestern Hokkaido, Japan, is characterized by numerous Quaternary volcanoes and geothermal areas. At the same time, the region hosts various critical infrastructures, and there is a need to assess the impact of volcanic hazards on them. Geophysics could provide scientific clues for the hazard assessment by elucidating the abundance of subsurface magma. To clarify the resistivity structure from the crust to uppermost mantle of the Toya caldera, a representative Quaternary volcanic area, a wideband magnetotellurics survey of 117 points over land, sea, and lake areas, as well as 3D inversion, has been conducted. In combination with petrological and seismological findings, quantitative interpretation of the inverted model has found that conductive bodies in the uppermost mantle (14–68 [Formula: see text]) suggest the presence of melts (0.25 vol%–3.4 vol%) or fluids (0.068 vol%–0.45 vol%). An extremely conductive body (&lt;10 [Formula: see text]) at a depth of approximately 3–14 km in the eastern geothermal area could be interpreted as a hydrothermal reservoir; below this body, the conductive column (1.8–15 [Formula: see text]), rising from the uppermost mantle, suggests fluid upwelling. In contrast, high resistivity (&gt;100 [Formula: see text]) beneath Usu Volcano, the center of active volcanism, suggests that no mobile magma was present. A columnar-shaped region of slightly low resistivity (44 [Formula: see text] at minimum) is observed below the Toya caldera, which was inferred as cooling magma or an altered or heated upper crust attributed to past magma intrusion. A resistivity structure observed below the volcanic edifice is considered to reflect the steady state of the dormant volcanic system in this area, and there is likely no large amount of melt that would be deemed imminent for a caldera-forming eruption. This information could be a valuable scientific contribution to the volcanic hazard risk assessments currently being conducted in Japan.

Volatile organic compounds (VOCs) from diffuse degassing areas: Interstitial soil gases as message bearers from deep hydrothermal reservoirs

The chemical composition of volatile organic compounds (VOCs) in interstitial soil gases from hydrothermal areas is commonly shaped by both deep hydrothermal conditions (e.g., temperature, redox, sulfur fugacity) and shallow secondary processes occurring near the soil-atmosphere interface. Caldara di Manziana and Solfatara di Nepi, i.e., two hydrothermal systems characterized by diverse physicochemical conditions located in the Sabatini Volcanic District and Vicano-Cimino Volcanic District, respectively (Central Italy), were investigated to evaluate the capability of VOCs in soil gases to preserve information from the respective feeding deep fluid reservoirs. Hierarchical cluster analyses and robust principal component analyses allowed recognition of distinct groups of chemical parameters of soil gases collected from the two study areas. The compositional dissimilarities from the free-gas discharges were indeed reflected by the chemical features of soil gases collected from each site, despite the occurrence of shallow processes, e.g., air mixing and microbial degradation processes, affecting VOCs. Four distinct groups of VOCs were recognized suggesting similar sources and/or geochemical behaviors, as follows: (i) S-bearing compounds, whose abundance (in particular that of thiophenes) was strictly dependent on the sulfur fugacity in the feeding system; (ii) C4,5,7+ alkanes, n-hexane, cyclics and alkylated aromatics, related to relatively low-temperature conditions at the gas source; (iii) C2,3 alkanes, benzene, benzaldehyde and phenol, i.e., stable compounds and thermal degradation products; and (iv) aliphatic O-bearing compounds, largely influenced by shallow processes within the soil. However, they maintain a chemical speciation that preserves a signature derived from the supplying deep-fluids, with aldehydes and ketones becoming more enriched after intense interaction of the hypogenic fluids with shallow aquifers. Accordingly, the empirical results of this study suggest that the chemical composition of VOCs in soil gases from hydrothermal areas provides insights into both deep source conditions and fluid circulation dynamics, identifying VOCs as promising geochemical tracers for geothermal exploration.

Optimal Network Design for Microseismic Monitoring in Urban Areas - A Case Study in Munich, Germany

Well-designed monitoring networks are crucial for obtaining precise locations, magnitudes and source parameters, both for natural and induced microearthqakes. The performance of a seismic network depends on many factors, including network geometry, signal-to-noise ratio (SNR) at the seismic station, instrumentation and sampling rate. Therefore, designing a high-quality monitoring network in an urban environment is challenging due to the high level of anthropogenic noise and dense building infrastructure, which can impose geometrical limitations and elevated construction costs for sensor siting. To address these challenges, we apply a numerical optimization approach to design a microseismic surveillance network for induced earthquakes in the metropolitan area of Munich (Germany), where several geothermal plants exploit a deep hydrothermal reservoir. First of all, we develop a detailed noise model for the city of Munich, to capture the heterogeneous noise conditions. Then, we calculate the expected location precision for a randomly chosen network geometry from the body-wave amplitudes and travel times of a synthetic earthquake catalog considering the modeled local noise level at each network station. In the next step, to find the optimum network configuration, we use a simulated annealing approach in order to minimize the error ellipsoid volume of the linearized earthquake location problem. The results indicate that a surface station network cannot reach the required location precision (0.5 km in epicentre and 2 km in source depth) and detection capability (magnitude of completeness Mc = 1.0) due to the city´s high seismic noise level. In order to reach this goal, borehole stations need to be added to increase the SNR of the microearthquake recordings, the accuracy of their body-wave arrival times and source parameters. The findings help to better quantify the seismic monitoring requirements for a save operation of deep geothermal projects in urban areas.

The Muschelkalk aquifer of the Molasse basin in SW-Germany: implications on the origin and development of highly saline lithium-rich brines in calcareous hydrothermal reservoirs

Highly saline lithium-rich hydrothermal fluids (measured chloride concentration up to 44 g kg−1, lithium concentration up to 162 mg kg−1) occur in the deep calcareous Muschelkalk aquifer beneath the northern Alpine foreland (Molasse) basin. We have combined geologic, hydraulic, hydrochemical, and stress field data of the Triassic Muschelkalk aquifer beneath younger sediments of Triassic–Jurassic successions and the Cenozoic Molasse basin of SW-Germany for a synthesis to constrain the origin and development of these brines. In contrast to the regional southeast plunge of Jurassic and Cenozoic strata, low-gradient groundwater flow in the Upper Muschelkalk aquifer is to the north, induced by regional recharge from west, south, and east. The investigated area is seismically active and north trending maximum horizontal stress likely fosters development of necessary fracture permeability for northward flow in the competent carbonates of the Upper Muschelkalk aquifer. The highest lithium concentrations and total dissolved solids (TDS) can be found in the southern parts of the Muschelkalk aquifer. Here, the Muschelkalk Group overlays directly a crystalline basement swell separating two ENE-trending Permocarboniferous troughs. We argue that the highly saline lithium-rich fluids originate from fluid–rock interaction of meteoric water with Variscan crystalline basement rocks and entered the Muschelkalk aquifer on top of the basement swell by permeable faults and fractures. The marginal calcareous sand-rich facies of the Muschelkalk enables the inflow of brines from crystalline basement faults and fractures into the aquifer. We thus argue for an external origin of these brines into the aquifer and further intra-reservoir development by dilution with meteoric water.

Seismic Characterization of the Blue Mountain Geothermal Field

Subsurface characterization is crucial for geothermal energy exploration and production. Yet hydrothermal reservoirs usually reside in highly fractured and faulted zones where accurate characterization is very challenging because of low signal-to-noise ratios of land seismic data and lack of coherent reflection signals. We perform an active-source seismic characterization for the Blue Mountain geothermal field in Nevada using active seismic data to reveal the elastic medium property complexity and fault distribution at this field. We first employ an unsupervised machine learning method to attenuate groundroll and near-surface guided-wave noise and enhance coherent reflection and scattering signals from noisy seismic data. We then build a smooth initial P-wave velocity model based on an existing magnetotellurics survey result, and use 3D first-arrival traveltime tomography to refine the initial velocity model. We then derive a set of elastic wave velocities and anisotropic parameters using elastic full-waveform inversion, and obtain PP and PS images using elastic reverse-time migration. We identify major faults by analyzing the variations of seismic velocities and anisotropy parameters, and reveal mid- to small-scale faults by applying a supervised machine learning method to the seismic migration images. Our characterization reveals complex velocity heterogeneities and anisotropies, as well as faults, with a high spatial resolution. These results can provide valuable information for optimal placement of future injection and production wells to increase geothermal energy production at the Blue Mountain geothermal power plant.

The alteration of illite by Bad Nauheim and Gerolstein brine; Implications on fluid permeability in geothermal systems

Illite is one of the most abundant clay minerals on earth, yet its structure remains not fully solved. In hydrothermal reservoir sandstones, illites can display a fibrous growth within the pores and pore throats. The network created by them and the particles trapped therein during fluid flow lead to a dramatic decrease in permeability, which should be prevented if the reservoirs were to be used for geothermal energy extraction. In order to determine possible changes in structure, stacking or shape of the illite fibers, the interaction of a sandstone with two different brines was compared to an unaltered sandstone via environmental scanning electron microscopy and transmission electron microscopy. Moreover, three-dimensional electron diffraction experiments were performed using automated diffraction tomography. The 1Mtv structure of illite could be solved based on a single dataset of a 50 nm illite fiber. Illites that were altered with synthetic Gerolstein brine showed one-dimensional diffuse scattering, indicating a disorder in the stacking of the illite layers. The degree of disorder is dependent on the potassium content of the fluid, since the K+ ions of the interlayer can be released via the (010) facet. This causes a stability reduction of the structure, resulting in a high fragmentation and detachment of individual illite layers. Fluids interacting with the sandstone can then transport such mobile layer fragments, leading to entangling and interweaving of illite fibers. These new insights into the structure of illite fibers show a significant dependence of the arrangement of the fibers in the pores on the potassium content of the fluid. The increased migration and interweaving of illite fibers associated with a low potassium content can lead to severe clogging of the pores and thus to a reduction in fluid permeability. Altered sandstones with illitic pore filling are therefore less suitable for long-term geothermal projects.

Geophysically guided well siting at the Aluto-Langano geothermal reservoir

Volcano-hosted high-temperature geothermal reservoirs are powerful resources for green electricity generation. In regions where such resources are available, geothermal energy often provides a large share of a country’s total power generation capacity. Sustainable geothermal energy utilization depends on the successful siting of geothermal wells, which in turn depends on prior geophysical subsurface imaging and reservoir characterization. Electromagnetic resistivity imaging methods have proven to be a key tool for characterizing magma-driven geothermal systems because resistivity is sensitive to the presence of melt and clays that form through hydrothermal alteration. Special emphasis is often given to the “clay cap,” which forms on top of hydrothermal reservoirs along the flow paths of convecting geothermal fluids. As an example, the Aluto-Langano volcanic geothermal field in Ethiopia is covered with 178 densely spaced magnetotelluric (MT) stations. The 3D electrical conductivity model derived from the MT data images the magma body that acts as a heat source of the geothermal system, controlling geothermal convection and formation of alteration zones (commonly referred to as clay cap) atop the geothermal reservoir. Detailed 3D imaging of the clay cap topography can provide direct insight into hydrothermal flow patterns and help identify potential “upflow” zones. At Aluto all productive geothermal wells are drilled into zones of clay cap thinning and updoming, which is indicative of underlying hydrothermal upflow zones. In contrast, nonproductive wells are drilled into zones of clay cap thickening and lowering, which is an indicator for underlying “outflow” zones and cooling. This observation is linked to fundamental characteristics of volcano-hosted systems and can likely be adapted to other geothermal fields where sufficiently detailed MT surveys are available. Therefore, high-resolution 3D electromagnetic imaging of hydrothermal alteration products (clay caps) can be used to infer the hydrothermal flow patterns in geothermal reservoirs and contribute to derisking geothermal drilling projects.

Three-dimensional magnetotelluric inversion for the characterization of the Sol de Mañana high-enthalpy geothermal field, Bolivia

We describe the three-dimensional magnetotelluric modeling results of Laguna Colorada - Sol de Mañana geothermal field in SW Bolivia. It is located near Laguna Colorada in the foothills of the Western Cordillera of the Andes and at the margin of the Altiplano-Puna Magma Body (APMB), which is characterized by a vast electrical conductivity anomaly and corresponding seismic anomalies in the mid and deep crust. The magnetotelluric (MT) data comprising 70 stations were modeled by a 3-D inversion. The final model shows a broad conductor at a few hundred meters in depth, which we interpret as the confining cap layer. Beneath, equally widespread, a layer of intermediate resistivity marks the high-temperature hydrothermal reservoir. Detailed interpretation of MT data with information from existing wells suggests a potential for a high-enthalpy geothermal field.

Depositional and diagenetic controls on porosity evolution in sandstone reservoirs of the Stuttgart Formation (North German Basin)

The Mesozoic succession of the North German Basin comprises permeable sandstones operated by ‘conventional’ open doublet systems for geothermal heat production. The high permeability, exceeding 6 Darcy at individual localities, is the result of undercompacted grain fabrics, low abundance of authigenic minerals and large secondary porosity volumes. The discrepancy of undercompacted grain fabrics and porosity volumes of >20% at depth of up to 2500 m gave rise to a basin-scale study, in which classical petrographic methods were employed to reconstruct the diagenesis of sandstones on the example of the Stuttgart Formation (Schilfsandstein, Late Triassic). Associated to the deposition of sandstones in distinct fluvio-deltaic environments, four diagenetic pathways are indicated by systematic variations of detrital and authigenic assemblages. These four pathways are herein introduced as Delta Channel, Delta Plain, Fluvial Channel, and Floodplain Diagenesis Types. In response to the history of the North German Basin, high permeabilities of sandstones of the Delta Channel and Fluvial Channel Diagenesis Types are related to the subsequent control of depositional and diagenetic regimes on porosity evolution. Pervasive eogenetic calcite cementations contributed to preservation of pre-burial intergranular volumes and effectively prevented the mechanical compaction of grain fabrics during late Triassic–late Jurassic burial diagenesis. The uplift from deep to shallow burial depth, associated to latest Jurassic–Cenozoic structural differentiation and inversion of the North German Basin, triggered substantial dissolution processes that mainly affected carbonate cementations. This opened pores and contributed to the evolution of secondary porosity, in particular in sandstones of the Delta Channel and Fluvial Channel Diagenesis Types. Moderate post-inversion reburial rates resulted only in negligible mechanical compaction and reduction of secondary porosity. The improved knowledge of diagenetic pathways, herein exemplified by sandstones of the Stuttgart Formation, will significantly contribute to improved predictions of Mesozoic hydrothermal reservoir in the North German Basin.

Geophysical Imaging of the Shallow Geyser and Hydrothermal Reservoir Structures of Spouter Geyser, Yellowstone National Park: Geyser Dynamics I

AbstractYellowstone 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.

Hydrothermal Reservoir and Electrical Anisotropy Investigated by Magnetotelluric Data, Case Study of Asal Rift, Republic of Djibouti

At the center of the Republic of Djibouti, an eroded rift called Asal is located where tectonic and magmatic activities can be observed at the surface. Multiple studies were carried out with different exploration methods, such as structural, geophysical and hydrogeological, to understand rifting processes and characterize the subsurface of this rift. Among these subsurface exploration methods, the deep geoelectrical structures need to be better defined with the magnetotelluric (MT) method to better delineate the deep resistivity structures. With the objective of improving our understanding of the deep rift structure, magnetotelluric (MT) data acquired in the Asal rift were analyzed and inverted to build a 2D electrical conductivity model of the hydrothermal system. To achieve this, a dimensionality analysis of the MT data along a 2D profile perpendicular to the rift axis was carried out. Results of this analysis justify the approximation of 2D conductivity structure. Then, 2D inversion models were achieved to build models of the conductive structures. Dimensionality analysis results revealed the existence of electrical anisotropy. Consistent correlation between geoelectric strike and electrical anisotropy direction was suggested. Electrical anisotropy direction determined from the ellipticity of the phase tensor for the short periods was interpreted as the consequence of tectonic activity and horizontal deformation of the rift. Moreover, electrical anisotropy direction for the long periods was assumed to be related to the effects of combined magmatic-tectonic activities with predominant magma/dyke intrusion, which implies the vertical deformation and the subsidence of the rift and may imply the alignment of Olivine. Moreover, the variation and rotation of paleo and recent stress fields direction of plate motion in Asal rift located at the junction of three diverging plates—Arabia, Nubia and Somalia—over geological time can generate both magmatic and tectonic activities which in turn can induce a preferred direction of electrical anisotropy which is the direction of the highest conductivity. While the north-south electrical anisotropy direction is parallel to the direction of Red Sea Rift propagation, the north-east electrical anisotropy direction is aligned with the extension direction between Arabia and Somalia plates. Results of the 2D inversion models presented for the Asal rift allowed to identify two superimposed conductive units close to the surface and are interpreted as a shallow aquifer and a wide potential hydrothermal system. These conductive mediums are overlying a relatively resistive medium. The latter is associated with a magmatic system likely containing hot and/or partly molten rocks. The 2D conductivity model developed in this study could be considered as conceptual model of Asal rift prior to modeling multiphase fluid flow and heat transfer and/or could be used to identify the hydrothermal system for future drilling target depth of geothermal exploration.

Косейсмические временные вариации термофильного элемента Si подземных вод западного побережья оз. Байкал в 2012–2022 гг.

The results of monitoring concentrations of the thermophilic element Si in fresh subthermal and cold groundwaters from the Kultuk polygon with a temperature range at the output from ca. 0 to 20 °C are presented. A stepwise zonal increase in the Si concentration is recognized in groundwaters of the polygon centered at station 40. In this center of the Kultuk hydrothermal reservoir, the minimum temperature of 25 °С on September 17, 2014 (before the earthquakes of 2014–2015) and the maximum temperature of 60 °С on January 23, 2021 (11 days after the strongest Khubsugul earthquake Mw=6.8) is obtained using the chalcedony geothermometer. It is proposed that groundwater came from the deep Kultuk reservoir with elevated temperature during the strong Kultuk seismic reactivation (August 27, 2008 – January 04, 2011) and changed to those with a lower temperature during the weak Tolbazikha one (June 24, 2011 – October 11, 2012) reaching a temperature minimum by 2014. Then the inflow of groundwaters with elevated temperature was revived again during the preparation and implementation of the Baikal-Khubsugul seismic reactivation that was marked by strong earthquakes in 2020–2022.

Volatile characteristics and fluxes of He-CO2 systematics in the southeastern Tibetan Plateau: Constraints on regional seismic activities

The spatial and temporal evolutionary mechanisms of volatile components at plate boundaries and their potential as earthquake precursors are not well understood. Based on observations of He–CO2 from 19 hot-spring gases in the Lancang River fault zone, Yunnan Province, the results show that the volatiles mainly come from the crust. The combined He isotope, He–CO2 relationships, and δ13C–CO2 values of the hot-spring volatiles indicate that it is a mixture of mantle and crustal fluids. These are related to the radiogenic He production caused by crustal U and Th elements with <5% mantle-derived He, whereas C is mainly from the metamorphic decarbonization of Tethyan marine carbonate rocks. Based on the He–CO2 isotopic relationship, crustal volatiles flux with a significant proportion were determined within a non-volcanic area, which was estimated to be 9–132 mol a−1, and CO2 degassing was estimated to be 2–120 × 109 mol a−1. These results are comparable to those of some volcanic areas and large seismically-active fault zones (such as the San Andreas Fault zone). The compressional stress field of the rigid crust near the study area leads to the low permeability of large faults and CO2 degassing from the hydrothermal system, resulting in chemical precipitation that further blocks the shallow crust conduits and deduced overpressure below the hydrothermal reservoir, which may be the cause of the presence of earthquakes with magnitudes greater than 7 in the Lancang River fault zone. The results are significant for elements cycling of converging plate boundaries and earthquake hazards monitoring.

Structure and Present State of the Astroni Volcano in the Campi Flegrei Caldera in Italy Based on Multidisciplinary Investigations

AbstractDespite its known reconstructed volcanic history, the structural setting and present state of the Astroni Volcano of the Campi Flegrei caldera in Italy are still poorly defined. Through structural, geophysical, and geochemical investigations, we elucidate the structure and present volcanic activity of the Astroni Volcano, which hosts tuff cones, scoriae cones, lava domes, and lakes on the crater floor. A volcano‐tectonic analysis focused on the entire volcano edifice, coupled with electrical resistivity tomography of the shallower part of the Astroni crater, revealed the main rock formations, faults, and possible fluid patterns within the first 150 m depth. Two main NE–SW and NW–SE trending fault sets were imaged using electrical resistivity modeling and measurements along the wall of the volcanic edifice; they likely delimit a maar‐like structure resulting from the highest energetic subplinian Astroni 6 eruption event and acted as magma pathways during the late eruptive activity stage. A 3D view of the reconstructed resistivity model revealed both deep root‐conduit‐like structures and shallower dome‐like shapes for volcanic edifices on the crater floor. Gas and carbon compositions in the NNE sectors of the Astroni Lago Grande are similar to those of the Solfatara fumarole fluids, suggesting common hydrothermal origin and a possible link with a deep hydrothermal reservoir. This fluid‐emission area along the border of the younger volcanic structure exhibits a +40°C maximum soil‐temperature anomaly. The proposed volcano‐tectonic architecture should improve the unrest scenarios in case of reactivation in this Campi Flegrei caldera sector and the monitoring strategies for the Astroni Volcano.