Hydrothermal Reservoir Research Articles

Three-dimensional shallow velocity structure beneath Taal Volcano, Philippines

Based on its numerous historical explosive eruptions and high potential hazards to nearby population of millions, Taal Volcano is one of the most dangerous “Decade Volcanoes” in the world. To provide better investigation on local seismicity and seismic structure beneath Taal Volcano, we deployed a temporary seismic network consisting of eight stations from March 2008 to March 2010. In the preliminary data processing stage, three periods showing linear time-drifting of internal clock were clearly identified from noise-derived empirical Green’s functions. The time-drifting errors were corrected prior to further data analyses. By using VELEST, 2274 local earthquakes were manually picked and located. Two major earthquake groups are noticed, with one lying beneath the western shore of Taal Lake showing a linear feature, and the other spreading around the eastern flank of Taal Volcano Island at shallower depths. We performed seismic tomography to image the 3D structure beneath Taal Volcano using the LOTOS algorithm. Some interesting features are revealed from the tomographic results, including a solidified magma conduit below the northwestern corner of Taal Volcano Island, indicated by high Vp, Vs, and low Vp/Vs ratio, and a large potential hydrothermal reservoir beneath the center of Taal Volcano Island, suggested by low Vs and high Vp/Vs ratio. Furthermore, combining earthquake distributions and tomographic images, we suggest potential existence of a hydrothermal reservoir beneath the southwestern corner of Taal Lake, and a fluid conduit extending to the northwest. These seismic features have never been proposed in previous studies, implying that new hydrothermal activity might be formed in places away from the historical craters on Taal Volcano Island.

Geochemistry of hydrothermal fluids from the eastern sector of the Sabatini Volcanic District (central Italy)

This study reports a complete geochemical dataset of 215 water and 9 gas samples collected in 2015 from thermal and cold discharges located in the eastern sector of the Sabatini Volcanic District (SVD), Italy. Based on these data, two main aquifers were recognized, as follows: 1) a cold Ca-HCO3 to Ca(Na)-HCO3 aquifer related to a shallow circuit within Pliocene-Pleistocene volcanic and sedimentary formations and 2) a deep CO2-pressurized aquifer hosted in Mesozoic carbonate-evaporitic rocks characterized by a Ca-HCO3(SO4) to Na(Ca)-HCO3(Cl) composition. A thick sequence of low-permeability formations represents a physical barrier between the two reservoirs. Interaction of the CO2-rich gas phase with the shallow aquifer, locally producing high-TDS and low-pH cold waters, is controlled by fractures and faults related to buried horst-graben structures. The δ18O-H2O and δD-H2O values indicate meteoric water as the main source for both the shallow and deep reservoirs. Carbon dioxide, which is characterized by δ13C-CO2 values ranging from −4.7 to +1.0‰ V-PDB, is mostly produced by thermo-metamorphic decarbonation involving Mesozoic rock formations, masking possible CO2 contribution from mantle degassing. The relatively low R/Ra values (0.07–1.04) indicate dominant crustal He, with a minor mantle He contribution. The CO2/3He ratios, up to 6 × 1012, support a dominant crustal source for these two gases. The δ34S-H2S values (from +9.3 to +11.3‰ V-CDT) suggests that H2S is mainly related to thermogenic reduction of Triassic anhydrites. The δ13C-CH4 and δD-CH4 values (from −33.4 to −24.9‰ V-PDB and from −168 to −140‰ V-SMOW, respectively) and the relatively low C1/C2+ ratios (<100) are indicative of a prevailing CH4 production through thermogenic degradation of organic matter. The low N2/Ar and high N2/He ratios, as well as the 40Ar/36Ar ratios (<305) close to atmospheric ratio, suggest that both N2 and Ar mostly derive from air. Notwithstanding, the positive δ15N-N2 values (from +0.91 to +3.7‰ NBS air) point to a significant extra-atmospheric N2 contribution. Gas geothermometry in the CH4-CO2-H2 and H2S-CO2-H2 systems indicate equilibrium temperatures <200 °C, i.e. lower than those measured in deep geothermal wells (∼300 °C), due to either an incomplete attainment of the chemical equilibria or secondary processes (dilution and/or scrubbing) affecting the chemistry of the uprising fluids. Although the highly saline Na-Cl fluids discharged from the explorative geothermal wells in the study area support the occurrence of a well-developed hydrothermal reservoir suitable for direct exploitation, the chemistry of the fluid discharges highlights that the uprising hydrothermal fluids are efficiently cooled and diluted by the meteoric water recharge from the nearby Apennine sedimentary belt. This explains the different chemical and isotopic features shown by the fluids from the eastern and western sectors of SVD, respectively, the latter being influenced by this process at a lesser extent. Direct uses may be considered a valid alternative for the exploitation of this resource.

Shallow crustal velocities and volcanism suggested from ambient noise studies using a dense broadband seismic network in the Tatun Volcano Group of Taiwan

The Tatun Volcano Group (TVG) is situated adjacent to the Taipei metropolis and was active predominantly around 0.8–0.2Ma (Pleistocene). Various recent lines of evidence suggest that the TVG is a potentially active volcano and that future volcanic eruptions cannot be ruled out. Geothermal activities are largely constrained to faults, but the relationship between volcanism and detailed velocity structures is not well understood. We analyzed ambient seismic noise of daily vertical components from 2014 using a dense seismic network of 40 broadband stations. We selected a 0.02° grid spacing to construct 2D and 3D shallow crustal phase velocity maps in the 0.5–3s period band. Two S-wave velocity profiles transect Chishingshan (Mt. CS) in the shallow 3km crust are further derived.The footwall of the Shanchiao Fault is dominated by low velocity, which may relate to Tertiary bedrock buried under andesitic lava flows dozens to hundreds of meters thick. The hanging wall of the Shanchiao Fault is the location of recent major volcanic activities. Low velocity zones in the southeast of Dayoukeng (DYK) may be interpreted as hydrothermal reservoirs or water-saturated Tertiary bedrock related to Cenozoic structures in the shallow crust. High velocities conspicuously dominate the east of the TVG, where the earliest stages of volcanism in the TVG are located, but where surface hydro-geothermal activities were absent in recent times. Between the Shanchiao Fault and Kanchiao Fault high velocities were detected, which converge below Mt. CS and may be related to early stages of magma conduits that gradually consolidated. These two faults may play a significant role with the TVG. The submarine volcanism adjacent to the Keelung coastline also requires further attention.

Dual stable isotopes of CH4 from Yellowstone hot-springs suggest hydrothermal processes involving magmatic CO2

Volcanism and post-magmatism contribute significant annual methane (CH4) fluxes to the atmosphere (on par with other natural sources such as forest fire and wild animal emissions) and have been implicated in past climate-change events. The Yellowstone hot spot is one of the largest volcanic systems on Earth and is known to emit CH4 (as well as carbon dioxide (CO2) and other gases), but the ultimate sources of this CH4 flux have not been elucidated. Here we use dual stable isotope analysis (δ2H and δ13C) of CH4 sampled from ten high-temperature geothermal pools in Yellowstone National Park along with other isotopic and gas analyses to evaluate potential sources of methane. The average δ13C and δ2H values of CH4 emitted from hot springs (26.7 (±2.4) and −236.9 (±12.0) ‰, respectively) are inconsistent with microbial methanogenesis but do not allow distinction between thermogenic and abiotic sources. Correlation between δ13CCH4 and δ13C of dissolved inorganic C (DIC) is consistent with DIC as the parent C source for the observed CH4, or with equilibration of CH4 and DIC. Methane formation temperatures estimated by isotopic geothermometry based on δ13CCH4 and δ13CCO2 ranged from ~250–350°C, which is just below previous temperature estimates for the hydrothermal reservoir. Further, the δ2HH2O of the thermal springs and the measured δ2HCH4 values are consistent with equilibration between the source water and the CH4 at the formation temperatures. Though the ultimate origin of the CH4 could be attributed to either abiotic of themorgenic processes with subsequent isotopic equilibration, the C1/C2+ composition of the gases is more consistent with abiotic origins for most of the samples. Thus, our data support the hypothesis that subsurface rock-water interactions are responsible for at least a significant fraction of the CH4 flux from the Yellowstone National Park volcanic system.

Seismicity associated with magmatism, faulting and hydrothermal circulation at Aluto Volcano, Main Ethiopian Rift

The silicic volcanic centres of the Main Ethiopian Rift (MER) play a central role in facilitating continental rifting. Many of these volcanoes host geothermal resources and are located in heavily populated regions. InSAR studies have shown several are deforming, but regional seismic networks have detected little seismicity. A local network of 12 seismometers was deployed at Aluto Volcano from 2012 to 2014, and detected 2142 earthquakes within a 24-month period. We locate the events using a 1D velocity model that exploits a regional model and information from geothermal boreholes and calculate local magnitudes, b-values and focal mechanisms. Event depths generally range from the near surface to 15km with most of the seismicity clustering in the upper 2km. A significant amount of seismicity follows the Artu Jawa Fault Zone, which trends in alignment with the Wonji Fault Belt, NNE–SSW and is consistent with previous studies of strain localisation in the MER. Focal mechanisms are mostly normal in style, with the mean T-axes congruent to the orientation of extension in the rift at this latitude. Some show relatively small left-lateral strike-slip components and are likely associated with the reactivation of NE-ENE structures at the southern tip of the Aluto-Gedemsa segment. Events range from −0.40 to 2.98 in magnitude and we calculate an overall b-value of 1.40±0.14. This relatively elevated value suggests fluid-induced seismicity that is particularly evident in the shallow hydrothermal reservoir and above it. Subdividing our observations according to depth identifies distinct regions beneath the volcanic edifice: a shallow zone (−2–0km) of high seismicity and high b-values that corresponds to the hydrothermal system and is influenced by a high fluid saturation and circulation; a relatively aseismic zone (0–2km) with low b-values that is impermeable to ascending volatiles; a region of increased fluid-induced seismicity (2–9km) that is driven by magmatic intrusion from below and a deeper zone (below 9km) that is interpreted as a partially crystalline, magmatic mush. These observations indicate that both the magmatic and hydrothermal systems of Aluto volcano are seismically active and highlight the need for dedicated seismic monitoring at volcanoes in the MER.

Fluid geochemistry of a deep-seated geothermal resource in the Puna plateau (Jujuy Province, Argentina)

This study focused on the geochemical and isotopic features of thermal fluids discharged from five zones located in the high altitude Puna plateau (Jujuy Province between S 22°20′–23°20′ and W 66°–67°), i.e. Granada, Vilama, Pairique, Coranzulí and Olaroz. Partially mature waters with a Na+-Cl− composition were recognized in all the investigated zones, suggesting that a deep hydrothermal reservoir hosted within the Paleozoic crystalline basement represents the main hydrothermal fluid source. The hydrothermal reservoirs are mainly recharged by meteoric water, although based on the δ18O-H2O and δD-H2O values, some contribution of andesitic water cannot be completely ruled out. Regional S-oriented faulting systems, which generated a horst and graben tectonics, and NE-, NW- and WE-oriented transverse structures, likely act as preferentially uprising pathways for the deep-originated fluids, as also supported by the Rc/Ra values (up to 1.39) indicating the occurrence of significant amounts of mantle He (up to 16%). Carbon dioxide, the most abundant compound in the gas phase associated with the thermal waters, mostly originated from a crustal source, although the occurrence of CO2 from a mantle source, contaminated by organic-rich material due to the subduction process, is also possible. Relatively small and cold Na+-HCO3−-type aquifers were produced by the interaction between meteoric water and Cretaceous, Palaeogene to Miocene sediments. Dissolution of evaporitic surficial deposits strongly affected the chemistry of the thermal springs in the peripheral zones of the study area. Geothermometry in the Na-K-Ca-Mg system suggested equilibrium temperatures up to 200°C for the deep aquifer, whereas lower temperatures (from 105 to 155°C) were inferred by applying the H2 geothermometer, likely due to re-equilibrium processes during the thermal fluid uprising within relatively shallow Na-HCO3 aquifers. The great depth of the geothermal resource (possibly>5000m b.g.l.) is likely preventing further studies aimed to evaluate possible exploitation, although the occurrence of Li- and Ba-rich deposits associated may attract financial investments, giving a pulse for the development of this remote region.

Experimental interaction of hydrothermal Na-Cl solution with fracture surfaces of geothermal reservoir sandstone of the Upper Rhine Graben

The Upper Rhine Graben (URG) is a Tertiary rift structure in central Europe offering favorable conditions for geothermal energy utilization. Relatively high heat flow combines with sufficiently high permeability of the hydrothermal reservoirs. One of the target stratigraphic levels is the lower Triassic sandstone formation (Buntsandstein), where hot water resources at temperatures up to 250 °C can be utilized. Extensional neotectonics and hydraulic stimulation form new fracture surfaces in the reservoir rocks (enhanced geothermal system, EGS). The exposed fresh rock fracture surface reacts with the highly saline reservoir brines (Na-Cl up to 200 g l−1) with consequences for the permeability of the reservoir.In order to better understand the dynamic evolution of fault systems caused by fluid-rock interaction, we conducted batch-type experiments in a stirred autoclave system and reacted arkosic sandstone with synthetic 2 molal Na-Cl solution at temperatures of 200 °C and 260 °C. After 45–55 days reaction time altered rock samples were compared with the starting material and the geochemical-mineralogical processes were deduced with the help of XRD, SEM methods and EMP measurements. Fluid compositions were examined by ICP-MS, ICP-OES and IC.The arkosic sandstones show a surprisingly high reactivity during the experiments. Quartz grain surfaces show deep dissolution features and all reaction fluids were saturated with respects to quartz. Illite and kaolinite from the primary sandstone cementation completely dissolved from the sample surface. Perfectly euhedral crystals of metastable analcime formed during the experiment as separate crystals on quartz, as groups or clusters and as surface covering mats. The overall net transfer process dissolves quartz + illite + kaolinite ± K-feldspar and precipitates analcime + chlorite ± albite. The process is accompanied by a total volume increase of the solids of 20–30 vol%. K-feldspar dissolution is hampered by albitization rims shielding K-feldspar and efficiently preventing an equilibration of the Na–K exchange with the fluid. The experiments show changes on the rock surface, leading to an increase of the aperture of a single fracture during the early phases of reaction and later to a decrease as the fluid-rock reaction progresses. Alteration of the fracture surface also generates loose fragments and altered minerals. This fine material may efficiently reduce the fracture aperture at narrow points along the fracture.

Magmatic-like fluid source of the Chingshui geothermal field, NE Taiwan evidenced by carbonate clumped-isotope paleothermometry

The Chingshui geothermal field, a moderate-temperature and water-dominated hydrothermal system, was the site of the first geothermal power plant in Taiwan. Many geological, geophysical and geochemical studies using more than 21 drilled wells have been performed since the 1960s. However, there are still controversies regarding the heat and fluid sources due to the tectonically complicated geological setting. To clarify the heat and fluid sources, we analyzed clumped isotopes with carbon and oxygen isotopic compositions of calcite scaling in geothermal wells and veins on outcrops and calculated the δ18O values of the source fluids. Two populations of δ18O values were calculated: −5.8±0.8‰ VSMOW from scaling in the well and −1.0±1.6‰ to 10.0±1.3‰ VSMOW from outcropping calcite veins, indicative of meteoric and magmatic fluid sources, respectively. Meanwhile, two hydrothermal reservoirs at different depths have been identified by magnetotelluric (MT) imaging with micro-seismicity underneath this area. As a result, we propose a two-reservoir model: the shallow reservoir provides fluids from meteoric water for the scaling sampled from wells, whereas the deep reservoir provides magmatic fluids from deep marble decarbonization recorded in outcropping calcite veins.

Fluid geochemistry of the Mondragone hydrothermal systems (southern Italy): water and gas compositions vs. geostructural setting

The geochemistry of natural thermal fluids discharging in the Mondragone Plain has been investigated. Thermal spring emergences are located along the Tyrrhenian coast in two different areas: near Padule-S. Rocco (41°7.5′N 13°53.4′E) at the foot of Mt. Petrino, and near Levagnole (41°8.5′N 13°51.3′E) at the foot of Mt. Pizzuto. The water isotopic composition of both thermal discharges is lighter than the one of local shallow groundwater (δ18O ≅ −6.3‰ SMOW vs. ≅ −5.9‰; δD ≅ −40‰ SMOW vs. ≅ −36‰, respectively) as a consequence of inland higher altitude of recharge by rainfall, suggesting that thermal water undergoes a deep and long flow-path before emerging along the coast. The chemical composition of the highest temperature samples of two areas points that fluids in the hydrothermal reservoir(s) interact with similar lithologies, since they are both hosted in the lower sedimentary carbonate formations of the Campanian–Latial Apennine succession. However, the two spring systems are different in terms of temperature and salinity (Levagnole: ≅50 °C and 8.9 g/L vs. Padule: ≅32 °C and 7.4 g/L, respectively). The higher salinity of Levagnole springs is due to a longer interaction with evaporite material embedded in Miocene sedimentary formations and to the eventual mixing, during rising, with fresh seawater close to the seashore. The chemical and isotopic composition of the free gases associated with the springs, again suggests a different source of the two hydrothermal systems. Comparing the 3He/4He measured ratios with other gas emissions located NE and SE of Mt. Massico-Roccamonfina alignment, it is evident that the Levagnole thermal springs are related to the northern Latial mantle wedge where the 3He/4He is about 0.5 R/Ra, whereas the Padule-S. Rocco springs, although being only 3.5 km south of Levagnole, are related to the Campanian mantle wedge where R/Ra is always ≥2.0. Such a difference in 3He/4He ratio in a very short distance, clearly, suggests a different source of the Padule-S.Rocco gas phase rising to the surface through the deep transpressive regional fault(s) system related to the NE–SW Ortona–Roccamonfina tectonic alignment.

Horizontal sliding of kilometre-scale hot spring area during the 2016 Kumamoto earthquake

We report horizontal sliding of the kilometre-scale geologic block under the Aso hot springs (Uchinomaki area) caused by vibrations from the 2016 Kumamoto earthquake (Mw 7.0). Direct borehole observations demonstrate the sliding along the horizontal geological formation at ~50 m depth, which is where the shallowest hydrothermal reservoir developed. Owing to >1 m northwest movement of the geologic block, as shown by differential interferometric synthetic aperture radar (DInSAR), extensional open fissures were generated at the southeastern edge of the horizontal sliding block, and compressional deformation and spontaneous fluid emission from wells were observed at the northwestern edge of the block. The temporal and spatial variation of the hot spring supply during the earthquake can be explained by the horizontal sliding and borehole failures. Because there was no strain accumulation around the hot spring area prior to the earthquake and gravitational instability could be ignored, the horizontal sliding along the low-frictional formation was likely caused by seismic forces from the remote earthquake. The insights derived from our field-scale observations may assist further research into geologic block sliding in horizontal geological formations.

The acid crater lake of Taal Volcano, Philippines: hydrogeochemical and hydroacoustic data related to the 2010–11 volcanic unrest

Abstract Studies of the water chemistry of Taal crater lake and echo-sounding surveys have provided new insights into its chemical and physical dynamics. During the volcano-seismic unrest of April 2010–June 2011, the waters of Taal crater lake showed changes in chemical composition and increases in CO 2 emissions associated with the seismic unrest. The chemical and isotopic data show that the lake water has contributions from both seawater and meteoric water and receives injections of deep hydrothermal water and gases during periods of intense volcano-seismic unrest. These inflationary periods may lead to faulting of the impermeable cap rock that usually seals the deeper Taal hydrothermal reservoir in response to degassing and convective movements in the underlying Taal magma chamber.

Recharge Conditions of the Low Temperature Paratunsky Geothermal Reservoir, Kamchatka, Russia

The Paratunsky geothermal field is a classic example of a low temperature hydrothermal reservoir of meteoric origin. Hydrogeochemical zoning reflects a different intensity of water exchange in the Western and Eastern sites of the field. The hydrogeochemical history of operations 1966-2015 shows the involvement of chloride waters across the field's Eastern boundary. Water recharge to the Paratunsky geothermal field is provided mostly by meteoric waters from elevations above +1150 masl. The most likely area of water recharge to the Paratunsky geothermal fields is the Vilyuchinsky volcano (elevation 2173 m abs.).

Geophysical imaging of shallow degassing in a Yellowstone hydrothermal system

AbstractThe Yellowstone Plateau Volcanic Field, which hosts over 10,000 thermal features, is the world's largest active continental hydrothermal system, yet very little is known about the shallow “plumbing” system connecting hydrothermal reservoirs to surface features. Here we present the results of geophysical investigations of shallow hydrothermal degassing in Yellowstone. We measured electrical resistivity, compressional‐wave velocity from refraction data, and shear wave velocity from surface‐wave analysis to image shallow hydrothermal degassing to depths of 15–30 m. We find that resistivity helps identify fluid pathways and that Poisson's ratio shows good sensitivity to saturation variations, highlighting gas‐saturated areas and the local water table. Porosity and saturation predicted from rock physics modeling provide critical insight to estimate the fluid phase separation depth and understand the structure of hydrothermal systems. Finally, our results show that Poisson's ratio can effectively discriminate gas‐ from water‐saturated zones in hydrothermal systems.

Conceptual and 3D simulation modeling of the Sorgun hydrothermal reservoir (Yozgat,Turkey)

Since November 2007, the Sorgun geothermal field has been developed with two production wells and one reinjection well. Hot water produced from the geothermal wells is mainly used for district heating, greenhouse heating, and spa facilities. A total of 12 geothermal wells were drilled in this field to depths ranging from 90 to 444 mbgs, with measured borehole temperatures between 50 and 85.4°C. During the heating period between November and May, the average rates of production and reinjection were 75 and 35l/s, respectively. During the non-heating period, the production rate decreased to 45l/s and the reinjection rate to 5l/s. The measured water level was 110 mbgs during the heating period and rose to 96 mbgs during the non-heating period. A conceptual model of the geothermal field was created with defined model parameters and boundary conditions. This geothermal conceptual model was analyzed using the TOUGH2 numerical simulation program. A natural state model was developed, and simulated results were computed using static temperature data for the production wells before exploitation. After a good match between the modeled and measured temperature data was achieved, the natural state model was run with production/injection flow rate and enthalpy data from 2007 to 2014 for history matching. Results of this model were calibrated with measured water level and temperature data for the observation well. A good match was achieved between the measured values and the model's predicted values.

The hydrothermal system of the Domuyo volcanic complex (Argentina): A conceptual model based on new geochemical and isotopic evidences

The Domuyo volcanic complex (Neuquén Province, Argentina) hosts one of the most promising geothermal systems of Patagonia, giving rise to thermal manifestations discharging hot and Cl−-rich fluids. This study reports a complete geochemical dataset of gas and water samples collected in three years (2013, 2014 and 2015) from the main fluid discharges of this area. The chemical and isotopic composition (δD-H2O and δ18O-H2O) of waters indicates that rainwater and snow melting are the primary recharge of a hydrothermal reservoir located at relative shallow depth (400–600m) possibly connected to a second deeper (2–3km) reservoir. Reactive magmatic gases are completely scrubbed by the hydrothermal aquifer(s), whereas interaction of meteoric waters at the surface causes a significant air contamination and dilution of the fluid discharges located along the creeks at the foothill of the Cerro Domuyo edifice. Thermal discharges located at relatively high altitude (~3150ma.s.l.), namely Bramadora, are less affected by this process, as also shown by their relatively high R/Ra values (up to 6.91) pointing to the occurrence of an actively degassing magma batch located at an unknown depth. Gas and solute geothermometry suggests equilibrium temperatures up to 220–240°C likely referred to the shallower hydrothermal reservoir. These results, confirming the promising indications of the preliminary surveys carried out in the 1980′s, provide useful information for a reliable estimation of the geothermal potential of this extinct volcanic system, although a detailed geophysical measurements is required for the correct estimation of depth and dimensions of the fluid reservoir(s).

Relationships among volcanic and hydrothermal activity and faults from the Shunnan 3-D well zone in the Tadong Area (Tarim Basin, China)

Tarim Basin distributed widely two stages of Permian volcanic rocks. However, the location and timing of these rocks pinching out and the relationship between Ordovician carbonate hydrothermal reservoirs and fault activity remain unclear. The Shunnan (Shunnan here in after referred to as SN) 1 3-D well zone in the eastern Tarim Basin contains volcanic rocks, and the Ordovician layer contains a hydrothermal reservoir. This paper describes the lithology, stage, volcanic rock distribution, crater position and deposition pinch-out line of volcanic rocks based on drilling and 3-D seismic data. The Permian strata in the well zone contain only one stage of Kupkuciman Formation basalt and tuff, the Kaipeleicike Formation contains terrigenous clastic rock and the adjacent volcanic rocks of wells SN2 and GL (Gulong here in after referred to as GL) 2 exhibit an absence of deposition. Although the craters are distributed on both sides of the NNW faults that cross well SN4, their positions are also controlled by the intersection of earlier NEE and NW faults and later NNE faults. Furthermore, the Ordovician strata in the SN1 well zone developed abundant tubular high-amplitude anomalies and tabular high-amplitude anomalies via hydrothermal corrosion, and the anomaly distributions are similar to the positions of the craters, which are controlled by the intersection of multi-group faults and the density of the faults.

Muon dynamic radiography of density changes induced by hydrothermal activity at the La Soufrière of Guadeloupe volcano.

Imaging geological structures through cosmic muon radiography is a newly developed technique which shows a great potential in volcanology. Here we demonstrate that muon radiography permits to detect and characterize mass movements in shallow hydrothermal systems of low-energy active volcanoes like the La Soufrière lava dome. We present an experiment conducted on this volcano during the Summer 2014 and bring evidence that very important density changes occurred in three domains of the lava dome. Depending on their position and on the medium porosity the volumes of these domains vary from 1 × 106 m3 to 7 × 106 m3. However, the total mass budget remains approximately constant : two domains show a mass loss (Δm∈ [−0.8;−0.4] × 109 kg) and the third one a mass gain (Δm∈ [1.5; 2.5] × 109 kg). We attribute the negative mass changes to the formation of steam in shallow hydrothermal reservoir previously partly filled with liquid water. This coincides with the emergence of new fumaroles on top of the volcano. The positive mass change is synchronized with the negative mass changes indicating that liquid water probably flowed from the two reservoirs invaded by steam toward the third reservoir.

Numerical Simulation for Natural State of Two-Phase Liquid Dominated Geothermal Reservoir with Steam Cap Underlying Brine Reservoir

Hydrothermal reservoir which liquid-dominated hydrothermal reservoir is a type of geothermal reservoir that most widely used for power plant. The exploitation of mass and heat from the geothermal fluid will decrease the pressure in the reservoir over time. Therefore the pressure drop in the reservoir will have an impact on the formation of boiling zones or boiling will increase. The impacts are an increase in the fraction of steam, dryness, in the reservoir and with good vertical permeability will form a steam cap underlying the brine reservoir. The two- phase liquid dominated reservoir is sensitive to the porosity and difficult to assign average properties of the entire reservoir when there is boiling zone in some area of the reservoir. These paper showed successful development of two-phase liquid dominated geothermal reservoir and discussed the formation of steam cap above brine reservoir through numerical simulation for state natural conditions. The natural state modeling in steam cap shows a match with the conceptual model of the vapor-dominated developed. These paper also proofed the presence of transition zone, boiling zone, between steam cap and brine reservoir.

Causes of unrest at silicic calderas in the East African Rift: New constraints from InSAR and soil‐gas chemistry at Aluto volcano, Ethiopia

AbstractRestless silicic calderas present major geological hazards, and yet many also host significant untapped geothermal resources. In East Africa, this poses a major challenge, although the calderas are largely unmonitored their geothermal resources could provide substantial economic benefits to the region. Understanding what causes unrest at these volcanoes is vital for weighing up the opportunities against the potential risks. Here we bring together new field and remote sensing observations to evaluate causes of ground deformation at Aluto, a restless silicic volcano located in the Main Ethiopian Rift (MER). Interferometric Synthetic Aperture Radar (InSAR) data reveal the temporal and spatial characteristics of a ground deformation episode that took place between 2008 and 2010. Deformation time series reveal pulses of accelerating uplift that transition to gradual long‐term subsidence, and analytical models support inflation source depths of ∼5 km. Gases escaping along the major fault zone of Aluto show high CO2 flux, and a clear magmatic carbon signature (CO2‐δ13C of −4.2‰ to −4.5‰). This provides compelling evidence that the magmatic and hydrothermal reservoirs of the complex are physically connected. We suggest that a coupled magmatic‐hydrothermal system can explain the uplift‐subsidence signals. We hypothesize that magmatic fluid injection and/or intrusion in the cap of the magmatic reservoir drives edifice‐wide inflation while subsequent deflation is related to magmatic degassing and depressurization of the hydrothermal system. These new constraints on the plumbing of Aluto yield important insights into the behavior of rift volcanic systems and will be crucial for interpreting future patterns of unrest.

Turmoil at Turrialba Volcano (Costa Rica): Degassing and eruptive processes inferred from high‐frequency gas monitoring

Eruptive activity at Turrialba Volcano (Costa Rica) has escalated significantly since 2014, causing airport and school closures in the capital city of San José. Whether or not new magma is involved in the current unrest seems probable but remains a matter of debate as ash deposits are dominated by hydrothermal material. Here we use high‐frequency gas monitoring to track the behavior of the volcano between 2014 and 2015 and to decipher magmatic versus hydrothermal contributions to the eruptions. Pulses of deeply derived CO2‐rich gas (CO2/Stotal > 4.5) precede explosive activity, providing a clear precursor to eruptive periods that occurs up to 2 weeks before eruptions, which are accompanied by shallowly derived sulfur‐rich magmatic gas emissions. Degassing modeling suggests that the deep magmatic reservoir is ~8–10 km deep, whereas the shallow magmatic gas source is at ~3–5 km. Two cycles of degassing and eruption are observed, each attributed to pulses of magma ascending through the deep reservoir to shallow crustal levels. The magmatic degassing signals were overprinted by a fluid contribution from the shallow hydrothermal system, modifying the gas compositions, contributing volatiles to the emissions, and reflecting complex processes of scrubbing, displacement, and volatilization. H2S/SO2 varies over 2 orders of magnitude through the monitoring period and demonstrates that the first eruptive episode involved hydrothermal gases, whereas the second did not. Massive degassing (>3000 T/d SO2 and H2S/SO2 > 1) followed, suggesting boiling off of the hydrothermal system. The gas emissions show a remarkable shift to purely magmatic composition (H2S/SO2 < 0.05) during the second eruptive period, reflecting the depletion of the hydrothermal system or the establishment of high‐temperature conduits bypassing remnant hydrothermal reservoirs, and the transition from phreatic to phreatomagmatic eruptive activity.