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Controlled-source electromagnetic (CSEM) measurements are complementary data for magnetotelluric (MT) characterization although its methodology on land is not sufficiently developed and tested as in marine environments. Acquiring expertise in CSEM is crucial for surveys in places where MT cannot be performed due to high levels of cultural noise. To acquire that expertise, we perform CSEM experiments in the Vallès fault [Northeast (NE), Spain], where MT results have been satisfactory and allow us to verify the CSEM results. The Vallès basin is relevant for potential heat generation because of the presence of several geothermal anomalies and its nearby location in urban areas. In this article, we present the experimental setup for that region, a 2-D joint MT+CSEM inverse model, several 3-D CSEM simulations in the presence of metallic casing, and its comparison with real data measurements. We employ a parallel and high-order vector finite element algorithm to discretize the governing equations. By using an adapted meshing strategy, different scenarios are simulated to study the influence of the source position/direction and the conductivity model in a metallic casing presence. An excellent agreement between the simulated data and analytical/real field data demonstrates the feasibility of study metallic structures in realistic configurations. Our numerical results confirm that metallic casing strongly influences electromagnetic (EM) responses, making surface measurements more sensitive to resistivity variations near the metallic structure. It could be beneficial getting higher signal-to-noise ratios and sensitivity to deep targets. However, such a casing effect depends on the input model (e.g., conductivity contrasts, frequency, and geometry). This work was been developed in the frame of GEOURBAN project (PCI2018-092943 and PCI2018-093186). This work has received also partial funding from the European Union's Horizon 2020 programme under the Marie Sklodowska-Curie grant agreement N777778. Also, the research leading to these results has received funding from the European Union's Horizon 2020 programme, grant agreement N828947, and from the Mexican Department of Energy, CONACYT-SENER Hidrocarburos grant agreement N B-S-69926. Furthermore, this project has been 65% cofinanced by the European Regional Development Fund (ERDF) through the Interreg V-A Spain-France-Andorra program (POCTEFA2014-2020). POCTEFA aims to reinforce the economic and social integration of the French-Spanish-Andorran border. Its support is focused on developing economic, social and environmental cross-border activities through joint strategies favouring sustainable territorial development. The authors would like to thank the Editors-in-Chief and to both reviewers for their valuable comments and suggestions that helped to improve the manuscript’s quality. This work benefitted from the valuable suggestions, comments, and proofreading of Dr. Josep de la Puente (BSC).

Structural studies in active caldera systems are widely used in geothermal exploration to reconstruct volcanological conceptual models. Active calderas are difficult settings to perform such studies mostly because of the highly dynamic environment, dominated by fast accumulation of primary and secondary volcanic deposits, the variable and transient rheology of the shallow volcanic pile, and the continuous feedbacks between faulting, secondary porosity creation, and geothermal fluid circulation, alteration and cementation that tend to obliterate the tectonic deformation structures. In addition, deformation structures can be also caused by near- and far-field stress regimes, which include magmatic intrusions at various depths, the evolving topography and regional tectonics. A lack of consideration of all these factors may severely underpin the reliability of structural studies. By rebutting and providing a detailed discussion of all the points raised by the comment of Norini and Groppelli (2020) to the Urbani et al. (2020) paper, we take the opportunity to specify the scientific rationale of our structural fieldwork and strengthen its relevance for geothermal exploration and exploitation in active caldera geothermal systems in general and, particularly, for the Holocene history of deformation and geothermal circulation in the Los Humeros caldera. At the same time, we identify several major flaws in the approach and results presented in Norini and Groppelli (2020), such as (1) the lack of an appropriate ranking of the deformation structures considering an inventory method for structural analysis; (2) the misinterpretation and misquoting of Urbani et al. (2020) and other relevant scientific literature; and (3) irrelevant and contradictory statements within their comment.

In geothermal exploration, magnetotelluric (MT) data and inversion models are commonly used to image shallow conductors typically associated with the presence of an electrically conductive clay cap that overlies the main reservoir. However, these inversion models suffer from nonuniqueness and uncertainty, and the inclusion of useful geologic information is still limited. We have developed a Bayesian inversion method that integrates the electrical resistivity distribution from MT surveys with borehole methylene blue (MeB) data, an indicator of conductive clay content. The MeB data were used to inform structural priors for the MT Bayesian inversion that focus on inferring with uncertainty the shallow conductor boundary in geothermal fields. By incorporating borehole information, our inversion reduced nonuniqueness and then explicitly represented the irreducible uncertainty as estimated depth intervals for the conductor boundary. We used the Markov chain Monte Carlo and a 1D three-layer resistivity model to accelerate the Bayesian inversion of the MT signal beneath each station. Then, inferred conductor boundary distributions were interpolated to construct pseudo-2D/3D models of the uncertain conductor geometry. We compare our approach against deterministic MT inversion software on synthetic and field examples, and our approach has good performance in estimating the depth to the bottom of the conductor, a valuable target in geothermal reservoir exploration.

The value of information (VOI) metric requires geophysicists to clearly link their measurements to important subsurface parameters in geothermal exploration, such as temperature, permeability, and fluid information. The metric quantifies the increase in the expected (average) value outcome of a decision. The decision is made with a certain type of additional information, compared to if the decision was made without the information. VOI has challenges that are similar to those faced by machine learning enthusiasts. Labeled data sets are needed in which the labels are the decision variables and the features are the geophysical observables. This paper presents three examples of how the statistical relationship, or reliability, may be calculated for geothermal exploration applications. VOI provides a way to document successes and identify opportunities for improvement in derisking geothermal prospecting with geophysical information.

Faults and fractures represent unique features of the solid Earth and are especially pervasive in the shallow crust. Aside from directly relating to crustal dynamics and the systematic assessment of associated risk, fault and fracture networks enable the efficient migration of fluids and therefore have a direct impact on concrete topics relevant to society, including climate-change-mitigating measures like CO2 sequestration or geothermal exploration and production. Due to their small-scale complexity, fault zones and fracture networks are typically poorly resolved, and their presence can often only be inferred indirectly in seismic and ground-penetrating radar (GPR) subsurface reconstructions. We suggest a largely data-driven framework for the direct imaging of these features by making use of the faint and still often underexplored diffracted portion of the wave field. Finding inspiration in the fields of optics and visual perception, we introduce two different conceptual pathways for coherent diffraction imaging and discuss respective advantages and disadvantages in different contexts of application. At the heart of both of these strategies lies the assessment of data coherence, for which a range of quantitative measures is introduced. To illustrate the versatility and effectiveness of the approach for high-resolution geophysical imaging, several seismic and GPR field data examples are presented, in which the diffracted wave field sheds new light on crustal features like fluvial channels, erosional surfaces, and intricate fault and fracture networks on land and in the marine environment.

The borehole-to-surface electromagnetic (EM) method is a viable imaging and monitoring tool for energy reservoirs, geologic storage, geothermal exploration, fault zones, and other subsurface targets. Data interpretation typically requires considering steel-casing effects, but it is difficult and impractical to directly discretize arbitrarily oriented hollow steel-cased wells in a 3D reservoir-scale earth model because of their extremely high electrical conductivity and long hollow geometry. We have considered a borehole-to-surface EM configuration in which an electric dipole source is placed below the bottom of a steel-cased well. To practically simulate the casing effects on EM measurements, we develop a novel 3D finite-element EM algorithm using an unstructured tetrahedral mesh. To avoid excessive use of fine grids for modeling a steel-cased well, the well is replaced with the combination of a short solid conductive prism and a long linewise perfect electric conductor. We find that this combined structure can approximate casing effects at a small fraction of the computational cost required for modeling a complete hollow casing because the linewise structure is volumeless and does not require an excessive number of small elements. We also find that steel-cased wells distant from sources and receivers can be modeled as simple linewise perfect electric conductors, further improving the computational efficiency. Using this approximation, the 3D EM algorithm presented here is well-suited to modeling many arbitrarily oriented steel-cased wells. After verifying the accuracy and efficiency of this approach using various examples, we performed a 3D borehole-to-surface/surface-to-borehole and surface EM inversion and determine that the inversion can image a deep localized target in the presence of steel infrastructure.

Multiple geophysical techniques have been applied to geothermal exploration within the Teshikaga area. The three-dimensional inversion of HeliFalcon® airborne gravity gradiometry (AGG) data has enabled a detailed interpretation of subsurface features. The vertically elongated high-density anomalies beneath lava domes located near Wakoto Peninsula and Mt. Atosanupuri are interpreted as subsurface intrusive bodies, while low-density anomalous feature to the depth represents the Kutcharo caldera structure, which was filled with large scale of low-density pyroclastic materials. Helitem® electromagnetic data acquired in combination with the AGG data revealed a widespread conductive anomaly west of Mt. Atosanupuri, which may represent the cap rocks to a geothermal reservoir. The distribution of a low magnetic intensity zone matches that of a low-resistivity zone, suggesting that magnetite degradation and demagnetization possibly occur, most likely caused by hydrothermal activities. Integrated interpretation of airborne gravity gradiometer, electromagnetic and magnetic data helps with identification of prospective areas and planning of a drilling program for the development of geothermal energy with greater precision.

espanolPara el desarrollo de un campo geotermico, el conocimiento de la distribucion de fracturas, la hidrologia y la evolucion tectonica del sitio a traves de la caracterizacion estructural del sistema natural es un requisito previo a la perforacion de pozos exploratorios. Fallas y fracturas representan vias preferenciales por el flujo de fluidos en el subsuelo y pueden ser detectadas mediante investigaciones geologicas en la superficie y geofisicas en el subsuelo. Una anomalia magnetica positiva y de forma sub-circular en planta se encuentra en el medio del lago Cuitzeo, Mexico. A traves de un levantamiento geomagnetico dentro y alrededor del area que comprende la anomalia tratamos caracterizar el reservorio geotermico que, al sur del lago, esta notificado por manifestaciones hidrotermales. Utilizando filtros computacionales basados sobre el uso de operaciones con las derivadas de valores de campo a los datos magneticos grabados, hemos puesto en luz algunas de las estructuras que influencian la circulacion de los fluidos en el sistema geotermal. Nuestra atencion se focalizo sobre fallas ~N-S and E-W, pertenecientes respectivamente a la tectonica Basin & Range (B&R) y a la del Cinturon Volcanico Mexicano (CVM). Segun nuestra interpretacion, la interaccion de dos o mas estructuras de diferentes origenes (B&R y CVM), ademas del entorno geodinamico especifico (subduccion de zona de fracturas), facilito el surgimiento de cuerpos magmaticos basicos del profundo que, bloqueados por la capa argilosa de sedimentacion lacustre del Cuitzeo, dieron lugar, enfriandose lentamente, al sistema geotermal y contribuyeron a la formacion de la anomalia magnetica. EnglishFor the development of a geothermal field, the understanding of fracture distribution, hydrology and tectonic evolution of the site through structural characterization of the natural system is a prerequisite to the drilling of exploratory wells. In order to image the underground shape of approximately planar, relatively permeable, geologic features (fractured rock-volumes around fault planes) and to detect hot rock volumes, geologic and geophysical surveying is carried out. A strong positive magnetic anomaly, nearly circular in 2D, characterizes the middle of Cuitzeo lake, Michoacan, Mexico, apparently not related with a volcanic structure on the surface. It may instead be associated to the geothermal system in the area which yields at present hot springs at the southern shore of the lake, at boiling temperatures. In this study, we conducted a ground magnetic survey within and around Lake Cuitzeo with the aim of characterizing this anomaly, following one-kilometer-spaced survey-lines and covering an area of approximately 100 km2. To enhance interpretability, we applied computational filters based on directional derivatives (vertical and horizontal) to our reduced-to-pole magnetic-field raw data – illuminating underground faults and other permeable pathways for fluids and delineating contacts between differently magnetized rocks, through maxima and minima on maps. While most filters used were able to define the basic configuration of the system, we found that computational filters working with ratios of derivatives (vertical and horizontal) were able to better account for the depth of a magnetic source. Also, faults were more clearly imaged on maps. We could mainly highlight ~E-W and ~N-S striking fault-systems, respectively belonging to the ~N-S TransMexican Volcanic Belt extensional tectonics and to the ~E-W Basin & Range, both with lateral movements and still active today. We interpret the magnetic source as a magma body (related to the volcanism of Michoacan-Guanajuato Volcanic Field) that remained trapped during its ascent under Cuitzeo lacustrine sedimentation, during the last 500 ka, forming the once stronger geothermal system.

SUMMARY The Greater Geneva Basin (GGB), located in southwestern Switzerland and neighboring France, is enclosed by the rotating northwestern edge of the Alpine front and the Jura mountains chain. Recently, this basin has received increasing attention as a target for geothermal exploration. Historical and instrumental seismicity suggest that faults affecting the basin may still be active. Moderate-magnitude earthquakes have been located along the Vuache fault, a major strike-slip structure crossing the basin. Before geothermal exploration starts, it is key to evaluate the seismic rate in the region and identify possible seismogenic areas. In this context, we deployed a temporary seismic network of 20 broad-band stations (from September 2016 to January 2018) to investigate the ongoing seismic activity, its relationship with local tectonic structures, and the large-scale kinematics of the area. Our network lowered the magnitude of completeness of the permanent Swiss and French networks from 2.0 to a theoretical value of 0.5. Using a new coherence-based detector (LASSIE - particularly effective to detect microseismicity in noisy environments), we recorded scarce seismicity in the basin with local magnitudes ranging from 0.7 to 2.1 ML. No earthquakes were found in the Canton of Geneva where geothermal activities will take place. We constructed a local ’minimum 1-D P-wave velocity model’ adapted to the GGB using earthquakes from surrounding regions. We relocated the events of our catalogue obtaining deeper hypocentres compared to the locations obtained using the available regional velocity models. We also retrieved eight new focal mechanisms using a combination of polarities and waveform inversion techniques (CSPS). The stress inversion shows a pure strike-slip stress regime, which is in agreement with structural and geological data. Combining the background seismicity with our catalogue, we identified seismogenic areas offsetting the basin.

Understanding the anatomy of magma plumbing systems of active volcanoes is essential not only for unraveling magma dynamics and eruptive behaviors but also to define the geometry, depth, and temperature of the heat sources for geothermal exploration. The Pleistocene–Holocene Los Humeros volcanic complex is part of the eastern Trans-Mexican Volcanic Belt (central Mexico), and it constitutes one of the most important exploited geothermal fields in Mexico with ca. 90 MW of produced electricity. With the aim to decipher the anatomy (geometry and structure) of the magmatic plumbing system feeding the geothermal field at Los Humeros, we carried out a field-based petrological and thermobarometric study of the exposed Holocene lavas. Textural analysis, whole-rock major-element data, and mineral chemistry are integrated with a suite of mineral-liquid thermobarometric models. Our results support a scenario characterized by a heterogeneous multilayered system, comprising a deep (depth of ca. 30 km) basaltic reservoir feeding progressively shallower and smaller discrete magma stagnation layers and batches, up to shallow-crust conditions (depth of ca. 3 km). The evolution of melts in the feeding system is mainly controlled by differentiation processes through fractional crystallization (plagioclase + clinopyroxene + olivine + spinel). We demonstrate the inadequacy of the existing conceptual models, where a single voluminous melt-controlled magma chamber (or “Standard Model”) at shallow depths was proposed for the magmatic plumbing system at Los Humeros. We instead propose a magmatic plumbing system made of multiple, more or less interconnected, magma transport and storage layers within the crust, feeding small (ephemeral) magma chambers at shallow-crustal conditions. This revised scenario provides a new configuration of the heat source feeding the geothermal reservoir at Los Humeros, and it should be taken into account to drive future exploration and exploitation strategies.

Geothermal exploration will help move towards a low-carbon economy and provide a basis for green and sustainable growth. The development of new practical, reliable methods for geophysical characterization of a reservoir has the potential to facilitate a broader application of deep geothermal energy. At the Groß Schönebeck in situ laboratory, a unique vertical seismic profiling (VSP) dataset was recorded in two 4.3 km deep geothermal boreholes using fibre-optic cables in early 2017. The experiment set-up consisted of 61 vibrator points organized in a spiral pattern around the well site to ensure a proper offset and azimuth distribution in the target reservoir section. Data were processed using a standard workflow for VSP. As a result, a detailed three-dimensional 0.75km×1km×4.5 km image around the existing boreholes was created using the Kirchhoff migration algorithm with restricted aperture. The imaging resolved small-scale features in the reservoir essential for the future exploration of the geothermal research site. Borehole data with vertical resolution up to 16 m revealed the existing depth variations of the Elbe reservoir sandstone horizon at 4.08–4.10 km depth and indications of an unconformity in the area where we expect volcanic rocks. In addition, in the borehole data a complex interlaying with numerous pinch-outs in the Upper Rotliegend reservoir section (3.8 to 4 km depth) was discovered. Thereby, we demonstrate that wireline fibre-optic data can significantly contribute to exploration by providing an efficient and reliable method for deep geothermal reservoir imaging.

In autumn 2017 a network of 14 broadband seismic stations was deployed at the Theistareykir high temperature geothermal field (NE Iceland). This experiment was conducted as part of the current efforts to characterize the field's main structures, and possible short and long term stress variations due to the ongoing fluid injection and extraction operations which started in autumn 2017. In this work, we use two years of continuous seismic records (October 2017–October 2019) to compute a 3D shear wave velocity model of the geothermal field and to detect possible crustal stress changes related to the injection and production activities. From phase cross-correlations of the vertical component recordings, we measure the Rayleigh wave group velocity dispersion curves to obtain 2D group velocity maps between 1 and 5 s. Subsequently, we use a neighborhood algorithm to retrieve the 3D shear wave velocity model of Theistareykir. Mainly, two sets of elongated high and low velocity anomalies can be observed oriented in a NW/WNW direction, parallel to the lineaments of the active Tjörnes fracture zone. Velocity reductions west of Ketilfjall and at Baerjafjall could indicate the location of upflow zones of the magmatic reservoir or hydrothermal system. We analyzed the temporal evolution of phase and amplitude of phase auto-correlations using the stretching technique and discuss their behavior in relation to the geothermal field operations. We notice a slightly stronger long-term velocity decrease in the reservoir region compared to outer regions. This could be related to the mass depletion in that area (higher fluid extraction compared to the water reinjection). In summary, our findings show how a monitoring network can be set up to enable a detailed imaging and monitoring of reservoir behavior in general., Journal of Volcanology and Geothermal Research, 429, ISSN:0377-0273

The investigation of permeable hydrothermally altered and fractured zones and their distribution is a key issue for the understanding of fluid circulation in granitic rocks, on which the success of geothermal projects relies. Based on the use of short-wave infrared (SWIR) spectroscopy applied to rock cuttings coupled with interpretation of geophysical logs, we propose an investigation of the clay signature of fault and fracture zones (FZ) inside the granitic basement. This methodology was applied to two geothermal wells: GRT-2 from the Rittershoffen and GPK-1 from the Soultz-sous-Forêts (Soultz) geothermal sites, both located in the Upper Rhine Graben (URG). A total of 1430 SWIR spectra were acquired and analysed. Variations in the 2200 nm absorption band area are correlated with hydrothermal alteration grades. The 2200 nm absorption band area is found to reflect the illite quantity and its variations in the granitic basement. Low, stable values are observed in the unaltered granite facies, showing good reproducibility of the method, whereas scattered high values are associated with high hydrothermal alteration and FZs. Variations in the 2200 nm absorption band area were correlated with the gamma ray and electrical resistivity logs. This procedure allowed us to confirm that illite mainly controls the resistivity response except inside the permeable FZs, where the resistivity response is controlled by the geothermal brine. Thus, the architecture of these permeable FZs was described precisely by using a combination of the 2200 nm absorption band area data and the electrical resistivity log. Moreover, by correlation with other geophysical logs (temperature (T), porosity, and density), paleo-permeable and currently permeable FZs inside the reservoir were distinguished. The correlation of SWIR spectroscopy with electrical resistivity logs appears to be a robust tool for geothermal exploration in granitic reservoirs in the URG.

International audience; Due to its geodynamic location in the Lesser Antilles arc, Basse-Terre of Guadeloupe displays an active volcanism which is favourable to the development of high-enthalpy geothermal energy. An exhumed analogue of the Bouillante geothermal reservoir has been recently characterized in Terre-de-Haut island (Les Saintes archipelago), giving an opportunity to study the cooling of a natural geothermal system and the related alterations.By coupling chlorite geothermometry and primary fluid inclusions in associated zoned quartz crystals located in N70 trending fractures, new insights on the Terre-de-Haut paleo-geothermal evolution can be inferred by distinguishing three new different episodes during functioning and cooling of this system: (i) a temperature of at least 240–270 °C in fluid inclusions trapped in the quartz core, (ii) chlorite formation at about 120 °C, and (iii) temperatures of ~50 °C - or less - in fluid inclusions of the quartz outer growth zones. Fluid inclusions also reveal that the hydrothermal fluid is of low salinity (2 wt% NaCl), probably of meteoric and sea water origin. Additionally, a second type of fluid inclusions observed in a banded quartz vein indicates at least an episode of a CO2-(H2O) fluid, with traces of H2S. Hence, some similarities can be highlighted between Terre-de-Haut and the Bouillante active geothermal systems in term of the highest temperatures of circulation events and gas composition, despite a difference in fluid origin. This study shows that some alteration episodes also occur at low temperature during the cooling of the geothermal system. Moreover, the N70 trending fractures, that have been identified as drains for the hot temperature fluids, have to be considered for future geothermal exploration.

The present study interpreted high-resolution aeromagnetic data using spectral analysis interpretation technique to carry out a regional estimation of geothermal resources of the entire Benue trough, Nigeria. The data was partitioned into 46 overlapping blocks and the depths to top, bottom and center of the magnetic sources for each overlapping data were computed. The calculated depths were then used to determine curie-point depths, geothermal gradients, and heat flow in the basin subsurface. The geothermal gradient in the study area indicated a range of values from 12.5 to 38.3 °C/km with a mean value of 26.0 °C/km, while the curie-point depths vary from 15.0 to 45.3 km with a mean depth of 23.2 km. The study identified four separate geothermal zones namely A, B, D, and F. However, zone F before this study has been unknown. It, therefore, represents an additional geothermal zone to the previous works in the study area. The values of heat flow vary from 31.2 to 95.8 mW m−2 with a mean value of 65.1 mW m−2. This study provides new insights into the geothermal resources of the Benue trough. The high heat flow values in the entire region might be considered good for geothermal exploration and therefore, the geothermal zones identified should be considered for further investigations. The study identified four separate geothermal zones: A, B, D, and F. The geothermal zone F before this study has been unknown. It provides new insights into the geothermal resources of the Benue trough.

SUMMARYThe Greater Geneva Basin is one of the key targets for geothermal exploration in Switzerland. Until recently, information about the subsurface structure of this region was mostly composed of well-logs, seismic reflection lines, and gravity measurements. As part of the current effort to further reduce subsurface uncertainty, and to test passive seismic methods for exploration purposes, we performed an ambient-noise tomography of the Greater Geneva Basin. We used ∼1.5 yr of continuous data collected on a temporary seismic network composed of 28 broad-band stations deployed within and around the basin. From the vertical component of the continuous noise recordings, we computed cross-correlation functions and retrieved Rayleigh-wave group-velocity dispersion curves. We then inverted the dispersion curves to obtain 2-D group-velocity maps and proceeded to a subsequent inversion step to retrieve a large-scale 3-D shear-wave velocity model of the basin. We discuss the retrieved features of the basin in the light of local geology, previously acquired geophysical data sets, and ongoing geothermal exploration. The Greater Geneva Basin is an ideal natural laboratory to test innovative geothermal exploration methods because of the substantial geophysical data sets available for comparison. While we point out the limits of ambient-noise exploration with sparse networks and current methodology, we also discuss possible ways to develop ambient-noise tomography as an affordable and efficient subsurface exploration method.

We acquired and interpreted data from two controlled-source electromagnetic profiles, in the Gulf of California off the coast of Santa Rosalia, Baja California Sur, Mexico. This area is interesting from the geothermal point of view due to its proximity to the Guaymas Basin, the La Reforma Caldera, and the Las Tres Virgenes volcano, the first with intense submarine hydrothermal manifestations and the final one with a geothermal field in production. The survey consisted of the deployment of 10 Ocean Bottom Electromagnetic receivers, distributed along two ~14 km-long profiles. These sensors recorded the two horizontal components of the electric and magnetic fields. The source consisted of a horizontal 200 m electric dipole towed behind the vessel. The processed data were inverted to estimate the subsurface distribution of electrical resistivity using a finite-element two-dimensional multi-frequency program, resulting in adequate fits between observed and calculated data. To estimate how well the models are constrained by the data, sensitivity tests were applied to the models. The tests were based on the Jacobian matrix and a perturbation analysis of selected features of the model. They show that the model in its central part is well constrained by the data down to a depth of about 2.5 km. With the perturbation approach, 16 resistivity features out of the 19 tested turned out to be well constrained by the data. The estimated resistivity distribution was compared with the information obtained from other geophysical methods (bathymetry, gravity, and 2D multi-channel seismic reflection), resulting in reasonably good correlations between them. For the first profile, we find a remarkable similarity with the current conceptual models proposed in the literature for a volcanic geothermal system, where an altered conductive clay cap overlays another with higher electrical resistivity. The second profile shows agreements with anomalies detected with echo-sounder bathymetry suggesting the existence of hydrothermal manifestations.

In this study, Curie-point depth (CPD), geothermal gradient, radiogenic heat production, and heat flow maps were constructed based on different thermal conductivity coefficients using magnetic anomaly data for the Kars Plateau, which has the largest volcanic cover in Turkey. The bottom depths of the magnetic crust in the research area were revealed by the CPD map for the first time in this investigation. There are two apparent magnetic anomaly trends in the study area: the first is the Horasan–Senkaya–Sarikamis–Selim–Arpacay trend in the NE–SW direction, and the other is the Hanak–Ardahan–Arpacay trend in the NW–SE direction. Two other prominent elongations extend into the Ardahan–Gole–Senkaya and Kars–Digor axes. All these trends represent mountain chains and/or stratovolcanoes in the region, and no anomalies are observed around the non-volcanic outcrops. Curie depths are shallow, up to 14 km between Horasan and Kagizman towns, and 12 km in the northwestern part of the study area. Gradient values can reach 50 °C km−1 in the northwestern sector, together with the high heat flows represented by the 150 Wm−1 K−1 contours. The deepest CPD region lies between Gole and Susuz towns, where the geothermal gradient decreases to 27 °C km−1. Heat flows decrease 60 Wm−1 K−1 in the same area. An apparent gap around the Kars Plateau was observed in previous regional heat flow maps of Turkey by other authors (who used the bottom hole temperatures of boreholes and hot springs temperatures). This gap has been accurately filled from the results of this study, and geothermal exploration areas and the geothermal potential of the Kars Plateau have thus been determined for future exploration activity on the basis of the tectonic elements and earthquake data.

SUMMARY The growth of the geothermal industry sector requires innovative methods to reduce exploration costs whilst minimizing uncertainty during subsurface exploration. Until now geoelectrical prospection had to trade between logistically complex cabled technologies reaching a few hundreds meters deep versus shallow-reaching prospecting methods commonly used in hydro-geophysical studies. We present a recent technology for geoelectrical prospection, and show how geoelectrical methods may allow the investigation of medium-enthalpy geothermal resources until about 1 km depth. The use of the new acquisition system, which is made of a distributed set of independent electrical potential recorders, enabled us to tackle logistics and noise data issues typical of urbanized areas. We acquired a 4.5-km-long 2-D geoelectrical survey in an industrial area to investigate the subsurface structure of a sedimentary sequence that was the target of a ∼700 m geothermal exploration well (Geo-01, Satigny) in the Greater Geneva Basin, Western Switzerland. To show the reliability of this new method we compared the acquired resistivity data against reflection seismic and gravimetric data and well logs. The processed resistivity model is consistent with the interpretation of the active-seismic data and density variations computed from the inversion of the residual Bouguer anomaly. The combination of the resistivity and gravity models suggest the presence of a low resistivity and low density body crossing Mesozoic geological units up to Palaeogene–Neogene units that can be used for medium-enthalpy geothermal exploitation. Our work points out how new geoelectrical methods may be used to identify thermal groundwater at depth. This new cost-efficient technology may become an effective and reliable exploration method for the imaging of shallow geothermal resources.

A three-dimensional (3D) magnetotelluric (MT) survey was carried out to delineate subsurface structures and possible fractures for geothermal development around Ulleung Island, Korea. Good-quality ...

The Shahin Dezh Region is one of several areas prospective for geothermal energy in the West Azerbaijan Province of Iran. Hot spring spewing fluids at 30 °C is regarded as the first surface signature for the existence of a subsurface geothermal reservoir in this region. To determine the geothermal energy potential for the region we carried out geological, geochemical and geophysical surveys in the study area. Field observation, thin section studies and XRD analysis reveal the existence of a zone of pervasive hydrothermal alteration in this region. Based on the geochemical analysis, the hydrothermal spring fluids falls into the bicarbonate water family. The results of geothermometric calculations indicate that geothermal reservoir temperature ranges between 40 and 51 °C. Euler deconvolution of geophysical data over the region suggests the reservoir is shallow. This shallowness together with groundwater mixing are factors that played a role in decreasing the reservoir temperature. Integrating geological, geochemical and gravity studies revealed that geothermal reservoir contains 2.3 × 10 7 kJ thermal energy.

Ceboruco volcano is one of the largest volcanoes of the Trans-Mexican Volcanic Belt (TMVB), which extends along central Mexico. Among the western TMVB, Ceboruco Volcano comes in the second place after Colima Volcano in its activity. Also, it is considered as the only one that has historically-documented eruptions. Few geophysical studies have been published with the aim of studying the internal structure of the volcano. In the current work, and throughout the support of the CeMIEGeo-P24 geothermal exploration project, we aim to delineate the possible subsurface structural trends, to determine the approximate depth to the basement surface, and to provide an illustrative 3D model for its subsurface structure. To achieve such goals, a detailed analysis of the aeromagnetic data for the volcano area was performed. 2D interpretation was carried out for the aeromagnetic data. Edge detection filters, such as Tilt Derivative, and Analytic Signal, were used to map the contact/faults within the study area. In addition, 3D inversion was used to image the subsurface structure of the volcano. Aeromagnetic data was inverted using the GMSYS-3D inversion program by applying Parker algorithm. Four subsurface layers at depths of 250, 500, 750 and 1000 m were assumed during the inversion process. Results indicate that the limits “boundary” of the subsurface magma chamber of the volcano can be traced easily from the first subsurface layer. A secondary structure line appears towards the east from the volcano boundary and it can be observed from the inversion of the second and third layers. This indicates that both, the base of the volcano and the structure line, stem from the same source. Finally, they disappear in the inversion results for the last layer, which concludes that we reach the basement above which the volcano is formed. Combination of all obtained results with the geological information, helped to understand the main structure of the Ceboruco volcano.

Transient electromagnetics (TEM) is a well-established method for mineral, groundwater, and geothermal exploration. Superconducting quantum interference device (SQUID)-based magnetic-field receivers used for TEM have quantitative advantages and higher sensitivity compared with commonly used induction coils. Special applications are deep soundings with target depths [Formula: see text] and settings with conductive overburden. However, SQUIDs have rarely been applied for TEM measurements in environments with significant anthropogenic noise. We compared a low-temperature SQUID with a commercially available induction coil in an area affected by anthropogenic noise. We acquired four fixed-loop data sets with totally 61 receiver stations close to Bad Frankenhausen, Germany. The high sensitivity of the SQUID enables low noise levels, which lead to longer high-quality transient data compared with the induction coil. The effect of anthropogenic and natural noise sources is more critical for the coil than for the SQUID data. In the vicinity of the transmitter loop, systematic distortion of the coil signals occurs at early times, most probably caused by sferic interferences. We have developed 1D inversion results of both receivers that matched well in general. However, the SQUID-based models were more consistent and showed greater depths of investigation. This led to a superior resolution of deeper layers and even enabled a potential detection of thin conducting targets at up to a 500 m depth. Moreover, we find that the SQUID data inversion revealed multidimensional effects within the conductive overburden. In this regard, we applied forward modeling to analyze systematic differences between inversion results of SQUID and coil data. We determine that low-temperature SQUIDs have the potential to significantly improve the reliability of subsurface models in suburban environments. Nevertheless, we recommend combined application of both types of receivers.

International audience; We present integrated structural, petrological, mineralogical and geochemical investigations conducted on an ex-humed hydrothermal system identified in Terre-de-Haut Island (Guadeloupe archipelago). This work demon-strates, for the first time, the occurrence of ductile shear zones, marked by the development of spaced schistosity planes generated by pressure-solution processes. We unravel first that the development of spaced schistosity is coeval with high-temperature hydrothermalism (approximately 350 degrees C) and second that this hy-drothermal alteration occurred at a depth of less than 2 km. We highlight, at shallow crustal depths, the record of both vertical and lateral fluid pathways in the studied volcanic arc. Finally, combining this new dataset with the large number of published surface data obtained in the active geothermal system of Bouillante, we propose a conceptual scheme for the studied fossil reservoir to document the roots of magmatic arc-related active geo-thermal systems and to introduce new perspectives for deep geothermal exploration.

Summary Geothermal energy is the natural heat that exists within our planet. The potential for harnessing electricity and heat from geothermal energy has long been recognized. The amount of heat within 10,000 metres of the earth's surface is estimated to contain 50,000 times more energy than all oil and gas resources worldwide (Shere, 2013). The most challenging aspect of geothermal exploration is the quantification of the subsurface temperature conditions. Actual temperature conditions often remain very uncertain as it is difficult to remotely measure through several hundreds of metres of rock. Only drilling through the rock layers will give information on the existing subsurface temperatures. As drilling is very expensive (€1million to €15million), any low-cost pre-drilling temperature estimation can bring in huge added value. The Electromagnetic (EM) technology that Adrok has been developing aims to locate sources of geothermal heat prior to drilling. Through empirical fieldwork, the EM technology can non-invasively and digitally provide a proxy temperature measurement of the subsurface without physical drilling. Key aspects of the technology have been field tested, including depth and capacity to identify water. This embryonic EM technology merits further investigation to enable efficient and optimal exploration of the natural resources useful for geothermal energy generation.

Gravity data retrieved from the Bureau Gravimétrique International and the Gravimetric Atlas of Switzerland have been used to evaluate their applicability as a subsurface investigation tool to assess key geological features in support of the geothermal exploration in the Geneva Basin (GB). In this context, the application of an effective processing workflow able to produce reliable residual gravity anomalies was implemented as a crucial first step to investigate whether and to what level gravity anomalies can be correlated to geologic sources of geothermal interest. This study focusses on the processing workflow applied to publicly available gravity data, including the quantification of the uncertainty. This was then also used for first-order 2D forward gravity modelling. The resulting residual anomalies demonstrate the potential use of gravity investigations for geothermal exploration in sedimentary basins, and also reveal areas of significant, irreparable misfit, which calls for the use of complementary data and 3D subsurface structural knowledge. The results of such investigations will be presented in subsequent studies.

Summary Electromagnetic (EM) and Potential Field (PF) methods are extensively applied to geothermal exploration not only for exploration but also for reservoir characterization and monitoring. The magnetotelluric method (MT) provides useful information regarding the structure and the hydraulic conditions of geothermal systems, but it is affected by EM noise, generated by most of the human activities. Potential field data, such as gravity, are not affected by EM cultural noise and can map lateral extent of low density/high permeability areas, helping to resolve the geometries of geothermal reservoirs. The combination of the two geophysical measurements can reduce the intrinsic uncertainty of each method. In the presented case study, MT and gravity were jointly modelled via Simultaneous Joint Inversion (SJI) to assess the lateral extent of an existing geothermal field, providing mutually consistent 3D resistivity and density distributions, in agreement with independent geologic information and interpreted faults.

At the development stage of a geothermal field, a comprehensive survey is needed to obtain an accurate picture of the geothermal system to identify the subsurface condition on the prospect field. The geological structure is one of the important aspects of geothermal exploration. It creates permeability, which allows geothermal fluids to flow to the surface. Locating geological structures in the prospect area will be a powerful tool to determine the drilling target for exploration or production wells. One of the methods to identify geological structure on the surface is using satellite imagery. It is based on visual interpretation of the morpho-structural aspect combined with the lithological aspect and then confirmed with field data. The remote sensing data is built from DEMNAS data and a regional geological map. DEMNAS has spatial resolution 0,27-arcsecond, vertical datum EGM2008. While the subsurface geological structure can be identified by using the Gravity Second Vertical Derivative (SVD) and First Horizontal Derivative (FHD) method. Gravity method identifies structure by slicing the SVD grid, which has suspected structure and then confirmed with FHD slicing in the same suspected area. The geological structure will be confirmed if the value of FHD maximum and SVD value 0. The results of the data processing above are then integrated to obtain the distribution of geological structures as a permeable zone in the geothermal field which then can be used as a guide in determining the location of exploration and production wells.

This paper deals with the design, acquisition and processing of vertical seismic profiling data collected in 2016 at the directional Thonex‐01 geothermal well, drilled in 1993 in the Geneva Canton (Switzerland). The aim of the study was to obtain additional information from existing‐abandoned well for downhole geophysical characterization of potential geothermal reservoirs in the Geneva area. The main results obtained by this study allowed an improved subsurface image by multi‐offset acquisitions near and around the deviated well. Specifically we were able to (1) improve the velocity model of the Geneva Basin, which, at present, is only constrained by one unique deep exploration well, located at the south‐west of the basin, far away from the study area; (2) image the transition between the Tertiary Molasse siliciclastic sediments and the Mesozoic carbonates; (3) define an acquisition strategy for future vertical seismic profiling surveys in support of the geothermal exploration campaigns in the area. This study has demonstrated the value of vertical seismic profiling method for the refinement of the subsurface geology at local scale, and its usefulness to assist the planning of second well associated with geothermal doublets.

As many ocean basaltic volcanoes worldwide, Piton des Neiges (Reunion Island) hosts a hydrothermal system that represents a potential geothermal resource. Despite several prospection campaigns over the past decades, this potential has never been confirmed because many aspects of fluid supply and circulation remain unclear. To track the origin and fate of hydrothermal fluids, we analyzed the geochemistry of thermal springs (water and gas), by combining conventional tracers (major-trace elements and O, H, C, Sr, Li isotopes) with Cl isotopes, a geochemical tool under development. Added to literature data, our new results allow us to compare the composition of thermal springs with cold waters, rocks, gas, and fumarolic deposits from La Reunion and elsewhere, and to untwine the complex history of fluid mixing that constitutes the geothermal system of Piton des Neiges. delta O-18(H2O) and delta D-H2O values of springs (-8.67 to -4.9 and -56.3 to -23.5 parts per thousand VSMOW, respectively) are positioned close to the local meteoric water line, showing that thermal waters are of meteoric origin. However, their depletion in heavy isotopes relative to cold waters suggests a recharge strongly influenced by cyclones, with a possible contribution from high altitude (>= 2000m) rainfalls. Their absence of O-18 enrichment relative to the local meteoric water line indicates that the isotopic exchange between rocks and water is very limited, and therefore suggests that their deep temperatures are relatively low and/or the water/rock ratios are high. delta C-13 values in thermal waters (-8.0 to +3.2 parts per thousand PDB) and gases (-6.7 to -5.3 parts per thousand PDB) confirm that carbon is essentially of magmatic origin, which we interpret as supplied by regional degassing of La Reunion hotspot. Major-trace elements and Sr-87/Sr-86 (0.704142 to 0.704336), delta Li-7 (+2 parts per thousand and +34.8 parts per thousand LSVEC), and delta Cl-37(-0.35 parts per thousand and +0.40 parts per thousand SMOC) compositions show that three additional processes contribute to the mineralization of thermal waters: 1) interaction with basalt at temperatures >= 50 degrees C, 2) very limited seawater contamination (< 3 mol parts per thousand), and/or 3) leaching of trachyte or scrubbing of fumarolic gas or condensates emitted by a shallow trachytic magma in the volcanic edifice. The geothermal system of Piton des Neiges is thus the result of meteoric water interaction with a trachytic heat source (as a solidified intrusion or a magma pocket), unrelated to the CO2 flux. We also emphasize that thermal waters at Piton des Neiges are exceptionally isolated from seawater compared to other geothermal systems in ocean island volcanoes worldwide, which implies a heat source located above the seawater/freshwater interface. These inferences are of critical importance for the future geothermal exploration on the island, as they suggest that the geothermal resource is within reach of relatively shallow drilling depths (

Geothermal energy is one of the most promising renewable energies due to its high load factor. This work is devoted to presenting the conceptualizations and research advances made at Shenzhen University on the exploration and utilization of low/medium-temperature geothermal energy based on the geothermal resource potential and characteristics of the Guangdong-Hong Kong-Macao Greater Bay Area. The results indicate that the geological structure and lithology of this area are conducive to the formation of geothermal conditions. The geothermal flow and geothermal gradient in this area are higher than the average values in southern China. The total hydrothermal resources and hot dry rock geothermal resources amount to 1.8 × 1017 kJ and 4.0 × 1017 kJ, respectively. To enhance the permeability of geothermal reservoirs, especially hot dry rock reservoirs, the volumetric fracturing technique, which is based on three-dimensional fracture mechanics, and the fatigue pneumatic fracturing technique are proposed to stimulate geothermal reservoirs. The advances in theoretical and experimental research and some noteworthy future research topics on volumetric fracturing and fatigue pneumatic fracturing techniques are summarized. To increase the efficiency of low/medium-temperature geothermal power generation, conceptualizations of magnetic levitation power generation technology and thermovoltaic power generation technology for geothermal power generation are proposed. This study may provide new insights into geothermal exploration and low/medium-temperature geothermal power generation.

Among geothermal exploration methods, active surface seismic methods have played only a minor role to date. Especially in high-temperature volcanic systems, reflection seismic data often reveal poor delineation of volcanic features, due to the internal heterogeneity of volcanic sequences. To enhance the vertical resolution, one possibility is the application of downhole seismic methods like vertical seismic profiling (VSP). A test experiment was carried out in the Krafla high-temperature geothermal field, NE-Iceland, to assess the ability of VSP to image subsurface structures, such as fractures, zones of high permeability, magmatic bodies, and zones of supercritical fluids and steam. Logging in such hostile environments is technical challenging in many aspects, but mainly due to the high temperature impact on the downhole electronic components of the measuring equipment. This requires a thorough pre-examination and implementation of the measurement, especially to avoid delays and tool failures. This paper presents results of zero- and far-offset VSP data from the K-18 borehole from within the Krafla caldera, which reveal good correlation with the surrounding lithology. The raw three-component seismic data display a good signal-to-noise ratio and dominant signal frequencies between 20 and 40 Hz, down to c. 2200 m depth, for air gun and explosive sources, respectively. A zero-offset source comparison was also conducted to assess the use of different impulsive sources for future VSP surveys in similar settings. By applying a standard VSP processing, we identified stratigraphic boundaries between lavas, hyaloclastites, and intrusions, which are in good agreement with existing well data. For the zero-offset VSP, both P- and S-wave velocity models were calculated and a depth-converted corridor stack was determined. In addition, multicomponent Kirchhoff depth migration and Fresnel volume migration were tested around the borehole. The 3D results are promising, but the specific shape and lateral extent of the reflectors could not be determined due to the restriction to only two sources and the insufficient spatial coverage (aperture). Our study demonstrates that vertical seismic profiles can clearly detect variations in the subsurface volcanic stratigraphy in high-temperature geothermal fields. A more detailed reservoir characterization can be achieved by further data integration, enhanced survey design including more source positions, and improved processing and imaging techniques, such as full-waveform inversion.

A passive seismic experiment using 25 broadband and 20 short‐period stations was conducted between September 2017 and September 2018 at Los Humeros geothermal field, an important natural laboratory for superhot geothermal systems in Mexico. From the recorded local seismicity, we derive a minimum 1‐D velocity model and obtain 3‐D Vp and Vp/Vs structures of Los Humeros. We improved the classical local earthquake tomography by using a postprocessing statistical approach. Several inversions were computed and averaged to reduce artifacts introduced by the model parametrization and to increase the resolution of the investigated region. Finally, the resulting Vp and Vp/Vs structures and associated seismicity were integrated with newly acquired geophysical and petrophysical data for comprehensive interpretation. The recorded seismicity is mainly grouped in three clusters, two of which seem directly related to exploitation activities. By combining new laboratory measurements and existing well data with our Vp model, we estimate possible geological unit boundaries. One large intrusion‐like body in the Vp model, together with neighboring high Vp/Vs anomalies, hints at a region of active resurgence or uplift due to the intrusion of new magma at the northern portion of the geothermal field. We interpret high Vp/Vs features as fluid bearing regions potentially favorable for further geothermal exploitation. Deep reaching permeable faults cutting the reservoir unit could explain fluid flow from a deeper local heat source in the area., Journal of Geophysical Research: Solid Earth, 125 (12), ISSN:2169-9313, ISSN:0148-0227, ISSN:2169-9356

The Los Humeros geothermal field is the third most important producer of geothermal electricity (70 MW) in Mexico. Geothermal fluids are hosted in fractured andesitic lavas and mostly consist of high enthalpy steam with limited water content (vapor fraction > 0.9). Despite the high reservoir temperature (~300–400 °C), thermal manifestations at the surface are scarce and locally appear as steaming grounds, weak steam vents and advanced argillic alteration. Geothermal fluid upflow from the reservoir towards the surface is limited by welded ignimbrite deposits that act as a low-permeability barrier. In this study, we present the first measurements of CO2, CH4 and H2S degassing rates from the soil performed at Los Humeros. Flux measurements were complemented with δ13C composition of degassing CO2 and soil temperatures to discuss gas origin and thermal anomalies. We measured high soil degassing rates (up to 7530 g m−2 d−1 CO2, 33 g m−2 d−1 CH4 and 22 g m−2 d−1 H2S) in three localized areas (Humeros North – HN, Humeros South – HS and Xalapazco – XA) as well as high soil temperatures reaching the boiling temperature at the local altitude (90.6 °C). The particular location of these three areas suggests that the steam-dominated reservoir degases to the surface through permeable faults crossing the ignimbritic deposits. The remaining surveyed areas are characterized by weak CO2 fluxes (≤44 g m−2 d−1), non-detectable CH4 and H2S fluxes, and lower soil temperatures (5–21 °C). The compositions in δ13CCO2 from HN-HS-XA areas (δ13CCO2 = −7.94 to −2.73‰) reflect a magmatic source with some possible contribution from the sedimentary basement, as well as fractionation induced by boiling and CO2 dissolution in shallow water bodies. We also discuss the processes causing the spread in CO2/CH4 flux ratios. Finally, we estimate the heat output from the three high degassing areas to a value of 16.4 MWt.