How can experimental magnetotellurics and petrophysics provide critical constraints on the structure of magmatic plumbing systems? How can magnetotellurics contribute to volcanic monitoring? We present a workflow applied to two French volcanoes: Montagne Pelée (Martinique, West Indies) and Mayotte.Montagne Pelée volcano has experienced renewed seismic activity since 2019, with earthquakes occurring below 10 km depth and more superficial activity within the first few kilometers. In 2023, a broadband magnetotelluric (MT) survey was conducted, allowing the construction of both finite-difference and finite-element 3D electrical conductivity models down to 20 km depth. These models reveal key features of the magmatic plumbing system, constrained by the joint interpretation of MT data and experimental petrophysics, using high-pressure laboratory measurements of electrical conductivity as a function of temperature on lava samples. In June 2025, an experimental array of three continuous MT monitoring stations was installed in strategically selected locations to track fluid migration and the progressive development of partial melt within the plumbing system. We describe the complete workflow, from 3D imaging to characterization and monitoring.Since 10 May 2018, Mayotte has been experiencing one of the largest offshore seismovolcanic crises of the past three centuries. The MAYOBS1 scientific mission (2–19 May 2019, Marion Dufresne vessel) led to the discovery of a new 820-m-high volcanic edifice, named Fani Maore, with an estimated volume exceeding 6.55 km³. Between 2018 and 2021, geophysical observations revealed an eastward displacement of the island of 21–25 cm, combined with 10–19 cm of subsidence, as well as more than 100,000 earthquakes occurring at unusually large depths (22–45 km). A combination of broadband land and marine MT surveys enabled the construction of a 3D resistivity model down to 30 km depth, revealing two major conductive bodies at approximately 12 km and 22 km depth. The deeper conductor is interpreted as a magmatic mush zone with an estimated melt fraction of 22–42%. As part of the REVOSIMA (Réseau de Surveillance Volcanologique et Sismologique de Mayotte), a network of permanent MT stations is currently monitoring the magmatic plumbing system. The latest imaging results and monitoring developments will be presented.

Geothermal systems in the context of very active rifting, such as in the Republic of Djibouti, are complex to characterize. Despite the numerous geoscientific studies conducted over decades in recognized prospects such as Fiale Caldera and Gale-le-Koma in the Asal Rift, we do not understand well the mechanism of the geothermal resources, their relationships with the heat source at depth and the recharge of water. In an attempt to progress in this understanding, we present an analysis of 3-D gravity data inversion guided by a new three-dimensional electrical model to obtain constrained density models for each geothermal site. These models along with the resistivity model revealed highly fractured underground structures primarily influenced by two processes: geothermal activity from the rift and structural control from the active rift tectonics. These models are in good agreement with the lithology from boreholes, as well as with geochemical and hydrogeological data. The study presents revised conceptual models for the two geothermal systems. These conceptual models consist of a shallow reservoir and a deep reservoir at each site (Fiale Caldera and Gale-le-Koma geothermal prospects), primarily recharged by seawater from the Bay of Ghoubbet and possibly by regional meteorological groundwater recharges.

The exact nature of crustal magmatic reservoirs is elusive as they cannot be sampled in situ. The traditional view that magma chambers contain essentially molten material has recently been replaced by the transcrustal magmatic system (TCMS), in which reservoirs are mostly composed of immobile magmatic crystals with a minute fraction of more mobile melt1-3, creating a 'magmatic mush'3. Eruptions are possible if a significant portion of melt segregates into melt-rich lenses within the mush reservoir1-3. The TCMS concept is, however, a default model essentially justified by the absence of clear geophysical signatures of melt-rich magma chambers1,4, and by the rare and tentative estimates of the melt fraction in the crustal storage zones based on geochemical and textural analysis of eruptive products5,6. Here we image a bright electrical conductor at 23 ± 1 km below sea level beneath Mayotte island that we interpret as a magmatic reservoir, based on laboratory measurements of Mayotte's melt conductivity. This large magmatic reservoir (more than 200 km3) contains a high melt fraction (22-42%). Such a crystal-to-liquid ratio matches the reconstructed differentiation paths7-9 producing the melts that recently erupted at Mayotte. This reservoir is possibly connected to the system that fed the large submarine eruption of Fani Maoré in 2018-201910.

Electromagnetic properties of Earth materials are important geophysical parameters and serve as good proxies for fluid-related processes, as their variations are directly linked to changes in porosity, connectivity, fluid flow, or magnetization of the porous network, as well as to the composition of fluids and the properties of mineral surfaces. In the oceanic crust, electromagnetic imaging provides key insights into the geometry of hydrothermal conduits and the distribution of fracturing and alteration. In sedimentary environments, it can also reveal the geometry of sedimentary bodies, map fluid migration conduits, and detect the presence and distribution of natural gas and gas hydrates within the upper tens to hundreds of meters of the seafloor. Within the Deepsea'Nnovation program (DSN) [1] this project proposes an innovative approach: adapting an ROV-based electromagnetic imaging methodology developed in our laboratory, originally inspired by large-scale controlled-source electrical transmitter/receiver systems. This method has already demonstrated its effectiveness in shallow water environments (less than 100 m depth). For high-resolution electromagnetic imaging in the deep sea, using an ROV is essential, as it allows deployment of an electrical source with sufficient power. The project aims to develop an ROV-mounted electromagnetic source paired with electric potential receivers. The system will be designed to accommodate different receiver configurations—varying in number, type (fixed or mobile), and deployment strategy. It will enable profile acquisition at exploration scale, with a penetration depth of up to a few meters below the seafloor.

The Northern Tanzanian Divergence in the East Africa Rift is arguably the best place on Earth to study the controls on rifting of thick lithosphere. Here, where the East Africa Rift intersects the Tanzanian Craton and the Mozambique Belt, the relationships between volcanism, faulting, pre-existing structures and lithospheric thickness and composition can be observed. In this work, we carry out the first lithospheric-scale 3D magnetotelluric modeling of the Northern Tanzanian Divergence and combine the results with experimental electrical conductivity and petrology models to calculate mantle composition, which is also inferred in the craton from reanalysis of garnet xenocryst data. Our results show that metasomatic materials exist in the cratonic lithospheric mantle and the relatively undeveloped southern part of the rift zone. However, the lithospheric mantle of the Mozambique Belt and the more developed northern section of the rift is more resistive and does not contain metasomatic phases. Combined with geochemical data from erupted lavas, these results suggest that, in zones that have experienced voluminous Cenozoic magmatism, melting events have destroyed the metasomes and dehydrated the mantle. Since the presence of magma is a primary control of lithospheric strength, rifting may become limited as the lithospheric mantle becomes dehydrated and harder to melt. The motion of tectonic plates relies on a specific set of physical conditions. Continental breakup or rifting occurs when certain parts of the lithosphere are weak, and when stress applied to these regions is sufficient. Weaknesses in the lithosphere rely on its composition and pre-existing structures. We can image and analyze these features using the magnetotelluric method, a geophysical technique that maps electrical conductivity variations within the Earth. Our results show that compositionally weakening agents (metasomes) play an essential role in the development of the rift by making the mantle easier to melt. We also image some portions of the rift that do not contain such agents, suggesting that melts may have dried out these parts of the lithosphere, leaving a dry and resistive residue. This situation may indicate that melting in the region might be limited in the long run due to the absence of these materials. 3D magnetotelluric models of North Tanzanian Divergence are converted to water in mantle models to map metasomatism in the region Melting events in the Mozambique Belt caused metasomes to be destroyed and the lithospheric mantle to be dehydrated The rifting in the region might be limited if there is no supply of metasomatic material toward the rift zone

Before exploiting a geothermal resource in a volcanic setting such as the Asal rift, it is necessary to acquire a better knowledge of the subsoil, with the objective of locating the geothermal reservoir and evaluating the resource characteristic (permeability, temperature, etc.). For this type of resource, geophysical exploration methods are essential (such as gravimetry, magnetotellurics, etc.). However, a particular data type does not necessarily have the resolution and sensitivity. Furthermore, individual inversions of these geophysical data face the ambiguity of the non-uniqueness of the inverse solution. In this paper, we present a new linear approach of gravity data using the constraint of a MT resistivity model. We coupled the resistivity and density using inversion cross-gradients and the linear correlations. The approach was tested and validated on synthetic data and applied to gravity and MT data in the Asal Rift. Multiple inversions with different levels of coupling provided a series of density models. We applied the principal component analysis (PCA) technique to assess these models. We were able to define two dominant processes acting differently on the density and resistivity distribution at depth, namely the geothermal activity of the rift and the structural control of active tectonics. The inversion of geophysical data faces the problem of ambiguity or nonuniqueness. Constrained inversion can be an effective solution to reduce these ambiguities. Coupling several geophysical methods provides a priori information to help sample the space of acceptable models toward a reliable and realistic solution. In order to assess the resulting models, we use multivariate statistics. We present two types of coupling, cross-gradient and linear correlation between density and electrical resistivity. The approach was tested and validated on synthetic data and on data acquired in the Asal rift, Republic of Djibouti. The results suggest that both the geothermal activity of the rift and the structural control of active tectonics control the distribution of density and electrical resistivity. We developed a new constrained inversion using the cross-gradient and linear correlation couplings between density and resistivity The models resulting from these two inversions are synthesized using the principal component analysis method Resistivity and density models highlight that the Asal rift is dominated by more than one process (geothermal and tectonic)

Geophysical data integration covers a wide range of approaches, from visual interpretation of model presented side by side to sophistical statistical analyses such as automatic clustering. We present here a geophysical model integration based on principal component analysis (PCA), which allows to gain insight in a multivariable system. PCA for geophysical models integration define a new set of principal component (PC) models, distributed along new orthogonal axes by solving a eigenvalue problem. We show that PC component models patterns reflect different processes sensed by the geophysical observations. We applied this integration method to models obtained from constrained and joint inversion of gravity, ambient noise and MT data in the framework of unconventional geothermal exploration in Massif Central, France. PCA of the log-resistivity, the density contrast and the Vs velocity model has three independent components. The first one (PC1) representing 69 per cent of the total variance of the system is highly influenced by the parameter coupling enforced in the joint inversion process. PC1 allows to point to geophysical structures that may be related to the geothermal anomaly. The second component (PC2) represents 22 per cent of the total variance and is strongly correlated to the resistivity distribution. PC2 correlation to shallow fault structures suggests that it may be a marker of fracturing. The third component, PC3, accounts for 9 per cent of the total variance and is correlated with velocity structures and anticorrelated with density structures, respectively. The contribution of geophysical properties, mainly sensitive to the elastic properties of rock units, and the good agreement with shallow geological features make PC3 useful for the 3-D description of geological units. This statistical approach helps the interpretation of geophysical models into a limited number of geological processes, one possibly geothermal. Lithology may be derived from PC3, fracturing from PC2 and hydrothermal anomaly from PC1. Nevertheless PCA as a geophysical integration methodology is site-dependent and interpretation relies on a priori knowledge of the local geological mechanisms.

Three-dimensional geophysical exploration is essential in identifying favorable areas and derisking high-temperature geothermal projects. In particular, electrical resistivity imaging plays a key role in this exploration due to its sensitivity to the presence of alteration products, geothermal fluid circulation, and temperature. As 3D seismic exploration is rarely effective in volcanic environments, resistivity methods such as magnetotellurics (MTs) are often used to extract deep structural information. However, deep electromagnetic (EM) imaging in coastal areas of volcanic islands with MT can be challenging due to anthropogenic noise induced by urbanized areas concentrated around the coast. The use of active EM sources, such as airborne EM (AEM) and controlled-source EM (CSEM), as a complement to MT is a solution for overcoming anthropogenic noise. However, applying these methods in this context is still challenging due to the proximity to the sea/land interface, large variations in topography and nearshore bathymetry, and the heterogeneity of the near surface. Our approach outlines the challenges of acquiring, processing, and inverting nearshore and land 3D CSEM data in such complex environments. The CSEM data are part of a multimethod geothermal exploration on the island of Martinique in the French West Indies, and its 3D inversion result is compared with the previous regional 3D MT and AEM inversion results. In addition to MT, CSEM may highlight the location of a potential high-temperature paleo-reservoir at shallow depths under the urbanized area of Petite-Anse covered by a paleo-clay cap ranging from a few meters to several hundred meters in thickness. We conclude by proposing possible improvements for CSEM and multimethod methodologies to obtain more reliable exploratory models.

Seismic investigation in marine gas-bearing sediments often fails to get information below the acoustic mask created by free gas. To circumvent this problem, we combined collocated multichannel ultra-high resolution seismic imaging, marine electrical resistivity tomography and core sampling to study the physical properties of gas-bearing sediments in the Bay of Concarneau (France). We obtained sections of compression (P-) wave velocity (VP${V_P}$) from the multichannel processing and 2D resistivity models from the marine electrical resistivity tomography data inversion. We observed low resistivity (similar to 0.5 ohm center dot m) and low VP${V_P}$ (similar to 1200 m/s) values where free gas was identified in the seismic data. We tested a joint processing workflow combining the 1D inversion of the marine electrical resistivity tomography data with the 2D P-wave velocity through a structural coupling between resistivity and velocity. We obtained a series of 2D resistivity models fitting the data whilst in agreement with the VP${V_P}$ data. The resulting models showed the continuity of the geological units below the acoustic gas fronts, which are associated with paleo-valley sediment infilling. We were able to demonstrate relationships between resistivity and velocity differing from superficial to deeper sediments. We established these relationships at the geophysical scale and then compared the results to data from core sampling (VP${V_P}$ and porosity). We inferred the porosity distribution from the marine electrical resistivity tomography data. At the core locations, we observed a good agreement between this geophysical scale porosity and the core data both within and outside the gas-bearing sediments. This agreement demonstrated that resistivity could be used as a proxy for porosity where no VP${V_P}$ was available below gas caps. In these regions, the observed low resistivity showed a high porosity (60%-70%) down to about 10-20 m in depth, in contrast with the surrounding medium that has a porosity of less than 55%. These results support the hypothesis that failures inside the paleo-valley sediment could control the gas migration.

Summary Within the framework of the development of the exploration of geothermal energy resources in coastal areas, we tested the importance of nearshore magnetotelluric (MT) prospection at the land-sea interface (less than 1 km from the coast). Indeed, the current exploration methods in geothermal, in general exclusively terrestrial, do not allow to extend the knowledge of the environments for the evaluation of the energy resource, the coast being the physical limit of the methods used. The objective of these explorations is to better understand the volumetric extent of the resource and in particular its possible extension beyond the coast to optimize the positioning of geothermal drilling and increase the production of these renewable energies. In all coastal geothermal regions (seas but also lakes), without information beyond the terrestrial domain, subsoil models are poorly constrained and the resource poorly evaluated. We deployed new marine MT systems designed for shallow water and light deployments in Mayotte (Comoros Archipelago) and Guadeloupe (French West Indies) to investigate the impact of offshore MT sites on the resistivity distribution in depth.

ABSTRACTSubmarine karstic environments are complex and challenging to study. Seismic investigations usually have difficulty getting geological information because of a lack of penetration due to the high reflectivity of the calcareous substratum. To circumvent this problem, we studied how to combine marine electrical resistivity tomography (MERT) with geotechnical data to investigate the porosity structure from the geotechnical to the geophysical scale. We applied the technique to the submarine karstic plateau of Banc de Guérande (Saint‐Nazaire, France), which is mainly composed of hard calcarenite and sandy pockets. We obtained sections of two‐dimensional resistivity models from the MERT data inversion. We used existing geotechnical data on extracted cores at several boreholes close to the MERT profiles using a multi‐sensor core logging (MSCL) bench. We used porosity proxies derived from Archie's law and porosity data from the MSCL inferred from gamma density measurements on the core to combine the data of very different scales (metre for MERT and centimetre for MSCL). The comparison between measurements showed a good similarity between in situ MERT and borehole MSCL data at depths greater than ∼10 m below the seafloor. A larger difference was observed close to the seabed, where the MERT porosity was higher than the MSCL porosity. The extraction of water‐saturated cores and the numerous core fractures could explain this difference near the surface. The results were analysed with respect to the scale difference between geophysical and geotechnical data. The conclusions suggested that the difference between MERT and MSCL porosities could be testified from the local heterogeneity of the soil and indicated whether the surrounding substratum was more porous (and thus fractured or dissolved) than the core or vice versa. The study highlighted the necessity of an excellent collocation of the data to retrieve reliable information from the comparison between geophysical and geotechnical data.

The axial fault-bounded depression of the South Kenya rift (SKR) locally displays anomalously wide sectors resulting from the presence of one (or many) elevated and offset block(s) on the flanks of the main trough. Very little attention has been paid so far to the nature of the driving mechanisms responsible for these atypical rift patterns. New insights are supplied by the Natron-Ol Doinyo Ogol rift segment at the southern extremity of the SKR, immediately north of the North Tanzanian Divergence (NTD). On the basis of interpreted SRTM-30 satellite imagery and Digital Elevation Models, our work allows us: i) to depict the highly-segmented arrangement of the similar to 7 Ma-lasted SKR system, ii) to establish a two-stage kinematic rift model that emphasizes the role of an inherited transverse discontinuity on the arrest, as well as lateral jump and off-axis development of anomalously-propagating rift structures, iii) to define the relative contribution of border vs inner fault networks to the total extension, which is estimated at 7-6 km (11.6-9.2%), and iv) to emphasize that inner faulting was not the dominant mode of strain accommodation during recent inward focussing of strain, and that no sharp transition exists from border fault- to intra-rift fault-dominated strain accommodation over time in the SKR immature rift system.

To obtain the fullest picture of geothermal systems, it is necessary to integrate different types of data, for example, surface electromagnetic surveys, lithology, geochemistry, and temperature logs. Here, by joint modeling a multichannel data set we quantify the spatial distribution of heat transfer through the hydrothermally altered, impermeable smectite layer that has developed atop the Wairakei‐Tauhara system, New Zealand. Our approach involves first constraining magnetotelluric inversion models with methylene blue analysis (an indicator of conductive clay) and mapping these onto temperature and lithology data from geothermal wells. Then, one‐dimensional models are fitted to the temperature data to estimate heat flux variations across the field. As a result, we have been able to map the primary seal that insulates the geothermal reservoir and estimate the heat flow of the system. The approach could be applied in geothermal provinces around the world with implications for sustainable resource management and our understanding of these magmatic systems.

Electromagnetic geophysical exploration plays a key role in high-temperature geothermal projects to estimate the geothermal potential of a region. The objective of an EM campaign applied to high-temperature geothermal exploration is to obtain an image of the impermeable clay cap, the permeable geothermal reservoir, and the system's heat source at depth, as these three components of the overall geothermal system have distinct electrical signatures. However, deep electromagnetic imaging in the coastal areas of volcanic islands represents a major challenge due to the presence of strong cultural noise induced by urbanized areas concentrated around the coast, the proximity to the sea, strong variations of topography and bathymetry, the small size of targets and the heterogeneity of the near surface. Our objective is the multi-scale integration of airborne transient electromagnetism (ATEM), shallow marine and in land magnetotelluric (MT) and controlled source electromagnetism (CSEM) to improve the reconstruction of deep geological structures by inversion. The contribution of the CSEM method is the key to overcoming cultural electromagnetic noise and exploiting data acquired in urbanized areas. In order to study how to integrate the different EM data, we first apply our methodology to data from a geothermal exploration campaign carried out a few years ago in Martinique in the French West Indies. Then, we present results from runs with synthetic tests for a campaign planned this year in Guadeloupe, also in the French West Indie, whose objective is to increase the production capacity of an existing geothermal field.

A GEOMAR (Kiel, Germany) research team has developed a passive electric field acquisition system for Autonomous Underwater Vehicles (AUVs) to optimize seafloor massive sulfides exploration. This sensor was made of two perpendicular and horizontal pairs of electrodes, and was successfully tested over active basalt‐hosted hydrothermal site TAG (26°N, Mid‐Atlantic Ridge) and several inactive sites in its vicinity. The resulting data underline the efficiency of combining deep‐sea electric and magnetic measurements for searching for active and inactive hydrothermal vent fields. With these datasets, it becomes possible to determine the geological nature of the targets and to constrain the characteristics of fluid circulation at depth without involving costly and invasive underwater tools such as Remotely Operated Vehicles or even manned submersibles to collect samples. Data analysis also revealed that AUV attitude variations induce distortions of the electric signal. These distortions start prevailing for dives at altitudes higher than 90 m above the seafloor, as the distance between the AUV becomes too important to guarantee that the signal produced by the geological target still dominates. To improve the acquisition system and reduce the overall noise, we discuss solutions that limit the impact of such attitude variations. These solutions consist of minor adjustments, such as masts at AUVs stern to tow damping electrodes arrays. In such configurations, we believe that deep‐sea passive electric measurements combined with high‐resolution magnetic measurements can become a highly efficient seafloor exploration tool, including for sulfide deposits associated with inactive hydrothermal systems.

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.

A major seismovolcanic crisis has afflicted the islands of Mayotte, Comoros Archipelago, since May 2018, although the origin is debated. Magnetotellurics (MT), which is sensitive to hydrothermal and/or magmatic fluids and can map the subsurface electrical resistivity structure, can provide insight by revealing the internal structure of the volcanic system. In this paper, we report the results of a preliminary land and shallow marine MT survey performed on and offshore the island nearest the crisis. The 3D inversion-derived electrical resistivity model suggests that the island is underlain by a shallow ~500-m-thick conductive layer atop a deeper, more resistive layer, possibly associated with a high-temperature geothermal system. At depths of ~15 km, the resistivity drops by almost two orders of magnitude, possibly due to partial melting. Further petrophysical and geophysical studies are underway for confirmation and to map the geometry and evolution of the volcanic system.

Since May 2018, the Mayotte Island (Comoros archipelago) is ongoing the largest basaltic eruption of the three last centuries, with up to several km3 deduced from modeling and direct seafloor observations. During this volcano tectonic crisis, we performed a land and shallow marine Magnetotelluric (MT) survey on the island the closest to the new volcano. Initially designed for shallow geothermal exploration (<2km depth), we extended the duration of the measurements to perform deep MT soundings (>10km depth) and get some insight into the geo-electric structure of the Mayotte island. The analysis of the MT data shows a deep geo-electrical anisotropy in the W-NW E-SE direction that is coherent with the expected orientation of the oceanic ridge between the Somalian and the Lwandle plate. Additionally, the 3D inversion of the data shows that a massive conductive body is present at great depth (>15km), possibly related to the presence of partial melt. Interestingly, this conductor seems to become shallower in the direction of the new volcano. After the survey, we installed two permanent MT stations in Petite Terre and Grande Terre islands to monitor possible time-lapse conductive anomaly related to fluid migration. We will show the results and discuss the Time Lapse MT strategy, challenges and observations.