Magnetotelluric Research Articles

Comparative 3D inversion of magnetotelluric phase tensors and impedances reveals electrically anisotropic base of Gawler Craton, South Australia

Isotropic three-dimensional (3-D) inversion has become a standard tool in the interpretation of magnetotelluric (MT) data. 3-D anisotropic inversion codes are under development, yet the number of unknowns increases by a factor of 6 rendering the problem extremely ill-posed. The presence of anisotropy is usually inferred from (i) spurious sequences of conductive and resistive bodies or (ii) comparison with two-dimensional anisotropic modelling approaches. Here, we investigate the 3-D structure of the Gawler Craton down to ∼250 km depth using 282 sites of the AusLAMP array located in the southern half of South Australia. Inversions of the MT impedance as phase tensors and real and imaginary parts result in diverging structures at depths > 70 km. We demonstrate that a unifying model that explains all data types similarly well is suggestive of an anisotropic resistivity structure at the base of the Gawler Craton lithosphere at depths of 120–210 km. Depth location and orientation of the anisotropy agree well with results from the analysis of seismic receiver functions. We suggest that electric anisotropy in the Gawler Craton is a result of lattice-preferred orientation of olivine crystals and metasomatic processes with macroscopic preferential orientation. Our results illustrate that inversion of phase tensor data is superior for the direct imaging of anisotropic resistivity contrasts in otherwise isotropic resistivity models; inversion models obtained with impedances may miss such structures. “Comparable” overall RMS misfits are often meaningless when comparing inversion results for various data types since sensitivities differ between data types. Reliable inversion results consistent with the entire data set can only be recovered if data fits are assessed systematically for all data representations. We also discuss the influence of error settings for phase tensors on the inversion. Our study also revealed that, if persistent across large areas, (i) parallel orientation of phase tensor major axes, (ii) constantly high phase tensor maximum phases or (iii) diverging directions of phase tensor major axes and induction arrows are suggestive of anisotropic structures and corresponding hypotheses should be evaluated.

Metasomatism and depletion of the southern Gawler Craton from combined mantle xenocryst and AusLAMP magnetotelluric data

Metasomatised mantle is considered a prerequisite for the formation of many major ore deposits. Since magnetotellurics (MT) is strongly sensitive to mantle metasomatism, MT is a useful tool for analysis of mineral systems that incorporate a metasomatised mantle as a key ingredient. In this work, we combine recently reported kimberlite-hosted mantle xenocryst data from the southern Gawler Craton and Adelaide Superbasin with AusLAMP MT models. We find good correlations between mantle MT conductors and geochemical indicators of mantle metasomatism from clinopyroxene xenocrysts, with the most metasomatically enriched lithospheric mantle beneath the Adelaide Superbasin and more depleted mantle beneath the Gawler Craton. There is a significant step in lithospheric thickness across the Adelaide Superbasin, and particularly around the Terowie kimberlite cluster. Combined with the fertile mantle, this helps explain this region’s prospectivity for sediment-hosted copper deposits. High mantle resistivities and depleted geochemical compositions towards the central Gawler Craton on and north of the Eyre Peninsula are consistent with melting of previously metasomatised mantle, including during production of the Gawler Range Volcanics and Hiltaba Suite Large Igneous Province. Most of the major iron oxide copper gold (IOCG) and gold deposits in the region lie on the edge of this zone of mantle melting, consistent with mineral systems models that require melting of previously metasomatised mantle in the generation of these deposits.

Deep electrical imaging of the extinct oceanic ridge in the southwestern sub-basin of the South China Sea

The resistivity structure of an extinct mid-ocean ridge is significant in understanding the evolution of a mid-ocean ridge from its spreading phase to its dying phase. The magnetotelluric (MT) method is a crucial tool in studying the deep resistivity structure as it is sensitive to resistivity which is affected by heat and allows for imaging of the electrical properties of the mantle. While modern electromagnetic data has enhanced our understanding of the deep structure of rapidly expanding and ultra-slow expanding mid-ocean ridges, the deep electrical structure below extinct mid-ocean ridges has not been studied extensively.In July 2020, marine MT instruments were deployed in the southwest subbasin of the South China Sea to study the resistivity structure below a stalled mid-ocean ridge. The study found that the imaged thickness of the lithospheric lid (>100Ωm) varies between 20 and 90 km, exhibiting a positive correlation with its age. The melt ascent channel is closed below the stalled mid-ocean ridge, and the melt falls back and forms a small melt trap below the dead mid-ocean ridge. In the northwest survey line of oceanic ridge, huge low-resistivity anomalies (<1Ωm), located between 80km and 160 km depth. In the southeast survey line of oceanic ridge, there is a slightly smaller low-resistivity anomalies (<1Ωm). These results indicate that partial melt continues to exist after the cessation of spreading at the mid-ocean ridge. The lithosphere-asthenosphere boundary (LAB) and the ocean basin of the South China Sea experienced a certain magmatic transformation during the cooling and falling process. According to the resistivity, temperature, pressure, and SEO3 model, the melt content is estimated to be approximately 1-12%. The electrical structure of the mantle in the South China Sea is an important basis for studying the current state beneath the ceased spreading mid-ocean ridge.

Comparison of different geoelectric field methods to calculate geomagnetically induced currents in North China

Geomagnetically induced currents (GICs) are caused by the interaction between the geomagnetic storm and the Earth's conductivity structure, which can enter the power grid and cause blackouts in extreme cases. To avoid significant economic losses, it is essential to correctly estimate the intensity of GICs first and then take corresponding protection measures to the power grid. There are many methods to carry out this estimation, of which the results can be very different. To evaluate these differences, four estimates of GICs for the 1000-kV grid in North China were compared during the geomagnetic storms that occurred from November 7 to 9, 2004, based on the conductivity models and magnetotelluric (MT) data from previous studies. These four estimations include using field MT data (data-space method), a one-dimensional (1D) conductivity model (1D model-space method), and two 3D conductivity models (3D model-space method) in the same area from different research teams. In consideration of the complex topology of the power system and the interactions between different power lines, the estimation using field MT data obtains the highest accuracy. Those using the 3D conductivity models rank the second and the one using the 1D conductivity model ranks the third. Among all, the data-space method is sensitive to the so-called corner effect in some transformers, which infers that this method provides the most correct ground current path information. The crustal structures of the two 3D conductivity models are slightly different. This slight difference does not bring any troubles to the final geophysical purpose but leads to a butterfly effect in the estimation of GICs, causing the difference between the estimation results based on the two models to an extreme value of 267.2 A. Based on the statistical analysis of the maximum intensity and duration of GICs in 20 substations, we believe that the GICs of the North China 1000-kV power grid have reached a very high-risk level, and the assessment results are 2 ∼ 3 times higher than other Kp = 8 geomagnetic storm events. Previous studies have shown that there are four conductors in the North China region, namely C1, C2, C3, and C4. After analyzing these conductors, we have concluded that C3 and C4 have a greater impact on the geoelectric field and GICs. Ultimately, it has furnished advantageous guidance for the installation of GICs monitoring apparatus and the discernment of optimal substation siting.

AusLAMP shines a light on space weather hazards in the Australian high-voltage power grid

A geomagnetic storm, also known as a geomagnetic disturbance (GMD), is a major disturbance of the Earth’s magnetic field caused by solar activity. A geomagnetic storm induces electric currents in the Earth that feed into power lines through substation neutral earthing, causing instabilities and even blackouts in electricity transmission systems. The intensity of geomagnetically induced currents (GICs) is closely associated with the electrical conductivity of the surrounding geology. In this paper, we analyse one of the most well-known geomagnetic storms, the 1989 “Québec storm” and 688 magnetotelluric (MT) survey sites from the Australian Lithospheric Architecture Magnetotelluric Project (AusLAMP) to gain insight into the space weather hazard posed for Australia's modern-day power grids. Transmission lines may exhibit local maxima at differing times depending on their spatial orientation and length with respect to the time-varying magnetic field. Localised peak voltages over 100 V can be observed on some individual lines. This assessment identifies the distribution of GICs in south-eastern Australia for the 1989 Québec storm and transmission lines that are more vulnerable to GICs. It is relevant to national strategies and risk assessment procedures to mitigate space weather hazards in the Australian high-voltage power grid and the design of a more resilient power transmission system. We also analyse the 2015 “St Patrick’s Day storm” to study under-estimation of the space weather hazard associated with the band-limited geomagnetic data and MT data sets. Key points The subsurface geology has a great influence on the intensity of geomagnetically induced electric fields, potentially causing up to three orders of magnitude difference between conductive basins and resistive cratonic regions in south-eastern Australia. Analysis using the 1989 “Québec geomagnetic storm” and AusLAMP magnetotelluric data shows the intensity of the geoelectric fields in south-eastern Australia could reach up to 5 [V/km]. Geomagnetically induced voltages in the Australian high-voltage power grid could be in excess of 100 V in some transmission lines for a geomagnetic storm with intensity comparable with the 1989 Québec geomagnetic storm.

Magnetotelluric insights into the formation and reactivation of trans-crustal shear zones in Precambrian basement of the eastern U.S. Midcontinent

Three-dimensional inversion of regional long-period magnetotelluric (MT) data reveals the presence of two distinct sets of high-conductivity belts in the Precambrian basement of the eastern U.S. Midcontinent. One set, beneath Missouri, Illinois, Indiana, and western Ohio, is defined by northwest−southeast-oriented conductivity structures; the other set, beneath Kentucky, West Virginia, western Virginia, and eastern Ohio, includes structures that are generally oriented northeast−southwest. The northwest-trending belts occur mainly in Paleoproterozoic crust, and we suggest that their high conductivity values are due to graphite precipitated within trans-crustal shear zones from intrusion-related CO2-rich fluids. Our MT inversion results indicate that some of these structures dip steeply through the crust and intersect the Moho, which supports an interpretation that the shear zones originated as “leaky” transcurrent faults or transforms during the late Paleoproterozoic or the early Mesoproterozoic. The northeast-trending belts are associated with Grenvillian orogenesis and also potentially with Iapetan rifting, although further work is needed to verify the latter possibility. We interpret the different geographic positions of these two sets of conductivity belts as reflecting differences in origin and/or crustal rheology, with the northwest-trending belts largely confined to older, stable, pre-Grenville cratonic Laurentia, and the northeast-trending belts largely having formed in younger, weaker marginal crust. Notably, these high-conductivity zones spatially correlate with Midcontinent fault-and-fold zones that affect Phanerozoic strata. Stratigraphic evidence indicates that Midcontinent fault-and-fold zones were particularly active during Phanerozoic orogenic events, and some remain seismically active today, so the associated high-conductivity belts likely represent long-lived weaknesses that transect the crust.

Tectonic and metallogenic implications of a metasomatized mantle inferred from magnetotellurics and gravity data: An example of Tapajós Mineral Province, Amazon Craton

The Tapajós Mineral Province (TMP) contains evidence of subduction processes during the Paleoproterozoic. Despite well-documented arc magmatism, the lack of solid geological evidence indicating a collision raises uncertainties about whether there was an arc-continent collision in the Paleoproterozoic or the process of subduction continued, and the closing of an ocean basin occurred only in Mesoproterozoic times. Additionally, world-class deposits have been linked with lithospheric electrical conductors using magnetotelluric (MT) data. On the lithospheric scale, an assessment of the potential for more significant deposits or correlated commodities is absent in the TMP. Therefore, we acquired MT data to provide constraints for the lithosphere of the TMP. The electrical resistivity model reveals a high resistivity zone (> 10,000 Ω.m) interpreted as the emplacement zone of granitoids in the upper crust. The mid-lower crust presents resistivity between 465 and 45 Ω.m, corresponding to depths of 10–35 km. A low resistivity zone below 35 km (45–10 Ω.m) represents a hydrated upper mantle due to the long-lived arc processes. Based on gravity-forward modeling, we estimate that the degree of serpentinization is <10% due to post-subduction magmatism. The lack of a lower crust conductor and paired high and low long wavelength Bouguer anomalies corroborates that a cryptic suture is unlikely. A low-angle conductor matches the limit of different magnetic domains and reveals the exhumation of the western portion of TMP's crust. The high correlation between the position of the deposits and electrical conductors corroborates that MT is a valuable tool for detecting fossil fluid pathways relevant to finding new mineral deposits.

Deep electrical resistivity imaging of crustal structures along the Satluj River valley, northwest Himalaya, India

Magnetotelluric (MT) investigations along the Nahan- Kourick Chango transect in the Satluj river valley, northwest Himalayas, provide insight into the subsurface electrical properties. In the geoelectrical model, medium to highly metamorphosed crystalline rocks in the Lesser and the Higher Himalayas are observed as high resistive features. At a depth approximately 5–15 km, an intra-crustal low resistive layer is consistently present along the entire profile. The geometry of this dipping intra-crustal layer signifies the presence of the Main Himalayan Thrust, which represents the base of the accretionary wedge where all the major thrusts of the Himalayan tectonics sole down. The geometrical differences and distinct resistivity of this layer compared to the adjacent regions of Garhwal-Kumaon and Nepal Himalaya indicate the variability of tectonic processes in this part of the Himalaya. Another significant observation is the existence of a thin, highly conducting layer at a shallow depth (approximately 1–3 km). The spatial extent of this layer, along with the bending extent of the main central thrust toward the south, suggests its role in the southward translation of crystalline sheets during Himalayan tectonics.

Influence of Tectonic and Geological Structure on GIC in Southern South Island, New Zealand

AbstractAs part of a 5‐year project to assess the risk posed by geomagnetically induced currents (GIC) to the New Zealand electrical transmission network, long‐period magnetotelluric (MT) measurements have been made at 62 sites in southern South Island of New Zealand, a region where there was an absence of previous MT data. The data are largely 3‐dimensional in character, but show distinct features that can be related to the known tectonic and geological structure. In this work we focus on how the measured MT impedance tensors, and a simple interpretation of conductivity structure, can be used to assess the influence of tectonic and geological structure on GIC. We use the impedance tensors to calculate the magnitudes and orientations of induced electric fields in response to various orientations of inducing magnetic field. The electric fields so calculated are then used in a simplified model of the transmission network to calculate GIC at grounded substations. Our results confirm that tectonic/geological structure in the lower South Island and the resulting electrical conductivity variations have important impacts on the GIC magnitude. In the south‐west, smaller induced electric fields, associated with the higher conductivity in that region, lead to much reduced GIC at a substation in that area. In contrast, higher electric fields occurring in a NW‐SE band across the center of the region, contribute to much larger GIC in Dunedin city. Our results thus help explain the observed GIC reported at transformers in the region.

Geoelectric characterisation of the junction of seismically active Delhi Hardwar Ridge and Delhi Sargodha Ridge

A magnetotelluric (MT) geophysical survey for the first time has been conducted for the geoelectric characterization of the junction of the contact zone of NNE-SSW striking Delhi Hardwar Ridge (DHR) and NW–SE trending Delhi Sargodha Ridge (DSR) in the Rohtak area, Haryana which has experienced 15 earthquakes of M2.0–M4.4 from April to August 2020. A total of 08 MT sites are acquired along a NW–SE profile of length 50 km. From the 2D MT data inversion, the DHR and DSR are for the first time characterized by equal values of moderate resistivity of 100 Ohm m at two depths. The resistivity variation for DHR corresponds to 100 Ohm m from the surface to the depth of 20 km, whilst DSR is found associated with the same value of resistivity extending in the NW direction. The DHR has been found striking NE-SW with a very shallow central axis (less than 400 m) having a width of 12–15 km forming half grabens on both limbs supported by shallow faults. The DSR has been found bifurcated from DHR at a depth of 12–13 km and extended in the NW direction. The DSR has been generated due to flexure bulging caused by collision and anticlockwise rotation of the Indian plate in the Eocene period. A NE striking steep dipping reverse fault (F1) has also been identified about 15 km west of the DHR. It is inferred that the DSR got upthrusted along this fault and became shallower in the NW region. The seismicity in the Rohtak and surroundings is located at the bifurcation points of DHR and DSR and the contact zone of DSR and reverse fault F1. The reverse fault F1 is also active and has generated microseismicity in the past.

Data processing method for magnetotelluric sounding based on cepstral analysis

Magnetotelluric (MT) signals exhibit the characteristics of being weak and having a wide frequency band. The acquired field data are susceptible to various types of noise, which poses challenges in accurate identification and processing. Currently, there exist numerous MT data processing methods; however, they lack efficiency and physical meaning. To address this issue and improve the signal-to-noise ratio of the acquired data, this study proposes a MT data processing method based on cepstral analysis. By employing cepstral analysis on the MT data, the cepstrum is obtained, and an appropriate truncation position is selected for processing. The experimental results demonstrate that this method obtains smoother and more continuous apparent resistivity curves with fewer errors. Compared with other methods, the cepstral analysis method can effectively suppress different types of MT noise, and the method is simple and efficient with clear physical significance.

Fast 3D simulation of magnetotelluric data in anisotropic media using a rational Krylov method

This study introduces a computational method aimed at accelerating multifrequency 3D magnetotelluric (MT) forward modeling in 3D conductivity structures with general anisotropy. The approach combines an edge-based finite-element (FE) method with a rational Krylov subspace method. In this method, the MT source term is expressed as a planar current source, and the frequency-dependent electric field response is the product of a transfer function and a constant vector of the current source. Consequently, the rapid approximate electric field for multifrequency calculations can be obtained by constructing the orthogonal basis of the rational Krylov subspace. By applying this technique, the large sparse matrix resulting from FE discretization can be projected onto a much lower-order matrix, significantly reducing its size to tens or hundreds. Incorporating the direct solver PARDISO to construct the orthogonal basis enables a considerably faster solution compared with the previous FE method. Moreover, the algorithm is implemented using Julia, a high-level programming language known for its readability, maintainability, and extensibility. The validity of the algorithm is demonstrated through a 1D anisotropic model, and two additional 3D models are designed to further evaluate the applicability and effectiveness of our algorithm. Numerical experiments illustrate a notable increase in modeling speed compared with the traditional edge-based FE method.

Feature-based magnetotelluric inversion by variational autoencoder using a subdomain encoding scheme

Magnetotelluric (MT) data inversion aims to reconstruct a subsurface resistivity model that minimizes the discrepancy between inverted and measured electromagnetic data. Conventional pixel-based minimum-structure inversion often yields a smoothed-out reconstruction with a relatively low resolution. A priori geophysical knowledge can be embedded into inversion and improve the reconstruction resolution through proper reparameterization. However, existing reparameterization approaches, such as model-based and parametric transform-based inversion, have limited ability to incorporate various a priori information. The effectiveness of existing deep generative model-based inversion algorithms is still debatable when applied to scenarios with complex backgrounds. We develop a feature-based MT data inversion method based on a variational autoencoder (VAE) with a subdomain encoding scheme. Instead of encoding the entire domain of an investigation, we adopt a 1D subdomain encoding scheme to encode the 1D resistivity-depth models using a single VAE. The latent variables for the 2D model are a combination of the latent variables for 1D models, and the encoded region of interest (ROI) can be flexibly determined. The latent variables of ROI and the pixels outside the ROI are simultaneously inverted using the gradient-descent method. Our 1D subdomain encoding scheme reduces the complexity and diversity of the data set, and it can flexibly embed a priori knowledge with various uncertainties. Synthetic data inversion and inversion of the Southern African Magnetotelluric Experiment field data validate our method’s ability to effectively improve inversion accuracy and resolution.

Geothermal Resource Mapping in Northern Botswana Inferred from Three-Dimensional Magnetotelluric Inversion

A set of magnetotelluric (MT) data collected from 28 stations at the Kasane Hot Spring in northern Botswana was used to derive a 3-D model of the electrical resistivity distribution around deep geologic structures that we associated with geothermal resources. The dimensionality analysis results revealed that 3-D modeling should be used to determine the electrical structure. The resistivity models revealed a conductive layer with an average thickness of 200 m, representing the overlying sediments of the Proterozoic volcanic rocks. The thick high-resistivity zone (&gt;100 Ωm) below the conductive layer can be associated with Mesoproterozoic bedrock. The MT measurements in this area show a tube-shaped conductive anomaly that could serve as a fluid pathway feeding the hot spring. A fracture-controlled meteoric fluid circulation presumably determines the existence of the Kasane Hot Spring system.

Application of Electromagnetic Methods for Reservoir Monitoring with Emphasis on Carbon Capture, Utilization, and Storage

The Controlled-Source ElectroMagnetic (CSEM) method provides crucial information about reservoir fluids and their spatial distribution. Carbon dioxide (CO2) storage, enhanced oil recovery (EOR), geothermal exploration, and lithium exploration are ideal applications for the CSEM method. The versatility of CSEM permits its customization to specific reservoir objectives by selecting the appropriate components of a multi-component system. To effectively tailor the CSEM approach, it is essential to determine whether the primary target reservoir is resistive or conductive. This task is relatively straightforward in CO2 monitoring, where the injected fluid is resistive. However, for scenarios involving brine-saturated (water-wet) or oil-wet (carbon capture, utilization, and storage—CCUS) reservoirs, consideration must also be given to conductive reservoir components. The optimization of data acquisition before the survey involves analyzing target parameters and the sensitivity of multi-component CSEM. This optimization process typically includes on-site noise measurements and 3D anisotropic modeling. Based on our experience, subsequent surveys tend to proceed smoothly, yielding robust measurements that align with scientific objectives. Other critical aspects to be considered are using magnetotelluric (MT) measurements to define the overall background resistivities and integrating real-time quality assurance during data acquisition with 3D modeling. This integration allows the fine tuning of acquisition parameters such as acquisition time and necessary repeats. As a result, data can be examined in real-time to assess subsurface information content while the acquisition is ongoing. Consequently, high-quality data sets are usually obtained for subsequent processing and initial interpretation with minimal user intervention. The implementation of sensitivity analysis during the inversion process plays a pivotal role in ensuring that the acquired data accurately respond to the target reservoirs’ expected depth range. To elucidate these concepts, we present an illustrative example from a CO2 storage site in North Dakota, USA, wherein the long-offset transient electromagnetic method (LOTEM), a variation of the CSEM method, and the MT method were utilized. This example showcases how surface measurements attain appropriately upscaled log-scale sensitivity. Furthermore, the sensitivity of the CSEM and MT methods was examined in other case histories, where the target reservoirs exhibited conductive properties, such as those encountered in enhanced oil recovery (EOR), geothermal, and lithium exploration applications. The same equipment specifications were utilized for CSEM and MT surveys across all case studies.

Three-dimensional magnetotelluric imaging of the Edough massif and its overlying formations (Northeast Algeria)

The Edough massif (NE Algeria) is the easternmost coastal crystalline massif of the Maghrebide belt, whose current structural position remains poorly known. Magnetotelluric (MT) data from 15 stations were collected along three profiles across the Edough massif and its neighboring formations in the period range of 0.001–1000 s to assess the deep electrical resistivity structure. A phase tensor dimensionality analysis indicates that the measured data are dominated by 3-D effect of underlying geological structure, thus revealing a complex subsurface structure. A 3-D inversion model was obtained using the full impedance tensor and the vertical magnetic field transfer function including bathymetry and topography. Our preferred 3-D geo-electrical model shows heterogeneous electrical structures and are related to the known geological features of the region. We mainly observe a clear resistivity contrast characterizing the upper/middle crust. The Edough massif appears generally as a homogenous resistive structure. The shallow conductive structures are associated with the Mio-Plio-Quaternary deposits and Flysch units. Most of conductive bodies are likely related to the iron/magnetic anomaly. These may likely correspond to the suture zone. Under the Neogene basin, the high resistivities with a north dipping strong angle may correspond to the African basement.

Electrical Response of a Reactivated Ancient Suture Revealed by Three‐Dimensional Magnetotelluric Inversion Across the Jinsha River Suture, Northern Tibetan Plateau

AbstractThe northern Tibetan Plateau offers an excellent opportunity to investigate continental collision involving the Indian and Asian plates in this remote, high‐altitude region. However, the deep structure of the region remains incomplete and controversial, which limits our understanding of the processes of continental convergence. This study presents a new 350‐km‐long magnetotelluric (MT) profile across the Northern Qiangtang and Hoh Xil terranes in the western part of the northern Tibetan Plateau to elucidate the lithospheric structure of the reactivated Jinsha River suture. We carry out a three‐dimensional inversion to acquire the electrical structure beneath the Jinsha River suture, and reveal the presence of large‐scale conductive anomalies in the middle‐lower crust of the Northern Qiangtang and Hoh Xil terranes. The high‐conductivity anomaly beneath the Jinsha River suture extends to &gt;80 km depth and is coincident with a zone of high melt fraction (&gt;5%) in the middle‐lower crust beneath the Jinsha River suture, thereby indicating that this suture has become a rheological channel. The convergence of Indian and Asian lithosphere beneath the northern Tibetan Plateau may be the driving mechanism for the reactivation of this ancient suture. Continental subduction leads to enhanced convection in the mantle. This reactivated suture, which is a tectonically weak zone, provides a conduit for the upwelling of hot mantle material to result in the subsequent melting of the middle‐lower crust. Moreover, our findings provide valuable insights into the geodynamic evolution of the northern Tibetan Plateau, as well as contribute to our understanding of continental convergence processes.

Role of fluid on seismicity of an intra-plate earthquake zone in Western India: an electrical fingerprint from magnetotelluric study

The magnetotelluric (MT) investigation carried out in Koyna Seismogenic Zone (KSZ), an intra-plate earthquake region in Western India, along an E–W profile brings out moderately conductive (~ 700–1000 Ωm) near vertical features within the very high resistive (> 20,000 Ωm) granite/granite-gneiss basement. Occurrences of these anomalous moderate conductors are corroborated with sensitivity analysis. The alignment of earthquake hypocenters along the resistive–conductive boundary signifies the moderate conductor as basement fault. The conversion of resistivity values to the ratio of seismic P- to S-wave velocity (vp/vs) suggests that the moderate conductivity of the fault zone (as compared to the surrounding basement) appears due to the presence of fluid in the fault zone. Geophysical evidences reveal ~ 2.5–3.6 vol% fluid in the fault zone with ~ 1.8–2.6% interconnected porosity, which migrates along the structural boundary and invades the mechanically strong basement to nucleate the brittle failure within it. The present study proposes two mechanisms for the seismicity in the Koyna region. First: the meteoric water circulation due to the loading–unloading of nearby Koyna reservoir acts as potential fluid source for this triggered seismicity, which has also been suggested by previous studies. Second: the fluid circulation due to a deep-seated source. The present MT study brings out a conductive feature below 20 km depth which is thought to be emerged due to the dehydration of amphibole bearing rocks. The fluid generated from dehydration might act as a probable source to the triggered seismicity; since the conductive feature has a linkage to the upper crust.Graphical

Volcanic monitoring of the 2021 La Palma eruption using long-period magnetotelluric data

Between September and December 2021, the first subaerial volcanic eruption in the Canary Islands in 50 years took place on the island of La Palma. Since November 2021, we have been conducting a long-period magnetotelluric (MT) monitoring experiment at a site located 2.4 km east of the volcanic cone. Having continuously recorded data since then, the obtained dataset shows significant changes in resistivity over the fourteen months following the eruption: more than ± 20% in apparent resistivity and ± 2 degrees in phase. These temporal variations in electrical resistivity, recorded continuously using long-period MT during both the syn- and post-eruptive stages, have not been reported to date, making this dataset unique. Four estimated impedances have been selected as representatives of the major temporal changes observed and inverted to generate new 3-D resistivity models. The results provide novel key information on the spatiotemporal evolution of the subsoil's electrical resistivity, enabling the characterization of a set of structures acting as preferred magmatic fluid pathways. Therefore, our study highlights the strong potential of MT as a volcanic monitoring tool and provides new insights about the evolution of the fluid pathways during the post-eruptive stage. These findings enhance our understanding of the magmatic system and may contribute to volcanic hazard mitigation in the future.

Seismic-electromagnetic projection attribute: Application in integrating seismic quantitative interpretation and 3D controlled-source electromagnetic-magnetotelluric broadband data inversion for robust ranking and sweet spotting of hydrocarbon prospects in offshore northwest Borneo

We introduce a new seismic-electromagnetic (EM) projection (SEMP) attribute to improve the characterization of pay sands by integrating seismic P impedance and S impedance with electrical resistivity measurements leveraging on the high sensitivity of resistivity to fluid saturation and the capability of seismic data to delineate thin beds. Porosity is predicted using an extended elastic impedance (EEI) approach to reduce the risk of encountering SEMP anomalies related to low porosity. First, we determine the seismic-EM integration concept using well logs from a well that encountered hydrocarbons in a deepwater fold belt in offshore Borneo. The SEMP logs supported by the EEI log successfully delineate the pay zones in the well. Second, we extend the approach in three dimensions and evaluate its effectiveness using large-size seismic controlled-source EM (CSEM) and magnetotelluric (MT) data from the same area. The seismic data are inverted to produce P- and S-impedance volumes. EEI volume calibrated to total porosity also is produced. We then perform 3D anisotropic cross-gradient joint CSEM and MT inversion of a large-size survey comprising 647 receivers with receiver spacing of 1.5–3 km at prospect to regional scale. The SEMP volumes computed using the seismic and CSEM-MT inversion results are then interpreted with the EEI volume to identify possible geobodies of pay sand. Resistive geobodies with low porosity are assigned higher risks because they are likely due to compaction rather than hydrocarbon presence, whereas geobodies with high porosity are assigned lower risks and identified as potential sweet spots. The geobodies need to be corroborated with independent geologic studies to mature them into exploration prospects and assess their associated risk and potential. Although a vertical transverse isotropy approximation is used in our CSEM-MT study, the SEMP concept also is applicable to cases considering tilted transverse isotropy or arbitrary anisotropy where available and operational for such large-size exploration data.