Magnetotelluric Data Research Articles

Effect of Karst Geomorphology on the Sedimentary Mineralization and Geochemical Distribution of Bauxite: An Example from the Xiaoyuan Area in Qingzhen, Guizhou Province

Bauxite, in central Guizhou, is predominantly karst bauxite, but there is insufficient research on the effect of karst paleogeomorphology on bauxite development. Xiaoyuan bauxite is also a karst bauxite, and high- and low-iron bauxite deposits exist in the study area. This study conducts geological modeling of karst bauxite using controlled-source audio-frequency magnetotelluric (CSAMT) data and drill core data. The effects of karst paleogeomorphology on bauxite deposition and mineralization are evaluated by assessing karst paleogeomorphology, conducting a mineralogical analysis of drill cores at different locations, and determining the geochemical distribution characteristics of the elements in the horizontal and vertical directions. Combined with previous research results, we propose two metallogenic processes of high-iron and low-iron bauxite. The findings are significant for understanding the mechanism of bauxite formation.

Modelling of subsurface geological structure related to anomalous magnetotelluric phase data exceeding 90° in Central India

SUMMARY We note anomalous phase (>90°) in magnetotelluric (MT) measurements at several stations along the Narmada river course normal to the Tan Shear Zone (TSZ) in central India. We made efforts to derive a possible cause of the anomalous phase by comparing the results of numerical modelling based on the tectonics of the TSZ with the measured impedance tensor data. A zone of multiple reactivations of the TSZ leads to the formation of a damage zone and conjugate Riedel shear faults parallel and normal to the TSZ, respectively. The conjugate Riedel shear faults act as a pathway for the Narmada River course. The multiple tectonic activities associated with the TSZ induce surficial heterogeneity that results in distortion in the MT data. The Mohr circle and phase tensor analysis establish the effect of distortion in the data. To understand the cause of distortion as well as anomalous phase in the present data, we carried out synthetic 3-D modelling. We modelled the damage zone and conjugate Riedel shear faults by a cross shape. From the results, we noted that the near-surface 3-D heterogeneity across the TSZ critically distorted the YX component of the MT impedance tensor due to the strong current channelling. When the current flows across the conjugate Riedel shear zone (Narmada river course) charges are accumulated along the boundaries of near-surface heterogeneity. The charges associated with the current channelling brought out the reversion of the electric field direction that is reflected in the form of an anomalous phase in the YX component of the impedance tensor in measured data.

An electrical resistivity image of the Hikurangi subduction margin

SUMMARY Along the Hikurangi subduction margin, on the east coast of New Zealand's North Island, the interplate coupling changes from locked in the south to weakly coupled in the north. New magnetotelluric (MT) data from 151 locations linking previous MT surveys into a single contiguous data set that encompasses the weakly coupled part of the margin are analysed. By inverting the combined data we have constructed a 3-D image of the electrical resistivity of the subduction interface shear zone along a 300-km-long segment of the margin. Our results show that the electrical resistivity of the subduction interface shear zone is heterogenous; the degree of heterogeneity decreasing from north to south. The resistivity heterogeneities correlate well with the distribution of near-plate interface seismicity, Vp/Vs values and the pattern of areal strain rate derived from GPS data. These correlations are consistent with variations in the fluid content of the subduction interface shear zone. In the northern part of this segment, conductive areas adjacent to the interface are interpreted to be fluid rich areas where seismicity is sparse, Vp/Vs ratios are high and the areal strain rate is extensional. In contrast, where the areal strain rate is compressional the plate interface is more resistive, and seismicity is more abundant consistent with greater interplate friction. In the south, the resistivity of the plate interface is more homogenous, and the overlying plate is more resistive at shallower levels than in the north. Our results support the hypothesis that the fluid and/or hydrated clay content of the subduction interface shear zone are an important control on interplate coupling.

Deep-learning-based sferics recognition for audio magnetotelluric data processing in the dead band

In audio magnetotelluric (AMT) sounding data processing, the absence of sferic signals in some time ranges results in a lack of energy in the AMT dead band, causing unreliable resistivity estimations. To address this issue, we develop a deep convolutional neural network (CNN) to automatically recognize sferic signals from redundantly recorded data over a long time range and use these sferic signals to accurately estimate resistivity. The CNN is trained using field time-series data with different signal-to-noise ratios (S/Ns) acquired from different regions of mainland China. To solve the potential overfitting due to the limited number of sferic labels, we develop a training strategy that randomly generates training samples with random data augmentations while optimizing the CNN model parameters. The training process and data generation are stopped when the training loss converges. In addition, we use a weighted binary cross-entropy loss function to solve the sample imbalance problem to optimize the network better, use multiple reasonable metrics to evaluate the network performance, and perform ablation experiments to optimize the model hyperparameters. Extensive field data applications indicate that our trained CNN can robustly recognize sferic signals from noisy time series for subsequent impedance estimation. The results indicate that our method can significantly improve the S/Ns and effectively solve the lack of energy in the dead band. Compared with the traditional processing method, our method can generate smoother and more reasonable apparent resistivity-phase curves and depolarized phase tensors, correct the estimation error of the sudden drop in high-frequency apparent resistivity and abnormal behavior of phase reversal, and better estimate the real shallow resistivity structure.

Crustal structure of the Lazufre volcanic complex and the Southern Puna from 3-D inversion of magnetotelluric data: Implications for surface uplift and evidence for melt storage and hydrothermal fluids

Abstract The Central Andes are unique in the global system of subduction zones in that a significant, high-altitude plateau has formed above a subduction zone. In this region, both subduction and the associated magmatism have been shown to vary in both space and time. Geophysical data have been invaluable in determining the subsurface structure of this region. Extensive seismic studies have determined the regional-scale distribution of partial melt in the crust and upper mantle. Magnetotelluric studies have been effective in providing independent constraints on the quantity and composition of partial melt in the crust and upper mantle. Geodetic studies have shown that a small number of volcanic centers exhibit persistent, long-term uplift that may indicate the formation of plutons or future eruptions. This paper describes a detailed study of the Southern Puna using magnetotelluric (MT) data. This region is located at the southern limit of the Central Andes in a region where a recent transition from flat-slab subduction to normal subduction has caused an increase in magmatism, in addition to hypothesized lithospheric delamination. It is also a region where an extensive zone adjacent to the volcanic arc is undergoing surface uplift, located near Volcán Lastarria and Cordon del Azufre (collectively called Lazufre). The main goals of the work are to define the crustal structure and to investigate processes that may cause surface uplift of relatively large regions not associated with active volcanism. As part of the PLUTONS project, MT data were collected on an east-west transect (approximately along 25°S) that extended across the Southern Puna, from Lazufre to north of Cerro Galan. The data were combined with previously collected MT data around Lazufre and inverted to give a 3-D resistivity model of the crust. The low resistivity of the crust resulted in limited sensitivity to mantle structure. A number of major crustal conductors were detected and included (1) a mid-crustal conductor extending eastward from the volcanic arc as far as the Salar de Antofalla; (2) an upper- to mid-crustal conductor located north of Cerro Galan; and (3) a conductor that rises westward from (1) and terminates directly beneath the region of surface uplift at Lazufre. These conductors are broadly coincident with the location of crustal low shear-wave anomalies. The conductive features were interpreted to be due to zones of partial melt stored in the crust, and petrological data were used to estimate melt fractions. Below Lazufre, it is likely that aqueous fluids contribute to the high conductivity, which is observed within the depth range of the inflation source, giving evidence that the surface uplift may be associated with both magmatic and hydrothermal processes.

A top-down control on upper crustal inheritance on the south-eastern Tibetan Plateau

Mantle tectonics, such as asthenospheric upwelling and lithospheric decoupling, usually controls subsequent fluid formation and migration from deep to shallow levels in the lower lithosphere and promotes rock failure and deformation. Based on the concept whereby the fluid migration location defines the structural inheritance, we aim to relate crustal processes to mantle tectonics using fluids revealed by a new magnetotelluric (MT) array across the Ailao Shan-Red River belt. As imaged via a 3D resistivity model, two lower-lithospheric conductive anomalies are determined as consistent with the Dian-Qiong (DQ) suture and Song Da (SD) belt and interpreted to contain interconnected melt. Because potassic magmatism was enabled by partial melting in the lower lithosphere, we could infer that the DQ and SD have reworked as the major fluid migration channels. Fluid migration is considered to drive lithospheric decoupling at a low-viscosity conductive layer, which is inferred feeding by aqueous and melt fluids originating from the two channels and diffusing at depths from 15 to 20 km. Constrained by the geochronology results, this weak layer could have provided convenience to induce the entirely upper crustal translation-rotation within the time interval between potassic magmatism and strike-slip of the Red River fault. This translation-rotation process is inherited from the underlying mantle processes and may further be remotely affected by the upper crustal movement of the Tibetan Plateau, conforming with the crustal rotation observed to the north of our study region. This work provides a compelling example of the tectonic control of the mantle on inherited responses in the crust.Plain Language Summary Crustal deformation is usually controlled by mantle tectonic processes, especially the southeastern Tibetan Plateau. Some models have proposed that lithospheric shearing and lower-crustal flow have controlled the crustal deformation in the Cenozoic. However, crustal rotation models show less possibility of a channel flow and relate the crustal deformation to the remote effect of India-Asia collision. Our study investigates above hypotheses by imaging the electrical resistivity of rocks beneath the major Red River Fault area in southeastern Tibet, using magnetotelluric data with high quality in a dense array. The lithosphere of the study area is interpreted to have been divided into two horizontal systems by a fluid diffusion layer at the bottom of upper crust. Resistivity low in the lower lithosphere is inferred due to partial melting of deep lithosphere and reworking of the paleo sutures in the Late Cenozoic, which have spread at the bottom of upper crust. Rather than the channel flow, this fluid migration process sensitively reflected in our model relates the mantle tectonics to crustal rotation by providing rheological conditions. We, hence, propose an inherited structure model featured by lithospheric decoupling and upper-crustal translation-rotation that may occur before the major strike-slip event.

Three‐Dimensional Electrical Structure Beneath the Epicenter Zone and Seismogenic Setting of the 1976 Ms7.8 Tangshan Earthquake, China

AbstractBased on magnetotelluric array data and the three‐dimensional (3D) inversion algorithm, we obtain the 3D electrical structure of the 1976 Ms7.8 Tangshan earthquake zone. The seismogenic structure comprised a thick resistive anomaly located east of the Tangshan fault (TSF), a thin resistive belt extending westward, and the TSF zone. A geodynamic model is proposed based on the 3D electrical model and the other research findings. The model indicates that the primary cause of the Tangshan earthquake was the almost EW‐trending horizontal pushing from the west. The local uplift of deep thermal materials on the west side of TSF produces tensile deformation superimposed on the existing shear deformation. The combined effect of these two forces controls the Tangshan earthquake deformation characteristic. Prevailing horizontal pushing from the west and local uplift of hot material may be associated with the Tibet Plateau's eastward escape and the subduction of the Pacific plate.

The deep structure of the Kuznetsk deflection and Kamzhelinsky block junction zone of the Kuznetsk Alatau according to magnetotelluric sounding data

The deep structure of the Kuznetsk deflection and Kamzhelinsky block junction zone of the Kuznetsk Alatau according to magnetotelluric sounding data

Three-dimensional electrical structure model of the Yangbajain geothermal field in Tibet: Evidence obtained from magnetotelluric data

Three-dimensional electrical structure model of the Yangbajain geothermal field in Tibet: Evidence obtained from magnetotelluric data

Basement and lithospheric structure of the central Ganga Basin between the Bundelkhand craton and the Sharda Deep by magnetotellurics

The geological and tectonic fabric of the Indian plate in the Ganga Basin is masked by alluvial sediments. Geophysical exploration of the basin for hydrocarbons yielded information on the presence of transverse basement ridges and Cenozoic as well as some pre-collision stratigraphy of the basin. However, the depth of the basement especially in the northern part of the basin and the nature of the underlying crust and mantle lithosphere is largely unknown. We have carried out a magnetotelluric (MT) study along a 300-km-long profile between the Bundelkhand craton (BC) and the Sharda Deep (at the Himalayan foothills) across the central Ganga Basin to delineate the basement and the lithospheric structure. The electrical resistivity image obtained by 2-D inversion of broadband MT data reveals significant lateral heterogeneity in the subsurface structure along the profile. The model yields a super-thick sedimentary basin in the Sharda Deep consisting of multiple stages of basin formation, including a Proterozoic paleo-rift, and a highly heterogeneous lithospheric structure of the Indian plate north of the BC. The model consists of two major upper mantle conductors connected to the conductors within the lower crust. It is surmised that enhanced conductivity could be due to the presence of fluids and the upper mantle conductors are feeding to the crustal conductors. However, an in-depth analysis of the genesis of these conductors would require other independent geophysical information which is not available at present. This is the first reporting of such a heterogeneous nature of the Indian continental lithosphere beneath the Ganga Basin at a regional scale. Our model further supports the presence of the electrical Moho in the BC and, together with earlier published MT results along another profile, suggests that the BC extends much farther north than its presently known northern limit.

Geophysically guided well siting at the Aluto-Langano geothermal reservoir

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

Electrical image of magmatic system beneath the Xiangshan volcanogenic uranium deposit, southeast China: Linking magmatic evolution and uranium metallogenesis

Abstract The uranium deposit of the Early Cretaceous Xiangshan caldera (southeast China) represents the world’s third-largest volcanogenic uranium deposit. While the hydrothermal system defines uranium mineralization in shallow volcanic-intrusive complexes, we argue that the underlying magmatic system, identified in magnetotelluric data from the Xiangshan deposit, is equally crucial in controlling the origin and migration of ore-forming fluids and driving uranium element transport. As imaged by a three-dimensional resistivity model, a volcanic conduit containing ~4% saline fluid underlies the Xiangshan peak, interpreted to control the formation of alkali fluids that aid the early alkaline uranium mineralization. A deep-seated intrusion also imaged beneath the northwestern Xiangshan caldera likely facilitates the formation of later acidic uranium mineralization and provides the requisite heat flux. Its enhanced conductivity is attributed to graphite liberalization along grain boundaries caused by CO2-bearing volatiles upwelling or igneous carbon injection. The merged zone of the volcanic conduit and intrusion, which is located at a depth of ~10–15 km, functions as a high-level magma chamber. It is fed episodically by low partial melting of lower-crustal rocks and may contain significant uranium concentrations. Essentially, the transport and enrichment of uranium elements are enabled by such a vertical extensive trans-crustal magmatic system, resulting in the emplacement of the shallow volcanogenic uranium deposit. This work is a compelling example regarding the inherited structural control of the magmatic system on volcanic-related uranium metallogenesis.

A geophysical investigation of the urban-expanding area over the seismologically active Dahshour region, Egypt

Western Desert region between Cairo and Fayoum in Egypt is a geologically interesting area due to its active seismicity status and the continuous bidirectional urban expansions ongoing northward and southward growth. This work aims to develop a geological model of the region by combining magnetic intensity data (aeromagnetic) and potential field data (airborne gravity) with magnetotelluric (MT) data to offer decision-makers an additional evaluation tool for long-term future planning. Applying a number of filters to aerial gravity and magnetic data allowed us to calculate the subsurface density distribution and structure of the area between Cairo and Fayoum. In addition, the surface of the crust was imaged down to a depth of 6 km using a 3-D inversion of gravity data. Magnetotelluric data were gathered at eight locations over the NS-extended Cairo-Fayoum Road profile. MT data were inverted in 2-D to generate a resistivity model that defines the subsurface structure of the researched region. The subsurface geometry in the MT-derived resistivity model agrees with the results of airborne potential measurements, and no consideration was given to any faults that may have been undetected from the potential maps. Long-term seismological observations indicated that the likely active sources of earthquakes are limited to the recognized main faults and that the ongoing “1185 buildings” construction project did not and will not cause unanticipated seismicity. The paper concludes that urban growth is secure so long as the earthquake codes are rigorously considered while planning civil projects. The northern study region must be regularly monitored for induced seismicity. On the Cairo-Fayoum Road's western side, the middle sector of the study area is the safest place for future civil developments.

Neoproterozoic and Cretaceous crustal structures revealed by a 3-D magnetotelluric investigation of the onshore Espírito Santo Basin and the contiguous Araçuaí Orogen (SE Brazil)

SUMMARY The study region in southeastern Brazil is composed of two domains: the Cretaceous onshore Espírito Santo Basin and the Neoproterozoic Araçuaí Orogen. In this region, associations among geological evolution, observed structures and geophysical data are limited. Therefore, this study aimed to further investigate this region using magnetotelluric (MT) data obtained from 22 W-E spanning stations over the Espírito Santo State. Because the dimensionality analysis revealed a substantial 3-D characteristic of the data, a 3-D MT inversion using the ModEM code was performed to generate a regional geoelectric model, and potential and well data were used as qualitative support for the interpretation. The resistivity profile obtained from the model revealed a resistive pattern for the Precambrian terrain and a conductive pattern for the sedimentary rocks of the onshore Espírito Santo Basin. The basement of the basin was delimited in the resistivity model, revealing consistency with the basement depths from the well data. Furthermore, three crustal conductors inserted in a resistive Precambrian orogen were identified. These conductors may be related to the recent circulation of saline fluids associated with plume action during the Upper Cretaceous and Cenozoic. However, the structures in which these fluids circulate exhibit different genesis along the resistivity profile. The conductors to the west of the profile may be related to plutons and dykes associated with the NNW Colatina Lineaments generated by the gravitational collapse of the orogen (530–490 Ma). Conversely, the conductor to the east under the Espírito Santo Basin appears to be related to the primary detachment rift fault generated during the Cretaceous breakup of Gondwana.

Seismogenic Structure of the 2014 M6.5 Ludian Earthquake From Three‐Dimensional Joint Inversion of Magnetotelluric Data and Seismic Arrival Times

AbstractOn 3 August 2014, a destructive earthquake with a magnitude of 6.5 occurred in the Ludian region in Yunan, China, causing heavy casualties. However, the seismogenic structure of the Ludian earthquake is still unclear. To address this issue, we have developed a three‐dimensional joint inversion algorithm for magnetotelluric data and seismic body wave arrival times based on the cross gradient structural similarity constraint, which utilizes the complementary advantages of the two different datasets. We use arrival times of P‐ and S‐waves from 6,369 local earthquakes recorded by 30 seismic stations and magnetotelluric impedances from 127 MT stations to perform the joint inversion to construct a combined 3‐D P‐wave velocity (Vp), S‐wave velocity (Vs), and resistivity models and the locations of relocated aftershocks in the source region of the Ludian earthquake. Our models show significant low Vp/Vs ratios and low resistivity in the upper crust of the main shock zone, and the relocated aftershocks form an L‐shaped zone. Based on petrophysical studies and analysis of the cross‐plots of different physical properties, we infer the existence of cracks/fractures and fluids as well as high quartz contents in the source region. These fluids may play a key role in promoting the Ludian earthquake by reducing the effective normal stress of the fault zone, and the existence of high quartz contents further makes the crustal rocks in the source region more brittle. Our study provides a new evidence how the properties of the crust in the focal area have some control on the genesis of moderate to large earthquakes.

Crustal Imaging of Portugal Mainland Using Magnetotelluric Data

AbstractThe comprehensive mapping of resistivities in depth and its interpretation is of great importance to unravel the electrical properties of deep‐seated rocks, providing significant insights into the structure of the crust. The first 3D resistivity model for Portugal mainland is here reported using data from 31 broadband MT soundings spaced 50 × 50 km apart. The model shows large crustal volumes with contrasting resistivity values. The central and northern regions are characterized by a high resistivity crustal domain spreading in‐depth (103–105 Ω.m) that correlates well with voluminous granitoid bodies and high‐grade metamorphic rocks. To the west and the south, roughly corresponding to the Portuguese Western Shore and the South Portuguese Zone (SPZ) respectively, the crustal resistivity tends to decrease unevenly to values between 1 and 103 Ω.m. Interconnected graphite in structural discontinuities, possibly forming a regional mid‐crustal décollement at ≈13–15 km depth, are highlighted by multiple low resistivity crustal domains (1–100 Ω.m). Similarly, the model reveals a very large E‐W low resistivity crustal domain in SPZ possibly representing a deep‐seated major décollement. The present study aims to contribute to a better understanding of the physical properties characterizing the concealed crust in Portugal mainland, providing also indirect information on its composition and structure.

Insights on the Interplay of Rifting, Transcrustal Magmatism and Formation of Geothermal Resources in the Central Segment of the Ethiopian Rift Revealed by 3‐D Magnetotelluric Imaging

AbstractThe Main Ethiopian Rift is accompanied by extensive volcanism and the formation of geothermal systems, both having a direct impact on the lives of millions of inhabitants. Although previous studies in the region found evidence that asthenospheric upwelling and associated decompression melting provide melt to magmatic systems that feed the tectono‐magmatic segments in the rift valley, there is a lack of geophysical models imaging these regional and local scale transcrustal structures. To address this challenge, we use the magnetotelluric method and image subsurface electrical conductivity to examine the magmatic roots of Aluto volcano, quantify and interpret the melt distribution in the crust considering established concepts of continental rifting processes and constrain the formed geothermal system. Specifically, we combined regional (maximum 30 × 120 km2) and local (15 × 15 km2) magnetotelluric data sets and obtained the first multi‐scale 3‐D electrical conductivity model of a segment of the central Main Ethiopian Rift. The model unravels a magma ponding zone with up to 7 vol. % melt at the base of the crust (30–35 km b.s.l.) in the western part of the rift and its connection to Aluto volcano via a fault‐aligned transcrustal magma system. Melt accumulates at shallow crustal depths (≥4 km b.s.l.), thereby providing heat for Aluto's geothermal system. Our model suggests that different volcano‐tectonic lineaments in the rift valley share a common melt source. The presented model provides new constraints on the melt distribution below a segment of the rift which is important for future geothermal developments and volcanic hazard assessments in the region.

Integration of Geophysical Methods to Study Deep Structure of the Angara Fault, the Largest in the Baikal Rift

Abstract —The Angara regional fault which is transversal to the Baikal basin totals about 150 km in length and ranks as a major strike-slip fault with the normal component in the seismically active rift zone. Given that its vicinities represent an area with high population density, the emphasis needs to be placed primarily on the study of its structural features. At this, the Angara fault has been poorly studied by geophysical methods. Results of the specialized mapping carried out in the 1990s revealed the block structure of the Angara fault zone, however without a reliable identification of the fault plane, which leaves its position still to be debatable. To establish the Angara fault plane and studying its deep structure, the integration of such geophysical methods as magnetotelluric (MT) sounding, radon emanation and fieldwalking magnetic surveys was required. Their application in the study of the deep structure of the southern parts of the Angara fault allowed revealing anomalies in all of the measured fields, accordingly. The anomaly-forming object, which the authors associate with the deep penetrated Angara fault plane, was expressed in the most distinct way by the magnetotelluric data (as a high-resistivity region (ER = 8500 Ohm∙m) relative to the host rock) and radon emanation survey (radon volumetric activity index RAI ≥ 20). The fact that the magnetic field received only minor distortions from the object may indicate either moderate magnetic characteristics of the latter or a significant occurrence depth. The identified anomaly-forming object localized within the crystalline basement tends to be more sharply expressed in the left bank of the Irkutsk Reservoir, while in the upper part of the cross-section it is overprinted by rocks of the sedimentary cover.

Cooperative joint inversion of magnetotelluric and microseismic data for imaging The Geysers geothermal field, California, USA

The Geysers geothermal field located in northern California, USA, is the world’s largest electricity-generating geothermal facility. To delineate the spatio-temporal distribution of reservoir steam and recharge water, we have collected microseismic and magnetotelluric (MT) data using a dense array of stations in 2021. The microseismic and MT data have been inverted together using a 3D cooperative joint inversion workflow. The joint inversion exploits a cross-gradient structural constraint because electrical conductivity structures observed in the geothermal field are strongly correlated with [Formula: see text]/[Formula: see text] structures. To mitigate convergence issues associated with 3D large-scale joint inversion, the scheme is split into small manageable inversion subsets. By systematically performing the three inversion subsets and exchanging the structural information between velocity and conductivity models, the cooperative joint inversion mimics the full joint inversion. The 3D joint inversion results agree well with previous 3D microseismic and MT inversion studies. We find that the cooperative joint inversion improves overall MT images in terms of resolution and consistency with respect to the [Formula: see text]/[Formula: see text] model. The joint inversion also further reduces the MT data misfit. In contrast, the joint inversion does not significantly improve microseismic images because MT inversion produces low-resolution conductivity images with respect to microseismic images in depth of investigation (i.e., 1–5 km) and thus does not provide sufficient structural details that are required for improving microseismic images.

The deformation mechanism in the western Qiangtang terrane and its surroundings: evidence from magnetotelluric data

Located in the central part of the Tibetan Plateau, the Qiangtang terrane preserves important record of the uplift and deformation history of the Plateau, and therefore remains an attractive area of research. However, deep geophysical investigations of its western part are still limited. To further understand the deep structure of the western Qiangtang terrane and its surroundings, we use magnetotelluric array data to generate a 3D electrical structure. It reveals high resistivity anomalies in the upper crust and scattered high conductivity anomalies in the mid-lower crust. The electrical structure also suggests that the Longmu Co-Gozha Co fault once believed to be a major regional deformation boundary, may not have cut through the crust. The melt content and rheological parameters derived from the electrical structures show dominant ductile-type deformation in most of the study area, which contributes to block extrusion along the slip faults. Viscous deformation regions formed by mantle melt upwelling in the mid-lower crust may contribute to the formation of the N-S directed normal faults on the surface.