Magnetotelluric Data Research Articles

Deep Structure of the Santos Basin, Offshore Brazil From 3D Inversion of magnetotelluric Data

AbstractThe relationship between deep crustal structure and the deformation in the overlying sedimentary wedge in Santos basin, Brazil is not well understood, and the origin and evolution of the salt‐related “Albian Gap (AG)” remain a topic of debate. We investigate the deep structure using three‐dimensional inversion of full tensor marine magnetotelluric (MT) data (of 10−1 to 104 s period bandwidth) from 92 stations along three NW–SE lines in 50–1,700 m water depth, one crossing the Cretaceous hinge line (CHL), AG, and Cabo Frio Fault (CFF). The geological validity of the resulting MT resistivity models was determined using resistivity logs from 11 wells and seismic data. The model shows two regionally persistent electrically conductive layers (C2 and C3) related to key Cenozoic and Cretaceous unconformities in the upper part of the sedimentary wedge. Beneath this wedge, the resistive continental crust is ∼35 km thick across the CHL until the 200 m isobath and thereafter thins rapidly seaward to ∼21 km over a lateral distance of ∼80 km defining a domain of highly extended and faulted crust. Our models show a mantle‐associated basement high and evidence of significant uplift of the lower part of the sedimentary wedge at 100–150 km distance along our central profile which spatially coincides with the AG and a previously proposed Moho high. This implies a mantle‐driven deformation of the crust and basin fill. We propose that mantle flow and magmatism may have played a significant role in the inferred displacement at the AG.

Structure of the Upper Part of the Earth’s Crust in the Area of the Lena River Delta: The First Magnetotelluric Data

Structure of the Upper Part of the Earth’s Crust in the Area of the Lena River Delta: The First Magnetotelluric Data

Evidence for partial melting and earthquake activity in the crust from a 2-D electrical resistivity model in the vicinity of the Chayu region, southeastern Jiali-Chayu fault, Tibetan Plateau

The southeastern segment of the Jiali-Chayu fault is a prominent strike-slip fault that separates the eastern Lhasa terrane. Its formation and activity are influenced by the subduction of the Indian plate towards the Lhasa terrane and clockwise rotation in the Himalayan Eastern syntaxis region. Notably, the Chayu 1950 Ms 8.6 earthquake, triggered by this fault, remains the largest recorded inland earthquake to date. However, due to a lack of high-precision geophysical evidence, the deep crustal structure of the hypocenter area remains challenging to reconstruct, limiting our understanding of earthquake activity in the region. In this paper, the Magnetotelluric data is consisted of 70 sites spaced 2 km apart. Through a joint inversion of the transverse electric and magnetic modes, we recovered the 2-D resistivity model that is approximately 140 km long and 50 km deep. The resistivity model provides compelling evidence that the presence of magmatic rocks and metamorphic basement is responsible for the high resistivity anomaly. Conversely, the low resistivity anomaly observed in the northern region can be attributed to crustal melting. These results suggest that the variation in earthquake activity between the Gongrigabu fault and Guyu fault can be attributed to the contrasting characteristics of the crustal material. The Gongrigabu fault represents a brittle plate with low water content and high crystallinity, resulting in a distinctive high resistivity anomaly. On the other hand, the Guyu fault corresponds to a molten plate, exhibiting a low resistivity anomaly. We believe that the Chayu earthquake is a typical intraplate earthquake in the Lhasa terrane and its seismogenic fault is a deeper hidden fault located below the Gongrigab fault, challenging the belief held by other scholars that the Gongrigabu fault is solely responsible. We think the Gongrigabu fault is a high-risk zone for severe earthquakes that demands continuous vigilance in the future.

Three-dimensional magnetotelluric and gravimetric imaging of Mount Lawu geothermal prospect area, Indonesia

Mount Lawu is a stratovolcano located on the border of Central Java and East Java Provinces in Indonesia. At least eleven manifestations indicate its geothermal potential in the form of hot water and fumaroles. This study aims to describe a geothermal system based on an integrated analysis of magnetotelluric (MT) and satellite gravity data. Some additional geochemical and petrological data analyses were also used to explain the existing hydrothermal system in this area. It is inferred from the gravity derivative analysis that fault structures exist in a location with high permeability, which acts as a controlling structure for geothermal manifestations. The 3-D resistivity inversion was carried out using MT full-impedance tensor data, while the 3-D density model was reconstructed based on the residual anomaly computed from the topography-free gravity disturbance. The 3-D resistivity model, from magnetotelluric inversion, shows a clay cap distribution, corresponds to a low resistivity anomaly, centered at the southern part of the mountain peak with a thickness of about 1 kilometer, which narrows to the western region where the hot spring shows an outflow manifestation. The gravity inversion shows several low-density bodies in the same location as the low-resistivity anomaly attributed to a clay cap. Coincident resistivity and density anomalies constrain the horizontal location of the geothermal reservoir. Despite the incapability of the model to resolve the body of the geothermal heat source, it can be inferred from the inversion results that the heat possibly emanates through the thermal conduction passage below the reservoir zone from the source located deeper down than the reconstructed models.

Evaluating the groundwater quality in Northern Tarim Basin by resistivity inverted from magnetotelluric data

Groundwater is a potentially significant resource for agricultural and industrial uses in Tarim Basin, the largest arid endorheic basin in China. Hydrogeochemical and hydrogeologic researches have provided detailed information of groundwater chemical type, composition, ion content and flow. However, there is lacking in quantitative estimation of groundwater in whole basin scale subject to the limits of samples as well as the unclear underground structure. To address the problem, 3-D electrical resistivity model constructed from inversion of magnetotelluric (MT) data was used to calculate the Total Dissolved Solids (TDS) of groundwater based on previous experimental studies and empirical equations. The TDS model shows relatively high value in the east of the study area and in deep, primarily controlled by depth, flow direction, stratigraphy, and water table gradient. Groundwater can be classified into five categories in terms of TDS range in Standard for Groundwater Quality. Finally, the volume of each type and average available quantity of Classes I ∼ III (TDS < 1 g/L) per person were estimated by taking account of population distribution. Mapping groundwater TDS and volume reveals that human activities also play a role in groundwater salinity. The evaluation of groundwater quality and volume based on TDS provides a basis for future urban construction and development as well.

Magnetotelluric Data Inversion Based on Deep Learning With the Self-Attention Mechanism

Magnetotelluric Data Inversion Based on Deep Learning With the Self-Attention Mechanism

Low-Frequency Magnetotelluric Data Denoising Using Improved Denoising Convolutional Neural Network and Gated Recurrent Unit

Low-Frequency Magnetotelluric Data Denoising Using Improved Denoising Convolutional Neural Network and Gated Recurrent Unit

A Magnetotelluric Data Denoising Method Based on Lightweight Ensemble Learning

A Magnetotelluric Data Denoising Method Based on Lightweight Ensemble Learning

Multiscale resistivity mapping from an intracontinental hydrothermal mineral system, Adelaide Rift Complex, Australia

Porphyry mineral systems provide a significant source of critical minerals for modern society, that are predominantly formed in subduction related settings from magmatic hydrothermal fluids derived from an underlying reservoir. However, in post-subduction settings the processes that entrain and focus magmatic hydrothermal fluids throughout the crust, from source to deposit are often poorly constrained. In this paper, we have undertaken a magnetotelluric (MT) survey in an intracontinental tectonic setting across the Adelaide Superbasin, which is a Neoproterozoic passive margin formed during rifting of the supercontinent Rodinia. This region contains the Burra magmatic copper deposit, along with numerous sedimentary-hosted copper deposits. Systematic three-dimensional modelling was undertaken, initially at regional-scale, with 55 km gridded long-period AusLAMP data over a 500 × 500 km area, followed by infilled 10 km gridded broadband MT data, at the district-scale, centred over an area of copper deposits in the Adelaide Rift Complex. The modelling reveals an elongated zone of low electrical resistivity (∼100 Ω.m), in an otherwise resistive crust, at lower crustal depth (20–40 km) that is contiguous with the regional lineament of the Adelaide Rift Complex and copper and gold mineralisation. A narrow zone of low resistivity (∼100 Ω.m) branches through the upper crust (5–20 km) to the surface, that is remarkably aligned with the Burra magmatic mineral system, in an otherwise quite resistive crust (∼1000 Ω.m). Additionally, in the upper crust a zone of low resistivity (1–10 Ω.m at a depth of 5–8 km) extends from the Burra magmatic copper deposit southwards, constrained by anticlinal folding from the ∼500 Ma Delamerian Orogenic deformation of the Adelaide Rift Complex and spatially aligned with the Kapunda sedimentary-hosted copper deposit. We argue that the low electrical resistivity signature encapsulates a whole-of-lithosphere magmatically-hosted copper system, that is further mobilised by low-temperature fluids by mapping the footprint of ore-forming fluids from the source to associated mineralisation. Where later basin wide fluid recycling in a permeability enhanced basin, via neo-tectonism along pre-existing structures/trends, has likely remobilised copper, from a prior magmatic event, with copper being leached in chloride-rich saline fluids at low temperatures.

Deep structural controls on the distribution of carbonate reservoirs and overburden heterogeneity in Central Luconia province, offshore Borneo revealed by 3D anisotropic inversion of regional controlled-source electromagnetic and magnetotelluric profile data

Understanding the deep structural controls on reservoir distribution and overburden heterogeneity is important for near-field exploration and green-field development of the Miocene to Holocene carbonate buildups in Central Luconia province in offshore northwest Borneo, Malaysia. Data from 59 stations along an approximately 180 km long controlled-source electromagnetic (CSEM) and magnetotelluric (MT) survey line with three segments recorded using different CSEM transmitter-towing directions were available. We applied three-dimensional (3D) anisotropic resistivity inversion with cross-gradient constraint and verified the resulting models using resistivity logs from nearby wells and the acoustic basement interpreted from seismic data. Our anisotropic resistivity models reveal a fragmented carbonate-rich zone atop a segmented basement comprising electrically resistive horsts coinciding with the northerly Mega-platform, Central and Southern Field Highs, separated by steep conductive zones coinciding with the West, East, and Southeast Troughs previously interpreted from seismic data. The structural highs correlate with the spatial distribution of the known carbonate buildups implying a genetic link. The carbonate bodies are overlain and underlain by persistent layers (C2 and C3) of low resistivity and high anisotropy, which we interpret as indicating compressional deformation or detachment zones. Overburden layer C2 (whose base coincides with the top of a key sedimentary package, Cycle V in seismic data) is thinnest over the East Trough and is discontinuous at the eastern margin of the West Trough, which are locations where drilled wells did not find hydrocarbons implying that the seal rocks are inefficient or breached at those localities. We used these observations to refine the existing seismic-based interpretation of carbonate play-types along our transect indicating how 3D joint CSEM-MT imaging can potentially contribute to derisking or optimizing future exploration and/or development work in this province.

Crustal root shapes the plumbing system of a monogenetic volcanic field as revealed by magnetotelluric data

The plumbing systems of subduction-zone volcanoes consist of magma reservoirs with different degrees of evolution at multiple depths. The geometries of plumbing system of intraplate monogenetic volcanic fields are generally characterized by basaltic magma ascending from the mantle and/or a shallower reservoir along dikes. However, the shape and vertical extent of these plumbing systems are largely unknown due to the rareness of robust geophysical data. The possible role of faults and mountain chains in controlling the geometry of magma reservoirs is also poorly understood. To address these limitations, we present a three-dimensional resistivity structure beneath the Holocene Jingpohu Monogenetic Volcanic Field (JMVF) in the Zhangguangcai Range (ZGC), northeastern China. The resistivity model was derived from the magnetotelluric data in a dense net-like array. We identified an inverted cone-shaped high-resistivity (∼10,000 Ω m) anomaly below the ZGC. This anomaly traverses almost the entire crust. We interpret this high-resistivity anomaly to be the stiff basement of the ZGC. At the base of the resistive volume, we resolved a sill-like, low-resistivity anomaly that occurs at the top of the lithospheric mantle. Sheet-like low-resistivity anomalies surrounding the high-resistivity body depart from the lithospheric mantle. We interpret these sill- and sheet-like anomalies as a trans-crustal saucer-shaped magma plumbing system beneath the JMVF. This system involves multi-level magma reservoirs, including basaltic magma at its base, basaltic dikes/sills swarms in the middle and sill intrusions in the upper crust. We propose that the stiff crustal root and the topographic loading of the ZGC work together to deflect the upward magma propagation towards the edges of the ZGC. The magma ascent to the surface is probably controlled by pre-existing, lithospheric faults. Our data show how mountain chains, together with the crustal structure, play a key role in controlling the geometry of multi-level plumbing systems of intraplate monogenetic volcanic fields.

A multi-disciplinary geophysical approach reveals a Paleoproterozoic cryptic suture in the Amazon Craton – Implications for Proterozoic Supercontinent reconstructions

The Amazon Craton evolved in the Paleo-Mesoproterozoic through successive continental magmatic arcs, which ages decrease westwards. However, when and if these magmatic arcs collided with another continent, closing an ocean basin is a subject of ongoing debate. It is speculated that the Baltica and Amazon cratons formed a purely long-lived accretionary margin for at least 900 Ma and only collided during the formation of the Rodinia Supercontinent at 1.2 – 1.0 Ga. Identifying relicts of suture zones is challenging since Proterozoic orogens tend to preserve the continental arc and the reworked components so that less than 10 % of the orogens are composed of oceanic evidence. Moreover, the Phanerozoic cover overlays more than 50 % of the proterozoic rocks in the Amazon Craton. Therefore, we propose a robust geophysical assessment to investigate the upper and lower crusts. We acquired new magnetotelluric data and integrated it to the available potential field (satellite, airborne, and ground), geochronological data (magmatic and metamorphic ages), crustal thickness and depth to Curie temperature estimates. A major crustal-scale structure (more than 1000 km in extension) separates two tectonic fragments with remarkable differences in the magnetic gradient and structure trend. We also find along this major boundary high and low Bouguer anomalies of long-wavelength paired, a step in the Mohorovic discontinuity, and a lower crust conductor. This evidence supports the hypothesis of a collision in Paleoproterozoic times. Our findings suggest that the orogen did not remain purely accretionary and evolved to a collisional type between 1.63 and 1.69 Ga. This contrasts with the purely accretionary orogens described in the Baltica Craton, thereby undermining the hypothesis that the Amazon and Baltica cratons shared the same peri-Nuna subduction system from 2.0 to 1.3 Ga.

Self-organizing Competitive Neural Network Based Adaptive Sparse Representation for Magnetotelluric Data Denoising

The existing sparse decomposition denoising methods for magnetotelluric (MT) data need to set the iterative stop condition manually, which not only has a large workload and high difficulty, but also easily causes subjective bias. To this end, we propose a new adaptive sparse representation method for MT data denoising. First, the data to be processed is divided into high-quality segments and noisy segments by machine learning algorithm. Then, the characteristic parameters of high-quality segments are calculated, and the boundary value of the characteristic parameters is taken as the threshold. The threshold has two functions, one is as a criterion for signal-to-noise identification, and the other is as an iterative stop condition for subsequent sparse decomposition. Finally, the optimized orthogonal matching pursuit algorithm is used to separate the signal and noise of the noisy segments, and the denoised segments and high-quality segments are combined to obtain the complete denoised MT data. The field data processing results show that this method is a fully automatic and intelligent MT data denoising method. It greatly improves the signal-to-noise ratio and the apparent resistivity-phase curves.

Estimation of Groundwater in Tarim Basin from Three-dimensional Resistivity Model Constructed by Magnetotelluric Data

Groundwater is an important resource of irrigation and domestic water in Tarim Basin (TB), the largest arid inland basin in China. Hydrogeologic models and hydrogeochemical analysis have provided information of groundwater flow, salinity and hydrochemical type in TB. However, there is lacking in quantitative estimation of groundwater volume due to that the underground aquifer structure is unclear. To address the problem, magnetotelluric (MT) data were collected in TB and subsurface electrical resistivity were produced by MT inversion. To estimate the porosity of aquifers by resistivity, we should determine the resistivity of groundwater which is controlled by the Total Dissolved Solids (TDS) in water. From the previously published data, TDS increases from piedmont plain to alluvial plain along groundwater flow direction in TB and other similar settings. In terms of spatial distribution of TDS and 3-D resistivity model, the effective porosity of aquifers in the study area was calculated. Finally, by taking account of the water table and thickness of Quaternary sediments collected by BGP Inc., the total groundwater volume was estimated in northern parts of TB.

Application of GeoGME Software in Shallow Oil and Gas Target Exploration

GeoGME is a software that can run on Linux and Windows operating systems and has the function of processing and interpreting the whole process of gravity, magnetic and electromagnetic data from field data to final results. In terms of preprocessing of magnetotelluric (MT) data, GeoGME provides several single point denoising methods, in which spatial filtering can be used to achieve good results. For static correction, the influence of static displacement can be suppressed by using man-machine interaction and 2D filtering static correction technique. In terms of inversion, the software also has a variety of inversion processing modules of 1D and 2D. The 2D regularization inversion method is used to process the field MT data from ES basin, and the inversion results show that the electrical characteristics of the shallow layer in resistivity profile are in good agreement with the known electrical logging curves. According to the electrical characteristics of petroliferous formation, the potential target oil and gas enrichment area is preliminarily inferred. The results of test line show that using GeoGME software in shallow target exploration can achieve good geological result.

Compression and Reconstruction of Magnetotelluric Data Based on Convolutional Neural Network

Magnetotelluric method is one of the important geophysical methods, and its signal acquisition requires more stacking times and longer stacking time. With the development of instruments, the acquisition time becomes longer and the amount of data becomes larger, which brings new challenges to data storage and transmission. Aiming at the above problems, a compression and reconstruction technology of magnetotelluric time series based on convolutional neural network is proposed. This paper introduces two convolutional autoencoders based on convolutional neural networks, which can effectively compress data space, improve transmission efficiency, and have high data reconstruction accuracy. Using the measured magnetotelluric time series, two autoencoder models based on convolutional networks are verified in this paper, which proves the feasibility of convolutional autoencoders in magnetotelluric data compression; the results show that model 2 can better reproduce It constructs the magnetotelluric time series, and has high training efficiency and good generalization ability.

Multi-station Magnetotelluric Data Processing Based on Singular Value Decompositionl

Singular value decomposition is an efficient matrix analysis method. This paper intends to use SVD to process magnetotelluric data from multiple stations. The data of multiple magnetotellus observation stations can make the main reactance of magnetotellus obtained by data processing more robust, suppress the interference signal, improve the data quality, and effectively increase the signal strength. The obtained results are more in line with the actual situation, and the decomposition by singular value method can greatly improve the efficiency of electromagnetic signal processing. Better access to underground information, geological resource exploration and deep earth research.

Fast 3D Magnetotelluric Forward Modeling Based on a Regularization Approach in Spherical Coordinate

The earth’s curvature shouldn’t be ignored for the large-scale magnetotelluric (MT) modeling. A forward code of spherical coordinate MT method based on staggered grid finite difference method is developed. In addition, the forward problem is usually solved numerically, mostly based on iterative solution. Krylov subspace method is widely used to solve electromagnetic forward modeling problems in frequency domain. However, as the period increases, the convergence of these methods becomes significantly slower (especially for long-period MT data). We added a scaled grad-div operator to the double curl equation of the electric field to perform the nondivergence condition (CCGD). The results show that CCGD method is more efficient than other methods, which is an efficient method for our code.

Research and Application of Deep Learning Selection Method for Magnetotelluric Power Spectrum Based on Rhoplus Correction

Magnetotelluric (MT) is an electromagnetic exploration method using natural field sources, which are unstable, irregular, and vulnerable to interference from electromagnetic signals. The total power spectrum of the magnetotelluric signal is superimposed by a sub-power spectrum of segmented time series. To reduce the influence of electromagnetic interference, the pre-processing of magnetotelluric data will delete the severely interfered sub-power spectrum. The selection of power spectrum often requires human-computer interaction, which is time-consuming and laborious, heavily dependent on personal experience, and has great randomness. This article innovation is put forward based on the classification of Rhoplus correction and deep learning automatic power spectrum method, first of all, the MT power spectrum impedance estimation results are Rhoplus one dimensional inversion and obtain fitting, using neural network learning fitting differential characteristics and classification, and then for the power spectrum of MT again after low-quality quantum power spectrum to eliminate overlay, Impedance estimation is performed, and the above process is repeated to find the optimal power spectrum combination. Through the comparison and verification of measured MT data, the automatic power spectrum selection method based on Rhoplus correction and deep learning classification achieve similar results to manual editing.

Analysing the Relationship Between the Fault’s Distribution and the Earthquake Actives Near Gongxian China Based on Gravity Data

Huaying Mountain fault zone, the largest fault zone in the Sichuan Basin, is playing an important role to geological studies, such as the characterization of structural features, earthquakes occurrence, oil and gas reservoir distribution. However, the complexity and lack of consensus on deformation mechanisms necessitate further investigation of fold and thrust belts in the SB. Therefore, based on the real areal gravity exploration data and magneto-telluric (MT) data, it characterizes the spatial distribution of HMFZ in this region. Further, it gives the analysis combining the gravity anomaly, fault distribution data, and frequent earthquake activity around the study area, which provides insights that can be exploited for the evaluation of seismogenic tectonics in this region.