Magnetotelluric Data Research Articles

A novel method for processing noisy magnetotelluric data based on independence of signal sources and continuity of response functions

Various schemes for processing magnetotelluric (MT) data have been reported aiming at suppressing the strong effect of artificial electromagnetic noises, especially coherent noise that is correlated between electric and magnetic time series. Many of the recent denoising schemes are based on decomposing MT data into the responses of the natural signals and noises. However, a steady differentiation of the natural signals from noises independent of any empirical choice of parameter setting is critically important. In addition, improper subtraction of values from the separated signals can lead to losing useful values of the natural signals or missing noise-affected values, which may result in failure in deriving the true MT responses. We propose a novel data-processing method that applies frequency-domain independent component analysis (FDICA) to both the local MT data and the reference magnetic data. Among the separated signals, the proposed method can quantitatively distinguish the natural signals from the noise-affected components by calculating the ratio of the cross-power spectrum with the reference data to the auto-power spectrum for each component. When determining which values to subtract from the separated signals, we introduce an evaluation index with respect to two characteristics of the MT response function: stationarity in the time domain and smoothness in the frequency domain. We conducted two types of experiments: with MT time series severely contaminated by synthetic coherent noises and with MT field data interfered with direct-current (DC) railways. Consequently, we confirmed the noise-suppression performance superiority of the proposed method over the conventional methods of MT data processing.

Three-dimensional anisotropic inversion and electrostratigraphic imaging of marine magnetotelluric data to understand the control of crustal deformations by pre-existing lithospheric structures in the Mexican Ridges Fold belt, Southwestern Gulf of Mexico

SUMMARY3-D imaging of the lithosphere in the Mexican Ridges fold belt is important for understanding how the crustal deformations in this basin relate to deep tectonic processes and structures inherited from extinct Jurassic seafloor spreading. Here, we use broad-band (0.0001–0.4642 Hz) marine magnetotelluric data from the basin to reconstruct the 3-D anisotropic resistivities of the lithosphere and their spatial gradients. The resistivity gradients maxima enabled independent definition of important geological boundaries (seen on collocated seismic reflection data) and estimation of crustal thickness. We found anomalous layered zones of low resistivity and high electrical anisotropy at 5–8 km depth (coinciding with the regional detachment zone in Eocene shales in 3-D seismic data) and in the upper mantle which we interpret as indicating intense deformation and/or recent magmatic influence. We also found a banded crystalline basement structure across the fossil spreading centre comprising WSW–ENE trending, 6–10 km wide, electrically resistive subvertical sheets with conductive and anisotropic borders, which merge into a basal resistive stock-like body at 15–20 km depth. These are cut or bounded by later NNW trending major faults. These WSW and NNW structural trends correlate with the previously interpreted transform and normal faults that formed during the Late Jurassic opening of the Gulf of Mexico only if rotated clockwise by 25–30°. Surprisingly, the rugged thrust-related seabed is offset at the projected positions of the steep resistive-conductive basement sheets (which also have spatially coincident high magnetic intensity and seismicity) enabling us to infer they represent magmatic intrusions facilitated by pre-existing faults. Their conductive borders spatially coincide with possibly fluid-filled vertical fracture-sets in the overlying sediments seen in seismic data which we interpret as hydrothermal fluid pathways. We infer that a magmatic body recently intruded our study area, its ascent controlled by pre-existing basement structures, and influenced the deformation of the Neogene sequences and the seafloor topography.

Geoelectric Model of the Central Part of the North Caucasus: Three-Dimensional Inversion

Abstract ––Interpretation of magnetotelluric data within the North Caucasus marginal massif, the uplift of the Main Range, and the Stavropol Arch, performed on the basis of one- and two-dimensional inversions of the impedance tensor components and three-dimensional mathematical modeling, makes it possible to compile the starting and test models that are necessary for a three-dimensional inversion. It is shown by the three-dimensional inversion of all the impedance tensor components, carried out on a test three-dimensional model, that it is possible to estimate the parameters of conducting model blocks. The three-dimensional inversion of all the experimental impedance matrix components, performed with account for the results obtained on the test three-dimensional model, significantly corrects the parameters of the conducting blocks identified at the stages of one- and two-dimensional inversions, as well as the three-dimensional mathematical modeling of magnetotelluric fields in the central part of the North Caucasus. In the resulting three-dimensional geoelectrical model of the region, the position of low-resistance blocks correlates with the location of suture zones, deep faults, volcanic chambers, and domains with converted earthquake waves, whose velocities are reduced and whose absorption is increased. This can be explained by the dependence of the conductivity of crustal blocks on water saturation. Earthquake hypocenters are grouped near low-resistance anomalies.

Geoelectric Model of the Northwestern Caucasus: Three-Dimensional Inversion

Interpretation of magnetotelluric observations made within the Northwestern Caucasus, by means of one-dimensional, two-dimensional inversion and three-dimensional mathematical modeling programs, has helped build testing and starting geoelectric models that are necessary to test and adapt the program of three-dimensional inversion of the impedance tensor components. The performed three-dimensional inversion of the experimental magnetotelluric data has significantly changed the parameters of conducting blocks within the folded structures of the region, identified at the previous stages of MT data interpretation. In the resulting three-dimensional geoelectric model, the position of low-resistance blocks correlates with: suture zones, deep faults, mud volcanoes, and domains characterized by an increased absorption of transverse and longitudinal seismic waves. The electrical resistivity of the most low-resistance anomalies is explained by the degree of their saturation with the water fraction of the fluid.

Undulating electrical lithosphere–asthenosphere boundary beneath Northeast China; as revealed by long–period magnetotelluric data

The geometry of continental lithosphere–asthenosphere boundary (LAB) significantly affects the movements of tectonic plates and flow of the upper mantle. However, the properties and geometry of this boundary beneath tectonically active regions in Phanerozoic continents are mysterious and elusive. Ninety–three long–period magnetotelluric (LMT) observations across Northeast China (NEC) provide a unique opportunity to image the shape of the LAB. Probabilistic resistivity–depth model inversion was employed to obtain the one–dimensional resistivity structure beneath each LMT site. The depth of the electrical LAB is constrained by the most probable interface at which the resistivity changes rapidly from high to low. The inversion results show that the electrical LAB is undulating beneath NEC. The North–South Gravity Lineament marks the transition from deep to shallow LAB. The thickest lithosphere occurs beneath the Great Xing'an Range (GXAR), where the LAB is ∼200 km deep. The electrical lithosphere is relatively thin in the east, with the thinnest lithosphere (∼50 km) being found beneath the Changbai Mountain volcano. We speculate that melt depletion, together with possible mantle downwelling are the likely causes for the formation of the thick GXAR lithosphere. The unresolved lithosphere beneath the northern Songliao Basin could be ascribed to the strong attenuation of electromagnetic waves in highly conductive sedimentary layers or the weak difference in the resistivity of the lithosphere and asthenosphere. The lithosphere steps that can form the edge-driven convection in the asthenosphere may play key roles in forming the widely-spread intraplate magmatism in NEC.

New constraints on the structure and composition of the lithospheric mantle beneath the Slave craton, NW Canada from 3-D magnetotelluric data – Origin of the Central Slave Mantle Conductor and possible evidence for lithospheric scale fluid flow

Previously collected magnetotelluric data were used to produce an improved 3-D resistivity model of the lithosphere beneath the Slave craton. A region of low resistivity (<10 Ωm) at ∼100 km depth was imaged beneath the central Slave craton, and identified as the Central Slave Mantle Conductor (CSMC) reported in previous studies. Earlier studies interpreted the CSMC as graphite films but new analysis from laboratory experiments indicates that it is unlikely that graphite films are stable/continuous at this depth. Using recent laboratory experiments we review alternative conduction mechanisms for the CSMC including (a) metasomatic phlogopite, (b) grain boundary sulphides and (c) high density fluids including hydrous carbonatite melts or brines. None of these hypotheses are without flaws requiring either unreasonably high-volume fractions or have petrological challenges. However, brines are the preferred explanation as they (1) can explain geophysical observations, (2) are observed in fibrous diamonds coincident with the CSMC and (3) require small volume fractions (<1%). We hypothesize their emplacement was related to a Mesozoic subduction event. This implies that upper mantle conductors should not be used as an indicator of carbon concentration in the mantle to prospect for diamondiferous regions. Our resistivity model indicates a lithospheric thickness of 210 ± 10 km beneath the Slave craton, consistent with seismic and xenolith studies. A water content of 10–150 ppm at 100–170 km depth in the lithospheric mantle is inferred from resistivity, broadly agreeing with estimates from mantle xenoliths from the central Slave craton lithosphere. Below 170 km the bulk lithosphere is "dry" (<10 ppm), which may explain why the cratonic root has been resistant to erosion by the underlying asthenosphere. Low resistivity fingers extend to surface in multiple locations that may represent paleo-fluid flow pathways through the lithosphere, broadly resembling those observed beneath Olympic Dam, Australia.

Three-dimensional audio magnetotelluric imaging of the Yangyi geothermal field in Tibet, China

The Yangyi geothermal field in southern Tibet has enormous potential for geothermal energy development. It is the high-temperature geothermal field at the highest elevation in the world, and the second largest high-temperature geothermal field in China. The total planned installed capacity of the geothermal power station is 32 MW. At present, no detailed three-dimensional (3D) geophysical survey has been carried out in the Yangyi geothermal field, and in-depth research is urgently needed. The main goal of our geophysical exploration is to study the geothermal structure of this region. In this study, the audio magnetotelluric (AMT) data of 85 stations were collected in the Yangyi geothermal field. The average distance between the stations was about 500 m, and the dimensions of the exploration area were 6 × 7 km. First, we analyzed the magnetotelluric data using the phase tensor method to qualitatively estimate the dimensionality of the subsurface resistivity structure. Then, by performing 3D inversion of 85 AMT data with relatively high quality, the 3D resistivity model from the surface to a depth of 2 km was determined, the high-temperature hydrothermal system under the Yangyi geothermal field was displayed for the first time. In addition, the main characteristics of the geothermal system were identified, and the genetic mechanism of the geothermal system was discussed based on the geological information and borehole data.

Integrated subsurface investigation for magmatic sulfide mineralization in Betul Fold Belt, central India

Magnetotelluric (MT), Electrical Resistivity Tomography (ERT), Time-Domain Induced Polarization (TDIP), Geochemical and Geological studies followed by drilling and down-hole logging were undertaken with in the Betul Fold Belt (BFB) in Central India, to demarcate zones of magmatic Ni-Cu-PGE sulfide mineralization. The BFB is predominantly composed of circular to elongate gabbro bodies of the Padhar Mafic-Ultramafic Complex, intruded into a sequence of bimodal volcanic rocks and quartzites. Near-surface samples of ultramafic rocks were subjected to precise geochemical analysis and scanned by an electron microscopy with an energy dispersive spectrometer (SEM-EDS). This work indicated the presence of pyrite, pyrrhotite, chalcopyrite, pentlandite, and minor amounts of W-Cd bearing boweiite and palladenite assemblage. These minerals are regarded as favorable to the occurrence of Ni-Cu-PGE sulfide mineralization. MT data derived from two profiles were analyzed and modeled using 2D and 3D inversion algorithms. The robust conductivity anomalies obtained from the MT model have been interpreted coupled with electrical tomography, geology, and geochemistry data. The near-surface shallow depth conductors observed in the ERT sections are interpreted as the sulfide mineralized zones. They corroborate the MT results. These conductive zones reflect the occurrence of the magmatic Ni-Cu-PGE bearing sulfide mineralization associated with rocks of the mantle-derived Padhar Mafic Ultramafic Complex. This geophysical data, in conjunction with petrological and geochemical analysis of drill core samples have allowed the identification of the origin and paragenesis of sulphide mineralization in the study area. Geochemical studies suggest that the parental magma was generated from a subduction modified, metasomatized and an enriched mantle source that was subsequently emplaced in a magmatic continental arc setting. The interpreted conductors, observed at shallow depths (∼200-300 m), have been generated by secondary hydrothermal fluid circulation leading to vein formation in the host Padhar Mafic-Ultramafic Complex. The MT and electrical tomography models delineate the geological boundaries of the sulfide-bearing mineralized deposits in the BFB.

Nature-inspired stochastic hybrid technique for joint and individual inversion of DC and MT data

Here, a new naturally-inspired stochastic nonlinear joint and individual inversion technique for integrating direct current (DC) and magnetotelluric (MT) data interpretation-based simulation of a swarm intelligence combo with specific capabilities for exploitation of the variable weight particle swarm optimizer (vPSO) and exploration of the grey wolf optimizer (GWO), vPSOGWO, is used. They are particularly notable for their capacity for information exchange while hunting for food. Through synthetic MT and DC data contaminated with various sets of random noise, the applicability of the anticipated vPSOGWO algorithm based joint and individual inversion algorithm was assessed. The field examples, collected from diversified different geological terrains, including Digha (West Bengal), India; Sundar Pahari (Jharkhand), India; Puga Valley (Ladakh), India; New Brunswick, Canada; and South Central Australia, have shown the practical application of the proposed algorithm. Further, a Bayesian probability density function (bpdf) for estimating a mean global model and uncertainty assessment in posterior; and a histogram for model resolution assessment have also been created using 1000 inverted models. We examined the inverted outcomes and compared them with results from other cutting-edge methodologies, including the GWO, PSO, genetic algorithm (GA), Levenberg–Marquardt (LM), and ridge-regression (RR). Our findings showed that the current methodology is more effective than the GWO, PSO, GA, LM, and RR techniques at consistently improving the convergence of the global minimum. In contrast to earlier approaches, the current cutting-edge strategy vPSOGWO offers an improved resolution of an additional significant crustal thickness of about 65.68 ± 1.96 km over the Puga Valley, in which the inverted crustal thickness determined by vPSOGWO agrees well with the published crustal thickness over the Puga Valley. The new technology brings simulations closer to genuine models by significantly reducing uncertainty and enhancing model resolution.

Geophysical Characterisation and Oil–Gas Resource Analysis of the Southern Huaying Mountain Fault Zone, Sichuan Basin, China

As the west rim of an ejective fold zone, the Huaying Mountain fault zone (HMFZ) in the eastern Sichuan Basin (SB) plays an important role in the tectonic evolution of the SB. The distribution and characteristics of HMFZ are strongly associated with tectonic activities and have greatly impacted the distribution of oil and gas reservoirs. However, its distribution and characteristics have remained poorly understood due to a lack of geophysical data, especially areal gravity survey and magnetotellurics (MT) survey, which are admittedly advantageous for detecting the edges of geological structures. Therefore, we carried out the ground geophysical surveys that areal gravity survey and MT survey, and acquired 1:250,000-scale real gravity data and MT data for the first time in this area. Optimized edge-detection methods were adopted to process the areal gravity data, allowing us to characterize the planar distribution of faults more reliably and convincingly. We found that the southern HMFZ is well developed and primarily trends in NNE and NE, whereas the subordinate faults trend in N-S and W-E. Vertical information for the faults extracted using the improved depth from the extreme points method revealed that the fault dominantly dipped to the SE, which was consistent with the results of MT inversion. Based on the spatial distribution of the faults, we further discussed the gravity anomaly, fault distribution, Luzhou palaeo uplift, and the distribution and characteristics of oil-gas resources, and found the convincing evidence to analysis the distribution of oil and gas resources in this region.

The Western Segment of the Precambrian Suture Between the Yangtze and Cathaysia Blocks: Constraints From Magnetotelluric Data in Southwest China

AbstractThe western segment of the suture zone between the Yangtze and Cathaysia blocks, which is the most important tectonic boundary related to the formation and rifting of south China, is enigmatic and not fully understood due to the sporadic exposure of Precambrian strata and ophiolites. Three‐dimensional electrical resistivity models derived from inversion of magnetotelluric data identified a lithospheric‐scale conductive zone extending northeastwards beneath the Youjiang basin, which was interpreted as the western segment of the suture zone. The high conductivity and coincident high magnetic anomalies closely match the location of Carlin‐type gold deposits, which can be explained by fluids and gold‐bearing sulfide minerals in a fossil suture zone. Inconsistent with the southeast‐dip resolved at the eastern segment of the suture zone (the Jiangshan‐Shaoxing fault) in the earlier study, the slightly north‐dipping geometry at the western suture zone implies the reactivation by northward subduction and closure of the Paleo‐Tethys Ocean.

Salair–Gornaya Shoria Junction (Northwestern Central Asian Orogenic Belt): Deep Structure and Tectonics from Magnetotelluric Data

Abstract —The Salair fold-thrust orogenic belt (Salair orogen, Salair) is located in the northwestern Altai–Sayan fold area within the Central Asian Orogenic Belt. The Salair orogen is an allochthon overriding the Kuznetsk Basin on a system of imbricate thrusts. The southern flank of the Salair thrust system is tectonically juxtaposed against the Gornaya Shoria terrane which differs markedly from Salair in its geological setting. The Salair and Gornaya Shoria terranes are separated by the Nenya-Chumysh Basin, a deep Mesozoic trough. The Salair orogen is composed of Cambrian–Early Ordovician island arc volcanic and sedimentary rocks, widespread garnet amphibolites and gneisses of the Angurep complex in its southern flank, and the Shalap subduction-related melange in the Alambai ophiolite suture. The southern Salair orogen and its junction with Gornaya Shoria have been imaged down to the lower crust by magnetotelluric (MT) soundings, which is an efficient tool for investigating the deep structure and tectonic history of orogenic areas. The MT surveys were performed at 25 stations on a 120 km long profile. MT data revealed an up to 70 km wide low-resistivity zone (a conductor) traceable till a depth of 20 km between the Salair and Gornaya Shoria terranes. The low-resistivity zone has a complex structure with its outer and interior boundaries dipping almost vertically. The conductor lies under several major geological structures: the Shalap melange, the Nenya-Chumysh Basin, and the NE trending Altai–Salair right-lateral strike-slip fault. The Altai–Salair fault, along which the Salair allochthon was displaced relative to Gorny Altai and Gornaya Shoria, joins the Salair system of imbricate thrusts. The Nenya-Chumysh Basin at the Salair–Gornaya Shoria junction is a deep trough having an asymmetric transversal profile with a steep western side and a shallower-dipping stepped eastern side. The southeastern flank of the basin is a wide area of thin sediments over the Paleozoic basement dipping gently in the northwestern direction. The revealed deep structure of the Nenya-Chumysh trough is consistent with its tectonic model implying an Early Cretaceous basin superposed over an early Jurassic pull-apart basin. Early Mesozoic motions on major faults is a regional-scale phenomenon known from many areas of southern West Siberia.

Crustal Structures of the Qimantagh Metallogenic Belt in the Northern Tibetan Plateau from Magnetotelluric Data and Their Correlation to the Distribution of Mineral Deposits

Crustal structure and fluid or melt originating in the deep crust and mantle are critical in regional magmatic mineral systems. However, the crustal structure and the processes that entrain and focus fluids from a deep-source region to a metallogenic belt remain relatively undisclosed. We present a magnetotelluric (MT) study of the eastern Qimantagh Metallogenic Belt (QMB) in the northern Tibetan Plateau. Data from 33 MT stations in two sections and 7 dispersed stations are acquired using a surface electromagnetic prospecting (SEP) system in frequency band ranges from 320 Hz to 0.00034 Hz. Data are converted by Bostick conversion and two-dimensional (2D) nonlinear conjugate gradient inversion. Our MT results reveal the geoelectrical crustal structure of the QMB, which consists of a southern low-resistivity domain that reflects the Kumukuri rift, a high-resistivity middle domain that represents the southern QMB in the central Kunlun belt, and a northern low-resistivity domain that covers the northern QMB and southwestern Qaidam block. We present a comprehensive tectonic and geophysical model of QMB based on the MT interpretation and geological analysis. We infer the high-resistivity domain as a reflection of a rigid crust and detached lithospheric mantle, this belt separate the QMB into northern and southern QMB. Most of the mineral deposits are found in the northern low-resistivity domain of QMB. Our study and findings provide an understanding of the tectonic evolution of the northern Tibetan Plateau, the crustal structure that controls the temporal and spatial distribution of magmatic rocks, and the geological signature associated with mineral deposits.

Smooth magnetotelluric impedance estimation by optimization

In urban and industrialised areas, magnetotelluric (MT) recordings often contain a significant amount of high-power near-field noise which, constituting non-Gaussian noise for most of the recording length, may lead to the failure of the common transfer functions (TF) estimation methods. The most evident consequence of this is the appearance of sharp points in the TF curves versus frequency, deteriorating their inherent smoothness. The TF's smoothness is in fact one of the main criteria adopted by MT practitioners to evaluate the data quality and effectiveness of the estimation procedure; when not reached by the estimation, it may be a-posteriori introduced by splines or filtering procedures, but these approaches lack physical soundness. We propose a new heuristic algorithm aimed at obtaining the maximum TF versus frequency smoothness, through an optimization scheme rejecting EM events in frequency-domain. The algorithm searches for frequency-dependent power thresholds to be applied to the events, rejecting those making the TF sharp. The TF's smoothness is the objective function to be minimized, and the model space is constituted by the infinite set of threshold vectors for each analysed frequency. After the process, the Ordinaly Least Squares (OLS) residual distribution is much closer to a Rayleigh one, allowing for an unbiased estimation. We applied the procedure both to synthetic and real MT data. Measured data has been collected for a geothermal exploration project in the East Africa Rift, where strong coherent noise affected the recordings. The combination of the technique with the remote-reference technique effectively reduced the bias due to the artificial signal. The physical consistency of the resulting TF has been tested by dispersion relationships.

Intra‐continental Orogeny: Insights from Magnetotelluric Data into the Mesozoic Uplift History of the Eastern Jiangnan Orogen in South China

AbstractDespite extensive efforts to understand the tectonic evolution of the Jiangnan Orogen in South China, the orogenic process and its mechanism remain a matter of dispute. Previous geodynamic studies have mostly focused on collisional orogeny, which is commonly invoked to explain the Jiangnan Orogen. However, it is difficult for such hypotheses to reconcile all the geological and geophysical data, especially the absence of ultrahigh‐pressure metamorphic rocks. Based on the magnetotelluric data, we present a group of resistivity models produced through the combination of two ‐dimensional and three‐dimensional inversions, revealing the geo‐electrical structures of Jiangnan and a typical collisional orogen. In our models, the resistive crust is separated into three parts by a prominent conductive layer with opposite dipping directions on both sides. A special thrust‐nappe system, which is different from that developed in a typical collisional process, is revealed in the Jiangnan Orogen. This structure suggests a process different from the simple collisional orogeny. To interpret our observations, an ‘intra‐continental orogeny’ is proposed to address the development of the Jiangnan Orogen in the Mesozoic. Furthermore, this ‘reworked’ process may contain at least two stages caused by the decoupling of the lithosphere, which is revealed by an extra conductive layer beneath Jiangnan.

One‐Dimensionl Inversion of Magnetotelluric Field Data by SIS and Occam's Inversion Technique: A Case Study from the Indian Himalaya

AbstractElectromagnetic method is one of the most effective geophysical methods in earth crust and mantle structure studies. Magnetotelluric (MT) data are used to obtain resistivity image of earth's surface on the depth scale of few hundred of meters to several tens of km. One‐dimensional (1D) MT inversion is carried out to estimate electrical resistivity variation of earth's surface with depth. In the present research paper one‐dimensional inversion of MT sounding curves is carried out on nine sites in foothill of Himalaya near Roorkee to Hardwar region. A non‐iterative and minimum norm estimator based algorithm Straight forward Inversion Scheme (SIS) is used to obtain continuous smoothest resistivity depth model. Another iterative and minimum norm estimator based algorithm Occam's inversion scheme is also used to obtain smoothest one‐dimensional inversion model. The data is inverted for both TE‐ and TM‐mode of polarization. The inverted data shows one‐dimensional behavior of subsurface resistivity up to depth about 0–15 km with resistivity variation ≈10–100 ohm‐m in both modes of polarization and after that the subsurface structure is two‐dimensional in both modes. 1D inversion of data provides a good starting model to reduce the computational cost of two‐dimensional inversion.

Electrical resistivity structure across the Late Cenozoic Abaga-Dalinor volcanic field, eastern China, from 3-D magnetotelluric imaging and its tectonic implications

The Abaga-Dalinor volcanic field (ADVF) in Inner Mongolia, China, located to the west of the North-South Gravity Lineament (NSGL), is the largest and less known area of Late Cenozoic intraplate volcanism in Northeast China. Knowledge of the subsurface structure beneath the ADVF will improve our understanding of its evolution process and formation mechanisms. New broadband magnetotelluric (MT) data were collected along a profile of about 300 km that crosses the major basaltic areas exposed at the surface in the ADVF and a series of east-north-east (ENE) trending faults. The electrical resistivity structure of the crust and uppermost mantle across the ADVF was generated by 3-D inversion of full impedance tensor and tipper data. This model reveals a thick high-resistivity layer (>1000 Ωm) in the upper crust that may represent the Precambrian basement and Late Paleozoic-Mesozoic granitic plutons. In contrast, high conductivity anomalies dominate the middle-lower crust. The northward-dipping high-conductivity feature at depths of 20–40 km below sea level under the Chagan Obo fault is suggested to be the involvement of the Early Paleozoic subducted oceanic crust. A remarkably high-conductivity zone (1–20 Ωm) exists in the middle-lower crust beneath the Abaga and Dalinor volcanic areas. It is explained to be a saline fluid zone of ∼0.45%–0.48% that exsolved from the partial melts as magma ascent rapidly. The impermeable upper crust traps these fluids in the middle-lower crust. A moderately high-conductivity anomaly in the uppermost mantle below near the Solonker Suture Zone (SSZ) is attributed to a minimum partial melt of ∼3–4% associated with the localized asthenospheric upwelling, which feeds the Abaga and Dalinor volcanos as a common magma source. Integrating our resistivity model with previous geophysical and geochemical studies, we suggest that intraplate volcanism in the ADVF is mainly controlled by the pre-existing tectonic weak zones. The lithosphere within the SSZ is susceptible to thermal perturbation from the upwelling mantle, providing the preconditions for melt generation. A set of ENE trending faults and an inferred NW-SE trending fault jointly dominated the vertical transport of magma and lateral extension along the faults, which may have influenced the spatial distribution of basalts and volcanic cones at the surface in the ADVF. In addition, we tend to suggest that Late Cenozoic intraplate volcanism in the ADVF could be the result of decompressing melting within the localized small-scale asthenospheric upwelling induced by the edge-driven convection near the NSGL, which is indirectly related to the westward subduction and retreat of the (Paleo-)Pacific plate during the Late Mesozoic to Cenozoic.

Geophysical data interpretation technologies in the study and exploration of oil-and-gas deposits

The article presents the results of scientific research on the creation of computer technology for the interpretation of geophysical field data in the exploration of oil-and-gas deposits of Ukraine. The theoretical, informational, technological and methodical foundations for increasing the efficiency of geological exploration work due to the in-depth extraction of information from geological and geophysical data based on their complex interpretation within the framework of new mathematical models have been developed. Three-dimensional gravity and magnetometric modeling can be directed to the detection of densification zones and the tracing of tectonic disturbances in the consolidated crust, without which the existence of hydrocarbon transportation channels is impossible. The practical application of examples of quantitative interpretation of three-component magnetic survey data is given, which will significantly help in the detection and localization of hydrocarbon deposits. Development and expansion of the software complex for the interpretation of magnetotelluric data based on the use of impedance-type boundary conditions. The technique is intended for visualization of MTS data at the stage of qualitative interpretation in parallel with the method of the impedance tensor and Wiese vectors. The absolute advantage of this approach is its independence from the condition of a plane incident wave and the use of all six components of the electromagnetic field (including the Z component of the electric component of the MT field). The integration of statistical and deterministic methods during the inversion of geophysical data will increase the re-liability of the obtained geological results. The relevance and importance of the results presented in the work is determined by the conceptual novelty of methods and tools for forecasting new promising areas, as well as the reassessment of reserves of known deposits.

3D analysis of the MT data for resistivity structure beneath the Ashute geothermal site, Central Main Ethiopian Rift (CMER)

The Ashute geothermal field (around Butajira) is located near the western rift escarpment of the Central Main Ethiopian Rift (CMER), about 5–10 km west of the axial part of the Silti Debre Zeit fault zone (SDFZ). Several active volcanoes and caldera edifices are hosted in the CMER. Most of the geothermal occurrences in the region are often associated with these active volcanoes. The magnetotelluric (MT) method has become the most widely used geophysical technique for the characterization of geothermal systems. It enables the determination of the subsurface electrical resistivity distribution at depth. The high resistivity under the conductive clay products of hydrothermal alteration related to the geothermal reservoir is the main target in the geothermal system. The subsurface electrical structure of the Ashute geothermal site was analyzed using the 3D inversion model of MT data, and the results are endorsed in this work. The ModEM inversion code was used to recover the 3D model of subsurface electrical resistivity distribution. According to the 3D inversion resistivity model, the subsurface directly beneath the Ashute geothermal site can be represented by three major geoelectric horizons. On top, a relatively thin resistive layer (>100 Ωm) represents the unaltered volcanic rocks at shallow depths. This is underlain by a conductive body (< 10 Ωm), possibly associated with the presence of clay horizon (smectite and illite/chlorite zones), resulting from the alteration of volcanic rocks within the shallow subsurface. In the third bottom geoelectric layer, the subsurface electrical resistivity gradually increases to an intermediate range (10–46 Ωm). This could be related to the formation of high-temperature alteration minerals such as chlorite and epidote at depth, suggesting the presence of a heat source. As in a typical geothermal system, the rise in electrical resistivity under the conductive clay bed (products of hydrothermal alteration) may indicate the presence of a geothermal reservoir. Otherwise, no exceptional low resistivity (high conductivity) anomaly is detected at depth.

Hilbert transform of unequally sampled data: Application to dispersion relations in magnetotellurics

The real and imaginary parts of the Fourier transform of a causal signal can be estimated from one another using the Hilbert transform. The numerical computation can be carried out by a fast Fourier transform or the convolution of the data with an appropriate Hilbert kernel. However, magnetotelluric (MT) data are unequally sampled because logarithmic frequency axes are conventionally used. A method has been developed for the Hilbert transformation of unequally sampled data and applied to the real and imaginary parts of the frequency normalized impedance to examine if the dispersion relations are satisfied for a given impedance tensor. A version of the Shannon sampling theory is generalized for the unequally sampled data. The method involves the reconstruction of the measured data by a linear combination of an analytic interpolation function distributed along the horizontal axis. Then, the decomposition coefficients that provide a fit between the measured and reconstructed data are solved using the linear least-squares method with singular-value decomposition. Finally, the discrete Hilbert transformed data are constructed at any desired abscissa values by the sum of the analytic Hilbert transform of the interpolation function using the decomposition coefficients solved in the previous step. Examples from the MT synthetic and field data, including out-of-quadrant phase cases, are given. In principle, the method also can be applied to other integral transforms.