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SUMMARY Joint inversion for the same or different geophysical parameters is proved to be an effective technique for obtaining high-resolution solutions. Thus, comprehensive geophysical interpretation based on joint inversion has been widely concerned and applied in recent years. To realize joint inversion conveniently and efficiently, we proposed a new inversion strategy based on the alternating direction method of multipliers. In this regard, three optimization algorithms were presented respectively to attain the joint inversion of body wave traveltime and surface wave dispersion data, to obtain the joint inversion of magnetotelluric and seismic data with cross-gradient constraints, and to acquire gravity constrained inversion. A complex model with inconsistent structures in terms of resistivity, velocity and density was designed to evaluate the accuracy and effectiveness of the multiparameter joint inversion algorithms. In our joint inversion processes, each method was optimized independently and the jointly inverted results were significantly more accurate than those of separate inversions. Finally, we applied the algorithms to the field data involving gravity anomaly data, magnetotelluric data and Rayleigh wave dispersion data. The reliable underground structure was achieved by the joint interpretation of density, resistivity and velocity profiles, which verified the practicality of the inversion strategy in the actual data.

SUMMARY Presence of electrical anisotropy in the lithosphere can provide useful constraints on regional structure patterns and dynamics of tectonic processes, and they can be imaged by magnetotelluric (MT) data. However, Inversion of MT data for anisotropic structures using standard gradient-based approaches requires subjective choices of model regularization for constraining structure and anisotropy complexity. Furthermore, the ubiquitous presence of galvanic distortion due to small-scale near-surface conductivity inhomogeneities prevents accurate imaging of subsurface structures if ignored or not properly removed. Here, we present a transdimensional Bayesian approach for inverting MT data in layered anisotropic media. The algorithm allows flexible model parametrization, in which both the number of layers and model parameters of each layer are treated as unknowns. In this manner, the presence or absence of anisotropy within the layers, as well as the level of model complexity, is determined adaptively by the data. In addition, to account for the effects of galvanic distortion, three frequency-independent distortion parameters resulting from the distortion decomposition are treated as additional variables during the inversion. We demonstrate the efficiency of the algorithm to resolve both isotropic and anisotropic structures with synthetic and field MT data sets affected by galvanic distortion effects. The transdimensional inversion results for the field data are compatible with results from previous studies, and our results improve the constraints on the magnitude and the azimuth (i.e. most conductive direction) of electrically anisotropic structures. For practical applications, the validity of 1-D anisotropic approximation should be first tested prior to the use of our approach. Otherwise it may produce spurious anisotropic structures due to the inapplicability of the anisotropic 1-D inversion for MT data affected by 2-D or 3-D electrical resistivity structures.

SUMMARYMagnetotelluric (MT) observations have revealed subvertical electrical conductors that extend from shallow depths into the mid-crust at various geothermal zones, active volcanoes and active faults worldwide. These deeply rooted subvertical conductors have typically been interpreted to represent entire zones of dedicated fluid transport through the crust. We estimate the high-resolution 3-D crustal resistivity structure below the Kuju Volcanoes, Japan, using dense observations from 153 broad-band MT measurement sites and 40 telluric measurement sites. The resistivity structure highlights subvertical conductors that merge into a deep conductor to the north of the volcanoes, with deep low-frequency earthquakes occurring near the southeastern edge of this subvertical conductor at 10–30 km depth. This deep conductor branches into several subvertical conductors at 2–10 km depth, coinciding with a shallow zone where tectonic earthquakes rarely occur. The surface expressions of active geothermal areas and past volcanic eruptions are all located above the edges of the conductors at 2–6 km depth. Widespread conductive layers exist around the volcanoes above 2 km depth, and their distribution approximately corresponds to a low-gravity-anomaly zone. We discuss the nature of these subvertical conductors, the potential causes of their complex structure and their relationship to local magmatic fluid transport. These subvertical conductors, a shallow clay-rich layer, developed fracture systems and high-strength solidified magma may all contribute to magmatic fluid transport to the surface at the Kuju Volcanoes. In this study, we add the possibility that the edges of these subvertical conductors act as important magmatic fluid pathways.

SUMMARY We present a new 2-D forward modelling and inversion scheme to interpret magnetotelluric/radio-magnetotelluric data by utilizing a novel meshless forward operator. We use this discretization technique within an inverse scheme to recover conductivity structures from given magnetotelluric data. To approximate solutions of the partial differential equations that describe the magnetotelluric experiment, we discretize the subsurface only in terms of nodes. These node sets, which are simple to generate, are used to derive the differential operators’ approximations in a generalized meshless framework. First, we study and compare forward modelling calculations to an analytical and known solution from the literature. Several example calculations are given, which validate the proposed meshless forward operator. We then formulate our inverse scheme for TE-mode data, which uses only subsets of the nodal subsurface parametrization to generate conductivity structures from this given data. The inverse scheme consists of a Gauss–Newton algorithm combined with the generalized meshless framework. To validate the algorithm, we present inversion results from synthetic and field data. We compare our results to conductivity models calculated by established, well-known inversion schemes and literature results. We report that our algorithm can accurately model magnetotelluric responses and recover meaningful conductivity models, explaining given magnetotelluric data.

SUMMARY Most of the existing 3-D electromagnetic (EM) modelling solvers based on the integral equation (IE) method exploit fast Fourier transform (FFT) to accelerate the matrix–vector multiplications. This in turn requires a laterally uniform discretization of the modelling domain. However, there is often a need for multiscale modelling and inversion, for instance, to properly account for the effects of non-uniform distant structures and, at the same time, to accurately model the effects from local anomalies. In such scenarios, the usage of laterally uniform grids leads to excessive computational loads, in terms of both memory and time. To alleviate this problem, we developed an efficient 3-D EM modelling tool based on a multinested IE approach. Within this approach, the IE modelling is first performed at a large domain and on a (laterally uniform) coarse grid, and then the results are refined in the region of interest by performing modelling at a smaller domain and on a (laterally uniform) denser grid. At the latter stage, the modelling results obtained at the previous stage are exploited. The lateral uniformity of the grids at each stage allows us to keep using the FFT for the acceleration of matrix–vector multiplications. An important novelty of the paper is the development of a ‘rim domain’ concept that further improves the performance of the multinested IE approach. We verify the developed tool on both idealized and realistic 3-D conductivity models, and demonstrate its efficiency and accuracy.

SUMMARY The largest earthquake recorded in South Korea occurred in Gyeongju on 2016 September 12. Since then, understanding the geological structures near the Gyeongju earthquake's epicentre has been demanded. As a preliminary step to understand the geological structures around the epicentre, a 3-D audio-magnetotelluric (AMT) survey was conducted. These 3-D AMT data were imaged through data processing and inversion. However, because the AMT data acquired in Gyeongju were very noisy, conventional imaging procedures did not yield reliable results. To obtain a reliable model inverted from such noisy data, we propose various imaging strategies: an additional data processing technique using the Nyquist diagram after conventional data processing and several inversion strategies related to the selection of data, the weighting of the data, the constraints on the model parameter vector and the Lagrange multiplier used for the regularization. By applying the additional data processing step and several inversion strategies, we were able to successfully invert the noisy field data. The inversion results verify that the data-screening procedure applying the Nyquist diagram remarkably improves the results compared with those obtained using only conventional data processing. The imaging strategies proposed in this case study can be used to image noisy MT/AMT field data for other regions.

SUMMARY The Bhuj area, in the Kutch region of western India, is a unique intraplate seismic zone in the world where aftershock activity associated with a large magnitude earthquake (7.7 Mw Bhuj earthquake on 26 January 2001) has persisted over a decade and up till today. We studied the lithospheric resistivity structure of the Bhuj earthquake aftershock zone to gain more insight into the structure and processes influencing the generation of intraplate seismicity in broad and, in particular, to detect the deep origin and upward migration channels of fluids linked to the crustal seismicity in the area. A lithospheric resistivity model deduced from 2-D and 3-D inversions of long-period magnetotelluric (MT) data shows low resistive lithospheric mantle, which can be best explained by a combination of a small amount of interconnected melts and aqueous fluid in the upper mantle. The MT model also shows a subvertical modestly conductive channel, spatially coinciding with the Kutch Mainland Fault, which we interpret to transport fluids from the deep lithosphere to shallow crust. We infer that pore pressure buildup aids to achieve the critical stress conditions for rock failure in the weak zones, which are pre-stressed by the compressive stress regime generated by ongoing India–Eurasia collision. The fluidized zone in the upper mantle beneath the area perhaps provides continuous fluid supply, which is required to maintain the critical stress conditions within the seismogenic crust for continued seismicity.

SUMMARY The Tuareg Shield was assembled by oceanic closures and horizontal movements along mega-shear zones between approximately 20 terranes during the Pan-African Orogeny (750–550 Ma). Although there is an ongoing debate about its origin, the exhumation of the Tuareg Shield is assumed to be related to Cenozoic intraplate volcanism. The Gour Oumelalen is a key region of the Tuareg Shield and is located in the northeastern part of the Egéré-Aleksod terrane, corresponding to the eastern boundary of the Archean–Palaeoproterozoic microcontinent LATEA (Central Hoggar). The eastern boundary of the study area corresponds to a Neoproterozoic suture zone separating two old microcontinents, LATEA and the Orosirian Stripe. We deployed two magnetotelluric (MT) profiles consisting of 33 broad-band MT stations and combined these with aeromagnetic data, aiming to define the crustal structure in detail. The resistivity cross-sections obtained from the 3-D inversion of full impedance tensor and tipper data from stations along the profiles, confirm the main Precambrian faults, some of which are covered by Quaternary sediments and hence, have not yet been deciphered. The cross-sections also highlight the Cretaceous–Quaternary sedimentary basins represented by low resistivities. The upper crust is typically cratonic with a high electrical resistivity. On the contrary, the lower crust shows a drastic drop in resistivity (

Simulation-based probabilistic inversions of 3D magnetotelluric (MT) data are arguably the best option to deal with the non-linearity and non-uniqueness of the MT problem. However, the computational cost associated with the modeling of 3D MT data has so far precluded the community from adopting and/or pursuing full probabilistic inversions of large MT datasets. In this contribution, we present a novel and general inversion framework, driven by Markov chain Monte Carlo (MCMC) algorithms, which combines i) an efficient parallel-in-parallel structure to solve the 3D forward problem, ii) a reduced order technique to create fast and accurate surrogate models of the forward problem, and iii) adaptive strategies for both the MCMC algorithm and the surrogate model. In particular, and contrary to traditional implementations, the adaptation of the surrogate is integrated into the MCMC inversion. This circumvents the need of costly offline stages to build the surrogate and further increases the overall efficiency of the method. We demonstrate the feasibility and performance of our approach to invert for large-scale conductivity structures with two numerical examples using different parameterizations and dimensionalities. In both cases, we report staggering gains in computational efficiency compared to traditional MCMC implementations. Our method finally removes the main bottleneck of probabilistic inversions of 3D MT data and opens up new opportunities for both stand-alone MT inversions and multi-observable joint inversions for the physical state of the Earth’s interior., Facultad de Ciencias Astronómicas y Geofísicas

SUMMARY Bayesian inversion of magnetotelluric (MT) data is a powerful but computationally expensive approach to estimate the subsurface electrical conductivity distribution and associated uncertainty. Approximating the Earth subsurface with 1-D physics considerably speeds-up calculation of the forward problem, making the Bayesian approach tractable, but can lead to biased results when the assumption is violated. We propose a methodology to quantitatively compensate for the bias caused by the 1-D Earth assumption within a 1-D trans-dimensional Markov chain Monte Carlo sampler. Our approach determines site-specific likelihood functions which are calculated using a dimensionality discrepancy error model derived by a machine learning algorithm trained on a set of synthetic 3-D conductivity training images. This is achieved by exploiting known geometrical dimensional properties of the MT phase tensor. A complex synthetic model which mimics a sedimentary basin environment is used to illustrate the ability of our workflow to reliably estimate uncertainty in the inversion results, even in presence of strong 2-D and 3-D effects. Using this dimensionality discrepancy error model we demonstrate that on this synthetic data set the use of our workflow performs better in 80 per cent of the cases compared to the existing practice of using constant errors. Finally, our workflow is benchmarked against real data acquired in Queensland, Australia, and shows its ability to detect the depth to basement accurately.

Geological disposal is a feasible and safe method for dealing with the high-level radioactive waste problem at present. The Beishan area is the key area preselected for high-level radioactive waste geological disposal in Gansu Province, China. The Jijicao rock block is currently the most extensively studied area, where there are several fracture zones in borehole BS15. In 2011, a remarkable conductive anomaly near the BS15 was detected using a 4-km-long audio-magnetotelluric (AMT) profile with 50 m station space crossing the borehole. To study the anomaly, a short seismic survey was shot along the part of the AMT profile. This paper presents an example for detection of the fracture zones in granite using AMT and seismic methods at Beishan. The AMT data inversion model agrees with the borehole well-logging data and weak reflections are related to the fractured zones. The seismic detection may help in interpreting the cause of the conductive anomaly, and the conductive anomaly could determine whether there are fracture zones within the weak reflection areas. The simultaneous surveys can complement each other for detecting fracture zones. Besides this, such joint detection may be used to estimate whether there exist aquiferous fracture zones at depth.

SUMMARYThis study focuses in the analysis of the internal structure of the upper 3 km of Los Humeros (LH) caldera and the relation of electrical and hydrothermal anomalies. For this purpose, we measured, processed and interpreted 78 broad-band magnetotelluric (MT) soundings. We performed a 3-D inversion of the data set (ModEM) using all MT soundings, although only half of the available frequencies per sounding due to limited computed power. We also carried out the 2-D inversions (NLCG) of the invariant determinant along two orthogonal profiles (EW and NS) crossing the caldera structure; their comparison yields similar resistivity and structural models results. The resistivity modelling is complemented with the results of a joint 3-D inversion of an accurate gravity database of 720 stations, and total field aeromagnetic data (SGM) from the caldera crater. The combined results provide novel details about the structure of the shallow geothermal reservoir of the resurgence caldera complex hosting the active hydrothermal system. Density and resistivity models show the existence of a composed crater basin structure separated by an EW high-density structure; the northern basin is associated to the LH crater, whereas the southern basin associates to the emergent Los Potreros (LP) caldera basin. The magnetization model indicates that there is a common source for the magnetic volcanic products observed at the caldera surface, and that the LP fault is the more magnetized fault of the geothermal system. The propylic zoning under the geothermal field, which according to the MT model results has resistivities above ∼100 Ω-m, was extrapolated using this and additional criteria to obtain the distribution of other hypothetical propylitic zones of hydrothermal potential.

SUMMARY Galvanic distortion of magnetotelluric (MT) data is a common effect that can impede the reliable imaging of subsurface structures. Recently, we presented an inversion approach that includes a mathematical description of the effect of galvanic distortion as inversion parameters and demonstrated its efficiency with real data. We now systematically investigate the stability of this inversion approach with respect to different inversion strategies, starting models and model parametrizations. We utilize a data set of 310 MT sites that were acquired for geothermal exploration. In addition to impedance tensor estimates over a broad frequency range, the data set also comprises transient electromagnetic measurements to determine near surface conductivity and estimates of distortion at each site. We therefore can compare our inversion approach to these distortion estimates and the resulting inversion models. Our experiments show that inversion with distortion correction produces stable results for various inversion strategies and for different starting models. Compared to inversions without distortion correction, we can reproduce the observed data better and reduce subsurface artefacts. In contrast, shifting the impedance curves at high frequencies to match the transient electromagnetic measurements reduces the misfit of the starting model, but does not have a strong impact on the final results. Thus our results suggest that including a description of distortion in the inversion is more efficient and should become a standard approach for MT inversion.

SUMMARY In this study, we investigated correlations between electromagnetic and seismic signals of the 2017 February 15 Veracruz, Mexico, earthquake (Mw = 4.8). We carried out a time–frequency misfit analysis based on the continuous wavelet transform in order to compare electric, magnetic and seismic records accurately. This analysis was performed for horizontal and vertical components separately. Our results from time–frequency misfit and goodness-of-fit criteria confirm the general similarity between seismic and electromagnetic signals both in frequency and time. Additionally, we studied the behaviour of peak amplitudes of seismoelectromagenetic records as a function of magnitude and distance. Our observations are in good agreement with previous studies, confirming scaling with magnitude and attenuation with distance. Radiated seismic energy estimations were performed with two methods: integration of velocity records and empirical Green function, respectively. Estimated energy magnitudes (4.35

SUMMARY We present a new approach that allows for the inversion of quantities derived from the observed data using non-diagonal data covariance matrices. For example, we can invert approximations of apparent resistivity and phase instead of magnetotelluric impedance using this methodology. Compared to the direct inversion of these derived quantities, the proposed methodology has two advantages: (i) If an inversion algorithm allows for the specification of a full data covariance matrix, users can invert for arbitrary derived quantities by specifying the appropriate covariance matrix instead of having to rely on the inversion code to have implemented this feature. (ii) It is fully compatible with the assumptions of least-squares optimization and thus avoids potential issues with bias when inverting quantities that are nonlinear functions of the original data, We discuss the theory of this approach and show an example using magnetotelluric data. However, the same method can be applied to other types of geophysical data, for example gravity gradient measurements.

SUMMARY Electrical anisotropy in the oceanic upper mantle can only be imaged by seafloor magnetotelluric (MT) data, and arguably provides important clues regarding the mantle structure and dynamics by observational determinations. Here, we attempt to probe the electrical (azimuthal) anisotropy in the oceanic mantle by analysing recent seafloor MT array data from the northwestern Pacific acquired atop 125–145 Ma seafloor. We propose a method in which an isotropic 1-D model is first obtained from seafloor MT data through an iterative correction for topographic distortions; then, the anisotropic properties are inferred as deviations from the isotropic 1-D model. We investigate the performance of this method through synthetic forward modelling and inversion using plausible anisotropic 1-D models and the actual 3-D bathymetry and topography of the target region. Synthetic tests reveal that the proposed method will detect electrical anisotropy in the conductive upper mantle or electrical asthenosphere. We also compare the performance of the proposed scheme by using two rotational invariant impedances and two topographic correction equations. The comparison reveals that using different rotational invariants and correction equations provides relatively consistent results, but among the rotational invariants, the sum of squared elements (ssq) impedance yields better recovered results for topographically distorted data than the determinant impedance. An application of the method to seafloor MT array data sets from two areas in the northwestern Pacific reveals the possible presence of two layers of electrical anisotropy in the conductive mantle (

SUMMARY Ten years (2008–2017) of continuous measurements from 40 electric and 36 magnetic sites collected in China for earthquake prediction research represent a unique EM array data set, which can be used to explore the challenging problem of very long-period MT data acquisition, to study source characteristics, and ultimately to learn about electric conductivity of Earth's mantle beneath East Asia. In this study, we focus on basic noise and signal characteristics in this data set, and on estimation of the MT impedances. We report a novel method to fix the numerous timing errors in the electric data caused by limitations in instrumentation and data acquisition. Then, we use multivariate array analysis to study signal and noise characteristics for periods from 250 s to 3.5 × 105 s (4 d). Signal-to-noise ratios (SNR) are above 30 dB in magnetic fields for the first two dominant modes, which correspond roughly to N–S and E–W quasi-uniform sources. SNRs for electric fields are lower, especially at very long periods, and especially for N–S electric components. There are clear peaks in signal strength at the daily variation (DV) periods, but source structure becomes more complex, and significant biases in MT impedance tensors are more often seen at these periods. The MT quasi-impedance, computed using the closest magnetic site for each electric site, is estimated by robust remote reference techniques (RR) and by using linear combinations of PCA (principal component analysis) modes that best approximate a uniform or plane-wave source (PW). For almost all sites, smooth impedances are obtained for periods up to 104 s using either approach. This result, and a more detailed analysis of impedances estimates obtained with shorter-wavelength (gradient) sources extracted from the array, suggests that source effects in MT impedances are minimal for periods below 104 s, at least at the latitude of China. At many sites curves can be extended a decade further, to 105 s, but here results are improved by carefully omitting DV bands and (at a few sites) with the PW approach. For longer periods (>105 s) SNR is very low in electric field channels at most sites, making estimation challenging. However, at a few sites, even some near big cities (e.g. including a site within 40 km of Beijing) smooth impedance components related to N–S magnetic sources (Zxx and Zyx) are obtained to periods to 3.5 × 105 s (4 d). This result suggests that cultural noise may not be the main impediment to collecting very long-period MT data.

SUMMARY Multiphysics imaging for data inversion is of growing importance in many branches of science and engineering. Cross-gradient constraint has been considered as a feasible way to reduce the non-uniqueness problem inherent in inversion process by finding geometrically consistent images from multigeophysical data. Based on OCCAM inversion algorithm, a direct inversion method of 2-D profile velocity structure with surface wave dispersion data is proposed. Then we jointly invert the profiles of magnetotelluric and surface wave dispersion data with cross-gradient constraints. Three synthetic models, including block homogeneous or heterogeneous models with consistent or inconsistent discontinuities in velocity and resistivity, are presented to gauge the performance of the joint inversion scheme. We find that owning to the complementary advantages of the two geophysical data sets, the models recovered with structure coupling constraints exhibit higher resolution in the classification of complex geologic units and settle some imaging problems caused by the separate inversion methods. Finally, a realistic velocity model from the NE Tibetan Plateau and its corresponding resistivity model calculated by empirical law are used to test the effectiveness of the joint inversion scheme in the real geological environment.

SUMMARY The magnetotelluric (MT) impedance tensor has a nil diagonal when one of the axes of the coordinate system coincides with the strike of a 2-D structure. In general, real data are full tensors either because of 3-D effects or measurements not aligned to the geological strike. The usual practice to adapt the field tensor to the 2-D assumption is to rotate to a new system of coordinates. In most cases, there is no single angle of rotation that warranties that the diagonal elements become zeros as in the ideal 2-D case. Even maximizing the off-diagonal elements does not necessarily produce a nil diagonal. Consequently, the 2-D inversions proceed by neglecting whatever there is left in the diagonals. In this work, we explore an alternative that places no constraints on direction but assures a nil diagonal. We use two rotational invariants that compact the four elements of the tensor into only two and reduce in 2-D to the TE and TM impedances. These are obtained readily by solving a quadratic equation. We explore four different scenarios: (1) using the invariants, (2) rotating the tensor perpendicular to the profile, (3) rotating to the average maximum orientation for each station and (4) maximizing the off-diagonal elements of the tensor for each site, frequency to frequency. These approaches were applied to 3-D synthetic and field data. The field data correspond to two marine magnetotelluric surveys in the Gulf of California. In one of them, there is no information on the instrument orientation because the compasses failed. In this case, the rotational invariants come handy to overcome the problem. In the other survey, there was orientation information and the 2-D inversions illustrate the better performance of the invariants relative to the traditional approaches.

SUMMARY The magnetotelluric sounding (MTS) conducted on drifting ice floes in the Arctic is of particular interest for detection of 3-D inhomogeneities in crustal conductivity. Their manifestations in behaviour of standard magnetotelluric curves are shielded by the well conducting layers of seawater and sediment. As target objects, these inhomogeneities are shown as small changes (only hundredths of per cent) in apparent resistivity, and as very weak variations of vertical magnetic field component. Therefore, accounting for additional data on the vertical electric field component or the electric field potential of the transverse magnetic (TM) mode is of interest. For recording the TM-mode potential, we offer to use the circular electric dipole (CED) located on the ice surface as a receiver in MTS. We investigate possibilities of crustal 3-D inhomogeneity detection using the TM-mode potential received with the CED. A numerical experiment showed that the potential of electric field of the TM-mode on the ice surface reaches values suitable for detection. Examples of the inverse problem solution using synthetic data on the TM-mode field are presented. We found that the information on the depth of 3-D object could be obtained by the analytical continuation of the profile data on the TM-mode potential. Using the MTU-5 Phoenix Geophysics stations for the CED system, we carried out preliminary measurements on the ice surface on the Ob river. We checked the measurement results of the TM-mode potential using the MTS data obtained earlier near the CED system. We compared results of the TM-mode potential calculation for the found 3-D-medium model to the CED experimental data. Our results showed that the potential dependences on the period measured by the СED and those obtained for the 3-D model have similar characteristics. We believe recording the TM-mode potential in the Arctic allows to reveal the existence and characteristics of crustal conductivity 3-D heterogeneities.

Summary Magnetotelluric (MT) data allow for electrical resistivity probing of the Earth's subsurface. Integration of resistivity models in passive margin studies could help disambiguate non-unique interpretations of crustal composition derived from seismic and potential field data, a recurrent issue in the distal domain. In this contribution, we present the first marine MT data in the Barents Sea, derived from industrial controlled-source electromagnetic (CSEM) surveys. We characterize data quality, dimensionality, depth penetration and elaborate an analysis strategy. The extensive MT database consists of 337 receivers located along 7 regional transects, emanating from ∼70,000 km2 of 3D CSEM surveys acquired for hydrocarbon exploration from 2007 to 2019. High-quality MT data are extracted for periods ranging from 0.5 s to 5000 s. The data show no apparent contamination by the active source nor effects related to large time-gaps in data collection and variable solar activity. Along receiver profiles, abrupt lateral variations of apparent resistivity and phase trends coincide with major structural boundaries and underline the geological information contained in the data. Dimensionality analysis reveals a dichotomy between the western domain of the SW Barents Sea, dominated by a single N-S electromagnetic strike, and the eastern domain, with a two-fold, period-dependent strike. 35 receivers show 3D distortion caused by nearby bathymetric slopes, evidenced by elevated skew values. We delineate geographical areas where the 2D assumption is tenable and lay the foundation for future MT modelling strategies in the SW Barents Sea. We performed 2D MT inversion along one of the regional transects, a ∼220 km-long, E-W profile encompassing a major structural high and sedimentary basin approaching the continent-ocean transition. The resistivity model reveals low crustal resistivity values (1–10 Ω.m) beneath the deep sedimentary basins, in marked contrast with high resistivity values (1000–5000 Ω.m) of the thick crystalline crust on the structural high. We interpret this abrupt lateral resistivity variation as a rapid transition from a thick, dry continental crust to a hyperextended and hydrated crustal domain. Integration of resistivity with seismic velocity, density and magnetic susceptibility models may further refine these structural models and the underlying tectonic processes in the SW Barents Sea margin. Our methodology is applicable globally where 3D CSEM surveys are acquired and has a large potential for harvesting new knowledge on the electrical resistivity properties of the lithosphere.

SUMMARYTectonic activity is very difficult to study in the Santorini volcanic complex (SVC) as it comprises a cluster of small/awkwardly shaped islands covered by pyroclastic deposits from which tell-tale markers are swiftly erased, while seismicity is generally absent. We address the problem by combining geophysical exploration methods to evaluate the long-term effects of tectonic deformation and time-lapse differential GPS to directly evaluate the magnitude and kinematics of present-day deformation. The former comprise 3-D gravity modelling to investigate the footprint of tectonics on the pre-volcanic Alpine basement and natural-field EM induction to map conductivity anomalies epiphenomenal to fluid circulation in faults. Our analysis identified the following principal tectonic elements:The Trans-Santorin Divide (TSD), a segmented NNW–SSE dextral strike-slip fault splitting the SVC sideways of the line joining Cape Exomytis, the Kammeni Islets and the Oia–Therassia Strait. It is collocated with a major vertical conductive zone and forms a series of dents and depressions in the basement. The Columbo Fault Zone (CFZ) is a pair of parallel NE–SW subvertical normal-sinistral faults straddling the northern SVC and terminating against the TSD; it may be associated with fluid injection into the shallow crust but appears to have limited effect on crustal conductivity (compared to TSD). The Anhydros Fault Zone (AFZ) is detected by its footprint on the basement, as a set of parallel northerly dipping NE–SW faults between the Athinios–Monolithos line and Fira. If it has any heave, it is left-lateral. It does not have distinguishable electrical signature and does not contribute to present-day horizontal deformation. The CFZ and AFZ are antithetic and form a graben containing the volcanic centre of Kammeni Islets.E–W extension was identified lengthwise of a zone stretching from Cape Exomytis to Athinios and along the east flank of the caldera to Imerovigli. N–S normal faulting confirmed therein, may have contributed to the localization of the east caldera wall. NNE–SSW compression was observed at SW Thera; this may have produced E-W failure and contributed to the localization of the south caldera wall. The footprint of the caldera on the basement is a parallelogram with N–S long and WNW–ESE short dimensions: if the east and south flanks collapsed along N–S normal and E–W inverse failures, then the west and north flanks may have formed analogously. Present-day deformation is localized on the TSD and CFZ: this can only be explained if the former is the synthetic (dextral) Riedel-R shear and the latter the antithetic (sinistral) Riedel-R′ shear, generated by N–S σ1 and E–W σ3 principal stress axes. Accordingly, NW–SE right-lateral shearing of the broader area is expected and indicated by several lines of indirect evidence. The geographic extent of this shearing and its role in the regional tectonics of the south Aegean remains to be confirmed and appraised by future research.Contemporary volcanic centres develop at the interface of the TSD with the CFZ/AFZ graben; volcanism appears to be controlled by tectonics and the SVC to be shaped by tectonic rather than volcanic activity.

For magnetotelluric (MT) data with a large coverage, inversion interpretation for the whole area is usually impossible, due to the physical limitation of a computer. However, for each MT station, it is only sensitive to a region (called MT footprint) localized immediately underneath the station, much smaller than the whole survey area. Thus the whole large survey area can be subdivided into many sub-areas and inverted using the moving footprint technique efficiently. Therefore, to understand the MT footprint is of great importance for developing such an inversion algorithm. In this paper, we study the influence of frequency, resistivity and existence of anomalous structure in the MT footprint. We first use the finite difference method to evaluate the total electromagnetic (EM) fields, then apply the numerical volume integration of tensor green's function to calculate the total induced fields at the receiver (summation of the contribution for each cell). The footprint is obtained based on the contribution of each cell to the total induced fields. Different models are designed to understand the footprint for MT problems. An increase in frequency or conductivity results in a decrease in the size of footprint. Existence of anomalous bodies within the footprint affects the MT footprint size to a degree, dependent on the overall resistivity. Outside the footprint, the existence of anomalous bodies have little effect on the footprint size and shape.

SUMMARY The investigation of using a novel radial-basis-function-based mesh-free method for forward modelling magnetotelluric data is presented. The mesh-free method, which can be termed as radial-basis-function-based finite difference (RBF-FD), uses only a cloud of unconnected points to obtain the numerical solution throughout the computational domain. Unlike mesh-based numerical methods (e.g. grid-based finite difference, finite volume and finite element), the mesh-free method has the unique feature that the discretization of the conductivity model can be decoupled from the discretization used for numerical computation, thus avoiding traditional expensive mesh generation and allowing complicated geometries of the model be easily represented. To accelerate the computation, unstructured point discretization with local refinements is employed. Maxwell’s equations in the frequency domain are re-formulated using $\mathbf {A}$-ψ potentials in conjunction with the Coulomb gauge condition, and are solved numerically with a direct solver to obtain magnetotelluric responses. A major obstacle in applying common mesh-free methods in modelling geophysical electromagnetic data is that they are incapable of reproducing discontinuous fields such as the discontinuous electric field over conductivity jumps, causing spurious solutions. The occurrence of spurious, or non-physical, solutions when applying standard mesh-free methods is removed here by proposing a novel mixed scheme of the RBF-FD and a Galerkin-type weak-form treatment in discretizing the equations. The RBF-FD is applied to the points in uniform conductivity regions, whereas the weak-form treatment is introduced to points located on the interfaces separating different homogeneous conductivity regions. The effectiveness of the proposed mesh-free method is validated with two numerical examples of modelling the magnetotelluric responses over 3-D conductivity models.

SUMMARY Joint inversion of multiple electromagnetic data sets, such as controlled source electromagnetic and magnetotelluric data, has the potential to significantly reduce uncertainty in the inverted electrical resistivity when the two data sets contain complementary information about the subsurface. However, evaluating quantitatively the model uncertainty reduction is made difficult by the fact that conventional inversion methods—using gradients and model regularization—typically produce just one model, with no associated estimate of model parameter uncertainty. Bayesian inverse methods can provide quantitative estimates of inverted model parameter uncertainty by generating an ensemble of models, sampled proportional to data fit. The resulting posterior distribution represents a combination of a priori assumptions about the model parameters and information contained in field data. Bayesian inversion is therefore able to quantify the impact of jointly inverting multiple data sets by using the statistical information contained in the posterior distribution. We illustrate, for synthetic data generated from a simple 1-D model, the shape of parameter space compatible with controlled source electromagnetic and magnetotelluric data, separately and jointly. We also demonstrate that when data sets contain complementary information about the model, the region of parameter space compatible with the joint data set is less than or equal to the intersection of the regions compatible with the individual data sets. We adapt a trans-dimensional Markov chain Monte Carlo algorithm for jointly inverting multiple electromagnetic data sets for 1-D earth models and apply it to surface-towed controlled source electromagnetic and magnetotelluric data collected offshore New Jersey, USA, to evaluate the extent of a low salinity aquifer within the continental shelf. Our inversion results identify a region of high resistivity of varying depth and thickness in the upper 500 m of the continental shelf, corroborating results from a previous study that used regularized, gradient-based inversion methods. We evaluate the joint model parameter uncertainty in comparison to the uncertainty obtained from the individual data sets and demonstrate quantitatively that joint inversion offers reduced uncertainty. In addition, we show how the Bayesian model ensemble can subsequently be used to derive uncertainty estimates of pore water salinity within the low salinity aquifer.

Geophysical Journal International, 215 (3), ISSN:0956-540X, ISSN:1365-246X

In this paper, the authors present the result of three-dimensional (3D) direct inversion of gravity data to determine the Moho depth along the Red River Fault (RRF) zone and adjacent areas in the land territory of Vietnam and the Gulf of Tonkin. The Moho depth was calculated by 3D Parker inversion supplemented with the gravity anomaly power density spectrum. The given Moho depth and the available teleseismic and magnetotelluric Moho depth are well matched. A structural Moho map of the study region was constructed from the interpretation results. The Moho surface was uplifted along the RRF zone. The shallowest Moho depth was determined in the Red River Basin (RRB), where it ranges from 22 ± 1.0 km to 24 ± 1.0 km in the depocenter of the RRB in the Gulf of Tonkin to 27 ± 1.0 km to 29 ± 1.0 km in the Red River Delta. It is 29 ± 1.0 km to 32 ± 1.0 km from Vietri to Laocai provinces. The deepest Moho depth was found in the northwesternmost and northernmost land territory of Vietnam, where it ranges from 33 ± 1.0 km to 35.8 ± 1.0 km. A percentage of the Earth's crustal compensation is also investigated. The obtained crustal thickness and compensation state are useful for their implications regarding the geodynamics and tectonics of the RRF and surrounding region.

We present a multi-resolution approach for 3-D magnetotelluric forward modelling. Our approach is motivated by the fact that fine-grid resolution is typically required at shallow levels to adequate ...

In order to understand directly and analyze accurately the detected magnetotelluric (MT) data on anisotropic infinite faults, two-dimensional partial differential equations of MT fields are used to establish a model of anisotropic infinite faults using the Fourier transform method. A multi-fault model is developed to expand the one-fault model. The transverse electric mode and transverse magnetic mode analytic solutions are derived using two-infinite-fault models. The infinite integral terms of the quasi-analytic solutions are discussed. The dual-fault model is computed using the finite element method to verify the correctness of the solutions. The MT responses of isotropic and anisotropic media are calculated to analyze the response functions by different anisotropic conductivity structures. The thickness and conductivity of the media, influencing MT responses, are discussed. The analytic principles are also given. The analysis results are significant to how MT responses are perceived and to the data interpretation of the complex anisotropic infinite faults.

International audience; S U M M A R Y Accurate geophysical imaging of shallow subsurface features provides crucial constraints on understanding the dynamics of volcanic systems. At Furnas Volcano (Azores), intense circulation of volcanic fluids at depth leading to high CO 2 outgassing and flank destabilization poses considerable threat to the local population. Presented is a novel 3-D electrical resistivity model developed from 39 magnetotelluric soundings that images the hydrothermal system of the Furnas Volcano to a depth of 1 km. The resistivity model images two conductive zones, one at 100 m and another at 500 m depth, separated by a resistive layer. The shallow conductor has conductivity less than 1 S m −1 , which can be explained by clay mineral surface conduction with a mass fraction of at least 20 per cent smectite. The deeper conductor extends across the majority of the survey area. This deeper conductor is located at depths where smectite is generally replaced by chlorite and we interpret it as aqueous fluids near the boiling point and infer temperatures of at least 240 • C. The less conductive layer found between these conductors is probably steam-dominated, and coincides within the mixed-clay zone found in many volcanic hydrothermal systems.

This paper analyses the 3D magnetotelluric (MT) response in the presence of resistivity frequency dispersion. The aim is to give further insight into this topic, already approached in previous papers dedicated to 1D and 2D cases. We show the MT diagrams along three parallel profiles, normal to the longitudinal axis of a dispersive conductive prism of finite horizontal and vertical extent, buried in a non-dispersive resistive half-space. The Cole–Cole dispersion law has been assumed to represent the dispersion features of the prism. The MT responses along the same profiles in the complete absence of dispersion effects are also provided for reference. The results confirm that the TE mode, as in the 2D case, is mostly affected by dispersion. Compared with the non-dispersive responses, a notable increase in the amplitude of the anomaly is observed along the profile passing through the center of the prism, while an increase in its width is the effect along the profile above the edge of the prism. As a field example, the MT profile in the eastern Snake River Plain geothermal area is considered. The profile was already dealt with in a previous paper by a 2D dispersive MT modeling approach, by which a dispersive slab of infinite length in the direction normal to the MT profile, immersed in a 1D layered host, was modeled. A finite length of the dispersive conductive slab is now derived, validated by the increased goodness of fit between the field TM and TE pseudosections and the synthetic ones from the 3D model compared with the misfit previously obtained by the 2D approach. Finally, an explanation of the chargeability and main values of the time constant assumed to fit the field data is attempted in terms of the abundance of hydrothermal alteration products and temperature, respectively, inside the dispersive slab.

Author(s): Lindsey, NJ; Kaven, JO; Davatzes, N; Newman, GA | Abstract: Previous magnetotelluric (MT) studies of the high-temperature Coso geothermal system in California identified a subvertical feature of lowresistivity (2-5Ohmm) and appreciable lateral extent (g1 km) in the producing zone of the East Flank field. However, these models could not reproduce gross 3-D effects in the recorded data. We perform 3-D full-tensor inversion and retrieve a resistivity model that out-performs previous 2-D and 3-D off-diagonal models in terms of its fit to the complete 3-D MT data set as well as the degree of modelling bias. Inclusion of secondary Zxx and Zyy data components leads to a robust east-dip (60°) to the previously identified conductive East Flank reservoir feature, which correlates strongly with recently mapped surface faults, downhole well temperatures, 3-D seismic reflection data, and local microseismicity. We perform synthetic forward modelling to test the best-fit dip of this conductor using the response at a nearby MT station. We interpret the dipping conductor as a fractured and fluidized compartment, which is structurally controlled by an unmapped blind East Flank fault zone.

We report on a study to explore the deep electrical conductivity structure of the Dead Sea Basin (DSB) using magnetotelluric (MT) data collected along a transect across the DSB where the left lateral strike-slip Dead Sea transform (DST) fault splits into two fault strands forming one of the largest pull-apart basins of the world. A very pronounced feature of our 2-D inversion model is a deep, subvertical conductive zone beneath the DSB. The conductor extends through the entire crust and is sandwiched between highly resistive structures associated with Precambrian rocks of the basin flanks. The high electrical conductivity could be attributed to fluids released by dehydration of the uppermost mantle beneath the DSB, possibly in combination with fluids released by mid- to low-grade metamorphism in the lower crust and generation of hydrous minerals in the middle crust through retrograde metamorphism. Similar high conductivity zones associated with fluids have been reported from other large fault systems. The presence of fluids and hydrous minerals in the middle and lower crust could explain the required low friction coefficient of the DST along the eastern boundary of the DSB and the high subsidence rate of basin sediments. 3-D inversion models confirm the existence of a subvertical high conductivity structure underneath the DSB but its expression is far less pronounced. Instead, the 3-D inversion model suggests a deepening of the conductive DSB sediments off-profile towards the south, reaching a maximum depth of approximately 12 km, which is consistent with other geophysical observations. At shallower levels, the 3-D inversion model reveals salt diapirism as an upwelling of highly resistive structures, localized underneath the Al-Lisan Peninsula. The 3-D model furthermore contains an E–W elongated conductive structure to the northeast of the DSB. More MT data with better spatial coverage are required, however, to fully constrain the robustness of the above-mentioned off-profile features.

Mid-crustal conductors are a common phenomenon in magnetotelluric studies. In the Andean Cordillera of southern Chile, they appear to concentrate along major fault zones. A high-resolution, broad-band magnetotelluric survey including 31 stations has been

We present a general finite-element solver, escript, tailored to solve geophysical forward and inverse modeling problems in terms of partial differential equations (PDEs) with suitable boundary conditions. Escript's abstract interface allows geoscientists to focus on solving the actual problem without being experts in numerical modeling. General-purpose finite element solvers have found wide use especially in engineering fields and find increasing application in the geophysical disciplines as these offer a single interface to tackle different geophysical problems. These solvers are useful for data interpretation and for research, but can also be a useful tool in educational settings. This paper serves as an introduction into PDE-based modeling with escript where we demonstrate in detail how escript is used to solve two different forward modeling problems from applied geophysics (3D DC resistivity and 2D magnetotellurics). Based on these two different cases, other geophysical modeling work can easily be realized. The escript package is implemented as a Python library and allows the solution of coupled, linear or non-linear, time-dependent PDEs. Parallel execution for both shared and distributed memory architectures is supported and can be used without modifications to the scripts.

Geophysical Journal International, 205 (2), ISSN:0956-540X, ISSN:1365-246X