Analysis Of Magnetotelluric Data Research Articles

Geoelectrical image of the Sabalan geothermal reservoir from magnetotelluric studies

The magnetotelluric (MT) survey of the Sabalan geothermal zone, which includes 22 stations along three traverse profiles across the main prospect zone, was conducted in 2007. The geophysical investigation utilized sharp boundary inversion of processed MT data due to the high contrast of electrical resistivity between embedded layers. This method was preferred to suppress the smeared-out impact of the smooth inversion algorithm on retrieving geological layering. The analysis of MT data led to the use of a 2D inversion algorithm to visualize the geothermal reservoir. Additionally, the geoelectrical strike of the main geothermal target was determined, showing some inconsistency across the profiles. The simulation of synthetic numerical models is conducted to evaluate the effectiveness of sharp boundary inversion compared to a smooth algorithm in sharp gradient mediums, specifically in terms of electrical characteristics, for plausible geothermal scenarios. The Finite Element method (FEM) was employed to construct these models, followed by running the 2D forward and inversion processes. The meshing and initial parameters of the modeling were set based on information obtained from prior geological and geophysical investigations. A sharp boundary inversion can be accurately implemented by introducing layering constraints along each profile surrounding the probable sought geothermal reservoir target. These layering constraints encompass the conductive cap, topsoil, and basement rocks, which are the key components of a plausible geothermal configuration. The results of the comparison indicate that the sharp boundary inversion technique effectively restored the boundary between neighboring layers that had significant differences in electrical resistivity. This was achieved when the smoothing algorithm failed to produce an accurate representation of the layered model. Ultimately, conceptual geological models were developed to illustrate the plausible structures corresponding to each profile, focusing on the sought geothermal reservoir targets. It can be deduced that many misunderstandings in the smoothing inversion algorithm will be omitted if the sharp boundary algorithm is used in highly electrical gradient structures. Based on the outcomes, a trachyandesite, andesite, and altered andesite reservoir exists at an elevation of 1600 to −2000 m. The reservoirs were recovered precisely with sharp and discriminable interfaces. Therefore, accurate interfaces led us to determine the conductive cap, heat source, and topsoil stacking as the main components of the geothermal system.

Retrieval of Subsurface Resistivity from Magnetotelluric Data Using a Deep-Learning-Based Inversion Technique

Inversion is a fundamental step in magnetotelluric (MT) data routine analysis to retrieve a subsurface geoelectrical model that can be used to inform geological interpretations. To reduce the effect of non-uniqueness and local minimum trapping problems and improve calculation speeds, a data-driven mathematical method with a deep neural network was developed to estimate the subsurface resistivity. In this study, a deep learning (DL) inversion technique using a revised multi-head convolutional neural network (CNN) architecture was investigated for MT data analysis. We created synthetic datasets consisting of 100,000 random samples of resistivity layers to train the network’s parameters. The trained model was validated with independent noised datasets, and the predicted results displayed reasonable accuracy and reliability, which demonstrates the potential application of DL inversion for real-world MT data. The trained model was used to analyze MT data collected in the southwestern Athabasca Basin, Canada. The calculated results from the DL method displayed a detailed subsurface resistivity distribution compared to traditional iterative inversion. Since this approach can predict a resistivity model without multiple forward modeling operations after the CNN model is created, this framework is suitable to speed up the computation of multidimensional MT inversion for subsurface resistivity.

Potential of the magnetotelluric data analysis block of the GIS INTEGRO complex

The article considers the possibilities of the electroprospecting complex for processing and analyze of magnetotelluric sounding data, implemented on the platform GIS INTEGRO. It performs input of magnetotelluric data in standard formats, their visualization in the form of frequency dependences and profile distributions, as well as analysis and primary interpretation. The complex allows to handle quickly the materials of modern geological exploration using the method of magnetotelluric sounding and to obtain information on the electrical properties of rocks in sections along a network of profiles.

Integrated Magnetotelluric (MT), Gravity and Seismic Study of Lower Kutai Basin Configuration

This work describes a subsurface basin configuration of the Lower Kutai Basin (hereinafter LKB) in East Kalimantan, Indonesia, as inferred from combination of magnetotelluric (MT), seismic, and gravity data. LKB is structurally controlled mainly by the Samarinda Anticlinorium extending in a NNE-SSW direction and is one of the most prolific hydrocarbon basins in Indonesia. The phase tensor analysis of MT data from most stations and frequencies exhibited a 2D character with a relatively low skew (-3° < β < 3°). The geo-electrical strike direction was estimated at N30°E, which is in good agreement with the regional geological strike with a NNE-SSW direction. 2D MT inversion modeling was performed to infer the subsurface resistivity distribution associated with LKB’s configuration. From the integration of MT, seismic and gravity models it was shown that LKB’s configuration is composed mainly of sandstone, black shale, claystone, and basement rocks. The conductive zones of the MT models are associated with thermal alteration of black shale, which changes its mineralization, leading to lower resistivity. Hence, the black shale may be interpreted as potential hydrocarbon source rock in LKB.

Magnetotelluric data analysis using 2D inversion: A case study from Al-Mubazzarah Geothermal Area (AMGA), Al-Ain, United Arab Emirates

Geothermal manifestations (hot springs) emerge in the Al-Mubazzarah Geothermal Area (AMGA), Al-Ain city, Abu Dhabi Emirate, United Arab Emirates. This paper presents the application and results of a Magnetotelluric (MT) survey, which was carried out in 2017 at the AMGA geothermal field. The MT method was used to investigate the variations in the electrical conductivity beneath the AMGA. This study focuses on characterizing the patterns of subsurface electrical conductivity of the AMGA geothermal reservoir. Dimensionality analysis of the measured MT data indicate that 2D inversion is appropriate for the subsurface resistivity interpretation. The inversion results support a model consisting of three resistivity-defined layers; from top to bottom they are: (1) a shallow layer with resistivity ranging from 10 to 20 Ωm, representing recent alluvial and windblown deposits, (2) a second conductive layer with resistivities less than 10 Ωm, beneath the first layer. This layer is recognized as the Tertiary carbonate sequence in the region, (3) a deep, moderate to relatively high resistive zone, 10–30 Ωm beginning at 800 m depth and reaching 4 km depth in the northern part of the profile, representing Mesozoic basement rocks. The observed moderate to high resistivity zone (10–30 Ωm) in the 2D model may represent a region where the hot groundwaters originated (geothermal reservoir), with the hottest geothermal located at a depth greater than 4 km. The geothermal reservoir zone is also represented by a low to high density contrast and a low to moderate magnetic susceptibility, as indicated in the inverted potential field data models, and confirmed the existence of a north dipping major fault.

Dimensionality and directionality analysis of magnetotelluric data by using different techniques: A case study from northern part of Saurashtra region, India

Dimensionality and directionality analysis of magnetotelluric data by using different techniques: A case study from northern part of Saurashtra region, India

A Comparison of impedance invariants and GB decomposition method for magnetotelluric data analysis; a case study from shallow sedimentary basins of SW Iran

Dimensionality and distortion analysis of the measured magnetotelluric (MT) data are challenging procedures to conclude the main properties of regional geo-electric structure (e.g., its strike direction) and quantify the influence of superficial distorting bodies. In this paper, we applied several analysis methods to the same data set from southwest Zagros to determine the dimensionality and degree of the inherent distortion in the data and to recover the regional responses. We conclude that the regional 2-D responses can be better retrieved in a two-step procedure: we first classify the dimensionality and distortion by impedance invariants and then the Groom-Baily (GB) method is applied to remove distortion effects and estimate the regional strike and principal impedances.We investigate an Audio-magnetotelluric (AMT) data set acquired at 19 stations along a profile in the southwest of simply folded belt (SFB) of the Zagros Mountain. The Bahr and WAL invariants confirm regional 1D and 2D structures with local galvanic distortion, at most periods. The Groom-Bailey (GB) decomposition of MT impedance data reveals approximately period-independent distortion parameters. The strike analysis on all data whose principal phase differences are greater than 5˚ reveals a regional trend of about 30˚. 2-D inductive effects were also retrieved by removing distortion effects from the measured data

Magnetotelluric Data Analysis Using Phase Tensor and Tipper Strike to Determine Geoelectrical Strike in “DKH” Geothermal Field

The magnetotelluric (MT) method is passive geophysical method that measures natural electromagnetic field to estimate the subsurface resistivity structure. Before conducting 2D MT data inversion, data analysis, dimensionality, including determining the direction of the geoelectrical strike must be performed. It is needed to fulfill the assumption of 2D MT modeling. 2D modeling of magnetotelluric data is an important part because the results can determine the quality of the subsurface interpretation. Therefore, this paper describes the determination of the dimensonality and geoelectrical strike using the phase tensor and tipper strike parameters. The invariant parameters of phase tensor including skew angle (β) and ellipticity are used to get the information about the dimensionality and geoelectrical strike. The results from the phase tensor are plotted in a rose diagram to determine the main direction of the geoelectrical strike. The phase tensor analysis contains ambiguity 90°, therefore tipper strike used to validate the analysis. From the analysis, the result shows the 1D or 2D structures of the MT data in high frequency, meanwhile the lower frequency, the majority of the data show 3D structures. The dominant geoelectrical strike is around N15°E and it is confirmed with the regional structure of the area. Determination of dimensionality and geoelectrical strike are applied to MT data from the geothermal field.

Subsurface geoelectrical structure from 3-d inversion of magnetotelluric data of Gisenyi geothermal field, western part of Rwanda

In this paper, a set of Magnetotelluric (MT) data collected from 69 stations at the Gisenyi geothermal field in the western part of Rwanda were used to derive a 3D electrical resistivity distribution model to investigate deep geologic structures related to the geothermal resources. The MT method is a geophysical technique commonly used in geothermal exploration projects. The aim of this study is to investigate subsurface electrical resistivity distribution in the Gisenyi geothermal area to gain an understanding of the process related to geothermal activities and to model deep crustal structures. MT data analysis using phase tensors revealed the nature of the subsurface structures of the geothermal area prior to performing a 3D MT inversion. The dimensionality analysis results revealed a 3D conductivity structure in the deeper portion. The ModEM 3D MT inversion code was used to infer the subsurface electrical distribution from the MT responses. The recovered 3D resistivity distribution model showed a conductive region (<10 Ω.m) at deeper depths beneath a thick high-resistivity zone with a resistivity more than 100 Ω.m. The 3D model indicates patches of high and low resistivity at shallow depths related the presence of recent volcanic lavas and basement rocks from the surface to depths of a few hundred meters. The MT model revels important subsurface structures that can be related to the geothermal resource in the area. The deep-seated conductive body is presumably associated to the heat source of the Gisenyi geothermal system.

Subsurface structure identification of ‘X’ geothermal prospect area based on gravity and magnetotelluric data

Exploration stage has the highest risk, due to discover the interest reservoir zone include high temperature and pressure, middle to high permeability, and non-acidic fluid. Permeability is one of the most difficult parameters to be determined in this stage. Fault zone could be able to control and provide fluid flow within reservoir. It is usually related to high permeability zone. Fault identification become an important and interesting parameter in to be explore. The aim of this research is identified subsurface structure to build up a conceptual model of geothermal system. The methods include magnetotelluric data analysis and 3D MT structure interpretation together with first horizontal derivative and second vertical derivative analysis of gravity data. Geochemistry and geology data are used as supporting data. There are structure types of normal and reverse fault identified in the research area. MT and gravity interpretation show that structures are direct to northeast-southwest and northwest-southeast. Up-flow zone is predicted between TPP-TPB manifestations. Structures supposed to highly contribute in providing permeability zone in the research area. Manifestations fluid have the characteristics of meteoric and magmatic fluid (volcano-hydrothermal system). The clay cap thickness average is predicted between 500–700 m. The reservoir thickness average is predicted between 1500–2000 m. Temperature estimation of the reservoir is 240–280 °C. Heat source might be located between TPB and TPP manifestations. Subsurface structure analysis might be useful to increase success ratio in drilling.

Implications for the lithospheric structure of Cambay rift zone, western India: Inferences from a magnetotelluric study

Broad-band and long-period magnetotelluric (MT) data were acquired along an east-west trending traverse of nearly 200 ​km across the Kachchh, Cambay rift basins, and Aravalli-Delhi fold belt (ADFB), western India. The regional strike analysis of MT data indicated an approximate N59°E geoelectric strike direction under the traverse and it is in fair agreement with the predominant geological strike in the study area. The decomposed transverse electric (TE)- and transverse magnetic (TM)-data modes were inverted using a nonlinear conjugate gradient algorithm to image the electrical lithospheric structure across the Cambay rift basin and its surrounding regions. These studies show a thick (~1–5 ​km) layer of conductive Tertiary–Mesozoic sediments beneath the Kachchh and Cambay rift basins. The resistive blocks indicate presence of basic/ultrabasic volcanic intrusives, depleted mantle lithosphere, and different Precambrian structural units. The crustal conductor delineated within the ADFB indicates the presence of fluids within the fault zones, sulfide mineralization within polyphase metamorphic rocks, and/or Aravalli-Delhi sediments/metasediments. The observed conductive anomalies beneath the Cambay rift basin indicate the presence of basaltic underplating, volatile (CO2, H2O) enriched melts and channelization of melt fractions/fluids into crustal depths that occurred due to plume–lithosphere interactions. The variations in electrical resistivity observed across the profile indicate that the impact of Reunion plume on lithospheric structures of the Cambay rift basin is more dominant at western continental margin of India (WCMI) and thus support the hypothesis proposed by Campbell & Griffiths about the plume–lithosphere interactions.

Dimensionality analysis of MT data using Mohr circle: A case study from Rewa–Shahdol region, India

Mohr circle in magnetotelluric (MT) is being used to represent dimensionality of subsurface structure. Mohr circle plot on individual axes for each frequency represents the dimensionality concerning frequency, whereas plotting of Mohr circle on common Cartesian coordinates for all frequencies displays the effects of noise on the impedance tensor. Here, we examine the Rewa–Shahdol region MT data with the Mohr circle approach to understand the sub-surface dimensionality structure, but the presence of noise in the signal has randomized the Mohr circle response. Hence, we made an effort to obtain the subsurface dimensionality using Mohr circle properties and derive two new invariants such as tan ϕ and sin θ. The two invariants represent the dimensionality and anisotropy nature of the subsurface structure. Results from the Mohr circle together with the new angles brought out the 1-D graben structure of the Gondwana and Vindhyan basins, 2-D/3-D nature of the underlying basement structure and the crustal structure below the Narmada–Son Lineament (NSL) zone. 2-D/3-D nature of the NSL zone represents the basement uplifted horst structure between Gondwana and Vindhyan basins. Further, the horst-graben structure of NSL zone is evident from the Mohr circle analysis suggesting of rifting and block movement.

3D inversion of MT data from northern Mexico for geothermal exploration using TEM data as constraints

San Felipe's area has geothermal surface manifestations such as hot springs and high temperature in drinking water wells. As a first stage of the geothermal exploration program, a regional geophysical survey has been carried out where 17 MT (Magnetotelluric) and TEM (Transient Electromagnetic) soundings were recorded along 4 different profiles. Both techniques were applied in the same spots and the TEM data are used to correct for static shift in MT data. Transient responses were measured from 1 to 70 ms, using a 50 x 50m2 single-loop configuration. MT signals were recorded with Metronix systems in the period range from 0.001 to 100 s. A robust processing scheme is applied to the MT time series and estimated Transfer Functions are appraised. The dimensionality analysis of MT data suggests a 1D and 3D subsurface structure. Thereby, a three-dimensional inversion of MT data is later carried out. The information obtained from the inversion modeling of TEM data is incorporated into the 3D MT inversion scheme. Thus, unconstrained and constrained inverse models are achieved and appraised. The preferred 3D inversion model is correlated to geological information of the survey area and a conductive structure is interpreted as a sedimentary basin at its shallow part and as a fault zone with geothermal fluids at depths >1.5 km.

Geophysical survey of geothermal energy potential in the Liaoji Belt, northeastern China

The Liaodong area which lies in the Liaoji Belt of northeastern China is rich in geothermal resources, but locating the resources is challenging. Here non-seismic geophysical data, existing geological data and the physical properties of rocks are examined to locate and characterise potential geothermal resources. High-precision gravity, aeromagnetic and magnetotelluric data are analysed to determine the characteristics of the hidden faults in the region and map the spatial distribution of the subsurface rock mass and its properties. The distribution of hidden faults and the basic characteristics of the rock mass were determined via Euler deconvolution of the gravity and aeromagnetic data. The subsurface extension of concealed faults, the spatial distribution of intrusive rocks, and the specific locations of underground thermal storage structures were determined from magnetotelluric data analysis, thereby enabling geothermal energy targets to be identified. The results highlight four potential sites of geothermal energy development in the study area, which indicate that non-seismic geophysical data can provide reliable clues to the geothermal energy potential in a given region.

Development of the MTpy software package for magnetotelluric data analysis

The magnetotelluric (MT) method is becoming more widely used in the geoscience community as it becomes increasingly recognised as a useful exploration tool. However, while the analysis and inversion tools available to the MT community have increased over recent years, the software available to work with these tools is still somewhat limited and often costly in comparison to some of the more mature techniques like gravity, magnetics and seismic. The MTpy python library is open source software that aims to assist MT practitioners in carrying out the processing and analysis steps that need to be carried out with MT data and in working with the various inversion codes that are available. However, MTpy still contains coding issues, bugs and gaps in functionality, which have limited its use to date. We are currently developing MTpy to rectify these problems and expand the functionality, and thus facilitate the use of MT as an exploration technique. Key improvements include adding new functions and modules, refactoring the code to give better quality and consistency, fixing bugs and adding new Graphic User Interfaces.

The Los Humeros (Mexico) geothermal field model deduced from new geophysical and geological data

The Los Humeros volcanic complex, a 21×15km diameter caldera edifice nesting volcanic domes and a complex faulting structure, is located at the eastern edge of the Trans-mexican volcanic belt (TMVB). It is a young edifice (<0.5Ma) that hosts one of the five main geothermal fields of Mexico with still an important energy production (∼65kW installed capacity). Being one of the more studied producing fields, the geothermal system of the caldera is largely unknown at depths greater than ∼2.4km, which is the approximate penetration range of the available geothermal wells. Here we present the results of a geophysical survey in Los Humeros caldera and surroundings with the aim to provide further insight on the physical characteristics of the geothermal system at depths greater than 2.4km. The survey comprised 70 broadband magnetotelluric (MT) soundings distributed within and in the periphery of the caldera edifice of which we present here three EW profiles. We also accomplished a mesh of 718 accurately leveled gravity stations. These data sets were complemented with 13 TDEM soundings for static shift control, as well as with the aero-magnetic digital chart of the area (#E14-3, SGM, 2004). The MT data analysis yielded an average electric azimuth of N23W for the central profile where the field production is concentrated, which is quite consistent with the mapped NW-SE geological structure. However, at individual frequency ranges the strike follows the local faulting structures, most of which are apparently controlled by the deeper crater structure. At the production zone, the conductivity model reveals the existence of an eastward dipping resistive body, which follows the isotherms registered at the wells. The inclined conductivity interface above it seems to play an important role in controlling the heat and fluid flow towards shallow depths. Petrographic studies of well samples provide evidence of mineralogical assemblages that suggest magnetite-metasomatose hydrothermal alteration. The production zone coincides with maximum gravity and magnetic gradients, at the western edge of the well-defined circular crater anomaly. At this point the central MT section (profile 1) shows the shallower depth (2–3km) to the relatively high resistivity (∼400–500ohm-m) and magnetized intrusive-like body. The interpreted geophysical and surface geological data backed by well data support a reservoir and plume model structure consisting of a resistive propylitic core that feds the geothermal field through fractures and deep- seated faults. Surface conductors associated with stratified mineralization produced from lixiviation of geothermal fluids are well differentiated from deeper conductors. The better preserved northern sector of the Humeros caldera yields an anomalous deep conductor at depths of 6–7km below sea level (profile 5) as well as at the central production sector (profile 1) whereas the anomalous conductivity zone at the southern profile (profile 6) could be as shallow as 5km below sea level. These anomalous conductivity zones are expected to be associated to the primary energy source of the Los Humeros geothermal system in the form of partial fusion or hypercritical trapped fluids within the upper crust, conditions that have been concluded to prevail along cordilleras of the American continent (e.g. Hyndman, 2017).

Use of ssq rotational invariant of magnetotelluric impedances for estimating informative properties for galvanic distortion

Several useful properties and parameters—a model of the regional mean one-dimensional (1D) conductivity profile, local and regional distortion indicators, and apparent gains—were defined in our recent paper using two rotational invariants (det: determinant and ssq: sum of squared elements) from a set of magnetotelluric (MT) data obtained by an array of observation sites. In this paper, we demonstrate their characteristics and benefits through synthetic examples using 1D and three-dimensional (3D) models. First, a model of the regional mean 1D conductivity profile is obtained using the average ssq impedance with different levels of galvanic distortion. In contrast to the Berdichevsky average using the average det impedance, the average ssq impedance is shown to yield a reliable estimate of the model of the regional mean 1D conductivity profile, even when severe galvanic distortion is contained in the data. Second, the local and regional distortion indicators were found to indicate the galvanic distortion as expressed by the splitting and shear parameters and to quantify their strengths in individual MT data and in the dataset as a whole. Third, the apparent gain was also shown to be a good approximation of the site gain, which is generally claimed to be undeterminable without external information. The model of the regional mean 1D profile could be used as an initial or a priori model in higher-dimensional inversions. The local and regional distortion indicators and apparent gains could be used to examine the existence and to guess the strength of the galvanic distortion. Although these conclusions were derived from synthetic tests using the Groom–Bailey distortion model, additional tests with different distortion models indicated that these conclusions are not strongly dependent on the choice of distortion model. These galvanic-distortion-related parameters would also assist in judging if a proper treatment is needed for the galvanic distortion when an MT dataset is given. Hence, this information derived from the dataset would be useful in MT data analysis and inversion.Graphical abstract1D models obtained by inverting the average det (left) and ssq (right) impedances from the 1D datasets distorted with different galvanic distortion strength. The synthetic profile is also shown by a dashed line for comparison. Note that all profiles inverted from average ssq impedances at any distortion strength are almost identical.

Reply to “Three-dimensional galvanic distortion of three-dimensional regional conductivity structures: Comment on Three-dimensional joint inversion for magnetotelluric resistivity and static shift distributions in complex media” by A. G. Jones

[1] Static shift and spatial aliasing are the bane of magnetotelluric exploration and Jones [2011] provides two useful comments on the issue of accounting for static shift in 3-D inversion, one concerning Sasaki and Meju [2006] and another concerning Zhdanov et al. [2011]. This is followed by an overview of well-known impedance tensor relationships in magnetotelluric data analysis (equations (1)–(9)), on which he based his comments. The main tenet of his discussion is that in a fully 3-D situation, all the elements of the impedance tensor (Zxy, Zxx, Zyx and Zyy) are nonzero and should be considered in any 3-D inversion approach. In practice, for geological interpretation, we commonly define apparent resistivities and phases using Zxy and Zyx, which permit an assessment of whether they are influenced by galvanic distortion (i.e., static shift) from their expected background levels especially by comparison with data from coincident alternative EM soundings which are less affected by small size near-surface heterogeneities such as the transient EM method [e.g., Sternberg et al., 1988; Meju, 1996, 2005]. There are other ways of determining and removing static shifts using only MT data as mentioned by Jones [2011] and these have their strengths and weaknesses. Whatever the adopted approach, for a long time, static shift is commonly identified on apparent resistivity sounding curves while the phase curves are assumed to be unaffected, even though Jones [2011] use physical arguments to suggest that phase mixing rather than geometric amplitude shifts obtains in complex 3-D environments at all spatial scales. Regional galvanic distortions are fully accounted for in 3-D inversions within the accuracy of the forward modeling unless the model discretization is inadequate. However, there is one type of galvanic distortions that cannot be handled in the conventional 3-D MT inversion. That is the distortions due to near-surface inhomogeneities located just below the receiver sites, because such inhomogeneities cannot be accurately represented by the given model discretization in the inversion. These include random conductive weathered patches (model A of Sasaki and Meju [2006]), elongate conductors such ancient stream channels (model B of Sasaki and Meju [2006]) and covered waste dump sites, all common in real life. The Sasaki and Meju [2006] approach to removing static shifts was introduced to tackle this type of galvanic distortions, assuming that the distortions can be represented by static shifts (and it was shown in Figures 3 and 5 that the phases are unaffected for model A and are partially affected at higher frequencies for model B). [3] It is important to stress that the purpose of inversion is to find a model that explains the observed data quantitatively. In the first experiment in our paper (model A), we show that joint estimation of resistivities and static shifts improve the inversion image in the presence of surficial inhomogeneities that are too small to be recovered by the conventional 3-D inversion. In the next experiment (model B), we show that our method is still effective in recovering resistivity distribution even if the effects of near-surface structure include frequency-dependent shifts. The necessary constraint, which serves as a form of a priori information, is provided by the assumption of a Gaussian or a zero-sum distribution of static shifts (justified by numerous alternative EM field measurements) in our inverse problem formulation (i.e., the β2S term in equation (1) above), not by the assumption that the MT phase is unaffected by static shift. While we put emphasis on static shifts arising from the weathered surficial layer, deeper structures may yield static effects if the period range starts at, say, 10 s. This is just a matter of the spatial scale of operation. Our 3-D algorithm is full domain and can handle such a situation. It should also be borne in mind that the mathematical representations of the complex subsurface process on which Jones [2011] based his comments are necessarily simple approximations of physical reality.

Possibilities of interpretation of the magnetotelluric data, obtained on a single profile over 3D resistivity structures

Magnetotelluric soundings are frequently carried out on a single profile or on profiles remote from each other. Interpretation of the obtained data is difficult in the presence of spatially heterogeneous geoelec� tric structures. We evaluate its capabilities on the basis of the synthetic data, that correspond to a geoelectric model, which consists of a threelayered section in th e background and three rectangular prisms, differently arranged relative to the profile. Using the simple methods of analysis of magnetotelluric data, we succeeded in allocating all three heterogeneities over the area that surrounds the profile of observations. As a result of the fast smoothedstructure 1D and 2D inversion of differen t components of data, taking into account their spe� cific features, the depths of the occurrence of anomalies and the order of the values of their electrical resistiv� ity were evaluated, and the background section was also reconstructed. On this basis, and, also, with the use of a priori geological-geophysical information, the construction of a 3D model in a more or less broad band around the profile and its correction with the aid of 3D data inversion are possible.

Dimensionality imprint of electrical anisotropy in magnetotelluric responses

Dimensionality analysis of magnetotelluric data is a common procedure for inferring the main properties of the geoelectric structures of the subsurface such as the strike direction or the presence of superficial distorting bodies, and enables the most appropriate modeling approach (1D, 2D or 3D) to be determined. Most of the methods currently used assume that the electrical conductivity of individual parts of a structure is isotropic, although some traces of anisotropy in data responses can be recognized. In this paper we investigate the imprints of anisotropic media responses in dimensionality analysis using rotational invariants of the magnetotelluric tensor. We show results for responses generated from 2D synthetic anisotropic models and for field data that have been interpreted as showing the effects of electrical anisotropy in parts of the subsurface structure. As a result of this study we extend the WAL dimensionality criteria to include extra conditions that allow anisotropic media to be distinguished from 2D isotropic ones. The new conditions require the analysis of the strike directions obtained and take into account the overall behavior of different sites in a survey.