Interpretation Of Magnetotelluric Data Research Articles

Three dimensional topography correction applied to magnetotelluric data from Sikkim Himalayas

Magnetotelluric (MT) method is one of the powerful tools to investigate the deep crustal image of mountainous regions such as Himalayas. Topographic variations due to irregular surface terrain distort the resistivity curves and hence may not give accurate interpretation of magnetotelluric data. The two-dimensional (2-D) topographic effects in Transverse Magnetic (TM) mode is only galvanic whereas inductive in Transverse Electric (TE) mode, thus TM mode responses is much more important than TE mode responses in 2-D. In three-dimensional (3-D), the topography effect is both galvanic and inductive in each element of impedance tensor and hence the interpretation is complicated. In the present work, we investigate the effects of three-dimensional (3-D) topography for a hill model. This paper presents the impedance tensor correction algorithm to reduce the topographic effects in MT data. The distortion caused by surface topography effectively decreases by using homogeneous background resistivity in impedance correction method. In this study, we analyze the response of ramp, distance from topographic edges, conductive and resistive dykes. The new correction method is applied to the real data from Sikkim Himalayas, which brought out the true nature of the basement in this region.

Interpretation of magnetotelluric data from the Gachsaran oil field using sharp boundary inversion

The magnetotelluric (MT) method is used in hydrocarbon exploration in situations where seismic exploration gives low quality data, or is logistically difficult. Static effects caused seismic data collected in the Gachsaran oil field to be of low quality, so a magnetotelluric survey was used to investigate the subsurface structures of this important reservoir. A total of 79 MT sites were collected on a 16.5km long profile that was oriented perpendicular to the main geological strike direction. Dimensionality analysis indicated that the resistivity structure of the region is 1D or 2D mixed with some local 3D structures at shallow depths. It was found to be mainly 3D at greater depths. Dimensionality analysis, phase tensor and tipper all showed that the prevalent geoelectrical strike in the Gachsaran oil field is in a NW-SE direction. The time domain electromagnetic (TEM) method was utilized to remove the effects of static shifts. To obtain physically reasonable 2D resistivity models of the Gachsaran oil field, the nonlinear conjugate gradients (NLCG) and sharp boundary inversion algorithms were used. In the inversion of the Gachsaran MT data, the NLCG method recovered the general resistivity distribution of the subsurface structures, but the sharp boundary inversion recovered a layered resistivity model with substantially less internal variation within the layers. The model obtained by the sharp boundary inversion showed that the top of reservoir formation (Asmari) in the study area was located at a depth of 1400–1900m below sea level. The inversion results were in reasonable agreement with mapped geological features and well logs. The results of this study show that the magnetotelluric method can be utilized effectively in hydrocarbon exploration in the Gachsaran oil field, and similar types of hydrocarbon reservoir elsewhere in the world.

수직 횡등방성 전기적 이방성을 고려한 자기지전류탐사 모델링

Magnetotelluric (MT) survey investigates electrical structure of subsurface by measuring natural electromagnetic fields on the earth surface. For the accurate interpretation of MT data, the precise three-dimensional (3-D) modeling algorithm is prerequisite. Since MT responses are affected by electrical anisotropy of medium, the modeling algorithm has to incorporate the electrical anisotropy especially when analyzing time-lapse MT data sets, for monitoring engineered geothermal system (EGS) reservoir, because changes in different-vintage MT-data sets are small. This study developed a MT modeling algorithm for the simulation MT responses in the presence of electrical anisotropy by improving a pre-existing staggered-grid finite-difference MT modeling algorithm. After verifying the developed algorithm, we analyzed the effect of vertical transversely isotropic (VTI) anisotropy on MT responses. In addition, we are planning to extend the applicability of the developed algorithm which can simulate not only the horizontal transversely isotropic (HTI) anisotropy, but also the tiled transversely isotropic (TTI) anisotropy.

3-D analysis and interpretation of magnetotelluric data from the Aluto-Langano geothermal field, Ethiopia

The Main Ethiopian Rift Valley encompasses a number of volcanoes, which are known to be actively deforming with reoccurring periods of uplift and setting. One of the regions where temporal changes take place is the Aluto volcanic complex. It hosts a productive geothermal field and the only currently operating geothermal power plant of Ethiopia. We carried out magnetotelluric (MT) measurements in early 2012 in order to identify the source of unrest. Broad-band MT data (0.001-1000 s) have been acquired at 46 sites covering the expanse of the Aluto volcanic complex with an average site spacing of 1 km. Based on this MT data it is possible to map the bulk electrical resistivity of the subsurface down to depths of several kilometres. Resistivity is a crucial geophysical parameter in geothermal exploration as hydrothermal and magmatic reservoirs are typically related to low resistive zones, which can be easily sensed by MT. Thus by mapping the electrical conductivity one can identify and analyse geothermal systems with respect to their temperature, extent and potential for production of energy. 3-D inversions of the observed MT data from Aluto reveal the typical electrical conductivity distribution of a high-enthalpy geothermal system, which is mainly governed by the hydrothermal alteration mineralogy. The recovered 3-D conductivity models provide no evidence for an active deep magmatic system under Aluto. Forward modelling of the tippers rather suggest that occurrence of melt is predominantly at lower crustal depths along an off-axis fault zone a few tens of kilometres west of the central rift axis. The absence of an active magmatic system implies that the deforming source is most likely situated within the shallow hydrothermal system of the Aluto-Langano geothermal field.

Petrology‐based modeling of mantle melt electrical conductivity and joint interpretation of electromagnetic and seismic results

AbstractThe presence of melt in the Earth's interior depends on the thermal state, bulk chemistry, and dynamics. Therefore, the investigation of the physical and chemical properties of melt is a probe of the planet's structure, dynamics, and potentially evolution. Here we explore melt properties by interpreting geophysical data sets sensitive to the presence of melt (electromagnetic and seismic) with considerations of petrology and, in particular, peridotite partial melting. We present a petrology‐based model of the electrical conductivity of fertile and depleted peridotites during partial melting. Seismic and magnetotelluric (MT) studies do not necessarily agree on melt fraction estimates, a possible explanation being the assumptions made about melt chemistry as part of MT data interpretation. Melt fraction estimates from electrical anomalies usually assume a basaltic melt phase, whereas petrological knowledge suggests that the first liquids produced have a different chemistry, and thus a different conductivity. Our results show that melts produced by low‐degree peridotite melting (< 15 vol %) are up to 5 times more conductive than basaltic liquids. Such conductive melts significantly affect bulk rock conductivity. Application of our electrical model to magnetotelluric results suggests melt fractions that are in good agreement with seismic estimates. With the aim of a simultaneous interpretation of electrical and seismic data, we combine our electrical results with seismic velocity considerations in a joint model of partial melting. Field electrical and seismic anomalies can be explained by ~1 vol % melt beneath Hawaii and ~1–8 vol % melt beneath the Afar Ridge.

Two‐Dimensional Simulation of Magnetotelluric Anisotropy and Case Analysis

AbstractAfter more than half century development, remarkable achievements have been made in the study of earth's interior electrical structure by using the magnetotelluric (MT) method. The majority of these research results are based on the electrical isotropic assumption. However, the electrical anisotropy phenomenon is prevalent in the earth interior, and the presence of such anisotropy in the crust and upper mantle is an important link factor among geoelectric models, underground structure and tectonic models. In this paper, the tensor conductivity is first introduced by starting from the Maxwell equations, then we deduce a set of partial differential equations with respect to Ex and Hx according to the characteristic of two‐dimensional electrical anisotropic structure. By using the partial differential equations, the appropriate solutions of Ex and Hx are solved, and other field components are obtained on the basis of Ex and Hx. In order to recognize the characteristics of electromagnetic propagation under common and special geological conditions, the influence on the measured MT data is studied by the forward simulation on ordinary and special two‐dimension anisotropic electrical structure, which lays theoretical foundation for the subsequent processing and interpretation of MT data. Finally, the electrical anisotropy theory is introduced into processing and interpretation of MT data based on the results in the paper, and the widespread existence of electrical anisotropy is illuminated, and the exactness of this theory and practicability of the algorithm are verified by means of the two‐dimension forward fitting interpretation on the MT data from Xinjiang. This work would provide theoretical basis and technical guidance for analyzing and explaining the electrical anisotropy in MT data and help opens approaches for processing of measured MT data.

On the Causes of Electrical Conductivity Anomalies in Tectonically Stable Lithosphere

Magnetotelluric (MT) data can image the electrical resistivity of the entire lithospheric column and are therefore one of the most important data sources for understanding the structure, composition and evolution of the lithosphere. However, interpretations of MT data from stable lithosphere are often ambiguous. Recent results from mineral physics studies show that, from the mid-crust to the base of the lithosphere, temperature and the hydrogen content of nominally anhydrous minerals are the two most important controls on electrical conductivity. Graphite films on mineral grain boundaries also enhance conductivity but are stable only to the uppermost mantle. The thermal profile of most stable lithosphere can be well constrained, so the two important unknowns that can affect the conductivity of a lithospheric section are hydrogen content and graphite films. The presence of both of these factors is controlled by the geological history of the lithosphere. Hydrogen in nominally anhydrous minerals behaves as an incompatible element and is preferentially removed during melting or high-temperature tectonothermal events. Grain-boundary graphite films are only stable to ~900 °C so they are also destroyed by high-temperature events. Conversely, tectonic events that enrich the lithosphere in incompatible elements, such as interaction with fluids from a subducting slab or a plume, can introduce both hydrogen and carbon into the lithosphere and therefore increase its electrical conductivity. Case studies of MT results from central Australia and the Slave Craton in Canada suggest that electrical conductivity can act as a proxy for the level of enrichment in incompatible elements of the lithosphere.

Three-dimensional inversion of magnetotelluric phase tensor data

SUMMARY Recent increase in the application of 3-D inversion of magnetotelluric (MT) data is facilitated by the availability of several 3-D inversion codes, which led to improved interpretation of MT data. However, still the galvanic effects continue to pose problems in interpretation of the MT data. We have addressed this problem using the phase tensor (PT) concept and developed a scheme based on a modification of the sensitivity matrix in the 3-D inversion code of Siripunvaraporn et al., which enables us to directly invert the phase tensor elements. We have used this modified code for PT inversion of MT data and evaluated its efficacy in reducing the galvanic effects through a few examples of inversion of synthetic data and its application on real data. The synthetic model study suggests that the prior model (mo) setting is important in retrieving the true model. Comparison of results obtained from conventional impedance inversion and the proposed PT inversion method suggests that, even in the presence of the galvanicdistortion(duetonear-surfacecheckerboardanomaliesinourcase),thenewinversion algorithmreliablyretrievestheregionalconductivitystructurewhenthepriormodelorregional resistivity value level can be estimated with sufficient accuracy. Data errors were propagated to the PT elements using delta method while inverting the real data from USArray. The PT results compare very well with those from full tensor inversion published earlier, signifying the efficacy of this new inversion scheme.

Interpretation of complex geological structures with the use of forward and inverse modeling of magnetotelluric field

General assumptions of 1D and 2D modeling and inversion were presented in this paper. Methodology of magnetotelluric data interpretation was described including analysis of dimensionality of geoelectrical medium and its influence on choosing the proper interpretation model. Relations between initial model and final results of inversion were shown based on magnetotelluric (AMT and CSAMT) data from Grabownica oil and gas field. The second example was from the Lodyna oil field. A possibility of interpretation of thin vertical layers, which can approximate the Lodyna field by 1D and 2D MT inverse procedures was analyzed. Dependences between final geological models and MT curve type (TE mode or TE mode) taken to interpretation were shown. Finally, methodology of interpretation was proposed.

Electromagnetic soundings of the sedimentary cover and consolidated crust in the transition zone from the Moscow syneclise to the Voronezh anteclise: Problems and prospects

The methods and results of electromagnetic soundings (EMS) performed in the transition zone from the Moscow syneclise to the Voronezh anteclise in the vicinity of the MSU geophysical base are considered. This base is located in the village of Aleksandrovka in the Yukhnov district of Kaluga area. The composite EMS curves characterizing rock complexes composing the sedimentary cover are constructed, and changes in these complexes within the specified transition zone are traced. The standard curves of magnetotelluric (MT) and magnetovariational (MV) soundings are constructed from the results of long-term measurements at the ALX observation point located at the Moscow State University’s (MSU) geophysical base. The maps of thickness and total longitudinal conductance of the sedimentary cover are constructed from the results of interpretation of MT data obtained in the region. A conductor in the consolidated Earth’s crust is identified within the Voronezh anteclise. Prospects for further investigations of the region are associated with the tracing of the crustal conductor within the Voronezh anteclise, as well as with the organization of an observatory at the MSU’s geophysical base in order to perform long-term measurements of the electromagnetic (EM) and other geophysical fields.

Stochastic interpretation of magnetotelluric data, comparison of methods

Global optimization and stochastic approaches to the interpretation of measured data have recently gained particular attraction as tools for directed search for and/or verification of characteristic structural details and quantitative parameters of the deep structure, which is a task often arising when interpreting geoelectrical induction data in seismoactive and volcanic areas. We present a comparison of three common global optimization and stochastic approaches to the solution of a magnetotelluric inverse problem for thick layer structures, specifically the controlled random search algorithm, the stochastic sampling by the Monte Carlo method with Markov chains and its newly suggested approximate, but largely accelerated, version, the neighbourhood algorithm. We test the algorithms on a notoriously difficult synthetic 5-layer structure with two conductors situated at different depths, as well as on the experimental COPROD1 data set standardly used to benchmark 1D magnetotelluric inversion codes. The controlled random search algorithm is a fast and reliable global minimization procedure if a relatively small number of parameters is involved and a search for a single target minimum is the main objective of the inversion. By repeated runs with different starting test model pools, a sufficiently exhaustive mapping of the parameter space can be accomplished. The Markov chain Monte Carlo gives the most complete information for the parameter estimation and their uncertainty assessment by providing samples from the posterior probability distribution of the model parameters conditioned on the experimental data. Though computationally intensive, this method shows good performance provided the model parameters are sufficiently decorrelated. For layered models with mixed resistivities and layer thicknesses, where strong correlations occur and even different model classes may conform to the target function, the method often converges poorly and even very long chains do not guarantee fair distributions of the model parameters according to their probability densities. The neighbourhood resampling procedure attempts to accelerate the Monte Carlo simulation by approximating the computationally expensive true target function by a simpler, piecewise constant interpolant on a Voronoi mesh constructed over a set of pre-generated models. The method performs relatively fast but seems to suggest systematically larger uncertainties for the model parameters. The results of the stochastic simulations are compared with the standard linearized solutions both for thick layer models and for smooth Occam solutions.

A realistic 3-D resistivity model explaining anomalous large magnetotelluric phases: the L-shaped conductor model

SUMMARY Anomalous large off-diagonal phases exceeding 90 ◦ pose a major issue impeding the accurate interpretation of magnetotelluric data due to the inadequacy of 1-D or 2-D resistivity models in explaining them. In this study, we propose a realistic 3-D resistivity model to explain anomalous phase data through the use of an L-shaped conductor model. The model presented here consists of a large, elongated conductor (a regional conductor) and a small, square conductor attached to a longer side of the regional conductor. In this model, telluric current is locally reversed and hence out of the ordinal quadrant phase is derived at the edge of the local conductor. Although conventional distortion analyses for galvanic distortion cannot remove anomalous phase problem, we showed that some impedance parameters are not distorted by the anomalous telluric current. Because the ‘L’ shaped conductor model is simple and broadly applicable to realistic resistivity distributions, it represents an important contribution to the construction of an appropriate model to explain various magnetotelluric data, including anomalous phase data.

Three-dimensional inversion of magnetotelluric data including sea effects obtained in Pohang, Korea

Magnetotelluric (MT) surveys were conducted in Pohang, Korea, for low-temperature geothermal exploration in 2002 and 2003. Pohang is located in the southeastern part of the Korean Peninsula and close to the East Sea. In the interpretation of MT data from a coastal environment, sea effects must be correctly included because seawater is a strong conductor. We first constructed a five-layered earth model with a realistic coastline and bathymetry to investigate sea effects on MT data measured in Pohang. This model clearly shows that the Pohang data are significantly influenced by sea water at frequencies blow 1 Hz at the whole measurement sites. Next, we utilized a three-dimensional inversion algorithm based on the Gauss–Newton approach to produce a reliable resistivity model. Seawater is excluded from the inversion domain to fix the resistivity, while included in the modeling domain to simulate sea effects on MT responses. Blocks for the sub-seafloor are included in unknown parameters since they are sufficiently close to the survey area to affect MT responses in Pohang. Static shifts are also considered in inversion for more accurate interpretation. The rms data misfit is smoothly reduced from 11.2 to 1.87 after 7 iterations. The resulting resistivity model shows a pattern of low–high–low resistivity with depth. The model is compatible with resistivity logs obtained from four boreholes in the survey area, and can explain major geological features in Pohang.

The Jeju dataset: Three-dimensional interpretation of MT data from mid-mountain area of Jeju Island, Korea

Magnetotelluric (MT) surveys have been performed along five lines in the Jeju volcanic island located at South Sea of Korea. The purposes of the MT surveys are to see if there still remains thermal regime and if there exist deeply extended fractures or an aquifer system beneath the mid-mountain region of Mt. Halla in Jeju. By performing Audio-frequency MT (AMT) surveys together with MT surveys at each site and by operating far-remote reference site of approximately 480 km apart, we could get a broadband (10 − 4 –10 4 Hz) MT dataset of totally 108 sites with very good quality. 3D inversion of MT data from 88 measurement sites gives a reasonable estimation of the subsurface structures beneath the island. An induction vector analysis and 3D inversion results commonly indicate the existence of a conductive anomaly extended down to a few-kilometer depth beneath the central part of the island, which can be related to ancient volcanic activities. Because the island is surrounded by electrically conductive sea and has steep topographic variation, measured MT data in terms of impedance is seriously biased by distortion of electric fields and is suffered from the static shifts. The Jeju MT dataset is valuable for the research purpose in the sense that the dataset contains so many problems, some of which can frequently be observed in every field data, and that the geology of Jeju Island is relatively simple.

The Magnetotelluric Impedance Tensor and its Properties

A consistent definition of the linear relationship between the electric field true vector E and magnetic field pseudovector H yields a true magnetotelluric impedance tensor T. The true tensor nature of T admits diagonalisation in terms of classical eigenstate decomposition. Eigenstate analysis of T is in agreement with biorthogonal methods. Physical properties of the electromagnetic energy at the surface of the earth define hermitian forms that can be viewed as surface functions of the field polarization parameters. Geometrical and physical properties of these surfaces establish the result that the eigenvectors of all the hermitian forms have the same principal directions. This property links the vertical magnetic field with T, reduces the degrees of freedom of T from eight to six, provides a single principal direction that facilitates the interpretation of magnetotelluric data and shows that when T is defined, defining the tipper is unnecessarily restrictive. Synthetic data from a 3D model is used to illustrate the main result.

The use of Mohr circles in the interpretation of magnetotelluric data

One of Ted Lilley?s many original contributions to electromagnetic geophysics was his introduction of the Mohr circle as an aid in the analysis of the magnetotelluric (MT) impedance tensor. Although well known as a representation of the stress tensor in elasticity theory, the usefulness of Mohr circles was virtually unrecognised by the MT community until the pioneer paper of Lilley (1976). An important difference between the stress tensor and the MT tensor is that the former is real while the latter is complex, which means that the MT tensor must be represented by two Mohr circles rather than one. In his early treatments of MT data, Lilley bypassed this complication by concentrating solely on the real part of the MT tensor, and was able to identify various invariants of the real tensor geometrically on the Mohr circle diagram. In later discussions of the physical interpretation of the seven independent invariants of the complex MT tensor, however, it became necessary to consider both real and imaginary Mohr circles together when seeking a geometrical representation of all the invariants. A significant advance was made with the introduction of the (real) phase tensor by Caldwell, Bibby and Brown (2002). Although the phase tensor has only three independent invariants, they retain the important physical properties of the seven invariants of the MT tensor, and can be displayed graphically in a single Mohr circle diagram. In particular, identification of the dimensionality of the regional conductivity structure becomes a straightforward matter whether or not the data are distorted by near-surface conductivity anomalies. An analysis of MT field data with error bars will be presented using the phase tensor and its Mohr circle representation in order to show when a two- or one-dimensional interpretation of the regional conductivity structure is appropriate and strike angles are calculated for two-dimensional structures.

Three-dimensional inversion of magnetotelluric data in complex geological structures

Interpretation of magnetotelluric data over inhomogeneous geological structures is still a challenging problem in geophysical exploration. We have developed a new 3-D MT inversion method and a computer code based on full nonlinear conjugate gradient inversion and quasi-analytical approximation for forward modeling solution. Application of the QA approximation to forward modeling and Frechet derivative computations speeds up the calculation dramatically. However, in order to control the accuracy of the inversion, our method allows application of the rigorous forward modeling in the intermediate steps of the inversion procedure and for the final inverse model. The 3-D magnetotelluric inversion code QAINV3D based on QA approximation, has been tested on synthetic models and applied to the practical MT data collected in an area with complex geology. The inversion of 3-D MT data can be done within a few minutes on a PC to generate a 3-D image of subsurface formations on a large grid with tens of thousands of cells.

VARNET MT: Interpretation of Magnetotelluric Data over the Killarney–Mallow Fault Zone in South-West Ireland

The south-west of Ireland is an area in the European Variscides where the surface geology can provide constraints on geophysical models of deep Variscan (and Caledonian) tectonic structures. The VARNET (VARiscan NETwork) project has acquired new wide-angle seismic reflection, magnetotelluric (MT) and gravity data collected in south-west Ireland in order to integrate these with surface geological observations and tectonic models from Ireland and other parts of the European Variscides. This paper is concerned with the interpretation of VARNET MT data collected across the major tectonic structure seen at the surface: the Killarney-Mallow Fault Zone. The MT data have identified a near-surface conductive zone associated with the east-west-trending Killarney-Mallow Fault Zone. The inversion of these data along three profiles across the Killarney-Mallow Fault Zone and the eastern end of Dingle Bay indicates the presence of steep, probably north-dipping structures, extending to depths of at least 2km. There is also prima facie MT evidence for a steep structure near the Cork-Kenmare Line and a steep, north-dipping structure associated with the Dingle Bay-Galtee Mountains Line. The locations of these structures appear to be consistent with those interpreted from the wide-angle seismic reflection data.

Varnet MT: Interpretation of Magnetotelluric Data over the Killarney-Mallow Fault Zone in South-West Ireland

The south-west of Ireland is an area in the European Variscides where the surface geology can provide constraints on geophysical models of deep Variscan (and Caledonian) tectonic structures. The VARNET (VARiscan NETwork) project has acquired new wide-angle seismic reflection, magnetotelluric (MT) and gravity data collected in south-west Ireland in order to integrate these with surface geological observations and tectonic models from Ireland and other parts of the European Variscides. This paper is concerned with the interpretation of VARNET MT data collected across the major tectonic structure seen at the surface: the Killarney-Mallow Fault Zone. The MT data have identified a near-surface conductive zone associated with the east-west-trending Killarney-Mallow Fault Zone. The inversion of these data along three profiles across the Killarney-Mallow Fault Zone and the eastern end of Dingle Bay indicates the presence of steep, probably north-dipping structures, extending to depths of at least 2km. There is also prima facie MT evidence for a steep structure near the Cork-Kenmare Line and a steep, north-dipping structure associated with the Dingle Bay-Galtee Mountains Line. The locations of these structures appear to be consistent with those interpreted from the wide-angle seismic reflection data.

Varnet MT: Interpretation of Magnetotelluric Data over the Killarney-Mallow Fault Zone in South-West Ireland

Varnet MT: Interpretation of Magnetotelluric Data over the Killarney-Mallow Fault Zone in South-West Ireland