Josef Pek

Magnetotelluric impedances and parametric sensitivities for 1-D anisotropic layered media

Anisotropy of the electrical conductivity within the Earth has recently provided a significant link between geoelectrical models and the underlying tectonic setting. Interpretation of magnetotelluric data for anisotropic conductivities suffers from inherent ambiguity, and even for theoretically distinguishable parameters the resolution pattern may be rather complex as compared to isotropic models. As a tool for the resolution and sensitivity studies, we present an algorithm for jointly evaluating the magnetotelluric impedances and their partial derivatives with respect to the parameters of a 1-D generally anisotropic layered medium. The algorithm is based on impedance propagation formulas through a stack of anisotropic layers and their direct differentiation with respect to the model parameters. By virtue of an equivalency principle, the situation of a generally anisotropic layered medium can be always reduced in magnetotellurics to a simpler model with horizontally anisotropic layers. By rotating the impedances into the local anisotropy strike, the two wave modes in an anisotropic layer can be quasi-separated, and elimination of the positive exponential wave factors in the impedance formulas is possible, which stabilizes both the impedance and sensitivity calculations. The performance of the algorithm is demonstrated on a simple qualitative sensitivity study for a four-layer model with depth-variable anisotropy parameters.

Numerical solution of the two-dimensional inverse geomagnetic induction problem

SummaryAn effective numerical approach to the solution of the two-dimensional inverse geomagnetic induction problem using the linearization method is presented. The numerical realization of the inversion is based on Marquardts algorithm, for which the solution of the direct problem and the partial derivatives of this solution with respect to the electrical parameters of the medium are computed by the finite difference method. Theoretical models are studied and numerical results are presented.

Deep electromagnetic soundings conducted in Trans‐European Suture Zone

A consortium of nine geophysical institutions recently carried out a large-scale geomagnetic experiment focused on revealing the deep electrical structure beneath central and eastern Europe around the Trans-European Suture Zone (TESZ), the regions first-order geological lineament. The TESZ is considered a broad zone of deformation that crosses all of Europe, from the British Isles in the northwest to the Black Sea area in the southeast, and it most likely continues in North America [Keller and Hatcher, 1999]. The geomagnetic experiment was called Central Europe Mantle Geoelectrical Structure (CEMES). Initiated by Polish scientists, geophysicists from eight other countries joined the project during a NATO Advanced Research Workshop held in the spring of 2001 in Belsk, Poland. The experimental phase of the project was held from 2001 through 2002; and altogether, 12 geomagnetic observatories, the international codes of which are shown among others in Figure 1, took part in acquiring the data. They will serve the objective of inferring information on the mantle conductivity structures beneath the region of TESZ, as well as beneath surrounding units, specifically the western part of the East European Craton (EEC), Variscides, including the Bohemian Massif, the Carpathians, and the Pannonian Basin.

Numerical inversion of 2D MT data by models with variable geometry

Abstract A linearization optimization procedure for interpreting 2D MT data is presented which allows optimization of both the electrical and geometrical parameters of a hypothetical model of the structure in the course of the iteration process. As a methodical basis theoretical relations for the partial derivatives of the MT frequency characteristics with respect to the electrical and geometrical parameters are derived and numerically approximated by the finite difference method. As a demonstration of the practical use of the developed interpretational algorithm several numerical tests with artificially generated MT E -polarization data are presented.

Resolution of anisotropic and shielded highly conductive layers using 2-D electromagnetic modelling in the Rhine Graben and Black Forest

Abstract Anisotropy in magnetotelluric (MT) data has been found very often and has been explained as the result of local structures of different conductivities. In this paper, an observed anisotropy in MT data is not interpreted qualitatively in terms of local structures but is modelled quantitatively by a quasi-anisotropic layer. Besides the MT transfer functions, measurements of the vertical magnetic component are required. The second goal of this paper is to describe a method which permits the resolution of mid-crustal conductive layers in the presence of an additional high-conductivity layer at the surface. This method is possible in a two-dimensional (2-D) situation that limits the spatial extension of the surface structure. Again, vertical magnetic field recordings are necessary, but the phase of the E-polarization with respect to the 2-D structure is the most sensitive parameter. Using two field sites in Southern Germany, it has been possible to give a quantitative explanation of anisotropy and an improved depth resolution, and to derive an integrated conductivity of the highly conductive mid-crustal layers using MT and geomagnetic depth sounding data. The anisotropic highly conductive layer is located 12 km beneath the poorly conductive Black Forest crystalline rocks, whereas it is at a depth of 6 km beneath the highly conductive Rhine Graben sediments.

Solution of the one-dimensional inverse magnetotelluric problem

SummaryMethods of solving the inverse magnetotelluric problem are compared. Basic relations for Newtons method, the least-squares method and Marquardts method are presented and the convergence properties of these methods are studied. The high effectiveness of Marquardts method is demonstrated and its application to practical magnetotelluric data is discussed.

Geoelectrical structure across the Bohemian Massif and the transition zone to the West Carpathians

Abstract Geoelectrical methods of magnetotelluric (MT) and geomagnetic deep soundings (GDS) were employed to infer the electrical conductivity structure at depths of specific regions of the Bohemian Massif (BM) and in the transition zone between the BM and the West Carpathians (WCP). Transfer functions between components of temporal variations of the natural electromagnetic field induced in the Earth by external sources were estimated and converted to electrical conductivity (resistivity) profiles. While the results from other regions of the BM are noted in references, the emphasis here is placed on the new analysis of MT/GDS data at field stations along the regional almost 500-km long Deep Seismic Sounding profile No. VI traversing the country from NW to SE. Inversion of its results suggests a complex geoelectrical structure, characteristic for each geological unit, with layers of increased electrical conductivity at crustal and uppermost mantle depths. The results also indicate strong fields of anomalous induction marking geoelectrical inhomogeneities at the eastern margin of the BM and in the foreland of the WCP.

A magnetotelluric profile across the German Deep Drilling Project (KTB) area: Two‐ and three‐dimensional modeling results

Previous interpretations of magnetotelluric data from the vicinity of the German Deep Drilling Project (KTB) revealed two major structures: a midcrustal layer of increased conductivity and large, regional extent and a highly anisotropic upper crust. Nevertheless, a satisfactory combination of both structures explaining all measurements has not yet been achieved, mostly due to incomplete and qualitatively poor data. Simplified superposition of both structures could not yield an explanation of the observations. On the basis of a carefully processed new data set we apply different modeling approaches to verify the existence of both structures. Models calculated with a two-dimensional modeling program, which allows for general anisotropy, as well as a full three-dimensional code show that an anisotropic upper crust is overlaying a regional east-west striking high-conductivity structure. Nevertheless, this continuous conductive midcrustal layer with a conductance decreasing from north to south must be replaced by a quasi-anisotropic one, at least in the region of the KTB. The final model may still be oversimplified, considering the complexity of the true but unknown geology in this particular area, but it demonstrates which major electrically effective structures could be resolved.

Electromagnetic measurements in the vicinity of the KTB drill site. Part II. Magnetotelluric results

SummaryIn addition to the magnetovariational measurements across an array in Western Bohemia, close to the KTB ultradeep borehole (Germany), discussed in part I of this paper [1], magnetotelluric results from pivot sounding point Ostrůvek within the array are presented here. Good quality of long-period magnetotelluric data (period range from 30 s to about 1 hour) allowed structural dimensionality of the medium to be analysed in detail. The geoelectrical structure was identified as a slightly distorted two-dimensional regional substratum, with dominating E - W strike, overlaid by a heterogeneous subsurface layer with extremely strong and anisotropic galvanic distortion effect on the magnetotelluric data. Estimating the total static shift distortion tensor by fitting the local magnetotelluric curve to the curve of the global magnetovariational soundings (for the European continent), the static distortions were identified as of generally multidirectional origin. The resulting telluric ellipse is, however, strongly anisotropic, indicating an approximately SW - NE apparent local strike, which is in the approximate agreement with remote reference magnetovariational results. Finally, the magnetotelluric results from the station Ostrůvek are compared with long-period data from the immediate neighbourhood of the KTB borehole on the German territory.

Geoelectrical and geological structure of the crust in Western Slovakia

Electrical resistivity of the Earth’s crust is sensitive to a wide range of petrological and physical parameters, and it particularly clearly indicates crustal zones that have been tectonically or thermodynamically disturbed. A complex geological structure of the Alpine nappe system, remnants of older Hercynian units and Neogene block tectonics in Western Slovakia has been a target of recent magnetotelluric investigations which made a new and more precise identification of the crustal structural elements of the Western Carpathians possible. A NW-SE magnetotelluric profile, 150 km long, with 30 broad-band and 3 long-period magnetotelluric sites, was deployed, crossing the major regional tectonic elements listed from the north: Brunia (as a part of the European platform), Outer Carpathian Flysch, Klippen Belt, blocks of Penninic or Oravicum crust, Tatricum and Veporicum. Magnetotelluric models were combined with previous seismic and gravimetric results and jointly interpreted in the final integrated geological model. The magnetotelluric models of geoelectrical structures exhibit strong correlation with the geological structures of the crust in this part of the Western Carpathians. The significant resemblance in geoelectrical and crustal geological structures are highlighted in shallow resistive structures of the covering formations represented by mainly Tertiary sediments and volcanics. Also in the deeper parts of the crust highly resistive and conductive structures are shown, which reflect the original building Hercynian crust, with superposition of granitoids or granitised complexes and lower metamorphosed complexes. Another important typical feature in the construction of the Western Carpathians is the existence of young Neogene steep fault zones exhibited by conductive zones within the whole crust. The most significant fault zones separate individual blocks of the Western Carpathians and the Western Carpathians itself from the European Platform.