Vertical Magnetic Transfer Functions Research Articles

Delineation of shallow volcanic structures from audio-frequency magnetotelluric data beneath Ulleung Island, East Sea (Sea of Japan)

As volcanoes are closely related to the living environment of humans, in particular via natural hazards and geothermal energy, scientific studies on volcanic edifice structures are required. Ulleung Island is a Quaternary volcanic island located in the mid-western East Sea (Sea of Japan). In this study, we conducted an audio-frequency magnetotelluric (AMT) survey to image the substructure of the Ulleung volcanic edifice. In addition, the electrical structure was interpreted from the geothermal system perspective, geochemical compositions of volcanic rocks, and the possibility of the presence of a magma reservoir. AMT data were obtained from 25 stations and processed using the remote reference technique. Then, the three-dimensional (3-D) approach was reasonably adopted according to dimensionality analysis. Before conducting the 3-D inversion, the effects on topography and ocean were analyzed using a simplified 3-D synthetic model, because Ulleung Island is surrounded by sea and the topography is undulating. Most AMT stations on Ulleung Island are distorted by topographical and oceanic effects; in particular, oceanic effects are significant at frequencies lower than 10 Hz. The 3-D inversion was conducted with full impedance components and vertical magnetic transfer functions in a frequency range of 10,000–1 Hz. The results show that the Ulleung volcanic edifice is characterized by two layers of electrical structures as follows: near-surface resistive anomalies and an underlying conductive layer. Considering the high geothermal gradient on Ulleung Island, we suggest that the conductive layer of the volcanic edifice is due to hydrothermal alteration of basaltic rocks, with a potential heat source underneath. Based on the geochemical characteristics of the Ulleung volcanic rocks, the possibility of heat transfer from a trachytic magma reservoir within the shallow crustal is suggested. In summary, this study presents a geological interpretation of shallow volcanic structures beneath Ulleung Island and the possibility of an active magma reservoir as a potential heat source for the volcanic edifice.Graphical

Enormously large tippers observed in southwest China: can realistic 3-D EM modeling reproduce them?

Vertical magnetic transfer functions (tippers) estimated at a global/continental net of geomagnetic observatories/sites can be used to image the electrical conductivity structure of the Earth’s crust and upper mantle (down to around 200 km). We estimated tippers at 54 geomagnetic observatories across China, aiming eventually to invert them in terms of subsurface three-dimensional (3-D) conductivity distribution. Strikingly, we obtained enormously large tippers at three inland observatories in southwest China. Large tippers are often observed at coastal/island observatories due to high conductivity contrasts between resistive bedrock and conductive seawater. However, tippers at those inland observatories appeared to be a few times larger than coastal/island tippers. As far as we know, such large tippers (reaching value 3) were never reported in any region worldwide. We perform electromagnetic simulations in 3-D conductivity models mimicking the geological setting and demonstrate that enormously large tippers are feasible and can be attributed to a current channeling effect.Graphical

Geoelectric studies of the Kozloduy nuclear power plant region, Bulgaria

The task of the work was geoelectrical studies using variations of the magnetotelluric (MT) field of the Kozloduy nuclear power plant (KNPP) region and the integration of its results with other geological and geophysical knowledge. This paper presents the determined interpretation parameters of the MT field. The KNPP is located on the right bank of the Danube River in close proximity to the river. This fact, together with the location of electrified railways determined the unique network of locations of observation points for MT field variations. Based on the analysis of Earthquake Catalogs of Bulgaria and international seismicity databases, a map of the seismicity of nuclear power plant areas was built. Over the past 50 years, about 750 earthquakes (mainly south of KNPP) have been recorded at a distance of 40—80 km from the KNPP. Two magnetotelluric stations GEOMAG-02 were used at measurement sites, but equipment for recording electrical channels was available only for one station (due to the lack of another set of non-polarizable electrodes). The MT field variations were observed at 21 points, which are located on the territory with sides approximately 30—35 km from east to west and 40—50 km from north to south. For all observation points on the profile, only the parameters of the vertical magnetic transfer function (VMPF) were determined, in the form of the real (Cu) and imaginary (Cv) parts of the induction vector. The steadily induction vector was defined for periods from 10—20 to 4900—10 800 s. For most points it was possible to estimate the values Cu, Cv with an error of 0.02—0.04 and AzCu, AzCv 3—5°. The analysis showed the presence of anomalous behavior of Cu, Cv in different intervals of periods at some points. In the shortest (about 20 s) and longest periods (600 to 1000 s), the Cu directions completely coincide and indicate the presence of anomalous conductivity of the quasi-longitudinal strike to the west of the study area. This behavior of the Cu vector is in good agreement with power isohypsum strike of the Cenozoic deposits. At intermediate periods of 50—200 s, the behavior of Cu is more complex. Approaching the zone of high seismicity, the direction of the Cu differs from the previous ones by almost 90°. On the Geoelectrical sections, obtained as a result of 1D inversions of MTS curves at 4 points located in the southern part of the region, anomalous layers are identified (ρ about 10 ohm · m, the depth of the center of the object is 15—20 km). It can be assumed that well-conducting objects in the Earth’s crust of the region, apparently, prevent the propagation of seismic waves from nearby earthquakes to the north towards the KNPP.

Exploring effects in tippers at island geomagnetic observatories due to realistic depth- and time-varying oceanic electrical conductivity

Vertical magnetic transfer functions (tippers) estimated at island observatories can constrain the one-dimensional (1-D) conductivity distribution of the oceanic lithosphere and upper mantle. This is feasible due to the bathymetry-dependent ocean induction effect (OIE), which originates from lateral conductivity contrasts between ocean and land and leads to non-zero tippers even for 1-D conductivity distributions below the ocean. Proper analysis of island tippers requires accurate three-dimensional (3-D) modeling of the OIE, for which so far was performed assuming constant sea water electric conductivity with depth. In this study, we explore—using rigorous 3-D electromagnetic modeling—to what extent realistic, depth-dependent, oceanic conductivity affects island tippers. The modeling is performed for 11 island observatories around the world in the period range 10^{-1} to 10^{4} s. We also investigate the effect of seasonal variations of the oceanic conductivity and to which extent this could explain the observed systematic seasonal variation of tippers. Our model studies suggest that for most of the considered island observatories the effect from depth-varying oceanic conductivity is tangible and exceeds the error floor of 0.025, which usually is assigned to tippers during their inversion. The effect varies significantly with location, depending on regional bathymetry. Contrarily, the effects from seasonally varying oceanic conductivity were found to be too small to be worth consideration.

The reduction of source effect for reliable estimation of geomagnetic transfer functions

SUMMERY We have analysed the literature suggestions regarding possible changes in vertical magnetic transfer function (VTF) over time. We have shown that for periods above 1500 s the observed changes in VTF are caused by the source effect and we proposed how to reduce this negative impact. For calculations we used 1-min recordings of geomagnetic variations registered between 2002 and 2017 in various geomagnetic observatories. In data processing we used frequency-domain Egbert's algorithm and our original algorithm based on the method of least squares in the time domain for some comparisons and tests. We have shown that for periods over 1500 s the VTFs calculated separately from summer and winter data are different. However, our analysis shows that the variability of the VTF values obtained is misleading and results from time-changing presence of magnetic field variations that do not fulfill the assumption of plane wave (there is a vertical component in the incident magnetic field). These variations are much more numerous in summer than in winter. More detailed analysis has shown also that they are usually small at night and big during the day. The vertical components of these variations constitute an error correlated with input signals (horizontal components), which alters the values of the determined VTF. Furthermore, error bars do not take this effect into account. It makes it impossible to improve the accuracy of calculations by increasing the amount of data. Analysing the estimated external parts of vertical components from the Central European observatories we noticed a great similarity of these signals even if the induction components were clearly different, which indicates that this is a regional effect. On this basis, we proposed a procedure to improve the accuracy of VTF determination by means of pre-selection of data.

Investigating karst collapse geohazards using magnetotellurics: A case study of M'rara basin, Algerian Sahara

The region of M'rara, which is located in the northeast Algerian Sahara basin, is seriously affected by several natural hazards. This region is exposed to risks such as the collapses and fissures around the water borehole that is situated in the center of the small town of M'rara. The present study was conducted in order to investigate the ground structures under eighteen (18) sites located near the above mentioned water borehole using the magnetotelluric (MT) method to collect the required data. The main objective of this research is to obtain a realistic three-dimensional (3-D) resistivity model of the structures, based on the 3-D inversion method. This was done using the ModEM package. The preferred model presented in this paper resulted from the total impedance as well as the vertical magnetic transfer functions were jointly inverted. Moreover, the dimensionality of the data was analyzed by two different methods, namely the Bahr method and the phase tensor method, which revealed that data at periods >0.5 s are relatively 3-D model. The outcomes of the 3-D inversion are sufficient to identify the M'rara basin; where reveal the folded resistive structure with an anticline form below the Senonian level and the great fractures of axial and transverse types that extend in the hinge zone of this structure. Moreover, the model was presented regarding the fault which is conductive zone. Finally, the interpretation results represent the reasons for the appearance of these collapses and fissures which are due to the process of karst dissolution of the limestone beds.

Magnetotelluric tensors, electromagnetic field scattering and distortion in three‐dimensional environments

AbstractThis paper describes how subsurface resistivity distributions can be estimated directly from the magnetotelluric (MT) tensor relationship between electric and magnetic fields observed on a three‐dimensional (3‐D) half‐space. It presents an inhomogeneous plane wave analogy where relationships between horizontal electric and magnetic fields, and an apparent current density define an apparent resistivity tensor constructed from a quadratic function of the MT tensor. An extended‐Born relationship allows the electric field to be normalized with respect to an apparent background current density. The model is generalized by including the vertical magnetic field in a 3 by 3 MT response tensor. A complex apparent wave number tensor, constructed from this tensor, has eigenvalues which, using the plane wave analogy, are the vertical wave numbers associated with the eigenpolarizations of propagating waves in the model half space. The elements associated with the vertical magnetic field transfer function define the horizontal wave numbers. An extended 3 by 3 phase tensor contains four elements of the conventional 2 by 2 phase tensor and two elements associated with the vertical magnetic transfer function. The extended phase tensor and a single real distortion tensor with six independent elements can be used to quantify static electric and magnetic field distortions. The approach provides a theoretical basis for visualization and migration of MT data, in comparison with results from other electrical and EM techniques, a starting point for constrained 3‐D inversions, and an assessment of results with other geophysical and geological data.

Understanding hydrothermal circulation patterns at a low-enthalpy thermal spring using audio-magnetotelluric data: A case study from Ireland

Kilbrook spring is a thermal spring in east-central Ireland. The temperatures in the spring are the highest recorded for any thermal spring in Ireland (maximum of 25°C). The temperature is elevated with respect to average Irish groundwater temperatures (9.5–10.5°C), and represents a geothermal energy potential, which is currently under evaluation. A multi-disciplinary investigation based upon an audio-magnetotelluric (AMT) survey, and hydrochemical analysis including time-lapse temperature and chemistry measurements, has been undertaken with the aims of investigating the provenance of the thermal groundwater and characterising the geological structures facilitating groundwater circulation in the bedrock.The three-dimensional (3-D) electrical resistivity model of the subsurface at Kilbrook spring was obtained by the inversion of AMT impedances and vertical magnetic transfer functions. The model is interpreted alongside high resolution temperature and electrical conductivity measurements, and a previous hydrochemical analysis.The hydrochemical analysis and time-lapse measurements suggest that the thermal waters have a relatively stable temperature and major ion hydrochemistry, and flow within the limestones of the Carboniferous Dublin Basin at all times. The 3-D resistivity model of the subsurface reveals a prominent NNW aligned structure within a highly resistive limestone lithology that is interpreted as a dissolutionally enhanced strike-slip fault, of Cenozoic age. The karstification of this structure, which extends to depths of at least 500m directly beneath the spring, has provided conduits that facilitate the operation of a relatively deep hydrothermal circulation pattern (likely estimated depths between 560 and 1000m) within the limestone succession of the Dublin Basin. The results of this study support the hypothesis that the winter thermal maximum and simultaneous increased discharge at Kilbrook spring is the result of rapid infiltration, heating and re-circulation of meteoric waters within this structurally controlled hydrothermal circulation system.This paper illustrates how AMT may be useful in a multi-disciplinary investigation of an intermediate-depth (100–1000m), low-enthalpy, geothermal target, and shows how the different strands of inquiry from a multi-disciplinary investigation may be woven together to gain a deeper understanding of a complex hydrothermal system.

Deep magnetotelluric signatures of the early Neoproterozoic Cariris Velhos tectonic event within the Transversal sub-province of the Borborema Province, NE Brazil

The Borborema structural province, in northeast Brazil, consists of a complex set of crustal blocks of different ages, origin and evolution, with final amalgamation during the late Neoproterozoic Brasiliano Orogeny. In the central part of the province, its Transversal sub-province is characterized by elongate NE-SW terranes largely affected by the Brasiliano metamorphic overprinting and by a suite of early Neoproterozoic metavolcanic and metaplutonic rocks, reflecting a distinct tectonic event (Cariris Velhos). Magnetotelluric (MT) data were collected at 64 stations along four profiles across this central region of the province to assess its deep electrical resistivity structure. Dimensionality analysis shows that the subsurface is electrically dominated by 3D structure, and 3D inversions of the full impedance tensor and vertical magnetic transfer function (tipper) were carried out. Data fit is relatively good with an acceptable distribution of the misfit between the sites and the dominant features in the final model are north-dipping crustal conductors, which can be tentatively correlated between the different MT profiles and cross contrasting terranes at the surface of the province. Considering the geometry and distribution of the conductors, it is proposed that they might reflect remnants of a north-directed subduction zone with enhanced conductivity associated with graphitized biogenic material in metasedimentary rocks originally deposited in a restricted oceanic environment and subsequently metamorphosed and introduced into the deep crust by the subduction of the oceanic lithosphere. Constrained by isolated outcrops of early Neoproterozoic rocks with oceanic affinity, the proposed subduction event may have taken place during the Cariris Velhos Orogeny with the crustal remobilization by the Brasiliano Orogeny being restricted to the brittle upper crust. This interpretation gives support for an accretionary model to the Neoproterozoic geodynamic evolution of the Borborema Province.

3-D joint inversion of the magnetotelluric phase tensor and vertical magnetic transfer functions

3-D joint inversion of the magnetotelluric phase tensor and vertical magnetic transfer functions

Pseudo‐remote reference processing of magnetotelluric data: a fast and efficient data acquisition scheme for local arrays

ABSTRACTThe basic physical properties of the magnetic source field, namely its homogeneity and spatial coherence, have been used for a variety of magnetotelluric processing techniques including remote reference processing. In the present work we propose a data acquisition and processing technique for a large number of stations distributed over a localized area ideally on a grid. For pseudo‐remote reference processing it is necessary to use the following station setup: five‐component MT data are only measured at some sites (base stations) while at the majority of sites (local stations) only the electric and vertical magnetic fields are recorded. The impedance tensor and vertical magnetic transfer functions at each local station are computed by assigning the magnetic fields of a base station to the local station as if they had been measured there. This approach can lead to biased or erroneous estimates of local transfer functions at stations in the vicinity of strong conductivity contrasts that can be corrected using the interstation transfer functions between the horizontal magnetic fields measured at the base station(s).We test this approach with a data set collected in the vicinity of the Groß Schönebeck geothermal test site. Magnetotelluric data were collected at 146 local and 5 base stations distributed over an approximately 5 km × 25 km wide grid with site spacing ranging from 500 m × 500 m to 1000 m × 1000 m in the frequency range 128–0.001Hz. The obtained pseudo‐remote reference transfer functions are generally smooth and consistent and conductivity models obtained from 2D inversion of the data are in agreement with previous conductivity models from the study area.

A hybrid regularization scheme for the inversion of magnetotelluric data from natural and controlled sources to layer and distortion parameters

Magnetotelluric (MT), radiomagnetotelluric (RMT), and, in particular, controlled-source audiomagnetotelluric (CSAMT) data are often heavily distorted by near-surface inhomogeneities. We developed a novel scheme to invert MT, RMT, and CSAMT data in the form of scalar or tensorial impedances and vertical magnetic transfer functions simultaneously for layer resistivities and electric and magnetic galvanic distortion parameters. The inversion scheme uses smoothness constraints to regularize layer resistivities and either Marquardt-Levenberg damping or the minimum-solution length criterion to regularize distortion parameters. A depth of investigation range is estimated by comparing layered model sections derived from first- and second-order smoothness constraints. Synthetic examples demonstrate that earth models are reconstructed properly for distorted and undistorted tensorial CSAMT data. In the inversion of scalar CSAMT data, such as the determinant impedance or individual tensor elements, the reduced number of transfer functions inevitably leads to increased ambiguity for distortion parameters. As a consequence of this ambiguity for scalar data, distortion parameters often grow over the iterations to unrealistic absolute values when regularized with the Marquardt-Levenberg scheme. Essentially, compensating relationships between terms containing electric and/or magnetic distortion are used in this growth. In a regularization with the minimum solution length criterion, the distortion parameters converge into a stable configuration after several iterations and attain reasonable values. The inversion algorithm was applied to a CSAMT field data set collected along a profile over a tunnel construction site at Hallandsåsen, Sweden. To avoid erroneous inverse models from strong anthropogenic effects on the data, two scalar transfer functions (one scalar impedance and one scalar vertical magnetic transfer function) were selected for inversion. Compared with a regularization of distortion parameters with the Marquardt-Levenberg method, the minimum-solution length criterion yielded smaller absolute values of distortion parameters and a horizontally more homogeneous distribution of electrical conductivity.

WSINV3DMT: Vertical magnetic field transfer function inversion and parallel implementation

We describe two extensions to the three-dimensional magnetotelluric inversion program WSINV3DMT (Siripunvaraporn, W., Egbert, G., Lenbury, Y., Uyeshima, M., 2005, Three-dimensional magnetotelluric inversion: data-space method. Phys. Earth Planet. Interiors 150, 3–14), including modifications to allow inversion of the vertical magnetic transfer functions (VTFs), and parallelization of the code. The parallel implementation, which is most appropriate for small clusters, uses MPI to distribute forward solutions for different frequencies, as well as some linear algebraic computations, over multiple processors. In addition to reducing run times, the parallelization reduces memory requirements by distributing storage of the sensitivity matrix. Both new features are tested on synthetic and real datasets, revealing nearly linear speedup for a small number of processors (up to 8). Experiments on synthetic examples show that the horizontal position and lateral conductivity contrasts of anomalies can be recovered by inverting VTFs alone. However, vertical positions and absolute amplitudes are not well constrained unless an accurate host resistivity is imposed a priori. On very simple synthetic models including VTFs in a joint inversion had little impact on the inverse solution computed with impedances alone. However, in experiments with real data, inverse solutions obtained from joint inversion of VTF and impedances, and from impedances alone, differed in important ways, suggesting that for structures with more realistic levels of complexity the VTFs will in general provide useful additional constraints.

Radiomagnetotelluric two-dimensional forward and inverse modelling accounting for displacement currents

SUMMARY Electromagnetic surface measurements with the radiomagnetotelluric (RMT) method in the frequency range between 10 and 300 kHz are typically interpreted in the quasi-static approximation, that is, assuming displacement currents are negligible. In this paper, the dielectric effect of displacement currents on RMT responses over resistive subsurface models is studied with a 2-D forward and inverse scheme that can operate both in the quasi-static approximation and including displacement currents. Forward computations of simple models exemplify how responses that allow for displacement currents deviate from responses computed in the quasi-static approximation. The differences become most obvious for highly resistive subsurface models of about 3000 Ω m and more and at high frequencies. For such cases, the apparent resistivities and phases of the transverse magnetic (TM) and transverse electric (TE) modes are significantly smaller than in the quasi-static approximation. Along profiles traversing 2-D subsurface models, sign reversals in the real part of the vertical magnetic transfer function (VMT) are often more pronounced than in the quasi-static approximation. On both sides of such sign reversals, the responses computed including displacement currents are larger than typical measurement errors. The 2-D inversion of synthetic data computed including displacement currents demonstrates that serious misinterpretations in the form of artefacts in inverse models can be made if displacement currents are neglected during the inversion. Hence, the inclusion of the dielectric effect is a crucial improvement over existing quasi-static 2-D inverse schemes. Synthetic data from a 2-D model with constant dielectric permittivity and a conductive block buried in a highly resistive layer, which in turn is underlain by a conductive layer, are inverted. In the quasi-static inverse model, the depth to the conductive structures is overestimated, artefactual resistors appear on both sides of the conductive block and a spurious conductive layer is imaged at the surface. High-frequency RMT field data from Avro, Sweden, are re-interpreted using the newly developed 2-D inversion scheme that includes displacement currents. In contrast to previous quasi-static modelling, the new inverse models have electrical resistivity values comparable to a normal-resistivity borehole log and boundaries between resistive and conductive structures, which correlate with the positions of seismic reflectors.

Mode separation of magnetotelluric responses in three-dimensional environments

SUMMARY Magnetotelluric (MT) responses in complex, 3-D terrains are in general characterized by (i) elliptical polarization states of horizontal electric and magnetic fields; (ii) the non-orthogonality of electric and magnetic fields and (iii) a coupling of the anomalous tangential-electric (TE) and tangential-magnetic (TM) modes, giving rise to a mode-mixed anomalous electric field at the surface. These 3-D effects are propagated into the MT impedance tensor, which is derived from horizontal electric and magnetic fields, recorded at the earth’s surface. The 2 × 2 impedance tensor is in general fully occupied, and each of its elements is a mode-mixed quantity. To study 3-D effects of MT (impedance) data, the TE and TM mode contributions must be separated. This becomes possible with the inclusion of the single-mode vertical magnetic transfer function (the ratio of vertical to horizontal magnetic fields). Then, the individual modes can be resolved without prior knowledge of the underlying 3-D conductivity structure. For this purpose, we consider (i) the spatial relations between electromagnetic field components recorded in an array of sites (Faraday’s law) and (ii) that the magnetic TE mode and electric TM mode fields are potential fields within the insulating air half-space above the earth’s surface. Based on these two dependencies, it is possible to reconstruct the entire electromagnetic field from (measured) mixed-mode impedances and vertical magnetic transfer functions and to separate it into TE and TM modes, and into normal and anomalous parts. Hereby, we cannot only study the contribution of the two modes on the observed impedance tensor but also quantify the influence of 3-D effects at each location and frequency of a particular data set. Results of a modelling study suggest, that (i) none of the elements of a 3-D impedances tensor can be regarded as favourable for a 2-D interpretation (only 3-D models can explain 3-D data), (ii) a heterogeneous crust can strongly obscure identification of responses originating from the lower crust or upper mantle even at very long periods and (iii) the TE mode magnetic transfer functions are most important for sensing deep anomalies.

On galvanic distortion of regional 3-D MT impedances On galvanic distortion of regional three-dimensional magnetotelluric impedances

Summary In magnetotelluric (MT) studies, the observed response function (the MT impedance) usually suffers from galvanic distortions due to near-surface inhomogeneities. Removal of these effects is essential to obtain an accurate model of the subsurface electrical conductivity structure. Galvanic distortion is usually expressed by a simple real tensor multiplying the undistorted regional impedance. The problem still remains of how to solve the ensuing linear equations in order to determine the distortion tensor and then to obtain the undistorted impedance. The methods presented and widely applied in previous works assume two-dimensionality for the undistorted impedance. This paper proposes a method that employs a relationship between the spatial derivatives of the horizontal electric field and the vertical geomagnetic component, which can be directly derived from Faraday’s law. The identity derived from the relationship is written using the vertical magnetic transfer function, the impedance, and the spatial derivatives of the impedance and horizontal magnetic transfer functions. The present method determines the real distortion tensor so that the identity is satisfied. Therefore, the method has two major advantages: (1) galvanic distortion that is to be removed from the impedance tensor is clearly defined, and (2) the method is applicable even when the regional structure is 3-D.

On the detectability of a highly conductive layer in the upper mantle beneath the Black Forest crystalline using magnetotelluric methods

SUMMARY Simultaneous geomagnetic and telluric variations (600 s-4 hr) were observed at two stations on the Rhine-Graben sediments and on the neighbouring Black Forest crystalline in order to extend the pulsation observations (10-600 s) of former magnetotelluric (MT) surveys to longer periods. These observations completely describe the anomalous magnetic and telluric fields in terms of a set of transfer functions using the Black Forest station as 'normal'. In the period range from 30 s to 4 hr the horizontal and vertical magnetic transfer functions of the Rhine-Graben anomaly are characterized by continuously decreasing amplitudes. The local Schmucker induction arrows point uniformly 45 south-east for all long-period geomagnetic transfer functions, and the NW-SE-striking direction in the upper mantle derived from the induction arrows is also consistent with the preferred telluric direction in the Black Forest. In both areas large offsets of apparent resistivity and phase curves for the E- and B-polarization directions are observed, except that the phase curves converge above 400s in the long-period range. The interpretation of the period dependences of the observed magnetic and telluric transfer functions from 600 s to 4 hr by a 2-D conductivity model requires a highly conductive layer in the upper mantle at a depth of 120 km beneath the Black Forest. This confirms Bahr's (1992) model for a highly conductive layer in the upper mantle that has also been found south of the Alps (Braitenberg et al. 1994) and which is interrupted by low conductive alpine roots. However, no information about such a layer beneath the Rhine Graben can be derived. 2-D model studies have shown that an improved depth resolution of highly conductive layers in the upper mantle is possible using telluric phases, but the transfer functions for the vertical magnetic fields are not sensitive enough for the determination of the depth of an upper mantle conductive layer.