Radiomagnetotelluric Data Research Articles

Exploration of the Near-surface Structure of Mud Volcanoes using Electromagnetic Techniques: A Case Study from Perekishkul, Azerbaijan

The general aim of this study is, for the first time, to investigate the inner structure of mud volcanoes in Azerbaijan using several near-surface electromagnetic methods. The central-loop transient electromagnetic (TEM) method and the radiomagnetotelluric (RMT) method were applied on mud volcanoes near Perekishkul, Azerbaijan. These methods complement each other in terms of depth of investigation. In particular, the RMT method generally resolves the very shallow resistivity structures (<10 m) directly beneath the mud volcanoes, whereas the TEM method provides a resolution of up to 100- to 150-m depth. The obtained data sets were processed and interpreted individually. The TEM data were interpreted by conventional 1-D inversion algorithms, i.e., Occam's and Marquardt inversion, and by a quasi-2-D laterally constrained inversion. The RMT data were interpreted by 2-D inversion. Subsequently, a combined model regarding the sediments and corresponding resistivities for the area around the mud volcanoes was derived. Overall, a resistivity range of 1–20 Ω-m was observed. The resistivity models obtained from TEM support the assumption of a three-layer subsurface for the resolved depth range. Also, a lateral resistivity variation caused by the mud volcanoes was observed. Furthermore, the RMT results indicate the presence of shallow low resistivity structures, which can be interpreted as mud reservoirs.

Joint Inversion of RMT and TEM Data for Mapping Active Faults in Northern Greece

Northern Greece is an area with one of the most seismically active region in Europe. Several earthquakes occurred during 20th century. The largest earthquake in recent time with magnitude of 6.5 happened in June 1978. The objective of the presented study is to investigate local geological structure and the top of the basement of the Volvi Basin, Northern Greece. The geophysical surveys are carried out using electromagnetic methods, Radiomagnetotelluric (RMT) and Transient electromagnetic (TEM). The investigations are limited to near surface studies with shallow depth (< 200 m). It is expected that the complex geological structure and the vertical distribution of resistivity layers can be defined by RMT and TEM methods. Joint inversion can increase the number of important model parameters and it can also decrease the ambiguity of the model. The result of joint inversion was in agreement with assumption as the RMT data are resolved the surface layer at depth down to 40 m, and TEM data is small enough to resolve the surface structure (z < 10 m), however it can resolve the deeper structure down to 200 m of depth, depending on the resistivity structure. The joint and sequential models can both represent 1-D single inversion of RMT and TEM data. The model parameters are well resolved in joint inversion in comparison with the single RMT and TEM inversions. Hence, the joint inversion of RMT and TEM data will produce good resolution of model parameters in shallow and deeper parts of fault structure.

Radio Magnetotelluric (RMT) measurement over lake Mälaren, Sweden a case study

Over the past few decades the application of frequency domain radio Magnetotelluric (RMT) have been well established and utilized in the studies of near surface geophysics. This method uses the distance radio transmitter in the frequency range 15 to 250 kHz as source and acts as a tool to model the variation of electrical resistivity of the subsurface. In practice, three component magnetic and two component electric data are measured and recorded. These measured quantities are then used to estimate the earth’s electromagnetic transfer functions called impedance tensor and tipper vector. The transfer function carries the information related to resistivity variation of the subsurface. The present study was carried out in an urban setting which is a challenging environment for this method. The Swedish Road Authorities (Trafikverket) has proposed and planned several infrastructure project to cater the immense overload of traffic in the Stockholm city due to growing population. One such project initiated by Swedish road authorities is the Stockholm bypass project that is a ten year construction plan of 21 km long motor way and to reduce the impact on the environment 18 km will be located in tunnel. The main objective of this study is thus to identify potential fracture zones and faults as well as the general geological settings. The proposed tunnel will be at a depth of 60m and will pass under Lake Malaren at three areas. This raised an opportunity and challenge to explore the applicability of the land RMT measurement system called EnviroMT (Bastani 2001). This acquisition system developed in Uppsala University has already proved to be a robust tool for land survey. To make it applicable in shallow water environment, an improvisation of existing EnviroMT system was pioneered by Geological Survey of Sweden (SGU). The prototype acquisition system was successfully tested in April 2012 by Geological Survey of Sweden (SGU) on Lake Malaren. The new system did not only turned out to be appropriate for such a challenging environment, but it was also much more efficient as compared with land surveys. The survey extensively covered all the three water passages. In three days, fifty two lines including 1160 stations with an average spacing of 15m were covered. Urban environment have its own adverse effect on the RMT data. Such cultural noises had to be identified and removed to obtain more accurate and realistic model. Parametric representation combined with a Truncated Singular Value Decomposition (TSVD) regularization described in Bastani and Pedersen (2001) was used to carry out the processing of the noisy data. Strike analysis of all the profile lines suggested the structure to be mostly 2D except for some region where multiple strike direction was observed suggesting a 3D structure. Thus the data were inverted to obtain 2D resistivity models using EMILIA (Kalscheuer et al., 2010) for all the lines and 3D models using WSINV3DMT (Siripunvaraporn and Egbert, 2000) for a few selected region. The resistivity models of all the lines had consistent features in term of water depth, sediments thickness and the depth to the bedrock (Mehta S et.al 2014). A coherent low resistivity is observed across all the lines and it coincides well with the low velocity zone suggest by the available seismic refraction survey in the same region. This is an indication of a possible fracture zone in the bedrock. The comparison of 3D slices with the 2D resistivity models along the lines further increases the certainty of the fracture zone.

Boat-towed radio-magnetotellurics — A new technique and case study from the city of Stockholm

We have developed a new data acquisition system and technique to measure the radio magnetotelluric (RMT) signals from distant radio transmitters with the objective of mapping and modeling electric resistivity structures below a river or lake. The acquisition system is towed by a boat; therefore, we call the technique boat-towed RMT. The data acquisition is fast with a production rate of approximately [Formula: see text] using a nominal sampling spacing of 10–15 m. Given the ample number of radio transmitters available in most parts of the world, the method can be used for near-surface studies of various targets. We have developed boat-towed RMT measurements on Lake Mälaren near the city of Stockholm in Sweden to determine the feasibility of the method. Approximately 15 km of RMT data were collected during three days above a planned 60-m-deep bypass tunnel with the goal of providing information on the bedrock depth and possible weak zones within the bedrock. The measured resistivity and phase data were of high quality with errors on the order of a few percent. The resistivity models from 2D inversion of the data showed a good correlation with available geologic data in resolving bedrock depth and also resistivity layering within the lake. Resistivity maps derived from the dense 2D models suggested a northeast–southwest-striking low-resistivity zone at less than a 30-m depth. The zone likely represents fractured crystalline bedrock. The boat-towed RMT technique is well suited for water bodies with moderate electric resistivity such as in brackish and freshwater environments.

Magnetic and electromagnetic data interpretation for delineating geological contacts in the Tomelilla area, Sweden

This paper describes an application of diverse geophysical methods to detect the geological contacts in the Tomelilla area, Sweden. Airborne magnetic data and the tipper and peaker maps derived from tensor very-low-frequency electromagnetic (TVLF-EM) data are interpreted in order to indicate most geological boundaries. The inverted airborne TVLF data along adjacent profiles over each contact show the resistivity variations at depth and laterally localize the geological contacts while delineating resistive bedrocks in all cases. To control the resistivity images across these profiles, the ground-based measurements of the radio magnetotelluric data combined with controlled-source magnetotelluric data (RMT/CSMT) have been performed to plot resistivity variations with higher resolution. The obtained resistivity images are in good agreement with the ones acquired from airborne TVLF data. A relatively conductive layer with an average thickness of 20 to 30 m of the Quaternary deposits, increasing in thickness from east to the west, and a resistive basement with much higher resistivity on the granites than the shales are resolved from the geoelectric sections. This paper confirms that the combined geophysical surveys (i.e., airborne magnetometry/TVLF along with ground-based RMT/CSMT) reduce the uncertainty arising from geological mapping. It is shown that the airborne TVLF data have high capability for image resistivity variation along geological contacts in the study area while its resolution has been confirmed by the ground-based RMT/CSMT survey.

Appraisal of a new 1D weighted joint inversion of ground based and helicopter‐borne electromagnetic data

ABSTRACTIn order to couple spatial data from frequency‐domain helicopter‐borne electromagnetics with electromagnetic measurements from ground geophysics (transient electromagnetics and radiomagnetotellurics), a common 1D weighted joint inversion algorithm for helicopter‐borne electromagnetics, transient electromagnetics and radiomagnetotellurics data has been developed. The depth of investigation of helicopter‐borne electromagnetics data is rather limited compared to time‐domain electromagnetics sounding methods on the ground. In order to improve the accuracy of model parameters of shallow depth as well as of greater depth, the helicopter‐borne electromagnetics, transient electromagnetics, and radiomagnetotellurics measurements can be combined by using a joint inversion methodology. The 1D joint inversion algorithm is tested for synthetic data of helicopter‐borne electromagnetics, transient electromagnetics and radiomagnetotellurics. The proposed concept of the joint inversion takes advantage of each method, thus providing the capability to resolve near surface (radiomagnetotellurics) and deeper electrical conductivity structures (transient electromagnetics) in combination with valuable spatial information (helicopter‐borne electromagnetics). Furthermore, the joint inversion has been applied on the field data (helicopter‐borne electromagnetics and transient electromagnetics) measured in the Cuxhaven area, Germany.In order to avoid the lessening of the resolution capacities of one data type, and thus balancing the use of inherent and ideally complementary information content, a parameter reweighting scheme that is based on the exploration depth ranges of the specific methods is proposed. A comparison of the conventional joint inversion algorithm, proposed by Jupp and Vozoff (), and of the newly developed algorithm is presented. The new algorithm employs the weighting on different model parameters differently. It is inferred from the synthetic and field data examples that the weighted joint inversion is more successful in explaining the subsurface than the classical joint inversion approach. In addition to this, the data fittings in weighted joint inversion are also improved.

Integrated 2D modeling and interpretation of geophysical and geotechnical data to delineate quick clays at a landslide site in southwest Sweden

Radio magnetotellurics (RMT), electrical resistivity tomography (ERT), and high-resolution reflection seismic data were collected along four lines to characterize the geometry and physical properties of geologic structures at a quick-clay landslide site in southwest Sweden. The site is situated in the Göta River valley where the normally consolidated materials mainly consist of glacial and postglacial sediments. Geotechnical data suggest the presence of quick clays above coarse-grained layers. These layers play a key role in the formation of quick clays and landslide triggering. The RMT and ERT data were individually and jointly inverted in 2D to study the resolution of resulting models for each data set. The resistivity models from the joint inversions demonstrate superior resolution and accuracy compared with individual ones. The geometry and location of shallower structures resolved in the 2D resistivity models from joint RMT&amp;ERT inversions correlated well with those imaged in the reflection seismic data and observed in the existing geotechnical boreholes. The models were poor in resolving deeper resistive bedrock at locations where the thickness of the conductive overburden exceeds a certain limit. However, information from the reflection seismic data could be used to estimate the depth to the top of the bedrock along all the four lines. Comparison between the geotechnical data and the resistivity models suggested that quick clays overlying the coarse-grained layer have higher electrical resistivity than the marine clays. We further validated and refined the obtained results by performing synthetic tests. We showed that integration of ERT and RMT data with reflection seismic data is ideal for quick-clay landslide studies especially when the clay materials are thick.

Integration of controlled-source and radio magnetotellurics, electric resistivity tomography, and reflection seismics to delineate 3D structures of a quick-clay landslide site in southwest of Sweden

Quick clay, which is the main cause of landslides that occur in the northern countries, liquefies easily, and its presence implies an increased risk of landslide. Geophysical methods have been increasingly used in landslide investigations. Three-dimensional electric resistivity tomography, radio magnetotelluric (RMT), controlled-source RMT (CSRMT), and high-resolution reflection seismic data were acquired at a quick-clay landslide site in the southwest of Sweden. The main objectives were to evaluate the capability of each method in delineating different subsurface geologic structures that controlled a peculiar and hazardous retrogressive-type landslide in the study area. A 3D resistivity model from the inversion of CSRMT data showed the best correlation with the reflection seismic data and borehole information, thanks to the broad frequency range of the data set. It better imaged the resistive crystalline bedrock underlying the marine conductive clays and showed considerable correlations with the 3D reflection seismic data in resolving a coarse-grained layer that was interpreted to act as a conduit directing freshwater into the clays under a confined pressure, leaching their salt and forming quick clays. The 3D CSRMT resistivity model and 3D reflection seismic data showed that the coarse-grained layer has a varying thickness. At some locations, it was too thin to be resolved by the methods used here. Combination of the CSRMT model, reflection seismic data, and the borehole data suggested that a layer with thickness of approximately 5 m and resistivity between [Formula: see text] was potentially quick clay, which probably extended laterally in the entire study area. These observations suggested that future developments should focus on joint inversion of such 3D data sets incorporating sharp boundaries as constraints in the inversion and particularly when quick clays were studied.

2D joint inversion of RMT and ERT data versus individual 3D inversion of full tensor RMT data: An example from Trecate site in Italy

Tensor radiomagnetotelluric (RMT) and electrical resistivity tomography (ERT) data were acquired along 10 parallel lines to image electrical resistivity of the vadose and the saturated zone in an area near Trecate, 45 km west of Milan in Italy. In 1994, the area was exposed to an oil contamination caused by a tank explosion and has since been subject to monitoring and remediation programs. For the first time, we have examined a 3D inversion of full tensor RMT data and have compared the results with 2D joint inversion of RMT and ERT data. First, a synthetic 3D resistivity model with similar variations close to those measured at the Trecate site was generated for the comparison. The synthetic tests showed that resistivity models from 2D joint inversion of ERT and RMT data contain more details closer to the surface compared to the models from the 3D inversion of tensor RMT data. High resistivity structures are better resolved by the 2D joint inversion, whereas the more conductive features are better recovered by the 3D inversion. In the next step, the ERT and RMT data collected in the Trecate site were modeled with the same approaches used in the synthetic modeling. Using the measured tensor RMT data, it was possible to carry out full 3D inversion to study the underlying geology. Comparison between the resistivity models from both inversions with the lithological data from the existing boreholes, resistivity models from the inversion of crosshole resistivity data, and water content models from magnetic resonance soundings measurements showed that the electrical resistivity, depth to the top and thickness of the water saturated zone is modeled more accurately with the 3D inversion.

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.

The electrical conductivity distribution of the Hallandsås Horst, Sweden: a controlled source radiomagnetotelluric study

ABSTRACTWe studied the behaviour of controlled source responses of 2½D synthetic models based on controlled source tensor magnetotelluric ( = 1–12 kHz) and radio magnetotelluric ( = 14–250 kHz) data sets collected along profiles on the Hallandsås Horst, Southern Sweden, with a view to study the depth extent of possible near‐surface fractures zones that might intersect a tunnel under construction. Based upon 2.5D forward modelling of the transverse electric (TE) and transverse magnetic (TM) mode radio magnetotelluric data, a simplified 2D model was constructed such that a dipping conductor was continued down to a depth of 140 m, which is the depth of the tunnel. Based on this extended model, we calculated both plane wave and controlled source responses for the frequency range 1–250 kHz corresponding to the source location in the study. Finally, we inverted both data sets under plane wave assumptions. When comparing synthetic controlled source and plane wave data in the frequency range 1–12 kHz, we find that the controlled source tensor magnetotelluric transfer functions generally deviate from the plane wave condition for frequencies less than 6 kHz. The phases are much more distorted than apparent resistivities. The TM mode phases, in particular, are generally distorted for frequencies less than 6 kHz. Regarding the vertical magnetic fields, we find that at all frequencies the real part of tippers are distorted by source effects but the tipper component along the profile direction is much less affected by the off‐profile lying controlled source tensor magnetotelluric source than the tipper component orthogonal to the profile direction. Based upon synthetic model studies of combined controlled source tensor magnetotelluric and radio magnetotelluric responses, we find that determinant data are less distorted than individual TE‐w and TM‐mode data. On the other hand, TE‐ and TM‐responses have greater resolving power and, when used with care (selecting data that are weakly distorted by source effects), the corresponding models can resolve the conductor down to tunnel depths.

Non-linear model error and resolution properties from two-dimensional single and joint inversions of direct current resistivity and radiomagnetotelluric data

SUMMARY For the first time, a comparative analysis of the resolution and variance properties of 2-D models of electrical resistivity derived from single and joint inversions of dc resistivity (DCR) and radiomagnetotelluric (RMT) measurements is presented. DCR and RMT data are inverted with a smoothness-constrained 2-D scheme. Model resolution, model variance and data resolution analyses are performed both with a classical linearized scheme that employs the smoothness-constrained generalized inverse and a non-linear truncated singular value decomposition (TSVD). In the latter method, the model regularization used in the inversion is avoided and non-linear semi-axes give an approximate description of the non-linear confidence surface in the directions of the model eigenvectors. Hence, this method analyses the constraints that can be provided by the data. Model error estimates are checked against improved and independent estimates of model variability from most-squares inversions. For single and joint inverse models of synthetic data sets, the smoothness-constrained scheme suggests relatively small model errors (typically up to 30 to 40 per cent) and resolving kernels that are spread over several cells in the vicinity of the investigated cell. Linearized smoothness-constrained errors are in good agreement with the corresponding most-squares errors. The variability of the RMT model as estimated from non-linear semi-axes is confirmed by TSVD-based most-squares inversions for most model cells within the depth range of investigation. In contrast to this, most-squares errors of the DCR model are consistently larger than errors estimated from non-linear semi-axes except for the smallest truncation levels. The model analyses confirm previous studies that DCR data can constrain resistive and conductive structures equally well while RMT data provide superior constraints for conductive structures. The joint inversion can improve error and resolution of structures which are within the depth ranges of exploration of both methods. In such parts of the model which are outside the depth range of exploration for one method, error and resolution of the joint inverse model are close to those of the best single inversion result subject to an appropriate weighting of the different data sets.

Aquifer characterization in the Ouarzazate Basin (Morocco): A contribution by TEM and RMT data

ABSTRACTIn this case study we present the first application of the radio magnetotelluric (RMT) technique in Africa. We applied the RMT and transient electromagnetic (TEM) methods to study an aquifer of the Ouarzazate Basin between the High Atlas and the Anti Atlas in Morocco. The main objective of the survey was to constrain the geometry of the near surface sediment layer by mapping the aquitard, an underlying clay layer.We collected RMT data in the frequency range of 16 kHz to 207 kHz and TEM data in the time range of 4 μs to 1 ms along an 8 km long profile in a desert area crossing an oasis region. After 2D inversion of the RMT data and 1D inversion of the TEM data, we carried out 1D joint inversion to enhance the resolution of the subsurface resistivity structure.From the inversions we derived the resistivity distribution down to a depth of about 100 m and identified a relatively resistive layer as the aquifer sediments, overlying a more conductive clay layer. Inversion statistics indicate that the depth of the clay layer is well resolved. Compared with the lithological logs of drillings located at distances of 150 m to 3 km from the profile, the depth of the clay layer beneath the aquifer in the south of the geophysical section agrees to ± 3 m. Furthermore, we observed a lateral resistivity contrast close to the surface separating the desert area and the oasis, due to differences in humidity.

Radiomagnetotelluric and Direct Current Resistivity measurements for the characterization of conducting soils

The application of geophysical techniques to hydrogeological questions has grown in importance due to the increasing concern for environmental issues. To improve the hydrogeological image of the subsurface we carried out electric and electromagnetic measurements using Radiomagnetotellurics and Direct Current Resistivity in East-Frisia, North-West Germany. The aim of our experiments was to compare the two methods and improve the hydrogeological image of the subsurface. This allows us to assess the threat groundwater exploitation poses to wetland habitats (biotopes). Pumping groundwater from deep aquifers can drain water from near surface aquifers. Hence it is vital to know ground water confining units. Prior to our measurements the hydrogeological image of those confining units was based on interpolation between boreholes. Some single Direct Current sounding measurements were also carried out and were used with the information from the boreholes to generate a hydrogeological model of the area. By combining the results from our measurement profiles with the hydrogeological model we were able to revise the hydrogeological model. By using the geophysical modeling results we could define regions where the hydrogeological model is inadequate due to errors in interpolation. We apply both Direct Current Resistivity and Radiomagnetotellurics and perform a practical comparison of the two methods. To ensure the comparability of the resulting resistivity models we examine the depth of exploration of the two methods and demonstrate the application of the depth of exploration index to Radiomagnetotelluric data. Both methods detect conducting structures, which correlate with low permeable clay structures in the hydrogeological models.

A comparison of different radiomagnetotelluric data inversion methods for buried waste sites

This study deals with two-dimensional (2D) inversions of synthetic and observed radiomagnetotelluric (RMT) data on typical buried conductive waste sites in Europe, and with the practical aspects of different inversion algorithms. In the inversion calculations, we used smoothing and L2-norm stabilizers and compared the results. The resolution of the geometry of the highly conductive waste site, in particular, was investigated. In the inversion with the L2-norm stabilizer, we used the least-squares solution with singular value decomposition (LSSVD) and conjugate gradient (CG), whereas only the conjugate gradient solver was used in the 2D-inversion with the smoothing stabilizer. The inversion results of the synthetic data showed a better resolution of the geometry of the highly conductive waste site when a L2-norm stabilizer was applied in the inversion; in particular, a better detection of the bottom of the waste deposit was achieved. Additional model studies were carried out using synthetic RMT data in order to investigate the 2D inversion of RMT data observed on a 3D structure; these studies showed that the use of TM mode data yields a better resolution of the structure than joint inversion of TE and TM modes. 2D inversions of RMT data on a waste site near Cologne showed that the inversion of the TM mode could resolve well the geometry, especially the bottom of the waste site, if information about the background conductivity structure was available. In this case study, inversion with the L2-norm stabilizer produced a sharper image of the waste site than inversion with the smoothing stabilizer, as indicated also by the inversion study that used synthetic data.

Comparison between VLF and RMT methods: A combined tool for mapping conductivity changes in the sedimentary cover

We compare the resolution power of single frequency very low frequency (VLF) electromagnetic data (real and imaginary parts of the tipper) and multi-frequency RadioMagnetoTelluric (RMT) data in delineating conductive structures typical for the sedimentary cover over crystalline basement in Scandinavia. Using VLF field data from five parallel profiles reveals that the estimated models have responses that fit the observed data well, and the models show an overall agreement with more detailed models derived from broadband RMT data. It is suggested that VLF data be used as a fast mapping tool to fill in the gaps between profiles along which more detailed RMT measurements are made. A generic model with conductive clay lenses and sandy formations over crystalline basement is used to generate synthetic data for the two cases. Using regularized inversion the corresponding estimated models clearly shows the strength and the weakness of both methods. Being inductive methods, they both have difficulties in clearly resolving the depth transition from conductive to resistive units. Especially the single frequency VLF data can be interpreted with very smooth models at depth. However, both methods resolve very well the lateral boundaries of the clay lenses and the RMT data also constrain the thickness of the clays quite well compared with the VLF models, which are less distinct at depth. Single frequency scalar VLF data emphasize those conductive structures that have dominant strikes in the direction of the transmitter. Multi-frequency VLF (tensor VLF) measurements provide the tipper vector which depends upon the underlying conductivity structure only. Real conductivity structures have significant 3D components which can be delineated easily by tensor VLF measurements. We propose that new VLF instrumentation be developed with this in mind.

Routine 2D inversion of magnetotelluric data using the determinant of the impedance tensor

Recent developments in the speed and quality of data acquisition using the radiomagnetotelluric (RMT) method, whereby large amounts of broadband RMT data can be collected along profiles, have prompted us to develop a strategy for routine inverse modeling using 2D models. We build a rather complicated numerical model containing both 2D and 3D elements believed to be representative for shallow conductors in crystalline basement overlain by a thin sedimentary cover. We then invert the corresponding synthetic data on selected profiles, using both traditional MT approaches, as well as the proposed approach, which is based on the determinant of the MT impedance tensor. We compare the estimated resistivity models with the true models along the selected profiles and find that the traditional approaches often lead to strongly biased models and bad data fit, in contrast to those using the determinant. In this case, much of the bias is removed and the data fit is improved. The determinant of the impedance tensor is independent of the chosen strike direction, and once the a priori model is set, the best fitting model is found to be practically independent of the starting model used. We conclude that the determinant of the impedance tensor is a useful tool for routine inverse modeling.

Characterization of a fractured granite using radio magnetotelluric (RMT) data

We applied tensor radio magnetotellurics (RMT) in the 10–250 kHz frequency range to study major fracture zones on Ävrö, a small island (1.6 × 1.2 km2) in southeastern Sweden with bedrock dominated by highly resistive granite. The interpretation of a 950‐m RMT profile was facilitated by seismic reflection and borehole data but was complicated (1) by possible 3D effects of the surrounding sea and (2) because the quasi‐static assumption is violated. Inversions based on the quasi‐static assumption give severely distorted models in this type of environment. Inversion codes that include displacement currents are restricted to 1D structures. Therefore, 2D inversions were applied to lower frequencies only. The central part of the inverted profile showed a 30–40‐m‐thick weathered layer over an almost intact bedrock down to a depth of at least 200 m, where higher salinity and/or fracturing yielded higher conductivities. The first 200 m of the profile revealed a major fracture zone, which coincided with a seismic reflector. We used 3D forward modeling to understand the sea effect and to model the conductor in three dimensions. We believe that 3D forward modeling is a highly valuable tool to distinguish known 3D effects (i.e., the sea) from regional 2D features of interest. We suggest that water flow at Ävrö is dominated by a few major fracture zones.

Evidence of electrical anisotropy in limestone formations using the RMT technique

Azimuthal resistivity surveys are often applied to complement hydrological information or to improve the location of observation boreholes in pump tests. Symmetric electrode configurations cannot distinguish anisotropy from lateral changes or dipping layers, but asymmetric arrays (e.g., the offset Wenner array) can. Tensor radiomagnetotellurics (RMT) is presented as an alternative method in studies of electrical anisotropy in the shallow subsurface. The electromagnetic and geomagnetic transfer functions provide information about the dimensionality of the data. These transfer functions can also be used to find the directions of anisotropy. Data with an anisotropic signature can be inverted for a one‐dimensional (1D) azimuthal anisotropy model. The method is faster than the azimuthal resistivity method. A 380‐m‐long profile of tensor RMT data (12.7–243 kHz) from limestones that overlie shale on the island of Gotland, Sweden, is used to show the merits of the method. The data have a clear anisotropic signature. The data are inverted for a three‐layer 1D model with azimuthal anisotropy using two different approaches: (1) a moving median filter of five neighboring stations and neglecting static shift parameters; and (2) treating each station separately and including static shifts of the electric field in the inversion. Both inversions show models having a marked anisotropy with anisotropy factors of 3.7 and 4.5, respectively, in the limestones. The second approach has a significantly better data fit. However, the first approach is preferred because the models are smoother from station to station.

Determination of the regularization level of truncated singular‐value decomposition inversion: The case of 1D inversion of MT data

ABSTRACTThe total mean‐square error (MSE) of the estimated model, defined as the sum of the standard model variance and the bias variance, is used to define the truncation level of the singular‐value decomposition to give a reasonable balance between model resolution and model variance. This balance is determined largely by the data and no further assumptions are necessary except that the bias terms are estimated sufficiently well. This principle has been tested on the 1D magnetotelluric inverse problem with special emphasis on high‐frequency radio magnetotelluric (RMT) data. Simulations clearly demonstrate that the method provides a good balance between resolution and variance. Starting from a homogeneous half‐space, the best solution is sought for a fixed set of singular values. The model variance is estimated from the sum of the inverse eigenvalues squared, up to a certain threshold, and the bias variance is estimated from the model projections on the remaining eigenvectors. By varying the threshold, the minimum of the MSE is found for an increasing number of fixed singular values until the number of active singular values becomes greater than or equal to the estimated number. As a side‐effect, the depth of penetration of a given set of measurements can be estimated very efficiently by simply noting at which depth the final model deviates little from the starting homogeneous half‐space model. A suite of synthetic data is inverted and an example of inversion of one site is shown to illustrate how the truncation is carried out as the non‐linear inversion process proceeds. A field example with a profile across a plume of contaminated groundwater in the Netherlands shows good agreement with the electrical resistivity obtained in a nearby borehole.