Radiomagnetotelluric Research Articles

Geophysical imaging of Silurian carbonates by use of ground and airborne electromagnetic and radiometric methods on the island of Gotland, Sweden

The island of Gotland in the Baltic Sea exhibits rock types dominated by limestone and marlstone, deposited in a Silurian carbonate platform environment. The strata are extensively exposed in outcrops, quarries, and coastal cliff sections. Low-lying marlstone-dominated areas are separated by highs dominated by limestone. The surface bedrock is well known, but the subsurface composition down to a 100-m depth has, until recent deployment of modern geophysical measurements, been largely unknown. Our geophysical surveys and methods were performed with the aim to evaluate their usefulness in characterizing the upper 100 m of the bedrock in a carbonate-dominated bedrock setting. The surveys were performed in connection to a bedrock mapping project including geologic and geophysical investigations. The airborne techniques complemented by ground measurements and correlation with outcrops and boreholes resulted in a definition of bedrock boundaries and especially subsurface characterization of bedding sequences. The airborne electromagnetic (very low frequency) survey was accompanied by radio magnetotelluric (RMT) measurement on the ground along several profiles. The difference in resistivity between high-resistive limestone overlaying a more conductive marlstone gave favorable conditions for this type of survey. The results have so far given new and valuable information on the thickness of the limestone as well as the distinction of lateral changes of the bedrock related to the depositional setting. The RMT measurements have made it possible in a new way to remotely map out bedding sequences in carbonates and to detect saline groundwater and fracture zones. In addition, a recently performed airborne transient electromagnetic survey contributed to the interpretation of the RMT measurements that we evaluated. The applied geophysical methodology and correlation with outcrops revealed carbonate bedrock with geophysical properties, enabling a better understanding concerning the groundwater conditions, limestone resources, and radon situation on the island of Gotland.

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.

The basement structure below the peat–lignite deposit in the Philippi sub-basin (Northern Greece) inferred by electromagnetic and magnetic methods

During 2009 and 2010 electromagnetic (EM) soundings and a high-resolution magnetic survey were conducted to study the deeper structure of the peat–lignite deposit in the Philippi sub-basin in Northern Greece. The primary intention of investigating the basement structure of the Philippi sub-basin is to propose the ideal location for a deep and continuous paleoclimate drill site.Data were collected along a 12km transect (NNE–SSW) through the largest extension of the basin from Krinides at the North to Eleftheroupolis at the South. We used a combined set of Radiomagnetotelluric (RMT), Time Domain Electromagnetic (TEM) and Audiomagnetotelluric (AMT) soundings to derive a 2D model of the electrical resistivity distribution versus depth using a joint inversion approach. This model was then cross correlated with a 2D forward model of magnetic anomaly data. The magnetic survey detected strong anomalies in the North that appeared to have been generated by the Philippi granitoid pluton. All three individual data sets support each other and have jointly been analyzed. From this study we yield an asymmetric graben model of the basin structure that shows maximum thickness (ca. 500m) in the northern part of the basin leading to a reduction of the thickness to the South. The interface between the basin fill and the bedrock ascend steeply in the North. The overall assessment of the deeper basin structure reveals a detachment system that is in good accordance with previous findings.

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.

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.

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.

Analysis of the Shallow Structures of Active Fault Zones Using 3D Radiomagnetotelluric Data Set

A radiomagnetotelluric (RMT) is an extension of the very low frequency (VLF) Electromagnetic induction method (10-30kHz) to frequencies as high as 1MHz. The field data set of RMT acquired in scalar mode has been successfully analysed shallow fault structure using a 3D modeling scheme with finite difference algorithm. Due to complex geological structure in the research study area, it is difficult to fulfil the 2D assumption regarding Transverse Magnetic (TM) and Transverse Electric (TE) Modes. Hence, the 3D modeling can provide adequate information on the active fault structure and gives a representative model for all conductivity structures. The 3D model is constructed using 2D conductivity model and the geology as priori information. The resulting model clearly detects the shallow fault structure and the 3D model response is in a good agreement with 2D models from observed data.

Research on technology of Controlled Source Radio MagnetoTelluric system’s transmitter

The basic theory and principle of the artificial Controlled Source Radio MagnetoTelluric (CSRMT) method is studied and a novel CSRMT transmitter in kHz frequency band is designed. The specific circuit and measured results of some key modules in transmitter are presented, and some outdoor experimental tests have been carried out, which shows that the completed prototype of transmitter can generate a continuous sine current with frequency up to 35.33 kHz, peak-to-peak amplitude up to 40 A, and realize a reliable transmitting mode with multi-frequency and high-current. The transmitter has a wide operating band, large magnetic moment and high waveform fidelity, and can meet the requirements of shallow geological exploration with high-resolution.

Radiomagnetotelluric mapping of marlstone and limestone in the Silurian bedrock of Gotland

The Silurian of Gotland is composed of a series of stacked carbonate platforms. Airborne electromagnetic measurements show areas with high electrical resistivity, dominated by dense limestone, and areas with lower resistivity, dominated by marlstone. On Gotland, three main regional, up to 80 m thick, limestone-dominated bedrock packages overlie marlstone-dominated strata. The thickness of the limestone and the subsurface relationship between limestone and marlstone was investigated in the area of Ale–Ardre on central Gotland by use of the radiomagnetotelluric (RMT) method. A significant difference in resistivity between the limestone (1000–10 000 Ωm) and the marlstone ( < 100 Ωm) renders favorable RMT conditions. Four profiles, up to 4.3 km long, were measured across the upper part of the Klinteberg Formation and lower part of the Hemse Group (late Wenlock–early Ludlow). The results show a distinct change in resistivity at 50–80 m depth, correlating with the transition between biostromal–biohermal limestone and shelf marlstone–wackestone lithofacies, as observed in boreholes. The measurements were used to create resistivity models that make it possible to map the subsurface lateral relationship between limestone- and marlstone-dominated sequences. The results also provide important information on distribution and bedding, which give supporting information regarding the sequence stratigraphic model for the Klinteberg Fm–Hemse Group in the Ala–Ardre–Sigsarve area. The RMT method utilized in this study proved to be a very useful method for mapping subsurface bedrock in carbonate-dominated geological settings such as those on Gotland.

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.

Delineation of a quick clay zone at Smørgrav, Norway, with electromagnetic methods under geotechnical constraints

In many coastal areas of North America and Scandinavia, post-glacial clay sediments have emerged above sea level due to iso-static uplift. These clays are often destabilised by fresh water leaching and transformed to so-called quick clays as at the investigated area at Smørgrav, Norway. Slight mechanical disturbances of these materials may trigger landslides. Since the leaching increases the electrical resistivity of quick clay as compared to normal marine clay, the application of electromagnetic (EM) methods is of particular interest in the study of quick clay structures. For the first time, single and joint inversions of direct-current resistivity (DCR), radiomagnetotelluric (RMT) and controlled-source audiomagnetotelluric (CSAMT) data were applied to delineate a zone of quick clay. The resulting 2-D models of electrical resistivity correlate excellently with previously published data from a ground conductivity metre and resistivity logs from two resistivity cone penetration tests (RCPT) into marine clay and quick clay. The RCPT log into the central part of the quick clay identifies the electrical resistivity of the quick clay structure to lie between 10 and 80 Ω m. In combination with the 2-D inversion models, it becomes possible to delineate the vertical and horizontal extent of the quick clay zone. As compared to the inversions of single data sets, the joint inversion model exhibits sharper resistivity contrasts and its resistivity values are more characteristic of the expected geology. In our preferred joint inversion model, there is a clear demarcation between dry soil, marine clay, quick clay and bedrock, which consists of alum shale and limestone.

A radio magnetotelluric study to evaluate the extents of a limestone quarry in Estonia

ABSTRACTElectromagnetic signals from distant radio transmitters in the frequency range 15–250 kHz were measured to model an electrical resistivity structure beneath 7 profiles in the vicinity of the Karinu limestone quarry in Estonia with the aim to map the extent of the economically exploitable limestone. The resistivity models from a 2D inversion of determinant resistivity and phase values using an Occam type of regularization contained reasonably accurate information about the geometry, namely depth to the top and the bottom of the target high‐resistivity limestone. The resistivity models correlated well with existing geological evidences as well as information from closely located boreholes. However, the sharp lithological boundaries seen in the boreholes were not resolved exactly in the resistivity models. This is probably because of the smoothing regularization used in the inversion process. Combined use of borehole data together with resistivity models resulted in two major geological interpretations; a) towards the western part of the existing limestone quarry there is a NNW to NS striking fault, covered by post‐glacial sediments, b) a potential cost‐effective exploitable area containing high quality highly resistive limestone is located south of the existing quarry. This case study shows the applicability of the reasonably fast radio magnetotelluric (RMT) method for the exploration of near‐surface resources.

Magnetic resonance sounding and radiomagnetotelluric measurements used to characterize a limestone aquifer in Gotland, Sweden

► We have tested MRS, RMT, VES and GPR to characterize a limestone aquifer. ► RMT locates the limestone–marl interface and helps to constrain the MRS inversion. ► The T 1MRS of MRS suggests the aquifer to be hosted by small fractures and vugs. ► No real correlation between water content and resistivity exist. ► MRS is hence the only method that measure water and hence the extension of the aquifer. Almost all drinking water in Gotland is groundwater and is mainly found in karst limestone. However, the unpredictable location and geometry of the karst cracks and caverns makes it very difficult to estimate groundwater storage and movement, as well as contaminant transport. The aim of this study was to test the performance of different geophysical techniques like Magnetic Resonance Sounding (MRS), Radiomagnetotelluric (RMT), Vertical Electrical Sounding (VES) and Ground Penetrating Radar (GPR) to characterize aquifers in Gotland, in respect to geometry and storage as well as connectivity over a wider area. The investigated area is located on the south-eastern part of Gotland. The geology here is dominated by 50–60 m thick successions of limestone that gradually turn into marlstone. The use of multiple techniques has shown to give a more coherent interpretation. However, the shallow penetration depth of GPR and the lack of soil cover in some places of the investigated area limit the use of geoelectrical methods and GPR. With MRS, water are found down to 60 m in depth, with a maximum water content at depths of 20–30 m. This coincides with the most resistive sections of the limestone. The water content varies between 0% and 3%, with a relaxation time ( T 1MRS ) less than 400 ms suggesting that the aquifer is hosted in small fractures, molds and vugs rather than larger karst fractures and caverns. Two potential aquifers were identified with MRS, possibly separated by marlstone. From modeling it can be seen that such boundary separating two aquifer apart can be more easily discriminated in the N/S-, than in the E/W direction. In summary, MRS is therefore the only method in this survey that can detect and determine the vertical and lateral distribution of water within the aquifer together with the total volume of free water. The RMT method has shown to be effective in characterizing the limestone/marl interface, but also to locate anomalous low resistive zones, possibly associated with salt water. RMT also helps to constrain the final MRS model by choosing a suitable regularization for the MRS 3D inversion. All together, the combination of MRS and RMT seems most efficient of the tested methods and therefore most promising for future groundwater explorations in geological environments like in eastern Gotland.

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.

Groundwater contamination in the Roorkee area, India: 2D joint inversion of radiomagnetotelluric and direct current resistivity data

Abstract The impact of sewage irrigation and groundwater contamination were investigated near Roorkee in north India using the Direct Current Resistivity (DCR) method and the Radiomagnetotelluric (RMT) method. Intensive field measurements were carried out in the vicinity of a waste disposal site, which was extensively irrigated with sewage water. For comparison a profile was investigated on a reference site, where no contamination was expected. In addition to conventional 1D and 2D inversion, the measured data sets were interpreted using a 2D joint inversion algorithm. The inversion results from the data obtained from the sewage irrigated site indicate a decrease of resistivity up to 75% in comparison with the reference site. The depth range from 5 to 15 m is identified as a shallow unconfined aquifer and the decreased resistivities are ascribed as the influence of contamination. Furthermore, a systematic increase in the resistivities of the shallow unconfined aquifer is detected as we move away from the waste disposal site. The advantages of both, the DCR and RMT methods, are quantitatively integrated by the 2D joint inversion of both data sets and lead to a joint model, which explains both data sets.

Investigation of groundwater resources using controlled-source radio magnetotellurics (CSRMT) in glacial deposits in Heby, Sweden

We have combined tensor radio magnetotelluric- (RMT, 15–250 kHz) and controlled source tensor magnetotelluric (CSTMT, 1–12 kHz) data for the mapping of aquifers in gravel formations lying in between crystalline bedrock and clay rich sediments in the Heby area some 40 km west of Uppsala in Sweden. The estimated transfer functions, the impedance tensor and the tipper vector generally satisfy 1D or 2D necessary conditions except for the lowest CSTMT frequencies where near field effects become more dominant. The data measured from 8 profiles were inverted with the Rebocc code of Siripunvaraporn and Egbert (2000) assuming plane wave conditions. This meant that only 12 frequencies in the range of 4–180 kHz could be used. The four lowest frequencies of CSTMT in the range of 1–2.8 kHz were excluded because of source effects. Data from all profiles were inverted with a starting model of 100 Ω-m and a relative error floor of 0.02 on apparent resistivity, corresponding to less than 1° on phase. Tipper vectors are generally small except when source effects become dominant in the lowest frequencies of CSTMT and were therefore not used for inversion. Comparing with models derived from vertical electrical soundings, refraction and reflection seismic data as well as ground truth from exploration wells assessed the reliability of the deep part of the models. Furthermore we carried out a non-linear resolution analysis to better quantify the depth extent of the aquifers. The inverted models from the Heby area show well the thickness variations of glacial deposits overlying crystalline bedrock. Generally, the upper 20 m of the models have resistivities below 40 Ω-m, taken to represent clay rich formations. Below the clay layer resistivities increase to about 40–400 Ω-m, interpreted to represent sand/gravel formations with a maximum thickness of about 40 m and a width of several hundred metres. This is a potential aquifer that extends in approximately N–S direction for some kilometres.

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.

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.

A new broadband radiomagnetotelluric instrument: applications to near surface investigations

ABSTRACTA new radiomagnetotelluric (RMT) instrument has been developed that can record time series of electric and magnetic fields in the frequency range from 10 kHz to 1 MHz. Military and civilian radio stations broadcasting in this frequency range are used as transmitters. Transfer functions (e.g., apparent resistivities and phases as a function of frequency) are determined by spectral analysis using a newly developed processing software. The transfer functions presented here are the first ones ever at frequencies above 300 kHz and they can lead to a better resolution of the shallow structure. The new RMT instrument enables quick and efficient mapping of shallow structures and supports tensor measurements thereby allowing a 2D or 3D interpretation of field data. The new RMT system was tested in the laboratory and then applied successfully in the field to several environmental and engineering problems. Case studies from applications in St. Petersburg, Russia and in Uzin, Ukraine will be presented. The observed apparent resistivities and phases are reliable and they show smooth frequency dependence. The data were inverted for 2D conductivity models under the assumption that the strike direction was known beforehand.