Radiomagnetotelluric Data Research Articles

2D Transdimensional Joint Inversion of Radio Magnetotelluric and Electrical Resistivity Tomography Data

Summary The joint inversion of radio magnetotelluric and electrical resistivity tomography data has the potential to reduce the uncertainties in the subsurface conductivity model. This is particularly beneficial when the datasets offer complementary information about the subsurface. However, the traditional gradient-based inversion methods pose challenges in quantifying uncertainty, as they yield a single model with limited appraisal of parameter uncertainty. The Bayesian inversion approach stands out for its capacity to provide quantitative assessments of uncertainty in the inverted model parameters. This is accomplished by generating an ensemble of models, leading to a posterior distribution that encapsulates both prior information concerning model parameters and the dataset information. We have implemented a transdimensional Markov chain Monte Carlo algorithm to perform the joint inversion of radio magnetotelluric and electrical resistivity tomography data. Through synthetic data studies, we illustrate how the inclusion of two complementary datasets can effectively reduce uncertainties in model parameters and how the model parameter uncertainties can be quantified. Subsequently, the developed algorithm is tested using exemplary field data from a waste site near Roorkee, India. Intensive prior geoelectric and transient electromagnetic as well as radio magnetotelluric studies investigated possible waste water seepage with a potential to contaminate the shallow aquifers. The derived subsurface structure from our transdimensional Bayesian results compare well with the deterministic results for the exemplary profile, but in addition provide comprehensive uncertainty estimates.

Constrained 2D inversion of radio-magnetotelluric and controlled-source audio-magnetotelluric data using high-resolution reflection seismic data: An example in groundwater surveying from Heby, Sweden

We advance a previously established method for 2D inversion of electromagnetic data, in which the smoothness constraints are locally reweighted according to the seismic envelope fractional gradients (SEFGs). As the first step of our modifications, seismic envelope values are edited and normalized. This results in the rejection of noise-contaminated parts, clipping the envelope outliers and increasing the reflectivity power of weak seismic signals. Second, we introduce a weighting matrix in the normalization process to incorporate prior information in the constrained inversion regarding the relative contrasts of electrical resistivity in the given parts of the model. Third, due to normalizing the SEFG, there is no need to search for an optimum stabilization factor to modify local smoothing weights, and hence, we set it to a constant value. Finally, an Occam inversion with additional Levenberg-Marquardt damping is used to mitigate possible artifacts in the resistivity model generated by reflection seismic constraints. In applying the proposed scheme to a synthetic example, interfaces of various geologic units are restored as sharp boundaries. In addition, artifacts generated in the original approach are effectively mitigated thanks to the applied normalization process. For the first time, we apply the method to radio-magnetotelluric (RMT) and controlled-source audio-magnetotelluric (CSAMT) field data acquired along a profile across a known aquifer in Heby, Sweden. Our inversion models compare favorably to previously presented results from seismic and geoelectric data. By modifying the smoothness weights based on SEFG, the depth to the bedrock is recovered well, being constrained by the corresponding interfaces in the seismic image. The resistivity models from seismically constrained inversions of RMT and CSAMT data reveal steeply dipping and possibly fractured bedrock underneath the valley-shaped aquifer in the area. This interpretation is verified by borehole logs.

A comparison of 2D inversion of RMT and 1D weighted joint inversion of ground-based and helicopter-borne electromagnetic data

A Pleistocene buried valley, the Cuxhaven Valley, located in Northern Germany, is extensively studied by three different electromagnetic methods. In order to enhance the resolution of the very shallow subsurface structures, the radiomagnetotelluric (RMT) measurements were conducted on the buried valley in addition to the helicopter-borne electromagnetic (HEM) and transient electromagnetic (TEM) data, which were available from the study area along the same profile. First, the RMT soundings are interpreted using the conventional 1D and 2D inversion algorithms. The RMT interpreted models inferred two-layer structure of the subsurface up to a depth of 30 m, which comprises a resistive layer at the top. Available lithological data is used for correlating the resistivity values derived from the 2D inversion of the RMT data with the subsurface formation. Subsequently, a 1D weighted joint inversion algorithm is utilized to interpret these three data sets jointly. The 2D interpreted models of RMT soundings are compared with the 1D weighted joint interpreted models of RMT, HEM, and TEM soundings.

Study of a Permafrost Area in the Northern Part of Siberia Using Controlled Source Radiomagnetotellurics

Controlled source radiomagnetotelluric (CSRMT) measurements with a horizontal electrical dipole were carried out for the planned construction of a railway bridge in the Northern part of Siberia (the Yamalo-Nenets region). The aim of this survey was the delineation of horizons of loams, which are mostly thixotropic, and the estimation of the depth of solid sands serving as a basement for bridge piles. The remote area of the investigation is characterized by a few number of broadcasting radio transmitters. Therefore, the possibilities of the conventional radiomagnetotelluric (RMT) method to study the geological structure are quite limited. Because of swamps, the construction area is only accessible in winter or early spring periods. Our CSRMT survey was carried out by using ungrounded (capacitive) electric lines. The measurements were conducted in the broadside configuration using a 580 m long grounded electrical line at a distance of about 640 m from the transmitter. At the first stage of the CSRMT data inversion, we used the 1D controlled source inversion code considering the geometry of the source and the effect of the transition zone. In addition, we selected frequencies associated with the far-field response of the electromagnetic field. To demonstrate the advantages of the controlled source, we compared the responses and inversion results obtained from both CSRMT and RMT data. The geoelectrical section obtained from CSRMT data is more detailed in the shallow part and allows to resolve the deep structures because of an extended frequency range and an increased depth of investigation. The conductivity model derived from the 2D inversion of the CSRMT data showed a good correlation with the borehole data. The CSRMT results were very useful for the construction of the railway bridge because of the reliable delineation of loams and sands horizons in the subsurface.

On the study of industrial waste sites on the Karelian Isthmus/Russia using the RMT and CSRMT methods

Radiomagnetotelluric (RMT) and Controlled Source Radiomagnetotelluric (CSRMT) measurements were carried out on landfills of industrial wastes located on the Karelian Isthmus/Russia. The RMT and CSRMT data were interpreted using the 2D inversion techniques, and the 2D conductivity structures of the investigated industrial waste sites were derived. The RMT method is based on measurements of electromagnetic fields of remote radio transmitters in a frequency range of 10–1000 kHz or in a frequency range of 1–1000 kHz by using a controlled source (CSRMT method). A landfill in the settlement Morozova near St. Petersburg was formed as a result of a long-term warehousing of toxic wastes by the sulfuric acid manufacture. Soils and ground waters were polluted with salts of heavy metals. In the adjacent to the landfill territory, investigations were realized by the RMT method and a polluted zone was determined. This zone is traced up to the coast of the Ladoga Lake and the Neva River. It constitutes a potential danger because the Neva River is used for the water supply of St. Petersburg. Another landfill of debris in a former sandy career is located in the Vyborg area. The high conductivity and a relative large thickness of the wastes did not allow us to map the bottom of the landfill by the RMT method using the frequency range of 10–1000 kHz. Therefore, CSRMT measurements with an extended frequency range up to 1 kHz were carried out on this landfill, and the 2D inversion of the CSRMT data clearly showed the lateral and vertical extension of it.

Imaging of groundwater contamination using 3D joint inversion of electrical resistivity tomography and radio magnetotelluric data: A case study from Northern India

ABSTRACTThe impact of untreated sewage irrigation and waste disposal practices on groundwater is investigated by 3D joint inversion of radio magnetotelluric and electrical resistivity tomography data. In this case study, electrical resistivity tomography and radio magnetotelluric field measurements were carried out on several profiles near a waste disposal site which was irrigated with untreated sewage water for agriculture purpose. In addition, radio magnetotelluric and electrical resistivity tomography measurements were carried out, far away from the waste site, to derive the uncontaminated geology. The data were analysed earlier using 2D inversion techniques. However, for the 2D inversion of the electrical resistivity tomography and radio magnetotelluric data, assumptions about the strike direction are required. As no clear strike direction is evident for the contamination, we considered the problem as 3D and interpreted the present data set using the 3D inversion algorithm ‘AP3DMT‐DC’. The inverted 3D resistivity model shows an unconfined aquifer of low resistivity which is overlain by an unsaturated slightly resistive near surface formation. With increasing distance from the waste sites, an increase in the resistivity of the shallow unconfined aquifer is observed. Furthest away from the waste site undisturbed geology is expected. We derived consistent and meaningful 3D resistivity models. The uncontaminated reference site indicates an increased resistivity for the aquifer layer. A synthetic 3D study was carried out to demonstrate and validate algorithm performance as well as convergence capabilities. The study demonstrates that the two methods, electrical resistivity tomography and radio magnetotelluric, complement each other. Besides, a better resolved inverted model is obtained through a 3D joint inversion, in comparison to individual 2D and 3D inversions.

Using GPR Data as Constraints in RMT Data Inversion for Water Content Estimation: A Case Study in Heby, Sweden

This study uses ground penetrating radar (GPR) data as constraints in the inversion of radio-magnetotelluric (RMT) data, to provide an improved model at shallow depth. We show that modification of the model regularization matrix using all GPR common-offset (CO) reflections can mislead the constrained inversion of RMT data. To avoid such problems, common mid-point (CMP) GPR data are translated to a resistivity model by introducing a new petrophysical relationship based on a combination of Topp’s and Archie’s equations. This model is updated through a semi-iterative method and is employed as an initial and prior model in the subsequent inversion of RMT data. Finally, a water content model that fits the GPR CMP and RMT data is derived from the resistivity model computed by the constrained inversion of RMT data. To assess the proposed scheme, it is applied to a synthetic data set. Then, real RMT data collected along an 870 m-long profile across a known aquifer situated in the north of Heby, central Sweden, are inverted. By removing the smoothness constraints across GPR CO interfaces or using CMP-based inversion, thick (> 10 m) vadose and saturated zones are resolved and shown to correlate with logs from nearby boreholes. Nevertheless, the application of our CMP-based inversion was the only efficient scheme to retrieve thin (~ 3 m) saturated zones and the water table at a depth of 7–15 m in the RMT models. The estimated models of water content are in good agreement with the available hydrogeological information in the study area.

Parallel Simulation of Audio- and Radio-Magnetotelluric Data

This paper presents a novel numerical method for simulation controlled-source audio-magnetotellurics (CSAMT) and radio-magnetotellurics (CSRMT) data. These methods are widely used in mineral exploration. Interpretation of the CSAMT and CSRMT data collected over an area with the complex geology requires application of effective methods of numerical modeling capable to represent the geoelectrical model of a deposit well. In this paper, we considered an approach to 3D electromagnetic (EM) modeling based on new types of preconditioned iterative solvers for finite-difference (FD) EM simulation. The first preconditioner used fast direct inversion of the layered Earth FD matrix (Green’s function preconditioner). The other combined the first with a contraction operator transformation. To illustrate the effectiveness of the developed numerical modeling methods, a 3D resistivity model of Aleksandrovka study area in Kaluga Region, Russia, was prepared based on drilling data, AMT, and a detailed CSRMT survey. We conducted parallel EM simulation of the full CSRMT survey. Our results indicated that the developed methods can be effectively used for modeling EM responses over a realistic complex geoelectrical model for a controlled source EM survey with hundreds of receiver stations. The contraction-operator preconditioner outperformed the Green’s function preconditioner by factor of 7–10, both with respect to run-time and iteration count, and even more at higher frequencies.

Two dimensional joint inversion of direct current resistivity and radiomagnetotelluric data based on unstructured mesh

Using the unstructured mesh, a new two-dimensional joint inversion algorithm has been developed for Radiomagnetotelluric and Direct current resistivity data. The unstructured mesh is generated with triangular cells, whose vertical and lateral lengths increase towards the depths. The Finite Element Method (FEM) has been used in the forward modelling part of the developed joint inversion algorithm. In the previous studies, structured grid-based joint inversion algorithms have been developed using the Finite Difference Method (FDM). In the structured grid-based algorithms, when the mesh is being generated with rectangular cells, the vertical lengths of the cells get bigger towards the depths while the lateral lengths remain constant. With the structured mesh, the undulated surface topography cannot be represented well enough. Also, because of the incompatible aspect ratio of model cell sizes in deeper model sections, the resolution of the model parameters will get smaller and cannot be resolved well with the structured grids. Imaging of surface topography and underground resistivity structures by the new algorithm requires fewer elements than those using structured grids. Therefore, the developed algorithm is faster than traditional 2D inversion algorithms. Furthermore, the resolution of the deeper model parameters has been increased by using the definition of the unstructured grid. A regularized inversion scheme with a smoothness-constrained stabilizer has been employed to invert the data. First, we have tested the developed joint inversion algorithm using synthetic data simplified from archaeological and mine site scenario and the results have been compared with the conventional algorithms using structured grids. We have also tested our algorithm with the real data which were collected from mineral investigation site at approximately 10 km east of the Elbistan district of Kahramanmaraş province, in the west of the Taurus Mountains, Turkey. The results show that the developed joint inversion algorithm is a powerful tool to detect both resistive and conductive targets.

Implementation and model uniqueness of Particle Swarm Optimization method with a 2D smooth modeling approach for Radio-Magnetotelluric data

We present a simple workflow for 2D smooth modeling of Radio-Magnetotelluric (RMT) data using Particle Swarm Optimization (PSO). The workflow aims for the general problems of the Global Optimization (GO) methods, encountered during multidimensional geophysical modeling; time consumption, and difficulties in solving large amount of model parameters. In our implementation of PSO, starting models are used and an optimum parameter change vector (Δm) is searched by the algorithm. Our initial studies showed, implementing a smooth modeling approach is necessary for modeling large amount of model parameters using PSO. To obtain smooth models and avoiding multi-objective optimization we used an operator to smooth the model parameters to the desired level. As result, we used the PSO to search for the Δm minimizing the functional when the model parameters are smoothed. The smoothness is gradually decreased when rougher models are wanted. As the smoothness decrease, artefacts are found to be emerging; especially at the low sensitivity zones. To prevent these artefacts, we added Minimum Gradient Support (MGS) to our functional with a multiplier. Since the value of this multiplier is necessarily small, the algorithm doesn't search for a balance between these objectives, and the contribution from the MGS acts as a threshold value to introduce a structure to the model. We tested this approach using a synthetic and a field dataset. In all cases the algorithm successfully minimized the functional and obtained geologically meaningful models. We also compared the uniqueness of the models recovered from the field dataset by multiple modeling realizations using smooth inversion and PSO. Our results suggest, when higher smoothness and minimum structures are aimed, the uniqueness of the models is significantly higher, however, for a fixed set of modeling parameters with moderate values, the non-uniqueness is observed to be higher than that suggested by the smooth inversions.

Boat-towed radio-magnetotelluric and controlled source audio-magnetotelluric study to resolve fracture zones at Äspö Hard Rock Laboratory site, Sweden

SUMMARY Boat-towed radio-magnetotelluric (RMT) measurements using signals between 14 and 250 kHz have attracted increasing attention in the near-surface applications for shallow water and archipelago areas. A few large-scale underground infrastructure projects, such as the Stockholm bypass in Sweden, are planned to pass underneath such water zones. However, in cases with high water salinity, RMT signals have a penetration depth of a few metres and do not reach the geological structures of interest in the underlying sediments and bedrock. To overcome this problem, controlled source signals at lower frequencies of 1.25 to 12.5 kHz can be utilized to improve the penetration depth and to enhance the resolution for modelling deeper underwater structures. Joint utilization of boat-towed RMT and controlled source audio-magnetotellurics (CSAMT) was tested for the first time at the Äspö Hard Rock Laboratory (HRL) site in south-eastern Sweden to demonstrate acquisition efficiency and improved resolution to model fracture zones along a 600-m long profile. Pronounced galvanic distortion effects observed in 1-D inversion models of the CSAMT data as well as the predominantly 2-D geological structures at this site motivated usage of 2-D inversion. Two standard academic inversion codes, EMILIA and MARE2DEM, were used to invert the RMT and CSAMT data. EMILIA, an object-oriented Gauss–Newton inversion code with modules for 2-D finite difference and 1-D semi-analytical solutions, was used to invert the RMT and CSAMT data separately and jointly under the plane-wave approximation for 2-D models. MARE2DEM, a Gauss–Newton inversion code for controlled source electromagnetic 2.5-D finite element solution, was modified to allow for inversions of RMT and CSAMT data accounting for source effects. Results of EMILIA and MARE2DEM reveal the previously known fracture zones in the models. The 2-D joint inversions of RMT and CSAMT data carried out with EMILIA and MARE2DEM show clear improvement compared with 2-D single inversions, especially in imaging uncertain fracture zones analysed in a previous study. Our results show that boat-towed RMT and CSAMT data acquisition systems can be utilized for detailed 2-D or 3-D surveys to characterize near-surface structures underneath shallow water areas. Potential future applications may include geo-engineering, geohazard investigations and mineral exploration.

Mapping of buried faults using the 2D modelling of far-field controlled source radiomagnetotelluric data

Controlled source radiomagnetotellurics (CSRMT) is a relatively new geophysical method for near-surface applications. A rectangular signal with base frequencies between 0.1 and 150 kHz is injected through a grounded electric dipole which is used as a transmitter. Electric and magnetic field components are observed at these frequencies and at their subharmonics, usually in the far-field zone so that apparent resistivities and impedance phases can be obtained in a broad frequency range between 1 and 1000 kHz. Inline or broadside configuration can be used for measurements. Similar to the controlled source audiomagnetotelluric method, tensor measurements are also possible when locating two transmitters perpendicular to each other. A scalar CSRMT survey was carried out on the buried faults in the Vuoksa region, 110 km north of St. Petersburg to test the applicability of this method to the mapping of near-surface faults. A 700 m electric dipole with base frequencies of 0.5, 11.3, 30 and 105 kHz was used as a transmitter. Smooth apparent resistivity and phase values as a function of frequency from 1 kHz to 1 MHz were observed in the far-field zone for the inline configuration at 57 stations using a station distance of 20 m. Electric fields observed in the direction of the transmitter were perpendicular to the assumed strike direction of the buried faults so that they could be associated with the TM mode. The observed apparent resistivity and phase TM mode data were interpreted using the 2D inversion algorithm, and a good data fitting could be obtained. The resistivity structure beneath the survey area (down to a depth of 80 m) could be derived and the buried faults could be mapped successfully. In addition to the CSRMT observations, a conventional radiomagnetotelluric (RMT) survey was also carried out on the same profile. An excellent correlation of the observed RMT and CSRMT transfer functions and 2D conductivity models was achieved.

Joint inversion of lake-floor electrical resistivity tomography and boat-towed radio-magnetotelluric data illustrated on synthetic data and an application from the Äspö Hard Rock Laboratory site, Sweden

Joint inversion of lake-floor electrical resistivity tomography and boat-towed radio-magnetotelluric data illustrated on synthetic data and an application from the Aspo Hard Rock Laboratory site, Sweden

Two-dimensional inversion of magnetotelluric/radiomagnetotelluric data by using unstructured mesh

We have compared structured and unstructured grid-based 2D inversion algorithms for magnetotelluric (MT) and radiomagnetotelluric (RMT) data in terms of speed and accuracy. We have developed a new 2D inversion algorithm for MT and RMT data by using a finite-element (FE) method that uses unstructured triangle grids. We compare the inversion results of our unstructured grid-based algorithm with those of the conventional algorithm, which uses either a structured FE or structured finite-difference (FD) numerical solution technique. The imaging of the surface topography and the underground resistivity structures by the new algorithm requires fewer elements than those that use FE and FD structured grids. We also find that when unstructured grids are used, the quality of the mesh is increased and the numerical errors are significantly reduced. Thus, the program runs faster and can simulate the complex surface topography in a more stable setting than the classic inversion algorithms. Furthermore, we implement a new smoothing matrix format for the unstructured triangle grids for the inversion procedure. We use two samples of synthetic data for the MT and RMT frequencies as well as a sample of field RMT data collected across a fault zone for comparison. In our synthetic data experiment, we find that the resistivity values and the boundaries obtained from the inversion of the unstructured mesh are closer to those of the true a priori synthetic model. Results of the synthetic and field data verify the computational advantages (speed and accuracy) of our inversion algorithm with respect to the conventional structured grid-based inversion algorithms.

Portable audio magnetotellurics - experimental measurements and joint inversion with radiomagnetotelluric data from Gotland, Sweden

Field setup of an audio magnetotelluric (AMT) station is a very time consuming and heavy work load. In contrast, radio magnetotelluric (RMT) equipment is more portable and faster to deploy but has shallower investigation depth owing to its higher signal frequencies. To increase the efficiency in the acquisition of AMT data from 10 to 300 Hz, we introduce a modification of the AMT method, called portable audio magnetotellurics (PAMT), that uses a lighter AMT field system and (owing to the disregard of signals at frequencies of less than 10 Hz) shortened data acquisition time. PAMT uses three magnetometers pre-mounted on a rigid frame to measure magnetic fields and steel electrodes to measure electric fields. Field tests proved that the system is stable enough to measure AMT fields in the given frequency range. A PAMT test measurement was carried out on Gotland, Sweden along a 3.5 km profile to study the ground conductivity and to map shallow Silurian marlstone and limestone formations, deeper Silurian, Ordovician and Cambrian sedimentary structures and crystalline basement. RMT data collected along a coincident profile and regional airborne very low frequency (VLF) data support the interpretation of our PAMT data. While only the RMT and VLF data constrain a shallow (~20-50 m deep) transition between Silurian conductive (<30 Ωm resistivity) marlstone and resistive (>1000 Ωm resistivity) limestone, the single-method inversion models of both the PAMT and the RMT data show a transition into a conductive layer of 3 to 30 Ωm resistivity at ~80 m depth suggesting the compatibility of the two data sets. This conductive layer is interpreted as saltwater saturated succession of Silurian, Ordovician and Cambrian sedimentary units. Towards the lower boundary of this succession (at ~ 600 m depth according to boreholes), only the PAMT data constrain the structure. As supported by modelling tests and sensitivity analysis, the PAMT data only contain a vague indication of the underlying crystalline basement. A PAMT and RMT joint inversion model reveals all the aforementioned units including the less than 80 m deep limestone and marlstone formations and the conductive sedimentary succession of Silurian, Ordovician and Cambrian units. Our test measurements have proven the PAMT modification to be time saving and easy to set up. However, PAMT data suffer from the same noise disturbances as regular AMT data. Since man-made EM noise can propagate over great distances through resistive underground, PAMT measurements are recommended to be carried out in areas with low resistivity. The PAMT method is proven to be applicable in shallow depth studies, especially in areas where normal AMT measurements are inconvenient and/or too expensive to carry out.

Interpretation of Very Low Frequency Measurements Carried Out with an Unmanned Aerial System by 2D Conductivity Models

Very Low Frequency (VLF) measurements were successfully acquired using an Unmanned Aircraft System (UAS) as the measurement platform at two sites. The first site was characterized by strong anthropogenic anomalies and was located in Switzerland. The second site was a saltwater-freshwater transition zone in Northern Germany. The induction coils were used to measure the three components of the magnetic field of VLF transmitters. The magnetic field sensors and the corresponding data logger were suspended below a 50 kg rotary wing UAS. Prior to the test survey, noise measurements were carried out to establish a proper distance for the logger and the sensor, which were found at 2 m and 4 m from the body of the UAS, respectively. A suspension system was developed to enable stable flight characteristics in combination with the VLF system. The test flights were carried out with low speed (1 m/s) and at low altitude (approximately 3 m) above ground. A processing algorithm was developed to analyze the time series of the measured magnetic fields. Transfer functions for the vertical magnetic field were derived using both a scalar and a bivariate analysis approach. After rotating the transfer functions into the strike direction of the 2D subsurface anomaly, a two dimensional inversion method was applied for the interpretation of the conductivity structure to a depth of 15 m. In addition, radiomagnetotelluric (RMT) measurements were carried out on the same UAS-VLF profiles. The RMT data were interpreted by 2D inversion for comparison and are used here to derive the background resistivity, which cannot be achieved by VLF measurements. As a result, good correlation was found between the UAS-VLF and RMT conductivity models.

Resolution and sensitivity of boat-towed RMT data to delineate fracture zones – Example of the Stockholm bypass multi-lane tunnel

The resolution and sensitivity of water-borne boat-towed multi-frequency radio-magnetotelluric (RMT) data for delineating zones of weaknesses in bedrock are examined in this study. 2D modeling of RMT data along 40 profiles in joint transverse electric (TE) and transverse magnetic (TM) as well as determinant mode was used for this purpose. The RMT data were acquired over two water passages from the Lake Mälaren near the city of Stockholm where one of the largest underground infrastructure projects, a multi-lane tunnel, in Europe is currently being developed. Comparison with available borehole coring, refraction seismic and bathymetric data was used to scrutinize the RMT resistivity models. A low-resistivity zone observed in the middle of all the profiles is suggested to be from fracture/fault zones striking in the same direction as the water passages. Drilling observations confirm the presence of brittle structures in the bedrock, which manifest themselves as zones of low-resistivity and low-velocity in the RMT and refraction seismic data, respectively. Nevertheless, RMT is an inductive electromagnetic method hence the presence of conductive lake sediments may shield detecting the underlying fractured bedrock. The loss of resolution at depth implies that the structures within the bedrock under the lake sediments cannot reliably be delineated. To support this, a synthetic data analysis was carried out providing useful information on how to improve and plan the lake measurements for future studies. Synthetic modeling results for example suggested that frequencies as low as 3kHz would be required to reliably resolve the bedrock and fracture zone within it in the study area. The modeling further illustrated the advantage of a fresh water layer that acts as a near-surface homogeneous medium eliminating the static shift effects. While boat-towed RMT data provided substantial information about the subsurface geology, the acquisition system should be upgraded to enable controlled-source data acquisition to increase the penetration depth and to overcome the shortcomings of using only radio-frequencies.

Combining electromagnetic measurements in the Mygdonian sedimentary basin, Greece

We present a novel approach where time-domain electromagnetic (TEM) data are transformed and subsequently used in two-dimensional (2-D) magnetotelluric inversion of the determinant of the impedance tensor. The main idea is to integrate TEM with magnetotelluric (MT) data to produce subsurface electrical resistivity models. Specifically, we show that 2-D MT data inversion of the determinant of the impedance tensor supported by inclusion of TEM–MT-transformed data has superior resolution at the near surface and at the same time static shift afflicting the MT data can be addressed. Thus, the approach allows for practical express integration of TEM data with MT measurements as opposed to a full combined 3-D inversion, which requires significant resources. The approach is successfully applied in the Mygdonian sedimentary basin located in Northern Greece. In addition to TEM and MT data, also controlled source — and radiomagnetotelluric data are available from the Mygdonian basin, which have been subjected to 2-D analysis previously. We have extended the analysis to a full 3-D inversion using ModEM code. All obtained models are analysed and are in a good agreement.

Multi-dimensional Interpretation of Radiomagnetotelluric and Transient Electromagnetic Data to Study Active Faults in the Mygdonian Basin, Northern Greece

The Mygdonian Basin is located about 45 km NE of the city center of Thessaloniki in northern Greece. We conducted an electromagnetic (EM) survey that covers parts of the epicenter area between Lake Volvi and Lake Langada of the local 1978 earthquake that caused structural damage in the city center of Thessaloniki. Ambient noise measurements in the basin strongly suggest a complex 3-D tectonic setting. Hence, near-surface electromagnetic measurements were carried out to map the local fault pattern in the research area. We conducted radiomagnetotelluric (RMT) and transient electromagnetic (TEM) measurements along eight profiles, giving a total number of 443 RMT and 107 TEM soundings to study the spatial electrical conductivity distribution. In this context, RMT data are used to study the shallow conductivity structure down to about 35 m, whereas the TEM data explore the conductivity distribution down to a depth of 200 m. The 1-D and 2-D interpretation of RMT and TEM data indicate a local fault pattern in the survey area that was also studied by 3-D modeling of the RMT data.

Geophysical characterization of areas prone to quick-clay landslides using radio-magnetotelluric and seismic methods

Landslides attributed to quick clays have not only considerable influences on surface geomorphology, they have caused delays in transportation systems, environmental problems and human fatalities, especially in Scandinavia and North America. If the subsurface distributions of quick clays are known, potential damages can be mitigated and the triggers of landslides can better be studied and understood. For this purpose, new radio-magnetotelluric (RMT) and seismic data were acquired in an area near the Göta River in southwest Sweden that contains quick clays and associated landslides. High-resolution data along 4 new lines, in total 3.8km long, were acquired and merged with earlier acquired data from the site. Velocity and resistivity models derived from first breaks and RMT data were used to delineate subsurface geology, in particular the bedrock surface and coarse-grained materials that overlay the bedrock. The latter often are found underlying quick clays at the site. Comparably high-resistivity and sometimes high-velocity regions within marine clays are attributed to a combination of leached salt from marine clays or potential quick clays and coarse-grained materials. The resistivity and tomographic velocity models suggest a much larger role of the coarse-grained materials at the site than previously thought, but they also suggest two different scenarios for triggering quick-clay landslides at the site. These scenarios are related to the erosion of the riverbank, increased pore-pressure and surface topography when close to the river and human activity when away from the river and where bowl-shaped bedrock surrounds the sediments.