Lithium (Li) is classified as a critical raw material by many countries and is an essential component of Li-ion batteries that underpin many fossil-free technologies. This study reports the integrated use of petrophysical data, 3D geophysical inversion modelling and geological observations for the J & auml;rkvissle Li pegmatite field and surrounding area in central Sweden. The work aims to better constrain geological controls on Li pegmatite mineralization, the geophysical signatures of the mineralization and host meta-supracrustal rocks and their 3D spatial distribution at depth. For the first time in Europe, the combined results of 3D inversions of airborne magnetic field data, very low frequency data, ground gravity data and resistivity models from laterally constrained 1D inversion of airborne Slingram measurements are integrated to characterize known Li pegmatite mineralization. It is done from a combined geological-geophysical perspective, with the aim to identify additional areas potentially favourable for Li pegmatites. At J & auml;rkvissle, Paleoproterozoic Li pegmatite mineralization occurs within a narrow deformation zone that is characterized by relatively high magnetic susceptibility (0.01-0.3 SI units), low resistivity (<2000 Omega m) and low rock density contrasts (<50 kg/m(3)) down to ca. 600 m, reflecting a combination of structural and host rock characteristics. Regional airborne radiometric data (K, U and Th) lack corresponding anomalies, although based on ground spectrometry measurements pegmatites show marginally lower Th/K and higher U/K values relative to surrounding granites and meta-supracrustal rocks. Petrophysical data for both Li-enriched and common pegmatites indicate overall lower magnetic susceptibility (

In the absence or unavailability of geometric information about underground structures, conventional quantitative indicators of Urban Underground Space (UUS) usage become challenging to obtain. Socio-economic proxy indicators, that are reportedly well correlated with these conventional indicators, are inherently unstable measures given their susceptibility to non-linear human decision-making processes. To address this limitation, we evaluated the potential of geophysical proxy indicators, specifically the magnetic 3D analytic signal (AS), as a basis for UUS assessment. We also assessed the suitability of the magnetic susceptibility model as a framework for three-dimensional subsurface planning. Stockholm's airborne magnetic data acquired in 1995 were processed to generate an AS map that delineated its metro system (tunnelbana) and similar subsurface structures. Observed signatures, correlated with the overlain metro layout, were independently validated on an electrical current density map derived from co-acquired tensor very low-frequency electromagnetic data. Because AS emphasizes the sharp edges of magnetic contrasts, it provides a direct geophysical marker of anthropogenic underground structures and, consequently, an effective proxy indicator for UUS utilization. We introduce two new indicators: analytic signal density (ASD, expressed in nT/m per hectare, interpreted as "the amount of underground structure from a certain depth range that fits a surface area of 1 ha"), and analytic signal per capita (ASPC, expressed in nT/m per 100 person, interpreted as "the amount of underground structure from a certain depth range per capita". Both demonstrate strong correlations, comparable with those of conventional indicators, with population density (r = 0.88 and r = -0.69, respectively). The 3D magnetic susceptibility inversion model further predicted known infrastructure with a vertical accuracy of similar to 20 m, which can be improved with better data quality. Our results indicate that central Stockholm exhibited significantly higher UUS usage in 1995 (> 1.2 nT/m per hectare and < 2.0 nT/m per 100 person) than other Stockholm areas in the same year. The model for 2023, based on population density, estimated a 20-60 % increase in UUS use in central Stockholm since 1995. This trend underscores the need to integrate UUS in planning and development in both the central districts and the development focus areas of Stockholm city plan, where substantial UUS use was also evident. We conclude that, pending the establishment and adoption of standards for UUS use, Stockholm municipality needs a rethink of its population-driven (coined as "first-full first-developed") underground development strategy, identified through the comparative analysis of the geophysical proxy UUS use indicators of 1995 and model estimates for 2023. Given the rapid and extensive coverage afforded by airborne magnetic surveys, ASD and ASPC provide a novel, scalable, and tangible proxy for estimating and comparing UUS use across cities, particularly in data-limited or data-denied environments.

We show how using different upward continuation (UC)levels during the separation of near-surface anthropogenic signals from regional geologic influence can affect the quality of the separation and how they can affect the inversion models of the residual data. This was demonstrated by modeling the magnetic susceptibility of Stockholm’s underground metro from extracts of a nationwide airborne magnetic dataset. We find that a residual magnetic field separated using an optimal UC height enables reliable modeling and investigation of targeted structures within its inversion model. Based on our results, we conclude that the upward continuation filter, which is widely used as a separation filter, could also be used to monitor the quality of residual signals prior to inversion.

The absence of the underground space and its usage in the current Stockholm city plan raises concerns about the urban underground space planning culture of Sweden. This issue becomes even more alarming because it is unclear, both in the literature and among practitioners, whether a map of Stockholm’s subsurface exists at a municipal or district scale for planning purposes, despite the extensive use of the subsurface. Therefore, we processed airborne geophysical datasets, namely magnetic and very-low-frequency (VLF) electromagnetic, acquired over Stockholm municipality, to produce municipal-wide geophysical signature maps of the underground infrastructures in Stockholm. Stockholm’s underground metro, a significant infrastructure across the city, served as a guide for the geophysical imaging, which included other underground infrastructures with similar geophysical signatures. Two maps were produced: the VLF electrical current density map, which was derived using a transformation from its raw data; and the magnetic anomaly map, which shows the residual between measured and upward continued (UC) fields. Our results show that Stockholm’s central area, known as the innerstaden in Swedish, has a significantly higher infrastructure density than other areas, necessitating a rethink for further underground development in this area. We also find geophysical signatures, typical of major infrastructures, beneath the development focus areas proposed in the city plan outside the innerstaden area, which should be incorporated into the developments in these areas. Although our results are presented as 2D maps and are thus limited in the vertical dimension, the transformation algorithms and filters applied during processing were in 3D. Inspite of the results’ dimensionality, they provide preliminary insights into the subsurface of Stockholm municipality at a resolution that may be suitable for strategic planning decision at a municipal scale. These results could be further developed into 3D models for a more holistic municipal-scale planning of Stockholm’s underground space.

The increasing need for mineral resources and critical rare earth elements (REE) due to the transition to clean energy has attracted interest in mine wastes as they may contain significant amounts of REE that were not of interest in the past but are today. Detailed knowledge about the 3D geometry and size of the waste deposits and their mineral content is important to understanding whether waste tailings can serve as a secondary resource, contributing to the energy transition, sustainability, and the circular economy, and promoting recycling. Geophysical methods can provide information on the geometry, and help to characterize and estimate the size of the mine waste. In the last two decades developments in sensor and computational technology have enabled cost-effective and environmentally friendly seismic ambient noise methods to be widely applied for imaging the subsurface. Among others, one of the ambient noise methods is the horizontal-to-vertical spectral ratio (HVSR) method, which is an efficient technique widely used for site characterization, estimating the thickness of overburden above bedrock, monitoring landslide, and examining the stability of tailing dams.In this study, ambient noise data and the HVSR method are used to estimate the thickness and delineate the 3D geometry of mine tailings. We use three-component (3C) ambient noise data that we collected with 50m spacing between the sensors and profiles in one of the non-active mine tailings of Nordic Iron Ore in Blötberget, Sweden, which might be a potential resource for REE. We process the 3C data and obtain the fundamental frequency at each receiver location. Moreover, one-component ambient noise data that we collected along two perpendicular profiles with a receiver spacing of 5m are used to estimate the surface wave velocity. Combining the fundamental frequency and velocity information, we calculate the depth of the contrasting interface. We show our preliminary results obtained from ambient noise data and compare them with the previous results from the radio magnetotelluric measurements conducted by Geological Survey of Sweden.This work is part of a project supported by the Geological Survey of Sweden. We gratefully acknowledge this support.

Recent years have witnessed an alarming increase in quick-clay landslides in Nordic countries, such as in Alta (Norway), Gjerdrum (Norway), and Stenungsund (Sweden), resulting in substantial damages and loss of lives. This study focuses on the application of geophysical methods, particularly the Controlled-source Radio-magnetotelluric (CSRMT) technique, to understand the characteristics and model the geometry and possibly dynamics of quick clay in these regions. The CSRMT method, combines Radio-magnetotelluric (RMT) and Controlled-source Magnetotelluric (CSMT) techniques and offers an innovative approach for investigating the electrical resistivity of subterranean structures, crucial for identifying quick clay zones. The Rissa region in Norway provides a unique opportunity for this research due to its historical context and existing infrastructure for geophysical studies. The catastrophic Rissa landslide of 1978 led to an extensive national quick clay mapping initiative, forming the basis for this study. We have also collected Distributed Acoustic Sensing (DAS) data at Rissa, intending to integrate it with CSRMT data for comprehensive analysis. Borehole analyses at the Rissa site reveal a relatively simple stratigraphy with a flat terrain and a marine clay stratum about 20 meters thick. Quick clay layers, identifiable due to their higher electrical resistivity compared to marine clays, are sandwiched in borehole samples. Our study utilizes (CS)RMT to model these layers and assess the impact of seasonal variations on their characteristics. Data collection involved a 250-meter long CSRMT profile with a 10-meter station spacing conducted in both summer and winter seasons. The EnviroMT instrument from Uppsala University was used for data acquisition. This time-lapse approach was critical to study the resistivity differences due to seasonal variation at the quick clay site. Results from modelling the CSRMT data show a four-layer model including L1-L4. L2 appeared thicker in winter, possibly due to reduced freshwater. Conversely, in some locations, L2 appeared thicker. These findings show that CSRMT data can distinguish resistivity differences at a quick-clay site due to seasonal variations. This research offers significant insights into the modelling of seasonal variations of the resistivity related to changes in the water content which in turn might lead to development of areas with quick clays. The integration of CSRMT and DAS data presents a novel approach to studying these phenomena, potentially aiding in better understanding and predicting quick-clay landslide triggering. The findings are not only crucial for academic research but also have profound implications for infrastructure planning and disaster management in regions prone to quick-clay landslides.

We develop a three-dimensional inversion code to image the resistivity distribution of the subsurface from frequency-domain controlled-source electromagnetic data. Controlled-source electromagnetic investigations play an important role in many different geophysical prospecting applications. To evaluate controlled-source electromagnetic data collected with complex measurement setups, advanced three-dimensional modelling and inversion tools are required.We adopt a preconditioned non-linear conjugate gradient algorithm to enable three-dimensional inversion of impedance tensor and vertical magnetic transfer function data produced by multiple sets of two independent active sources. Forward simulations are performed with a finite-element solver. Increased sensitivities at source locations can optionally be counteracted with a weighting function in the regularization term to reduce source-related anomalies in the resistivity model. We investigate the capabilities of the inversion code using one synthetic and one field example. The results demonstrate that we can produce reliable subsurface models, although data sets from single pairs of independent sources remain challenging.

Summary Mine tailings are potential secondary resources of critical rare earth elements (REE) that are worth investigating as they can contribute to sustainability and the circular economy. Due to the ongoing clean energy transition, mine tailings have attracted interest in meeting demand since opening new mines requires tremendous effort and may have negative effects on the climate and environment. To understand the performance of the ambient noise methods for 3D characterization and dimensioning the tailings, we collected data over a non-active mine tailing in Sweden. To estimate the fundamental frequency of the site we used three-component noise data and the Horizontal-to-Vertical Spectral Ratio (HVSR) method. The frequency-wavenumber (fk) beamforming method was applied to vertical one-component data to obtain the S-wave velocity of the overburden. Assuming the fundamental frequency is related to the tailings thickness, we calculate the depth to the base of the tailings using the shear wave velocity estimated from the one-component data. Interpolating the available elevation and depth information at each receiver location, we delineate the 3D geometry of the tailings. Moreover, we compare our results with previous results obtained by radio magnetotelluric measurements conducted by the Geological Survey of Sweden.

Summary Ground-based geophysical surveys were conducted at eight mine-tailings repositories during 2021 and 2022 to help preparing an inventory of potential sources of critical raw materials in mine-waste. Geophysical methods included ERT, DCIP, RMT, and tTEM. In this study, we show results obtained on two of the sites as an example. Integration of models of electrical properties, geological observations, historical orthophotos, and drillhole samples provided estimations of the thickness of tailings. The interpretation was extended to construct 3D models of the deposits. Moreover, the modelled electrical properties helped to study the variations in the grain size, porewater content and mineral chemistry. The used geophysical methods resulted in consistent estimations of the geophysical properties and the depth to the more resistive material below the tailings could therefore be determined with higher confidence. Variations in chargeability and phase response from inversion of DCIP data are generally more strongly related to variations in mineral content and grain size. A strong correlation was observed at one of the sites. Spectral IP measurements on samples showed Cole-Cole like spectra and a clear relationship between the frequency of the maximum phase angle and the grain size.

Summary The induced polarization (IP) effect can be studied using both galvanic and inductive methods. Here we study IP and its manifestation in transient electromagnetic (TEM) data from a tTEM system and in time-domain electric resistivity tomography (ERT) data. Since the two methods are sensitive to two different frequency ranges, a direct IP spectrum comparison is not possible. Instead, we assess the IP resolution and compare both resistivity and chargeability inversion results. We show that tTEM data recorded in highly resistive environments with high IP, suffer from fast-decaying transients and sign-changes, which makes it difficult to resolve the true resistivity and IP model due equivalences. A comparison of the maximum phase shifts from two mine tailing sites shows that the chargeability structures gained from the inversion of tTEM data to some degree follow the structures of the chargeability resolved from the ERT data. However, the magnitudes of the resolved phases are generally below 50 mRad for ERT, but up to 500 mRad for tTEM. This indicates that different polarization mechanisms are dominating within the different frequency ranges of the tTEM and ERT methods.

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.

Landslides may cause severe destruction that affects both the individuals and functions vital for society. Minor landslides in an area with quick clay may trigger secondary slides, influencing a much greater area compared to slides in areas with no quick clay. Today’s expanding societies demand new areas for exploitation. To effectively meet this demand, there is an increased need to identify areas where quick clay may occur. Direct or indirect methods for assessing the presence of quick clay have previously been presented as well as a strategy for site investigations in quick clay areas. In this article, a methodology for mapping quick clays for the Swedish conditions with methods commonly available in this area is presented. The methodology presented in the article is structured in steps with different levels of detail and visualized with two conceptual flowcharts. Depending on the stage of planning, different types of surveys are recommended. The methodology has been applied at four sites where integrated interpretation of airborne and ground geophysical measurements as well as geotechnical investigations have been carried out. The results from two of these sites are presented here. The study reveals that all the methods used have their advantages and limitations. However, a combined use of the information provides much more accurate interpretation that can be used for a more cost-effective future planning and decision-making.

ABSTRACTIn a novel approach, we have carried out controlled‐source and radio‐magnetotelluric measurements in the frequency range of 2–250 kHz on a frozen lake located over a planned major multi‐lane underground road tunnel near the city of Stockholm. The aim was to gain a better understanding of the resistivity variations above and, potentially, within the crystalline bedrock. Previous studies on the lake water using the boat‐towed radio‐magnetotelluric technique at the higher end of the frequency band lacked resolution at depth and could not provide conclusive information about bedrock level and potential fracture systems within the bedrock. Taking advantage of Nordic winters, we measured four profiles on the frozen lake complementing the previously acquired boat‐towed radio‐magnetotelluric data utilizing a double horizontal magnetic dipole transmitter that generated signals down to 1 kHz. The new resistivity models, incorporating the lower frequency data, show improvements and deeper penetrations based on a combined analysis of penetration depth, data misfits and sensitivity studies. The resistivity models also show better correlation with the available high‐resolution shallow water seismic reflection data and the geological observations. A potential fracture system within the bedrock can also be inferred better in the new models. The idea of running similar surveys on frozen lakes can be further exploited in similar conditions in countries such as Sweden, where approximately 7% of the land is covered by freshwater bodies and poorly explored for infrastructure planning projects.

Summary In the frame of an EU-funded project a data acquisition system has been developed to measure the electromagnetic (EM) signal from the distant radio transmitters in the frequency range 10–350 kHz onboard an Unmanned Aerial Vehicle (UAV). The system is composed of a data logger and a 3-component induction coil sensor, mounted in a costume-built bird, and records the magnetic field components of the EM signal in three perpendicular directions. The system is hung 10 m below the UAV. We present two case studies in Sweden where the system has been tested and utilized to map the electrical resistivity variations. The first area was located close to an iron ore mine and the resistivity maps from the recoded data revealed a very good correlation with the underlying structures such as landfills, glacial sands, and pits. The second area was near an abandoned gold mine where the conductive sulfide mineralization residing in greywacke rocks are mapped as low-resistivity zones. A semi-airborne test was also conducted using a grounded electric dipole transmitter at frequencies down to 1 kHz. The collected semi-airborne data are of high quality and show signal to noise ratio.

The geological survey of Sweden (SGU) has carried out several detailed airborne TEM (Transient Electromagnetic) surveys in recent years. The data collected in these surveys were inverted to provide models of the resistivity of the subsurface, down to a few hundred meters depth. These resistivity models together with the data from existing boreholes and ground observations offer an excellent basis for further 3D geological modeling. The airborne TEM data presented in this study were collected between 2013 and 2016, covering large areas of the islands of Öland and Gotland, in Sweden. Both islands face problems with water supply due to limited groundwater resources. The aim of the surveys was to identify new groundwater resources, specify the depth to saline groundwater and to improve the understanding of the geology of the islands. On Öland, the Paleozoic sedimentary succession reaches thicknesses of approximately 250 m and is composed of Lower Cambrian sandstone, Middle Cambrian siltstone, and claystone followed by the Alum Shales of Upper Cambrian and Lower Ordovician age. Above this lies an up to 40 m thick Lower Ordovician limestone succession, which forms the bedrock at the surface across much of the island. The entire sedimentary sequence rests on Precambrian crystalline rocks. On the Island of Gotland, Silurian bedrock represents the upper part of a 250-800 m thick Paleozoic sequence overlying the crystalline basement. The Silurian bedrock is dominated by interbedded layers of limestone and marlstone, where the interface between limestone and marlstone is often the primary hydraulic conductor. After acquisition, these data were processed and inverted (1D inversions with lateral constraints), to provide a series of large airborne datasets, providing a resistivity image down to depths of about 250 m in some areas. The considerable resistivity contrast between lithologies, e.g. limestone and marlstone on Gotland, provided an excellent opportunity to resolve boundaries between the different rock types. Borehole information, geological maps, ground geophysical data and the inversion results were incorporated in a 3D geological modelling software. On comparison of the airborne models, ground geophysical data and borehole information it was clear that the airborne resistivity models correlated well with the other available data. Hence, the resistivity models were used as the basis for constructing the 3D hydrogeological and geological models over significant parts of the islands. In this study we present the 3D geological models over the islands of Öland and Gotland which were constructed from the integrated interpretation of all the available data. The models are composed of voxels, each representing a certain lithology classified using a statistical approach. The classification is based on the resistivity range, distance to the neighboring wells/boreholes and the geological observations at the surface. The 3D voxel models will be/have been utilized in hydrological modelling, societal planning, and groundwater management.

Summary Inverse modeling of potential field data is a well-known ill-posed problem where many models exist that fit data to a desired level. This study presents an example of 3D inversion of airborne magnetic field data where various a-priori knowledge and other available geophysical data are utilized to improve validity of the final 3D susceptibility model. The data are collected in the Bergslagen mining district in Sweden and the study focuses in a 7.5X7.5 km2 area including Blötberget iron-oxide mine. The assay data including iron content and magnetic susceptibility of the drill-cores, geological field observations and measurements, drill-core lithology, and 2D and 3D reflection seismic data are jointly used to generate surfaces representing the geometry of the iron-oxide bodies. The surfaces are then used to build constraints such as reference susceptibility, reference gradient and gradient weighting models. Based on the observed geophysical borehole logs and measured susceptibilities of the drill-cores, the upper and lower susceptibility bounds are also applied to guarantee a realistic range of susceptibilities. The resulting model fits the data within an acceptable level and matches the geometry of the known ore bodies. Our results suggest that another deeper mineralization may exit underneath the partially already mined deposits.