Magnetotelluric (MT) survey results from the Late Archean Manica greenstone belt, an extension of the Odzi-Mutare greenstone belt of the Zimbabwe Craton, are presented. A total of 33 MT stations were acquired on an irregular grid with an average station spacing of approximately 5 km. This data set represents the first MT survey in Mozambique. The results from the 3-D modeling indicate a conductive mid-crustal structure in the central part of the surveyed greenstone belt and the presence of narrow sub-vertical conductive structures connecting the mid-crustal conductor with shallow structures. These sub-vertical conductive structures are tentatively interpreted as marking the location of fluid pathways for mineralizing fluids associated with gold occurrences. The modeled mid-crustal conductor mapped in the Manica greenstone belt does not have a western continuation toward the adjacent Odzi-Mutare greenstone belt in Zimbabwe.

Drone-towed scalar field gradiometry surveys conducted in windy conditions or under self-excited oscillations generate attitude-induced responses that can hinder the geological interpretation. Here, we present a gradiometric equivalent source method (GESM) to remove these attitude-induced responses by interpolating and continuing the measured gradiometry data to new idealized pseudo-sensor positions free of any attitude deviations. In addition, we present transverse horizontal difference (THD) data from a precisely positioned drone-towed horizontal gradiometry survey collected in Nautanen, northern Sweden. Analysing the Nautanen survey's positional data revealed that the gradiometer system exhibited directional-dependent yaw deviations with periods of unpredictable attitude deviations. Based on synthetic THD data created using the Nautanen survey's positional data, these deviations manifest as line-to-line striping and short-wavelength oscillations in the THD maps. Applying GESM to the synthetic THD data removes these attitude-induced THD responses with satisfactory accuracy compared to the true THD values. Furthermore, on the actual THD data collected in Nautanen, applying GESM improved the continuity of anomalies, significantly improving the interpretation of the data. The results suggest that applying GESM to drone-towed gradiometry surveys, given precise attitude information via an onboard GNSS-IMU system, maintains high quality even when surveying in windy conditions or in high-gradient areas. The results suggest that including GESM in the data processing of drone-towed gradiometry surveys, given precise positional information via an onboard GNSS-IMU system, ensures high-quality geological interpretation even in windy conditions or in high-gradient areas.

A model is developed to test the hypothesis that kimberlites can form by low-degree melting of asthenospheric mantle followed by entrainment and assimilation of lithospheric mantle. The developed model uses inversion calculations based upon rare earth and compatible trace elements. For kimberlites (s.s.), an equation describing mass balance between a melt of unknown composition and a contaminant end-member of xenocrystic/assimilated material from the lithospheric mantle is inverted. This allows calculation of the mass fraction of xenocrystic minerals from the lithospheric mantle (olivine, orthopyroxene, clinopyroxene, garnet, ilmenite) entrained in the kimberlitic magma, as well as the source mineralogy and melt degree in the source region.

The composition of the parental melt prior to interaction with the lithosphere is not assumed a priori but is calculated by the model. The CO2, H2O, K2O and P2O5 contents of the source are estimated assuming batch melting and the inversion models. The range and coupling of the model parameters are found using a non-linear most-squares inversion procedure, and the model space is visualised using a Self-Organising Map approach. Our earlier work supporting assimilation of xenocrystic opx is, however, not a precondition but provides a post-processing constraint, as well as the selection of a more likely set of solutions from the Self-Organising Map.

The calculation is applied to a data set from the Majuagaa kimberlite dyke (southern West Greenland) including added whole rock analyses for CO2 and H2O. Major variations in whole rock compositions are related to flow differentiation of olivine macrocrysts. The textures of opx, cpx, gt and ilm megacrysts show evidence for reaction with the transporting melt and physical erosion in the kimberlitic mush. Using the bulk rocks in our inversion scheme results in a silico-carbonatite parental melt with major element concentrations consistent with experimental melts. The ol, opx, and cpx mass fractions in the source are not well-resolved by this calculation, but the proportion of gt in the source is comparatively well defined at 15–22 wt% and cpx is constrained to less than 14 wt%. The source assemblage required is 36–80 wt% ol, 2–49 wt% opx, 0–6 wt% cpx, and 15–19 wt% gt. This suggests a peridotitic rather than an eclogitic source.

The inversion model gives an overall mass fraction of xenocrystic material in the Majuagaa kimberlite magma of 41–51 wt% The mass fractions of the xenocryst phases are as follows: 71–85 wt% ol, 0–13 wt% opx, 5 ± 1 wt% gt, and 10–14 wt% ilm. There is less than 3 wt% cpx in the xenocrystic and assimilated assemblage. These results agree with petrographic observations. Processing the model results using the Self-Organising Map clearly displays the extent and coupling within the statistically acceptable region of the model space and leads us to a preferred model of 49 wt% xenocrysts with a xenocryst assemblage of 71–76 wt% ol, 8–13 wt% opx, 4 wt% gt and 12 wt% ilm.

A source with a REE pattern similar to that of primitive mantle is sufficient to form the parental melt and consistent with generation of the initial kimberlite melt in the convecting mantle. Calculated CO2 and H2O concentrations in the source of the Majuagaa kimberlite of 230–860 μg/g and 223–741 μg/g, respectively, are within the range of independent convecting mantle estimates. This is equivalent to <0.17 wt% magnesite and the H2O budget of the mantle source can be accommodated via storage in nominally anhydrous silicate phases.

When applied to Majuagaa kimberlite, the inversions are consistent with a conceptually simple model of kimberlite formation: (1) low degree melting in carbonated asthenospheric peridotite, (2) melt extraction and concentration, and (3) entrainment and reaction with lithospheric mantle material.

The transient electromagnetic (TEM) method utilizes artificial transmitters and measures electromagnetic (EM) responses to reveal the resistivity information of the subsurface. The current waveform of transmitters has nonnegligible effects on induced fields. Therefore, 3-D TEM forward modeling algorithms need the capability of simulating arbitrary waveforms to obtain accurate responses. In time-stepping-based 3-D TEM forward modeling, the source term (ST) approach is frequently used, which employs the source current density to model the waveform variation during time-stepping. The ST approach, however, requires fine-time discretization to describe complex waveforms, which could significantly raise the computational cost. We present a robust convolution (Conv) approach that computes the convolution between the time derivative of the waveform and the step-off response to incorporate the waveform effects in 3-D TEM modeling. The Conv approach does not discretize the waveform using time steps. Hence, it is advantageous when modeling full-waveform cases. The developed algorithm is based on the finite-element (FE) method using unstructured grids and the implicit backward Euler approach. Both galvanic and inductive transmitters are incorporated. Ground and airborne TEM surveys are tested using an actual airborne TEM waveform, a full waveform of the 2(n) -sequence pseudorandom signal, and various synthetic waveforms. Accuracy is validated against the 1-D and 3-D solutions of published studies. The ST and Conv approaches are compared. Synthetic examples show that the latter approach simplifies the waveform incorporation in TEM modeling and substantially improves time-stepping efficiency without sacrificing accuracy.

Scalar magnetic surveying using unmanned aerial vehicle (UAV) platforms is slowly gaining momentum within geophysical applications. So far, only a handful of studies have dealt with UAV-Towed scalar field measurements, while even fewer have considered towed scalar difference measurements (or gradients). In this paper, we demonstrate the possibilities and benefits of deploying precisely positioned noise-minimized UAV-Towed scalar transverse horizontal difference (THD) measurements for mineral exploration purposes. UAV-Towed gradiometry bird data are presented from the Nautanen area in northern Sweden and compared with ground magnetic surveys. This area is known for its iron oxide copper-gold mineralizations. The UAV survey spans a total area of 2.5 km2. It was covered using an average line spacing of 30 m and a constant flight altitude above ground level of 30 m. High-quality scalar total-field and THD data were collected with a dynamic noise level of the raw scalar data of about ±0.05 nT. Comparison with the ground magnetic data shows a strong correlation between magnetic anomaly lows and highs across the survey areas. A map with new structural information is presented based on subtle magnetic structures identified in discrete derivatives of the total magnetic intensity anomaly and THD data. Such systems may replace high-quality heliborne systems and reduce costs of the geophysical exploration phase. However, mapping with UAV-Towed systems is not straightforward. With typical UAV flight speeds of only 10-12 m/s, the wind often disturbs the 3D attitude of the bird during flights. Hence, advanced processing is required to obtain the intended gradients. Similar challenges are less important in surveying, where the survey speed often greatly exceeds the wind speed.

Two geophysical field works were conducted during a feasibility study in the Smaltjärnen tailing repository of the abandoned Yxsjöberg mine, located in south-central Sweden. The aim of these studies was to evaluate the applicability of the geophysical methods in tailing characterizations i.e. to (I) identify the approximate level of the underground water table, (II) understand the vertical and lateral distribution of the tailings and (III) image the variations within the internal stratigraphy of the tailings. Ground Penetrating Radar (GPR) data with medium–low-frequency antennas (300, 250 and 100 MHz) and Self-Potential (SP) data using the Streaming Potential Phenomena (SPP) were collected to characterize the top few meters of the subsurface and understand the water flow direction. Results from the mineralogical and geochemical studies of the drill-core samples were incorporated in the study to complement the interpretations of the geophysical data. Three distinct layers had been earlier identified based on the interpretations of the geochemical data which agreed well with the GPR interpretations in this study: (I) oxidized tailing (new), (II), transition zone and (III) old tailing which is located under the water table. The SP data, unexpectedly, indicated that the groundwater flows from the lake i.e. lower altitudes, towards the higher altitudes which probably is related to the uncertainties resulted from 2D data while the actual water flow direction can be best studied in 3D, or, dominant effect from the metal contents. Complementary geophysical studies including a 2D-SP survey and Direct Current (DC) electrical resistivity measurements are suggested to improve the present understanding of the morphology of the site.

Potential field data in databases of the Geological Survey of Sweden (SGU) combined with newly acquired broadband magnetotelluric data are used to map and interpret geological units and structures of a 200 km by 250 km area in the Paleoproterozoic Norrbotten ore province (northern Sweden, latitudes 66°–68.5° and longitudes 19°–24°). In order to achieve this, a new approach is proposed with respect to extracting and analysing the possible correlation between modelled physical properties as well as their patterns with respect to depth variation within the crust. In this study, we propose the use of a neural net self-organising map procedure (SOM) for simplification, data reduction, and domain classification of the models derived from independent 3-D geophysical inversion of magnetotelluric, gravity, and magnetic data. The crustal model of the electrical conductivity structure was obtained from previous 3-D inversion of the magnetotelluric data. Processing and 3-D inversion of the regional magnetic and gravity field data were performed using an open-source object-oriented code called SimPEG. The input data to the SOM analysis contain resistivity, magnetic susceptibility, and density model values within the Norrbotten area for some selected depth levels of the entire crust. The domain classification is discussed with respect to the geological boundaries and composition of the crust. Consistency between model domain classification and geological boundaries is observed in general but an apparent discrepancy is noted for some areas. The reason for the apparent discrepancy is likely related to that most geological boundaries represent surface features whereas the geophysical data includes information at depth.