Modern measurement systems for geophysical exploration are increasingly based on uncrewed aerial vehicles. For performing semi-airborne electromagnetics, they offer a cost-efficient alternative to helicopters as carrier for receiver systems with reduced logistical efforts and a greater flexibility in survey layouts. We present inversion results of controlled-source electromagnetic data recorded with two different sensors, a scalar and a vector magnetometer, acquired in two mining areas, namely, the Hope deposit in Namibia and the Poderosa mine in the Eastern Iberian pyrite belt, Spain. The scalar magnetometer is more sensitive at lower frequencies while the vector magnetometer is more sensitive at higher frequencies. The Hope demonstration site is easily accessible with almost no vegetation and topography, allowing for optimized regular survey layouts and dense data sets to cover the region of interest. In contrast, the Poderosa test site is characterized by poor accessibility, topographic undulations up to 300m, and thick vegetation. For simulating and inverting scalar data projected to the local total magnetic field direction, we consider a resource-saving simulation approach using mesh rotation. We further extended the capabilities of the open-source tools custEM/pyGIMLi to invert the combined data sets of both receiver systems in both, 2.5D and 3D. As the two applied sensors are sensitive in different frequency ranges, combining the two data sets in inversion with a common frequency range of 1 to 1024 Hz provides the advantage of enhanced near-surface resolution together with investigation depths up to 1 kilometer. We assess the reliability of the inversion results by performing misfit and resolution analysis as well as comparing the inverted conductive zones with additional information about the two deposits. The results from both test sites demonstrate the capability of the proposed measurement design to recover comparatively small but elongated conductors associated with known massive sulfide deposits.

We are investigating the lithospheric properties and lithospheric architecture beneath Mongolia with three-dimensional models of the electrical resistivity generated from magnetotelluric measurements. In addition, thermo-mechanical numerical modelling, with geophysically-guided constraints, is being used to provide valuable insights by testing the mechanical viability of different hypotheses for the temporal evolution and dynamic processes within this region. Mongolia is located between the relatively stable Siberian craton and the extensional regime near the Baikal rift zone to the north and to the south the North China and Tarim cratons that have a northward-directed compressional regime. Due to its location, it is an excellent region to study intracontinental deformation. Furthermore, enigmatic continental intraplate basaltic volcanism of the Cenozoic age exists across Mongolia. In addition, this region contains economically important mineral zones (copper and gold), with the origin and evolution of the mineral systems linked to the whole-lithosphere architecture, crust-mantle interactions, and mantle convection dynamics. Magnetotelluric data has been collected across Western, Central, and Eastern Mongolia. Three field campaigns in 2016, 2017, and 2018 collected more than 328 sites on an array (50 km spacing) and along three dense profiles (3-15 km spacing) that focused on the Hangai Dome (plateau) and Gobi-Altai (Arkhangai, Bayankhongor) over an area of approximately 800 km (north-south) by 400 km (east-west). Between 2020 and 2022, the array was extended to the east with 77 sites collected across central-east Mongolia (Bulgan, Selenge, Tuv, Uvurkhangai, Dundgovi; 400 by 200 km), including 34 sites along an 810 km long north-south profile crossing the Mongol-Okhotsk suture zone. In late 2022, 79 measurements were acquired in northern Mongolia across the Hovsgol region and Darhad (200 by 200 km) with an array and several profiles, which connect to data west of Lake Baikal. In early 2023, 38 sites were collected in central-east Mongolia (Umnugovi; 200 by 200 km), completing the eastern array. Later in 2023, a major field campaign was launched that successfully collected 150 measurements in western Mongolia (Zavkhan, Uvs, Govi-Altai, Khovd) over an area of approximately 500 by 400 km. This included an array (50 km spacing) and three dense profiles (5-10 km spacing). This gives approximately 700 magnetotelluric measurements collected over a total area of approximately 1000 km (north-south) by more than 1150 km (east-west). This is a large area that approaches the scope of several other regional and national magnetotelluric survey programs. What’s more, this dataset fills an important gap between the existing magnetotelluric data across China and the Tibetan Plateau with several profiles across the Siberian Craton, in principle completing a remarkable transect of 4000 km across a variety of tectonic domains. In this presentation, we will report on the new measurements. They will be integrated into the previously collected dataset, and new models will be generated that incorporate all data. We will also present new models of western, central and eastern Mongolia that provide insights on the properties, structure, and evolution of the Hangai Dome, the Mongol-Okhotsk suture and the Central Asian Orogenic Belt.

Inversion of geomagnetic anomaly data poses an ill-posed problem, and extremal models such as equivalent source layers or point-source distributions can explain observations to the same degree as volumetric magnetization distributions. However, the spectral characteristics of magnetic anomalies provide fundamental constraints for magnetic source-depth estimation. Specifically, the maximum detectable depth of crustal magnetic sources is dictated by the longest wavelengths present in the field, which correspond to the low-wavenumber bands of the spectrum. This relationship is often analysed through the log power spectrum versus wave number plot, using the slopes of the linear segment for depth estimation. Methods aiming at reconstructing the depth to the bottom of magnetization from spectral field characteristics are commonly referred to as spectral methods. However, these methods are based on assumptions about the statistical properties of the source distribution and are prone to misinterpretations. Here, we apply sparsity-constrained 3-D inversion of magnetic data using an elastic net regularization to recover the susceptibility distribution and the bottom of magnetization. We claim that the elastic net (l(2)l(1) norm) regularization, when properly tuned to balance the solution's smoothness with sparsity, stabilizes the inversion, avoiding extremal magnetization distributions and generating a geologically plausible source depth distribution that is consistent with the expected source distribution. The l(1) norm brings sparsity and high resolution, while the l(2) norm brings inversion stability and structural continuity to the final model. From the recovered 3-D elastic net sparse inversion model, we extract the depths of all the deepest non-zero susceptibility values and suggest this to be an alternative estimate to the base of magnetization. Moreover, we suggest that the resulting 3-D model has a value in itself and may aid geological interpretation.

In recent decades, global technological expansion, alongside significant shifts in information technology, energy supply and mobility, has dramatically increased the demand for certain raw materials, especially minerals. To meet future demand, new strategies and solutions are being sought. Semi-airborne electromagnetics, as an emerging method to sense conductive subsurface structures, holds high potential for mineral exploration and can be applied to uncover untapped ore deposits or re-evaluate exploited ones. This technique has been successfully implemented in multiple studies and is a core part of the DESMEX (Deep Electromagnetic Sounding for Mineral Exploration) joint project. To date, promising sites are being surveyed either employing crewed aircraft or using ground-based methods, both of which come with limitations concerning site access, survey period, achievable resolution and cost. By utilizing uncrewed aerial vehicles (UAVs) some constraints and expenses can be overcome. Taking advantage of a battery-powered 25 kg maximum takeoff weight octocopter and two complementary magnetometers, an optically pumped total-field magnetometer and an induction coil triple, we have surveyed the Hope deposit, a known, unexploited massive sulfide mineralization in Western Namibia. Time-varying electromagnetic fields, excited by grounded electric-dipole transmitters, were measured and evaluated discretely in frequency domain. Based on two-dimensional inverse modeling, we were able to image the Hope ore body and to trace it down to a depth of more than 300 m. Combined sensor data inverted along adjacent profile lines reveal an imposing, contiguous dipping conductor that can be clearly assigned to the Hope structure. To assess the significance of our results, we inverted data from the two sensor systems individually as well as jointly and carried out detailed modeling studies. Our findings are supported by available resistivity models based on audio magnetotelluric data and yield an excellent match to existing borehole probes.

Since the Cenozoic, a series of extensional south-north normal faults and gneiss-granite domes evolved in the southern Tibetan Plateau, the formation mechanism of which is of scientific interest and which has implications for the tectonic dynamics of the plateau. Typical of such features are the Xainza-Dinggye rift and the Mabja gneiss dome, which are located in the Xainza-Xietongmen-Dinggye region in the central Tibetan Plateau. In this study, Magnetotelluric measurements across this region are used to generate a high-resolution 3-D electrical resistivity model of the subsurface and to analyze the cause of the conductive zones. The large-scale conductive zones identified in the middle-lower crust may result from aqueous melt partial melting, whereas the smaller-scale conductive zones in the upper-middle crust may result from saline fluids, possibly with varying minor volumes of melts. Subsequently, based on the electrical resistivity model and combined with the spatiotemporal coupling of the geological, geochemical, and geophysical data, the state and migration features of crustal materials are discussed.The results show that the upwelling of mantle materials along subduction channels and slab-windows related to the tearing of the Indian lithospheric plate contributed to the partial melting of the middle-lower crust in the Lhasa terrane. Furthermore, partial melting of the upper-middle crust in the Tethys-Himalaya terrane resulted from southern extrusion of crustal materials in the Lhasa terrane. These two mechanisms can significantly reduce the effective viscosity. We speculate that the deformation of the brittle upper crust that is controlled by large-scale ductile layers characterized by weak rheology is the main dynamic mechanism of rift evolution. Meanwhile, the metamorphism and anatexis in the upper-middle crust of the Tethys-Himalaya terrane related to the southern extrusion of materials contributed to the evolution of the Mabja gneiss dome. During the middle Miocene, the southern extrusion of crustal materials may have been influenced by the cooling events beneath the Mabja gneiss dome, which can explain why the deep areas beneath the Mabja gneiss dome have middle-high resistivity. In addition, our study region is located in the Mediterranean-Himalayan seismic belt, and mainly includes shallow-focus earthquakes and intermediate-depth earthquakes. In the north, shallow-focus earthquakes are mainly controlled by the accumulation of stress in the brittle layer of the overlying crust related to the ductile layer of the middle and lower crust. In the south, shallow-focus earthquakes（e.g., Dingri M S 6.8 earthquake） mainly occur in the rigid, resistive block, which is surrounded by conductive zones, possibly because fluid migration may be hindered by these resistive blocks. The intermediate-depth earthquakes are mainly controlled by the response in the subsurface area, which is related to the detachment of the Indian lithospheric mantle from the Indian crust.

Quantum sensing provides advanced technologies which significantly improve sensitivity and accuracy for sensing changes of motion, gravity, electric and magnetic field. Therein, quantum sensors for the detection of magnetic fields, so-called quantum magnetometers, are one of the most promising technological realizations. We firstly will provide a brief overview on methods in geophysical exploration benefitting from quantum magnetometers with resolution at the physical and technical limit. We will introduce recent developments on SQUID and OPM based sensors as specific implementations of a quantum magnetometer systems and application examples.

Abstract Electrodes are used to measure a potential difference between two points. In geophysical and geotechnical applications they are often in the form of non‐polarizable porous‐pot electrodes. Here we describe the design, construction, and testing of modular and refillable electrodes, which facilitates repair as the electrodes degrade over time. We use a chemical composition based on a metal in contact with an over‐saturated electrolyte that consists of a salt of that metal and an auxiliary salt. We compare characteristics when the electrolyte is stabilized in a clay or not, and with various states of ceramic porous plugs and two types of wood plugs. Next, we assess the long‐term stability (more than 1 month), noise (periods of 1 s to 1 hr), and temperature sensitivity of different types of electrodes. Electrodes with an electrolyte and clay formula showed lower noise (0.2–0.4 μV at periods of 1–120 s), greater long‐term stability (0.05–0.5 mV/month of smooth drift), and greater consistency between samples measured than those with no clay (noise and drift values up to four times larger). The effects from different porous plugs were negligible, with similar results for ceramic and wood types. The temperature sensitivity of the electric potential was assessed, from −3 to 35°C. All electrodes showed a temperature sensitivity of about −30 μV/°C. This is considered very low compared to some commercially available electrodes. Finally, continuous long‐term laboratory and field measurements of the potential highlight the application of the new electrodes.

The Mongol-Okhotsk suture and the Adaatsag ophiolite belt are associated with the closure of the Mongol-Okhotsk paleo-ocean and are located within the Central Asian Orogenic Belt (CAOB) and Mongolia. The suture zone is flanked by volcanic-plutonic belts that host significant metallogenic zones, containing deposits of copper and gold. The tectonic evolution of this region is not fully understood and the lithospheric structure has been poorly studied. We analyze magnetotelluric data and generate a model of the electrical resistivity distribution across this region. Whereas the northern segment has a sharp transition from a high-resistivity upper crust to a low-resistivity lower crust, as observed beneath the Hangai Dome, the southern segment does not show this transition. A wide, low-resistivity zone (1-100 Omega m) imaged in the crust and lithospheric mantle is coincident with the Mongol-Okhotsk suture and ophiolite, revealing a clear and significant lithospheric-scale feature. Across the profile, numerous narrow, vertically oriented, low-resistivity features (1-100 Omega m) are spatially associated remarkably well with the proposed boundaries of tectonic domains. These results confirm ideas about the development of the CAOB. Some of these low-resistivity features are beneath the surface locations of large mineral zones, and likely represent fossil fluid pathways. We show congruent seismic velocity models for comparison and the results show a large-scale low-velocity anomaly (decrease of 2%-3%) that correlates with the location of the low-resistivity anomaly below the Mongol-Okhotsk suture. The geophysical results, combined with geological and geochemical data, provide insights into the structure of this region and help shed light on unanswered questions. When the ancient Mongol-Okhotsk ocean closed, due to subduction from tectonic re-arrangement, it left the Mongol-Okhotsk suture zone and the Adaatsag ophiolite as a trace of its location. Similarly, other tectonic boundaries are hypothesized to exist from terrane accretion across the Central Asian Orogenic Belt (CAOB) and Central and Southern Mongolia, which is located between the Siberian and North China cratons. This region is also rich in economically significant copper and gold deposits. The tectonic evolution of this region and especially the lithospheric structure is not fully understood and has been poorly studied. We analyze magnetotelluric data and generate a model of the electrical resistivity distribution. Additionally, we show models of the seismic velocity for comparison. Examining multiple complementary geophysical models helps to reduce interpretation uncertainty. Anomalies are observed in both models (e.g., low resistivity and low velocity). The suture zone is proven to be a strong lithospheric-scale boundary. The proposed boundaries of tectonic domains are also imaged, confirming ideas about the development of the CAOB, and solving some controversies. Lithospheric-scale, wide, low-resistivity zone revealed below the ophiolite belt associated with the closure of the Mongol-Okhotsk ocean Vertical, narrow low-resistivity features aligned with proposed tectonic boundaries and locations of large mineral zones (copper and gold) The northern part of central Mongolia has a sharp mid-crustal transition from high to low resistivity, whereas the southern part does not

SUMMARY As deep-seated ore deposits become increasingly relevant for mineral exploration, the demand for time-efficient and powerful deep-sounding exploration methods rises. A suitable method for efficiently sensing ores at great depth is airborne electromagnetics (EM) using natural signal of atmospheric origin. The method relates airborne magnetic field recordings in the audio-frequency range to reference magnetic field recordings measured at a ground-based site and can achieve greater penetration depths when compared to controlled source airborne EM techniques. However, airborne natural source EM data are prone to noise caused by platform vibrations especially deteriorating data quality at low frequencies and thus narrowing the depth of investigation. Motional noise manifests as coherent noise on all airborne magnetic field components demanding for a powerful processing tool to remove such kind of noise. Unlike the bivariate approach, which is widely used in natural source EM, the multivariate approach is capable of detecting and reducing the effect of coherent noise. We introduce a robust multivariate processing for airborne natural source EM data and present the code implementation. The code was applied to a large-scale data set from the Kalahari–Copper–Belt in Namibia covering over 1000 km2. We obtained spatially consistent and smooth sounding curves in a frequency range of 10 to 1000 Hz including frequencies with prominent motional noise. Transfer functions are in good agreement with other geophysical and geological information.

>In the framework of a mineral system approach, a combination of components is required to develop a mineral system. This includes the whole-lithosphere architecture, which controls the transport of ore-forming fluids, and favorable tectonic and geodynamic processes, occurring at various spatial and temporal scales, that influence the genesis and evolution of ore-forming fluids(Huston et al., 2016; Groves et al., 2018; Davies et al., 2020). Knowledge of the deep structural framework can advance the understanding of the development of a mineral system and the emplacement of mineral deposits. Deep geophysical exploration carried out with this aim is increasingly important for targeting new ore deposits in unexplored and underexplored regions(Dentith et al., 2018; Dentith, 2019).

The transition toward renewable energies demands a secure supply of critical raw materials and requires efficient noninvasive methods for deep earth resource exploration. The novel, deep electromagnetic (EM) sounding for the mineral exploration semi-airborne EM (semi-AEM) exploration concept aims at the efficient exploration of resources down to a depth of 1 km. Here, we evaluate a large-scale semi-AEM exploration study in a graphite mining district in eastern Bavaria, Germany. The derived 3D semi-AEM model is based on approximately 70,000 complex-valued data points recorded using seven transmitters. Strong conductivity contrasts with dominant east–west-trending anomalies are visible. Shallow high-conductivity structures correlate well with the known occurrence of graphite and match existing helicopter-borne EM results. The main anomaly reaches down to several hundred meters depth. To investigate different interpretation scenarios for large-scale semi-AEM data, we determine the effect of topography and analyze the feasibility of fast 2D inversion applications. To validate the robustness of the 3D semi-AEM model, the data set is inverted with two different 3D inversion algorithms and the results are compared. The presence of graphite leads to significant induced polarization (IP) effects with considerably high chargeabilities superposing EM induction. We include these effects in a realistic 3D inversion using a synthetic data study to analyze if the IP effect alters the overall conductivity structure and demonstrate that the obtained 3D model is reliable.

>In order to attain good quality transfer function estimates from magnetotelluric field data(i.e., smooth behavior and small uncertainties across all frequencies), we compare time series data processing with and without a multitaper approach for spectral estimation. There are several common ways to increase the reliability of the Fourier spectral estimation from experimental(noisy) data; for example to subdivide the experimental time series into segments, taper these segments(using single taper), perform the Fourier transform of the individual segments, and average the resulting spectra.

The semi-airborne electromagnetic method (SAEM) has recently gained increased interest in geophysical exploration applications. For SAEM, ground-based transmitters are deployed within the area of interest or in its vicinity, and the induced magnetic is measured airborne. Because of the large footprint of extended transmitters, three-dimensional (3D) effects can have a significant impact on the gathered data and 3D modelling and inversion approaches must be employed for data analysis. Any 3D EM simulation is a resource-consuming task and efficient tools can therefore be useful to add data interpretation. In this study, 3D tomographic inversion of frequency-domain SAEM data was accomplished based on the integral equation (IE) method. The inversion was linearised by approximating the sensitivity matrix. We further employ the quasi-linear approximation to facilitate out quick forward modelling in the inversion which is tractable on field computers. While this approach sacrifices inversion accuracy for efficiency, we guide the inversion by introducing multinary constraints. These constraints impose a priori known conductivity values into the inversion process. Our results from synthetic and field data demonstrate the efficiency of our approach in achieving a coherent geophysical model that is consistent with geological data and findings from other geophysical techniques. This method provides a practical and capable solution for unveiling conductive 3D structures in the field.

Northeast Africa, which today includes the Arabian-Nubian Shield and the Saharan Metacraton, experienced a complex and long history of tectonic events. These include cratonization, which resulted in thickening of the lithosphere and formation of stable cratons, and decratonization, which occurred as a result of the remobilization and reactivation of the tectonic domains through subsequent orogenies, or destruction of the cratonic root during extensional events. One outstanding question is the present-day architecture of the lithosphere across this region, including the location of important tectonic boundaries. Several geophysical investigations have been conducted to study the lithosphere, including density and velocity modeling; however, they have mainly focused on the hydrocarbon-rich areas offshore and onshore close to the western coast of the Gulf of Suez, in addition to some small regional-scale studies.We present a tectonic model of the Arabian-Nubian Shield and Saharan Metacraton derived, in part, from a 3D electrical resistivity model generated from magnetotelluric measurements acquired along a 700 km long profile across the central part of Egypt. The profile, roughly west-east, consists of 57 measurements, a subset of a larger dataset acquired in the region. The profile crosses the main tectonic boundaries in Egypt: the Arabian Nubian Shield (ANS) in the eastern part, the Nile River in the central part, and the Saharan Metacraton (SMC), in addition to its cratonic remnants (Al-Kufra), in the western part. The profile runs approximately along a line from Dahkla to Kharga, across to Qena, and towards Hurghada on the coast. On average, the measurement spacing is approximately 10 km, although it is denser in some regions (e.g., near ANS) and sparser in others (e.g., near Qena) due to local conditions.The data were acquired in campaigns carried out in autumn 2019, spring 2020, spring 2021, and spring 2022. The measurements used Metronix data loggers (ADU07e) and Metronix induction coils along with locally developed copper-copper sulphate electrodes to measure the electric field. Most sites were recorded for 2-5 days. The sampling rate used was 512 Hz. Periods up to 1,000 – 5,000 s were recorded. The data are generally considered to be of good quality and had low noise; this is primarily due to the lack of urban electrical noise in most of the survey area.Dimensionality analyses suggest a 3D character for long-period data, particularly in the ANS area, that requires the use of full 3D inversion to properly describe all aspects of the data. Several sensitivity tests were carried out to validate the robustness of the features in the 3D electrical resistivity model. A comparison of the resistivity model with other geophysical models in this region (including density and velocity models) shows a good correlation for the location of the cratonic boundary, which has a clear resistive electrical signature.

Both low resistivity zones and low velocity zones are distributed in the middle-lower crust of the western Lhasa terrane, Tibetan Plateau, China. Some estimates from electrical resistivity data suggest large volume fractions of silicate melts that are difficult to reconcile with seismic velocity data that prefer lower volumes. A second conductive phase, such as saline fluids, that drastically reduces the conductivity but does not significantly affect the seismic velocity because of its low volume may be able to explain these differences. In this study, a 3-D model of the electrical resistivity structure is generated on a profile along longitude 85°E from a latitude of 29°N to 32.5°N. Based on experimental measurement of melts and alkali-rich fluids (e.g., H2O-NaCl), we estimate the volume fraction of each phase that is required to explain the conductive anomalies observed in the geophysical model. The model reveals that the maximum bulk conductivity of the mid-lower crust in the south (1.52 S/m) is much higher than the conductivity of the mid-lower crust in the north (0.18 S/m) when taking 31°N as a rough boundary, near Coqen region. We hypothesize that the conductive zones in the south of the Coqen region may result from a silicate melt and alkali-rich fluid (multicomponent) system. In contrast, partial melting alone can explain the conductive zones in the north. The hypothesis can reconcile the predictions from electrical resistivity data and seismic data, and it corresponds well with zircon Hf isotope data. For example, a combination such as the presence of <1% NaCl-bearing aqueous fluids in addition to 5-10% partial melt can reconcile electrical conductivity data and seismic data. We propose that the contributions from partial melt or saline fluids are controlled by the distinct tectonic dynamics in each region. Furthermore, the model compatible with the idea that the Indian lower crust subducted northwards beneath the Lhasa terrane and may not extend far beyond the Indus-Yarlung Zangbo suture (approximately 30-31°N). The widespread distribution and interconnection of crustal conductors at different depths is consistent with the lateral migration of materials. However, both geophysical data sets agree that some anomalies are discontinuous along the profile. Furthermore, the low-angle subducted Indian Plate with no obvious tearing feature and a low volume of melts may have contributed to the absence of long, continuous, N-S-trending normal faults in this region.

SUMMARY The analysis of controlled-source electromagnetic (EM) data recorded with semi-airborne exploration systems requires advanced simulation and inversion tools that are capable of handling realistic survey geometries. Semi-airborne EM setups with elongated transmitters deployed in mountainous terrain prohibit the exploitation of secondary-field formulations in numerical approximations without producing hardly quantifiable errors. Building upon the open-source software custEM for forward modeling and pyGIMLi for geophysical inversion, we present an inverse modeling procedure based on highly accurate second-order finite-element forward solutions on irregular grids and fast-converging Gauss–Newton minimization. Using the total-field formulation of the electric field approach in combination with a direct solver enables calculating explicit sensitivities with comparatively cheap back-substitutions for thousands of ground and airborne receiver stations in multiple flight areas. Second-order basis functions show general superiority over first-order basis-functions regarding the accuracy and performance of the forward problem. Beyond that, synthetic and real data inversion studies related to semi-airborne geometries indicate that second-order basis functions help particularly to avoid high modeling errors for the weakest field components and artifacts in the vicinity of transmitters or at the surface. This leads generally to a better convergence and final inversion results of higher robustness and quality. The presented tools are freely available such as the underlying software.

&lt;p&gt;Economies are critically dependent on the secure provision of raw materials. The EU commission currently lists 30 elements as critical, including base metals as well as high-tech metals. Projections indicate a significant increase in demand within the next decades, in parts due to the transformation of the energy sector and the digital revolution. Therefore, reuse of raw materials in the context of circular economies must be accompanied by the development of additional primary resources. Furthermore, to minimize dependencies from individual producing countries, the supply chains must be diversified, including the exploitation of domestic resources. Consequently, exploration activities - of which geophysics is a critical component - must be intensified today to increase the reserves for tomorrow.&lt;/p&gt; &lt;p&gt;New deposits are likely to be found under cover and at depths greater than has typically been exploited in the past. Both greenfield and brownfield environments, such as historic mining districts, have a potential for new discoveries. Recent developments in airborne geophysics aim at increasing the exploration depth and improving the imaging capabilities to detect targets that have previously remained hidden. In this lecture, I will discuss the challenges for airborne geophysical exploration, with a particular focus on semi-airborne electromagnetics, a hybrid approach that combines the benefits of powerful land-based transmitter deployments with the dense spatial coverage of overflights with passive airborne receivers. This concept has been implemented in the ongoing DESMEX project and has also received interest elsewhere. The hybrid approach can be shown to exhibit significantly increased penetration depth than classical airborne EM systems. Moreover, without the need to tow heavy transmitters airborne, the concept can be ideally transferred to unmanned platforms, reducing the costs significantly. Finally, three-dimensional inverse modelling becomes tractable with this setup; this is because the number of involved simulation steps per iteration scales with the number of transmitter installations (typically less than ten) instead of the number of measurement points with fixed transmitter-receiver geometries as in classical airborne measurements (typically several thousand). I will present case studies from demonstration measurements in Europe and beyond.&lt;/p&gt;

SUMMARY Various electromagnetic (EM) techniques have been developed for exploring natural resources. The novel frequency-domain semi-airborne controlled source electromagnetic (semi-AEM) method takes advantages of both ground and airborne techniques. It combines ground-based high-power electrical dipole sources with large-scale and spatially densely covered magnetic fields measured via airborne receivers. The method can survey the subsurface down to approximately 1000 m and is particularly sensitive towards conductive bodies (e.g. mineralized bodies) in a more resistive host environment. However, the signal-to-noise ratio of semi-AEM is lower than that of ground-based methods such as long-offset transient electromagnetics (LOTEM), mainly due to the limited stacking time and motion-induced noise. As a result, the semi-AEM often has reduced depth of investigation in comparison to LOTEM. One solution to overcome these flaws is to analyse and interpret semi-AEM data together with information from other EM methods using a joint inversion. Since our study shows that LOTEM and semi-AEM data have complementary subsurface resolution capabilities, we present a 2-D joint inversion algorithm to simultaneously interpret frequency-domain semi-AEM data and transient electric fields using extended dipole sources. The algorithm has been applied to the field data acquired in a former mining area in eastern Thuringia, Germany. The 2-D joint inversion combines the complementary information and provides a meaningful 2-D resistivity model. Nevertheless, obvious discrepancies appear between the individual and joint inversion results. Consequent synthetic modelling studies illustrate that the discrepancies occur because of (i) differences in lateral and depth resolution between the semi-AEM and LOTEM data caused by different measuring configurations, (ii) different measured EM components and (iii) differences in the error weighting of the individual data sets. Additionally, our synthetic study suggests that more flexible land-based configurations with sparse receiver locations are possible in combination with semi-AEM without a significant loss of target resolution, which is promising for accelerating data acquisition and for survey planning and logistics, particularly when measuring in inaccessible areas.

Some of the largest and most significant Miocene porphyry copper systems in China are within the Gangdese metallogenic belt on the southern Tibetan Plateau. It has been recognized that the crustal architecture and rheology, derived from regional tectonic events, has direct implications for the evolution and transport of fluids and magmas, and thus for the metallogenesis and prospectivity. Using data from a magnetetolluric array, which intersects the Lhasa-Mozugongka district of the Gangdese metal belt, a 3-D electrical resistivity model was generated, with the goal of investigating the tectonic and rheological controls on the magmatic mineral system. The lithospheric temperature distribution was estimated by applying the Arrhenius equation to conductivity profiles generated from 1-D Monte-Carlo models of long-period magnetotelluric data. The conductivity of partial melts in the lower and middle crust (30-60 km depth) was estimated for local conditions by applying the experimentally-derived equation of X. Guo et al. (2018). Subsequently, we estimated the melt fraction required to explain the observed bulk resistivity in each part of the study area. Variations in the effective viscosity of the lower and middle crust were constrained by the electrical resistivity model by applying the empirical relation of Liu and Hasterock (2016). Beneath the Miocene Cu-Mo deposits in the Lhasa terrane, conductive features in the lower and middle crust are attributed to partial melt fractions of more than 5% and viscosity reductions of 1-2 orders of magnitude. These conductive features may represent the signatures of (ore-controlling) melt/fluid migration channels and deeper melt/fluid source regions in the form of extensive crustal reservoirs of partial melt. Based on the interpretations of the geophysical model, and other available geological and geochemical evidence, a model of the metallogenic dynamics of the Miocene porphyry Cu-Mo deposits is proposed. Overall, the study highlights the applicability of electromagnetic geophysical methods to reliably link resistivity structures to melt/fluid transport channels and sources within a mineral system and supports the hypothesis that crustal rheology exerts a major control on the distribution of ore deposits.