Michael Becken

SUMMARY: Airborne natural source electromagnetics (EM) provide spatially densely gridded, frequency-dependent intersite transfer function estimates referring airborne vertical and horizontal magnetic field data to the horizontal components of a ground site. Common processing schemes for airborne natural source EM data are conceptually based on approaches developed for ground-based magnetotelluric sounding curve estimation and yield statistically independent transfer function estimates for selected times windows and separately for each component of the airborne recording. However, the magnetic field is spatially smooth and, moreover, its components are mutually dependent in space. Here, we propose to exploit spatial potential field relations directly in the processing workflow to refine transfer functions. The self-consistent response function estimator is applied after a first statistical estimation of transfer functions and supports the determination of a spatially smooth and self-consistent set of vertical and horizontal magnetic transfer functions that reflects physical constraints. Self-consistency is enforced by fitting sources distributed within an equivalent sheet to a preliminary set of transfer function estimates determined on a statistical basis. Smoothness is controlled by the depth to the equivalent sources. Once estimated, the distribution of vertical and horizontal magnetic transfer functions can be predicted from the equivalent sheet at any point in the air half-space. Additionally, the scheme allows for the identification of anomalous parts of the horizontal fields at the ground site. The procedure is demonstrated on a large-scale field data set from Gobabis (Namibia). We observe that transfer functions derived by using the equivalent source layer interpolator improve processing results when compared to purely statistically determined transfer function averages.

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.

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 (SAEM), 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 Deep Electromagnetic Sounding for Mineral Exploration joint project. To date, promising sites are being surveyed using either crewed aircraft or ground-based methods, both of which come with limitations concerning site access, survey period, achievable resolution, and cost. By using 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 surveyed the Hope deposit, a known, unexploited massive sulfide mineralization in Western Namibia. Time-varying EM fields, excited by grounded electric-dipole transmitters, are measured and evaluated discretely in the frequency domain. Based on 2D inverse modeling, we are able to image the Hope ore body and 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 invert data from the two sensor systems individually as well as jointly and carry 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.

Modern measurement systems for geophysical exploration are increasingly based on uncrewed aerial vehicles. For performing semi-airborne electromagnetics, they offer, with total payload limitations of commonly 25 kg, a cost-efficient alternative to helicopters as a carrier for receiver systems with reduced logistical efforts and greater flexibility in survey layouts. We evaluate the inversion results of the controlled-source electromagnetic (EM) data recorded with two different sensors, a scalar magnetometer (SM) and a vector magnetometer (VM), acquired in two mining areas, namely, the Hope deposit in Namibia and the Poderosa mine in the Eastern Iberian pyrite belt, Spain. The SM is more sensitive at lower frequencies, whereas the VM 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 300 m, 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 extend the capabilities of the open-source tools custEM/pyGIMLi to invert the combined data sets of both receiver systems in 2.5D and three dimensions. As the two applied sensors are sensitive in different frequency ranges, combining the two data sets in inversion with a common frequency range of 1–1024 Hz provides the advantage of enhanced near-surface resolution together with maximum investigation depths up to 1 km. 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 our measurement design to recover comparatively small but elongated conductors associated with known massive sulfide deposits.

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 Ωm) 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 Ωm) 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.

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 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.

Intracontinental deformation and intraplate volcanism, which occur far from tectonic plate boundaries, are not fully understood. Their origin and evolution are linked by crust-mantle interactions and mantle convection dynamics. Mongolia is an ideal natural laboratory for studying such processes because it is located far into the continental interior, several thousand kilometres from major tectonic margins. To the north is the Siberian craton, which is relatively stable, to the northeast is an extensional regime near the Baikal rift zone, which stretches for more than one thousand kilometres, and to the south are the North China and Tarim cratons, which have northward-directed motion creating a compressional regime. Central Mongolia, which contains a high plateau (with indications of vertical motion), is characterized by a shallow lithosphere-asthenosphere boundary that deepens at the edges, notably northwards towards the Siberian Craton. Continental intraplate basaltic volcanism of Cenozoic age exists across central and northern Mongolia, with several large concentrations within the Hangai region. As part of an ongoing project, we are investigating the lithospheric properties and lithospheric architecture beneath this region with magnetotelluric measurements and three-dimensional models of electrical resistivity. In addition, thermo-mechanical numerical modelling, with geophysically-guided constraints, is being used to provide valuable insight by testing different hypotheses for the temporal evolution and dynamic processes -- such as whether an upwelling asthenosphere and/or lithospheric removal could realistically be a consequence of delamination, edge-driven convection mechanisms from a lithospheric step, or some combination. Towards these goals, geophysical models that image the transition from thin lithosphere to thick lithosphere (and its geometry), believed to occur beneath northern Mongolia, are beneficial. There exists a wealth of recent geophysical data across central Mongolia, in addition to petrological data. This includes a temporary broadband seismic array that covers the Gobi, Hangai, and Hovsgol regions. In this presentation, we will report on 79 new magnetotelluric measurements acquired in 2022 in northern Mongolia across the Hovsgol and Darhad regions, as well as 77 new measurements acquired from 2020-2022 in central-east Mongolia (Bulgan, Arvaikheer). The acquired data are very good quality with low noise, a clear benefit of the remote location. Recordings were carried out at each location for approximately 1-5 days. The data typically had reliable periods up to 1,000 - 8,000 s. The new data will, ultimately, be integrated into the previously collected dataset across central Mongolia (Hangai, Bayankhongor, and Gobi-Altai), which consists of 328 measurement locations (thus approximately 500 total), which covers a total area of, currently, approximately 1000 km by 800 km. This is a notably large area, within the realm of several large regional and national magnetotelluric (and seismic) surveys. Furthermore, the data across northern Mongolia fill the last gap in a remarkable transect of existing magnetotelluric data that extends approximately 4,000 km from across the Siberian Craton to across the Tibetan Plateau.

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.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.