In this study, we conducted an extensive geophysical survey to explore the potential of electrical resistivity methods in delineating deep ore deposits within between Koillismaa Intrusion and Näränkävaara intrusion, northeastern Finland. Preliminary investigations in 2022, including magnetic, gravity and audio-magnetotelluric (AMT) methods, along with drilling, uncovered significant anomalous structures in the survey area. Subsequent drilling of an exploration well provided positive lithological indications of a ultramafic igneous rock at more than 1.5 km depth, which are very likely of the same age as the layered intrusions in the area. Borehole data indeed revealed that the Archaean basement gneiss extends down to approximately 510 m, underlain by a granite dyke with interspersed thin layers of pyroxenite and peridotite. Notably, peridotite layers around 1500 m depth exhibited distinct magnetic and IP responses in core data.We employed electrical methods at the site, including electrical resistivity tomography (ERT) and induced polarization (IP). To cover a large-scale area, 25 transmitter dipoles, each 1 km long and using three different transmitter systems, were deployed and data were recorded at 119 receiver stations. This work presents the acquisition and preliminary results from the ERT-IP surveys. During the processing of ERT and IP data, we utilized full time-series data recorded across the four lowest main frequencies (from 0.0625 Hz to 8Hz) to capture voltage data in a steady state. Apparent resistivity data were derived from the stacked voltage data, while IP data were initially extracted from these decay curves of these stacked voltage data and subsequently processed in the frequency domain (outphasing). Analysis of the resistivity and IP responses revealed notable IP signals at depths exceeding 1.5 km. Meanwhile, the resistivity data indicated generally very high values, around 10,000 ohm-m, with complex variations observed near the surface. This study demonstrates the efficacy of ERT and IP methods in delineating deep-seated mineral deposits, with the deep-depths IP responses being particularly noteworthy.

At GTK we are currently developing the entire workflow of the controlled source EM (CSEM) method, ranging from data acquisition to time series processing to modelling and inversion. Part of this work is the development of a 3D modelling and inversion framework, which is mostly done within the DroneSOM project. The flexible implementation allows not only for modelling and inversion of semi-airborne drone EM data, but also land-based CSEM/MT data. The forward problem is solved using the finite element method on hexahedral meshes. We separate forward and inverse mesh using octree mesh refinement. This helps in solving the trade-off between the required accuracy in the forward modelling and computational cost. It is also a great tool to combine different multiresolution EM data (e.g., CSEM and MT) in a single comprehensive inversion framework. This work will focus on first applications of land-based CSEM and (CS)MT. In 2022 we collected controlled source MT data using a grounded electric dipole transmitter along the Koillismaa ultra-mafic intrusion in North-Eastern Finland. Despite transmitter receiver offsets of 3-5 km far field condition does not apply for frequencies below 4kHz, which permits the use of standard MT inversion. Here we show first inversion results of these data using our new EM inversion routine taking the transmitter position into account. In addition to the active source MT data, we also collected conventional MT data on a larger scale crossing the Koillismaa intrusion. Our inversion routine also allows the inversion of MT data. We are thus showing first inversion results of joint inversion of both datasets.

The rapid development of geophysical systems utilizing drones facilitates mineral exploration with more efficient and economical data collection. To align the progress of hardware advancement and meet the model complexity needs for exploration, we aim to develop an efficient and robust 3D inversion code to interpret the drone-based EM data. Here, we present the framework of the implementation and show some preliminary results of the development.We use a total electric field formulation with curl-conforming Nédélec elements to solve Maxwell equations in the frequency domain. Octree grids are used to accommodate the meshing of even large models at adequate resolution, separately in forward and inverse domains. A direct solver (MUMPS) is applied to solve the linear system of equations of the forward problem. The code is implemented in C++ and allows for easy adaptation for various sources and data types.Currently, to solve the inverse problem, we minimize the misfit using a Gauss-Newton scheme with explicit computation of the Jacobian. The implementation was built on the deal.II library, where the interface wrappers allow to use MUMPS and PETSc for numerically intensive computations, such as system equation solving (MUMPS) and inversion model update (PETSC Conjugate Gradient solver). Currently, the code is parallelized using MPI throughout for both forward and inverse modeling and additionally OpenMP for MUMPS only.The code is planned to be a reliable and competent imaging tool, that can be applied for both commercial and educational use. Currently, the code is under the development and testing stage. The preliminary results will be shown on-site.

The transition towards carbon neutral transportation and energy sources increases the global demand for mineral raw materials while easy-to-find near-surface ( 200 m) ore deposits are unlikely discovered in well-explored areas such as Europe. In order to increase the mineral exploration success rate, the project SEEMS DEEP (SEismic and ElectroMagnetic methodS for DEEP mineral exploration) develops geophysical deep exploration workflow capable of imaging the bedrock from the surface down to several kilometres depth. In this paper, we present first results from ground electrical and electromagnetic surveys conducted at the SEEM DEEP geological test site, namely the Koillismaa Layered Intrusion Complex in north-eastern Finland. Here, a 1.7 km long hole drilled by GTK intersected mafic-ultramafic rocks with anomalous electrical and chargeability properties at  1400 m depth, making it an interesting case study to test the ability of such technologies for imaging resistivity and chargeability contrasts at several kilometre depth.

In this study, we conducted a comprehensive geophysical survey near Kuusamo (Finland) to assess the potential of electrical resistivity methods in delineating mineral deposits at depths greater than 1 km. Preliminary investigations, including magnetic and gravity methods as well as drilling, revealed significant anomalous structures in the survey area. We employed multiple electrical and electromagnetic methods at the site, comprising controlled-source electromagnetic (CSEM), magnetotelluric (MT), electrical resistivity tomography (ERT), and induced polarization (IP). To obtain the geophysical data in very large-scale area, we used a total of 25 transmitter dipoles with 1km long using three distinct transmitter systems and recorded data at 119 receiver stations. In this paper, we present the acquisition and preliminary results from ERT-IP. Analysis of the resistivity and IP responses revealed notable IP signals at depths exceeding 1.5 km. Meanwhile, the resistivity data indicated generally very high values, around 10,000 ohm-m, with complex variations observed near the surface.