To evaluate and upscale the feasibility of using exploration tunnels in an operating mine for active-source seismic imaging, a seismic experiment was conducted at the Neves-Corvo mine, in southern Portugal. Four seismic profiles were deployed in exploration drifts approximately 650 m beneath the ground surface, above the world-class Lombador volcanogenic massive sulfide deposit. In addition to the tunnel profiles, two perpendicular surface seismic profiles were deployed above the exploration tunnels. The survey was possible due to a newly developed prototype global positioning system (GPS) time transmitter enabling accurate GPS synchronization of cabled and nodal seismic recorders, below and on the surface. Another innovative acquisition aspect was a 1.65 t broadband, linear synchronous motor (LSM) driven - electric seismic vibrator (e-vib) used as the seismic source along two of the exploration tunnels. We have evaluated the challenges and innovations necessary for active-source tunnel seismic acquisition, characterized by high levels of vibrational noise from the mining activities. In addition, we evaluated the LSM vibrator's signal and overall seismic-data quality in this hard rock mining environment. Our processing results from the tunnel data and 3D reflection imaging of the Lombador deposit below the exploration tunnels were checked for consistency through constant-velocity 3D ray-tracing traveltime forward modeling. For imaging purposes, 3D Kirchhoff prestack depth and poststack time-migration algorithms were used, with both successfully imaging the targeted deposit. The results obtained show that active-source-seismic imaging using subsurface mining infrastructure of operational mines is possible. However, it requires innovative exploration strategies, a broadband seismic source, an accurate GPS-time system capable of transmitting GPS-time hundreds of meters below the surface, and careful processing. The results obtained open up possibilities for similar studies in different mining or tunneling projects.

Mineral exploration industry needs to push its technological advancement towards finding the so-called critical raw materials. These materials are fundamental for our green technologies and help accelerate the energy transition towards decarbonisation. While in-mine and near-mine exploration will be more convenient in the short term, providing fresh raw materials and mines in greenfield or brownfield areas must not be forgotten in the longer term. As the chase for mineral deposits becomes deeper, seismic methods play a greater role for exploring at depth. Through a series of experiments conducted within the EU-funded Smart Exploration project, we have innovated a number of hardware and methodological solutions for in-mine as well as brownfield seismic exploration. Along with these, legacy data have also been recovered, reprocessed and their values for mineral exploration illustrated. The legacy data examples are from the Ludvika Mines (Nordic Iron Ore AB) of central Sweden and Neves-Corvo (Somincor-Lundin Mining) of southern Portugal. In particular, through the development of a GPS-time system, we have managed to acquire a globally unique semi3D in-mine and surface seismic dataset at the world-class Neves-Corvo mine. This helped to utilize four exploration tunnels at 600 m depth and two receiver lines on the surface allowing over 1000 recorders to be synchronized for down-tunnel exploration. A broadband electromagnetic-based seismic source (7 kN or 1.5t), developed also in the project, was used as the seismic source. In central Sweden, at an iron-oxide mining site of Nordic Iron Ore company, 2D seismic profiles helped to suggest potential resources in the down-dip continuation of the known deposits but also in their footwall. A follow-up and more recent survey employed over 1250 seismic recorders and a 32t vibrator to acquire a sparse 2 by 2 km seismic dataset. The data show great quality and allow to image lateral extent of the deposits and crosscutting reflections that may be important factors for mine planning and understanding structural evolution of the deposits. The broadband seismic source was also tested at the site along the existing 2D profiles with raw data already showing a number of reflections interpreted to be from the mineralization. This survey further illustrates that the seismic source functions well and has a great potential for hard rock seismic applications. Acknowledgements: This work was supported by the Smart ExplorationTM project. Smart Exploration has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No. 775971.

Unmanned aerial vehicle (UAV)-based geophysical surveys are attractive for land mineral exploration and are gradually opening extraordinary opportunities in providing high-resolution definition of geologic structures and for direct targeting of mineral deposits. There are, however, challenges such as electromagnetic noise from the UAV, limited load capacity, and short flight times. If these are overcome, there will be a new era in using UAV-based geophysical systems for mineral exploration and for a number of mining-related purposes. In this study, we have tested the potential of rotary-wing UAV systems, given their flight flexibility and robustness for direct targeting of iron-oxide deposits in central Sweden. A walking-mode high-precision Overhauser magnetometer was reassembled so that it could be lifted by the rotary-wing system. Successful backyard tests were performed, but during the real experiment several issues related to high UAV noise level and extreme magnetic field from the mineralization delayed data acquisition. At the end, within nearly three hours and 10 sorties, approximately 20 km-line total-field magnetic data were collected covering an area of about 2 km2. Flight lines were designed perpendicular to the strike of the mineralization to maximize data sampling. Two distinct mineralized zones, magnetite- and hematite-rich and only 50–100 m apart, are notable in the magnetic data due to the fine sampling spacing provided by the UAV survey. Historical low-altitude (30 m above the ground) fixed-wing aeromagnetic data are available from the study area and are compared with the UAV data. Both data sets are consistent in delineating the mineralization, therefore demonstrating the potential of UAV-based surveys for mineral exploration in geologically and logistically challenging areas.

During the last few decades there has been an increased demand for infrastructure, along with a greater awareness of environmental issues in the construction industry. These factors have contributed to an increased interest in using seismic methods for near surface characterization, particularly in urban environments. Seismic sensors not affected by anthropogenic electromagnetic noise are therefore important, as well as an acquisition system that is easy to deploy, move and non-invasive. To address some of these challenges, a multicomponent broadband MEMS (micro-electro mechanical system) based landstreamer system was developed. The landstreamer system is fully digital, therefore it is less sensitive to electrical or electromagnetic noise. Crosstalk, leakage and tilting tests show that the system is superior to its predecessors. The broadband nature of the sensors (theoretically 0–800Hz), 3C (three-component) recording and the close spacing of the sensors enable high-resolution imaging. The current streamer configuration consists of 20 sensors 4m apart and 80 sensors 2m apart. The streamer can easily be combined with wireless recorders for simultaneous data acquisition. In this study, we present results from testing of the streamer with various sources, such as a shear wave vibrator and different types of impact sources. MEMS-sensors and their high sensitivity allowed recording clear reflections that were not observed with coil-based sensors. A complementary test was also carried out at a planned access ramp for an urban tunnel where potential poor quality rocks had been identified by drilling. First-break traveltime tomography showed that these poor quality rocks correlate with low velocity zones. The presented landstreamer system has great potential for characterizing the subsurface in noisy environments.

A set of three new tensor audiomagnetotelluric – magnetotelluric (AMT-MT) instruments have been assembled in Oulu. Their design is based on the MTU2000 magnetotelluric equipment that was developed earlier in the Uppsala University. The new instruments are broadband systems covering the period range from 0.001 s to d.c. Induction coil magnetometers are used to measure magnetic field in the period range of 0.001–1000 s and fluxgate magnetometers for periods longer than 10 s. The electric field is measured with two 50–200 m long orthogonal electric dipoles using non-polarisable Pb/PbCl2 electrodes. The MT systems are equipped with high dynamic range (24 bit ADC) data recorders having a maximum sampling frequency of 3000 Hz and possibility for dual band burst recording. Monitoring of the system health with SMS messages helps to maintain long period observations allowing continuous measurements and reducing the maintenance efforts and costs. The three new instruments extend the Fennoscandian pool of homogeneous MT equipments into 14 systems and allow synchronous array measurements. This makes it possible to use multisite data processing techniques and therefore considerably improve the data quality, especially in electromagnetically noisy regions. The instrument pool can be used in a broad range of applications to study the crust and upper mantle.

The radiomagnetotelluric (RMT) method utilizes man-made signals generated by distant transmitters or by dedicated local transmitters. Man-made electromagnetic (EM) signals in the 1-250-kHz frequency band come mainly from two sources: (1) distant radio transmitters operating in the 15-250-kHz band and (2) nearby or distant industrial sources emitting either transients or higher harmonics of 50 Hz. The natural or background EM signals form a kind of noise floor. In Europe for the 15-250-kHz band, there generally are sufficient transmitters available to estimate the EM transfer functions completely, i.e., the full impedance tensor and the tipper vector. We show examples of the variability of power spectra and azimuthal distributions of transmitters from sites in Sweden, Hungary, The Netherlands, and Spain. We also show that the estimated transfer functions are stable as a function of time; even under typical noisy conditions, simple stacking of spectra in narrow frequency bands provides good-quality estimates of transfer functions.