3D modelling of the dolerite dyke network within the Siilinjärvi phosphate deposit

Non peer reviewe

Tests of inter-hotspot motion and of hotspot motion relative to the spin axis

First, an updated Pacific paleomagnetic skewness pole for chron 32 (72 Ma) is presented. The updated paleomagnetic pole corrects for the spreading-rate dependence of anomalous skewness, a correction which hasn't been applied to Pacific skewness poles before. The presence of anomalous skewness is one of the main factors limiting the accuracy of paleomagnetic poles determined from the skewness data. Thus, successfully correcting for the anomalous skewness, as was done in this study, significantly improves the reliability of the skewness poles. The earlier assertions that the Hawaiian hotspot has shifted southward relative to the spin axis by 13° since ≈72 Ma are also confirmed. Second, updated reconstructions of the Pacific plate relative to the hotspots for the past 68 million years are presented, with the uncertainties in the reconstructions. Plate-circuit reconstructions are used to predict the tracks of some major Indo-Atlantic hotspots (Tristan da Cunha, Reunion and Iceland) from the Pacific-hotspot plate motion and the rates of relative motion between the Pacific and Indo-Atlantic hotspots are estimated. Within the uncertainties, motion between the hotspots is found insignificant for the past 48 million years. For earlier times, a systematic error in the plate circuit used to make the predictions is inferred and which may be due to unmodeled motion between East and West Antarctica. If the observed discrepancy can be shown to correspond to an error in the plate circuit, the southward motion of the Hawaiian hotspot of 13° since ≈72 Ma can likely be attributed to true polar wander. Building on the above-mentioned work, finally, for the first time, a globally self-consistent model of plate motions relative to the global hotspots for the past 48 million years is presented, and the implications of this model to the question of relative hotspot motion discussed. The provided globally self-consistent set of reconstructions can be used as a fixed frame of reference for absolute plate motions, and true polar wander, for the past 48 million years

Common Mid Point (CMP) surveying for depth calibration of ground penetrating radar data

Non peer reviewe

Semi-automated geological mapping and target generation from geochemical and magnetic data in Halkidiki region, Greece

This study focuses on using multivariate analyses to generate semi-automated geological maps and exploration targets associated with porphyry Au-Cu mineralization within the Kassandra mining district, Greece. We use principal component analysis (PCA) and self-organizing maps (SOM) to reveal variations in geochemical and magnetic signatures within the input datasets. We visualize the results as pseudo-geological maps reflecting the associated geological processes and their end products. In specific, we utilize the potential of these two methods through an integrated interpretation and comparison of the results. We test the validity of the unsupervised PCAand SOM-derived lithological and prospectivity models by comparing them with existing geological observations and interpretations. The results of this investigation show that both PCA and SOM are able to reproduce the key features of existing geological observations within the study area, but more importantly, also provide useful information that can be used to recognize prospective geological units and exploration targets from previously unknown locations.Peer reviewe

Predicting Missing Seismic Velocity Values Using Self-Organizing Maps to Aid the Interpretation of Seismic Reflection Data from the Kevitsa Ni-Cu-PGE Deposit in Northern Finland

We use self-organizing map (SOM) analysis to predict missing seismic velocity values from other available borehole data. The site of this study is the Kevitsa Ni-Cu-PGE deposit within the mafic-ultramafic Kevitsa intrusion in northern Finland. The site has been the target of extensive seismic reflection surveys, which have revealed a series of reflections beneath the Kevitsa resource area. The interpretation of these reflections has been complicated by disparate borehole data, particularly because of the scarce amount of available sonic borehole logs and the varying practices in logging of borehole lithologies. SOM is an unsupervised data mining method based on vector quantization. In this study, SOM is used to predict missing seismic velocities from other geophysical, geochemical, geological, and geotechnical data. For test boreholes, for which measured seismic velocity logs are also available, the correlation between actual measured and predicted velocities is strong to moderate, depending on the parameters included in the SOM analysis. Predicted reflectivity logs, based on measured densities and predicted velocities, show that some contacts between olivine pyroxenite/olivine websterite-dominant host rocks of the Kevitsa disseminated sulfide mineralization—and metaperidotite—earlier extensively used “lithology” label that essentially describes various degrees of alteration of different olivine pyroxenite variants—are reflective, and thus, alteration can potentially cause reflectivity within the Kevitsa intrusion

Reflection imaging of complex geology in a crystalline environment using virtual-source seismology : case study from the Kylylahti polymetallic mine, Finland

For the first time, we apply a full-scale 3D seismic virtual-source survey (VSS) for the purpose of near-mine mineral exploration. The data were acquired directly above the Kylylahti underground mine in Finland. Recorded ambient noise (AN) data are characterized using power spectral density (PSD) and beamforming. Data have the most energy at frequencies 25-90 Hz, and arrivals with velocities higher than 4km s(-1) have a wide range of azimuths. Based on the PSD and beamforming results, we created 10 d subset of AN recordings that were dominated by multi-azimuth high-velocity arrivals. We use an illumination diagnosis technique and location procedure to show that the AN recordings associated with high apparent velocities are related to body-wave events. Next, we produce 994 virtual-source gathers by applying seismic interferometry processing by cross-correlating AN at all receivers, resulting in full 3D VSS. We apply standard 3D time-domain reflection seismic data processing and imaging using both a selectively stacked subset and full passive data, and we validate the results against a pre-existing detailed geological information and 3D active-source survey data processed in the same way as the passive data. The resulting post-stack migrated sections show agreement of reflections between the passive and active data and indicate that VSS provides images where the active-source data are not available due to terrain restrictions. We conclude that while the all-noise approach provides some higher-quality reflections related to the inner geological contacts within the target formation and the general dipping trend of the formation, the selected subset is most efficient in resolving the base of formation.Peer reviewe

Hydrogeological Characterization Of Crystalline Bedrock Using Borehole Magnetic Resonance At A Mining Development Site, Northern Finland

The Borehole Magnetic Resonance (BMR) method was tested for measuring porosity and estimating groundwater flow parameters required for hydrogeological modelling in a crystalline rock setting at the Sakatti Ni‐Cu‐PGE mining project site in Northern Finland. The characterization of hydraulic properties of deep bedrock, shallow fractured bedrock, surficial deposits as well as groundwater and surface water flow patterns and interactions between them is informative already during ore prospecting phase. Although BMR is widely used for determining storage and flow properties in sedimentary rocks forming hydrocarbon reservoirs, there have been few studies in basement rocks as described here. The BMR method allows the detection of water in the subsurface by applied electromagnetic fields in a continuous manner with decimetre‐scale measurements along the boreholes. The measured response is inverted to give a continuous distribution of relaxation time T2 which is a direct measure of the amount of water, or pore volume. This can further be divided into bound water and free water using specific cut‐off values for each lithology, which can then be used to calculate estimates of hydraulic conductivity. The Sakatti Cu‐Ni‐PGE deposit, located several hundred meters below the Natura2000‐protected Viiankiaapa mire, displays complex and heterogeneous subsurface geology, hydrogeology and deformational history. The BMR data was acquired from six drillholes in order to obtain specific information about the structural heterogeneity and free water content within the depth interval of 50 to 360m, and thus estimations of the local flow parameters. Other available survey data, such as natural gamma ray, acoustic image data, core logging and packer tests were compared with the BMR data in order to compare and calibrate the groundwater flow parameters calculated based on these measurements. The initial results indicate that BMR is a suitable tool for studying lithologically and hydrogeologically heterogeneous fractured crystalline rocks. Flow parameters derived from the measured T2 distributions vary significantly throughout the intervals. In this crystalline bedrock setting, independently from the lithological composition, the measured intervals locally display relatively high hydraulic conductivities, and may be correlated to the more intensely fractured and/or brecciated zones. In addition, BMR may reduce more time‐consuming methods such as packer tests or replace them altogether in regions where borehole conditions prevent the likelihood of successful testing. Furthermore, the BMR log provides continuous data over fractured crystalline basement rocks. This will include questions dealing with interactions between surface water, shallow groundwater as well as groundwater in fractured and weathered bedrockPeer reviewe

Underground Vertical Seismic Profiling with Conventional and Fiber-Optic Systems for Exploration in the Kylylahti Polymetallic Mine, Eastern Finland

Seismic reflection methods have been used for the exploration of mineral resources for several decades. However, despite their unmatched spatial resolution and depth penetration, they only have played a minor role in mineral discoveries so far. Instead, mining and exploration companies have traditionally focused more on the use of potential field, electric and electromagnetic methods. In this context, we present a case study of an underground Vertical Seismic Profiling (VSP) experiment, which was designed to image a (semi-)massive sulfide deposit located in the Kylylahti polymetallic mine in eastern Finland. For the measurement, we used a conventional VSP with three-component geophones and a novel fiber-optic Distributed Acoustic Sensing (DAS) system. Both systems were deployed in boreholes located nearby the target sulfide deposit, and used in combination with an active seismic source that was fired from within the underground tunnels. With this setup, we successfully recorded seismic reflections from the deposit and its nearby geological contrasts. The recording systems provided data with a good signal-to-noise ratio and high spatial resolution. In addition to the measurements, we generated a realistic synthetic dataset based on a detailed geological model derived from extensive drilling data and petrophysical laboratory analysis. Specific processing and imaging of the acquired and synthetic datasets yielded high-resolution reflectivity images. Joint analysis of these images and cross-validation with lithological logging data from 135 nearby boreholes led to successful interpretation of key geological contacts including the target sulfide mineralization. In conclusion, our experiment demonstrates the value of in-mine VSP measurements for detailed resource delineation in a complex geological setting. In particular, we emphasize the potential benefit of using fiber-optic DAS systems, which provide reflection data at sufficient quality with less logistical effort and a higher acquisition rate. This amounts to a lower total acquisition cost, which makes DAS a valuable tool for future mineral exploration activities.Peer reviewe

COGITO-MIN seismic reflection profiling for mineral exploration in Polvijärvi, Finland

Non peer reviewe

3C Seismic Interferometry at the Polymetallic Kylylahti Deposit, Outokumpu District, Finland

Non peer reviewe