Browsing by study line "Den fasta jordens fysik"
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(2019)Tiivistelmä/Referat – Abstract This study investigates temperature data that Posiva Oy has from the Olkiluoto and ONKALO® sites. The aim of the study was to create a unifying data classification for the existing temperature measurements, give an estimate of the initial undisturbed bedrock temperature and temperature gradient and model the temperature profiles in 3D. The thermal related issues, which the repository will undergo once in operating are significant and have fundamental contribution to the evolution of the repository, creating a need in such a study. Posiva Oy has temperature data obtained with four main methods; Geophysical drillhole loggings, Posiva flow log (PFL) measurements, thermal properties (TERO) measurements and Antares measurements. The data classification was carried out by creating a platform of quality aspects affecting the measurements. The classification was then applied for all the available data by inspecting the measurement specifics of each configuration and by observing the temperature/depth profiles with WellCad software. According to the specifics of each individual measurement the data was classified into three groups: A= the best data, recommended for further use, and which fulfils all quality criteria, B= data that should be used with reservation and which only partly fulfils quality criteria, and C= unusable data. Only data that showed no major disturbance within the temperature/depth profile (class A or B) were used in this study. All the temperature/depth data was corrected to the true vertical depth. The initial undisturbed average temperature of Olkiluoto bedrock at the deposition depth of 412 m and the temperature gradient, according to the geophysical measurements, PFL measurements (without pumping), TERO measurements and Antares measurements were found to be 10.93 ± 0.09°C and 1.47°C/100m, 10.85 ± 0.02°C and 1.43°C/100m, 10.60 ± 0.08°C and 1.65°C/100m, and 10.75°C and 1.39°C/100m, respectively. The 3D layer models presented in this study were generated by using Leapfrog Geo software. From the model a 10.5 – 12°C temperature range was obtained for the deposition depth of 412 – 432 m. The models indicated clear temperature anomalies in the volume of the repository. These anomalies showed relationship between the location of the major brittle fault zones (BFZ) of Olkiluoto island. Not all observed anomalies could be explained by a possible cause. Uncertainties within the modelling phase should be taken into consideration in further interpretations. By combining an up-to-date geological model and hydraulic model of the area to the temperature models presented here, a better understanding of the temperature anomalies and a clearer over all understanding of the thermal conditions of the planned disposal location will be achieved. Based on this study a uniform classification improves the usability of data and leads into a better understanding of the possibilities and weaknesses within it. The initial bedrock temperature and the temperature gradient in Olkiluoto present thermally a relatively uniform formation. The estimates of the initial bedrock temperatures and the temperature gradient presented in this study, endorse previous estimates. Presenting the classified temperature data in 3D format generated good results in the light of thermal dimensioning of Olkiluoto by showing distinct relationships between previously created brittle fault zone (fracture zone) models. The views and opinions presented here are those of the author, and do not necessarily reflect the views of Posiva.
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(2021)A new ground motion prediction equation, named ON21, is solved for the ST1 Deep Heat enhanced geothermal system in Otaniemi, city of Espoo, Finland. The raw data from seismic events, that occurred during the stimulation of 2018, is processed, instrument response is removed, and frequency domain is used to obtain peak ground displacement, velocity, and acceleration. A database with 20,768 ground motion recordings from 204 events is compiled and used to solve a ground motion prediction equation for peak ground velocity and acceleration for vertical and horizontal movement. The model has a magnitude range from 0.0 to 1.8 on the scale of local magnitude used in Finland, and hypocentral distances of 0 km to 20 km. A 1σ value of 0.60 for vertical peak ground velocity model is lower than the 1σ of the models previously in use at Otaniemi and its surrounding areas. It is observed that the azimuth between the strike of the fault causing the earthquakes and the station recording the events seems to affect the peak ground motion values at a hypocentral distance of no more than 10 km, and beyond that the magnitude and distance are the dominant factors in the peak ground motion values. The new ground motion prediction equation model ON21 should be tested with ground motion data from earthquakes that occurred during the 2020 stimulations to assess its usefulness in predicting peak ground motion values, and to further study the effect of azimuth on the peak ground motion values.
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(2020)NMR Services Australia (NMRSA) Pty Ltd has developed a Borehole Magnetic Resonance (BMR) tool which is based on the principles of nuclear magnetic resonance (NMR). Drillhole NMR tools have been used mostly in sedimentary environments for oil and gas exploration while applications in hard, heterogeneous, crystalline bedrock are still lacking. This study aims to test the BMR method in a hard rock environment, and for determining hydrogeological parameters in the spent nuclear fuel disposal site, the Olkiluoto island. Essentially, the objective is to design an optimal BMR data processing workflow and calibrate the estimated hydrogeological parameters, currently optimized for data from sedimentary environments, to suit the crystalline bedrock. For testing the BMR method in hard, crystalline bedrock, Posiva Oy, the Finnish expert organization responsible of spent nuclear fuel disposal, made test measurements in the drillholes of the spent nuclear fuel repository site, island of Olkiluoto. The collected data was processed with WellCAD software using additional NMR module. The BMR tool derives T2 distribution (representing pore size distribution), total porosity, bound water and moveable water volumes and permeability calculated with two different models. Some processing parameters (main/burst sequence, moving averages, temperature gradient, cutoff values) were tested and adjusted to fit into crystalline bedrock. Magnetizing material of the surface environment strongly disturbed the uppermost ~20.0 m portions of the measurement data. Some noise was encountered also deep in bedrock, which was cut away from the signal. A list of criteria was created for recognizing noise. The BMR data was compared with other drillhole data acquired by Posiva, i.e. fracture and lithology logs, seismic velocities and hydrogeological measurements. It was observed that the T2 distribution and total porosity correlate rather well to logged fractures and seismic velocities. Lithological variations did not correlate to BMR consistently, mostly because of the strong dependency on fracturing. Permeabilities were compared to earlier conducted hydrogeological measurements, with an intention to calibrate the permeability calculation models. However, this proved to be challenging due to the significant differences of the BMR method and conventional hydrogeological measurements. Preferably, the permeability models should be calibrated by laboratory calibration of the drillhole core, and possibly a new permeability model suitable for crystalline bedrock should be created.
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(2020)Terrestrial cosmogenic nuclides can be used to date glacial events. The nuclides are formed when cosmic rays interact with atoms in rocks. When the surface is exposed to the rays, the number of produced nuclides increases. Shielding, like glaciation, can prevent production. Nuclide concentration decreases with depth because the bedrock attenuates the rays. The northern hemisphere has experienced several glaciations, but typically only the latest one can be directly observed. The aim of the study was to determine if these nuclides, produced by cosmic rays, can be used to detect glaciations before the previous one by using a forward and an inverse model. The forward model predicted the nuclide concentration with depth based on a glacial history. The longer the exposure duration was, the higher was the number of nuclides in the rock. In the model, it was possible to use three isotopes. Be-10, C-14 and Al-26. The forward model was used to produce synthetic samples, which were then used in the inverse model. The purpose of the inverse model was to test which kind of glacial histories produce similar nuclide concentrations than what the sample had. The inverse model produced a concentration curve which was compared with the concentration of the samples. The misfit of the inverse solution was defined with an “acceptance box”. The box was formed from the thickness of the sample and the corresponding concentrations. If the curve intersected with the box, the solution was accepted. Small misfit values were gained if the curve was close to the sample. The idea was to find concentration curves which have as similar values as the samples. The inverse model was used in several situations, where the number of limitations was varied. If the timing of the last deglaciation and amount of erosion were known, the second last deglaciation was found relatively well. With looser constraints, it was nearly impossible to detect the past glaciations unless a depth profile was used in the sampling. The depth profile provided a tool to estimate the amount of erosion and the total exposure duration using only one isotope.
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(2020)The Southern Andes is an important region to study strain partitioning behavior due to the variable nature of its subduction geometry and continental mechanical properties. Along the plate margin between the Nazca plate and the South American plate, the strain partitioning behavior varies from north to south, while the plate convergence vector shows little change. The study area, the LOFZ region, lies between 38⁰S to 46⁰S in the Southern Andes at around 100 km east of the trench. It has been characterized as an area bounded by margin-parallel strike-slip faults that creates a forearc sliver, the Chiloe block. It is also located on top of an active volcanic zone, the Southern Volcanic Zone (SVZ). This area is notably different from the Pampean flat-slab segment directly to the north of it (between latitude 28⁰ S and 33⁰ S), where volcanic activity is absent, and slip seems to be accommodated completely by oblique subduction. Seismicity in central LOFZ is spatially correlated with NE trending margin-oblique faults that are similar to the structure of SC-like kinematics described by Hippertt (1999). The margin-oblique faults and rhomb-shaped domains that accommodate strain have also been captured in analog experiments by Eisermann et al. (2018) and Eisermann relates the change in GPS velocity at the northern end of LOFZ to a decrease in crustal strength southward possibly caused by the change in dip angle. This project uses DOUAR (Braun et al. 2008), a numerical modelling software, to explore the formation of the complex fault system in the LOFZ in relation to strain partitioning in the Southern Andes. We implement the numerical versions of the analog models from Eisermann et al. (2018), called the MultiBox and NatureBox models to test the possibility to reproduce analog modelling results with numerical models. We also create simplified models of the LOFZ, the Natural System models, to compare the model displacement field with deformation pattern in the area. Our numerical model results in general replicate the findings from MultiBox experiment of Eisermann et al. (2018). We observe the formation of NW trending margin-oblique faulting in the central deformation zone, which creates rhombshaped blocks together with the margin-parallel faults. More strain is accommodated in the stronger part of the model, where the strain is more distributed across the area or prefers to settle on a few larger bounding faults, whereas in the weaker part of the model, the strain tends to localize on more smaller faults. The margin-oblique faults and rhomb-shaped domains accommodating strain is not present in the Natural System models with and without a strength difference along strike. This brings the question about the formation of the complex fault system in both the analog models and our numerical versions of them and hypothesis other than a strength gradient could be tested in the future.
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(2020)A seismic reflection survey was carried out at the Hannukainen-Rautuvaara Iron (Fe), Copper (Cu), and Gold (Au) deposits as part of the HIRE (High Resolution Reflection Seismics for Ore Exploration, 2007-2010) project. The main discovery from this survey was a regional structure showing three reflective layers dipping to the southwest. The top of this package of reflectors is currently planned to be mined at Hannukainen. The deeper parts of this package may have potential continuation of the economically viable deposits seen at shallow depths. In this work, a target-specific, amplitude-preserving workflow for profiles E1 and V5 of the Hannukainen-Rautuvaara HIRE seismic data will be formulated and applied. Then seismic amplitude vs. offset (AVO) and attribute analyses will be used to analyse the reflective layers and identify potential areas of interest for further study. This is a burgeoning area of seismic research, AVO analysis is typically used in hydrocarbon exploration and has only been sparsely used in hard rock settings for mineral exploration. Attribute analysis is more common in hard rock environments, but still underutilised. The seismic reflection data were re-processed focusing on retaining the high-frequency content of the seismic signal, this is key for further analysis. The results of the AVO analysis consist of determining the AVO class of the responses seen across the CMPs of two selected AVO horizons. AVO product and Poisson’s ratio change across the horizons were calculated, and an area of interest was identified from the correlation of these parameters. Attribute analysis was done using the seismic attributes envelope, first derivative envelope, Hilbert Transform, relative impedance, phase, weighted instantaneous frequency and dip. The amplitude attributes (envelope, first derivative envelope, Hilbert Transform, relative impedance) were useful in determining the areas of the reflector package that showed the strongest amplitudes and selecting horizons on the uppermost reflector for AVO analysis. Phase and weighted instantaneous frequency helped determine the continuity of the reflector package which revealed a clear four-layer signature, differing form the earlier three-layer interpretation. The dip attribute showed vertical anomalies, some of which correlated with mapped faulting in the area. Detailed interpretation of the geophysical results requires better borehole coverage, and petrophysical work, to tie in the seismic data results to the alteration and mineralisation. With open pit mine planning ongoing in the study area, the identification of deep-seated mineral deposits will have direct impact on the planning of the mine and the future of exploration in Hannukainen-Rautuvaara.
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(2021)Kappa-parameter (κ) is used to estimate the decay of seismic spectral amplitudes with frequency and is the sum of regional kappa (κr) and site-specific kappa (κ0). The site-specific kappa (κ0) parameter in Olkiluoto (Southwestern Finland) is generally small, approximately 0.002 to 0.004. These values, although smaller, are in the same range that have been found in Eastern North America, where kappa is around 0.006. In Western North America kappa is around 0.04. In Europe, e.g., in alpine region, kappa value is around 0.025. The kappa-value was studied by analysing microearthquake recordings gathered by Posiva Oy’s seismic monitoring network from 2016 to 2019. From these microearthquakes 51 microearthquakes were selected and used in the analysis. All these microearthquakes occurred relatively close to the monitoring stations, from tens of meters to few hundred meters. Each of the events were detected by multiple sensors and the total number of microearthquake registrations used in this study was 297. From these recordings the κ0 was calculated for each component (two horizontal and one vertical). Total number of calculated κ0 values was 473. The kappa-method used was the original introduced by Anderson and Hough in 1984. Besides using earthquake data, the site-specific kappa was also calculated from excavation blasts in Olkiluoto for comparison. Blasting related kappa was smaller than the one calculated from microearthquakes, with average values between 0.0012 and 0.0017. The number of blasts used to calculate κ0 was quite small and the results may not be statistically relevant. Results are in line with similar study areas around the world – harder rock has lower κ0 values
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(2020)Phosphate is reported to be subject to “high supply risk” by the EU Commission (European Commission 2017). At present, the Siilinjärvi mine in Finland is the only mine in the EU producing phosphate. Optimising the productivity of the Siilinjärvi mine is crucial to address the demand for phosphate within the EU. The current production prognosis of the mine is to the end of 2035. To improve the prognosis of the mine, an exploration program is being undertaken to investigate the extent of the deposit and possible locations for new pits. The main area of interest is the area south of the current Särkijärvi pit. Exploration drilling is limited in this area due to obstacles created by infrastructure of the mine, including the factory area and gypsum pile. To address this, 3D passive source seismic, 2D active-source reflection seismic, Ground Penetrating Radar (GPR) and magnetic surveys were conducted at the Siilinjärvi mine site as part of the H2020 Smart Exploration project. This study focuses on two of the acquired active-source seismic reflection profiles, SM2 and SM3. The aim of the study is to determine the depth and lateral extent southern continuation of the deposit in the area south of the Särkijärvi pit, next to the gypsum pile, and create a 3D model of the Siilinjärvi deposit based on the obtained results. In addition, obtaining information on waste rocks and zones of weakness, such as shear and fracture zones, is also of interest as this information is critical for mine planning. The main focus for seismic data processing was to improve the signal-to-noise ratio. Strong amplitude S-waves and unclear first-breaks were limitations found in the data. As a consequence, in addition to bandpass filtering, seismic line SM2 required a combination of attenuation and muting to supress the impact of S- waves. Seismic line SM3 had a lower data quality in comparison to that of SM2. The suppression of S- waves had a negative impact on the near-surface reflections along SM3 and therefore was not carried out. The GPR and magnetic data were processed using standard workflows. The active-source seismic survey was successful in determining the depth and the lateral extent of the southern continuation of the Siilinjärvi deposit. A 3D model of the deposit was created based on the obtained seismic images. This model expands on the previous model and indicates that the carbonatite- glimmerite deposit expands towards the W, beneath the gypsum pile. This information can be used as a guide for future drilling in the area. In addition, information was obtained on zones of weakness and the waste-rock dike network. Sub-horizontal to gently dipping reflections observed in the seismic data were interpreted as diabase dikes. On a smaller scale, GPR measurements detected shallower near-surface features which are also interpreted to possibly be dikes. For some features, a correlation could be made between the various geophysical measurements. The carbonatite-glimmerite deposit was found to be associated with elevated magnetic total field (nT) values.
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(2019)Siilinjärvi mine in Finland is the only mine within the European Union producing phosphate rock, a critical raw material for the European Union. With the current mining plans, the production is estimated to continue until 2035. The extent of the ore deposit and new locations for open pits are currently being investigated to ensure continuation of the mining operations also after 2035. The Siilinjärvi carbonatite-glimmerite deposit has been intruded by multiple waste-rock diabase dykes crosscutting the deposit and by a tonalite-diorite intrusion, creating a complex geological setting. To study the depth and lateral extent of the deposit, the diabase dykes, tonalite- diorite, major zones of weakness and the geophysical anomalies related to these features, active-source 2D reflection seismic, Ground Penetrating Radar (GPR) and magnetic surveys were conducted at the Siilinjärvi mine site in fall 2018 as part of the H2020 Smart Exploration project. Understanding the locations of the waste rocks and fracture and shear zones is crucial for mine planning and optimising the production prognoses. The interest of this study is in particular on imaging the sub-horizontal diabase dykes, whose locations and continuation are harder to predict. The focus area of this study is in the southern end of the Särkijärvi pit and the area just south of the pit, where the well-known geology of the pit can be used to constrain the interpretation. Processing of the reflection seismic data focused especially on the static corrections and the methods used to improve the signal-to-noise ratio. This was done so that the processed data could serve as a reference for new processing methods, focused on these aspects, developed within the Smart Exploration project and planned to be tested with the Siilinjärvi data. The static corrections were constrained by the limited number of first-break picks clear enough for picking from the data. In addition to the bandpass filtering, suppressing the S-wave arrivals was found to be crucial for increasing the signal-to-noise ratio, particularly in the near subsurface which is the main interest area of this study. The GPR and magnetic data were processed with standard processing workflows. The lateral and depth extent of the Siilinjärvi carbonatite-glimmerite deposit, the large-scale sub-horizontal waste- rock dykes and the major zones of weakness are imaged with the active-source reflection seismic data. The ore deposit is associated with a complex reflectivity pattern due to the intruded diabase dykes and tonalite-diorite, and the fracture and shear zones. The interpreted diabase dykes correlate with a large-scale sub-horizontal waste-rock dyke model created from the production drilling data as part of the Smart Exploration project, supporting the continuation of the sub-horizontal diabase dykes south of the pit. With GPR data, the smaller- scale sub-horizontal dykes within the shallow subsurface (<30 m) are imaged. The GPR data correlates with a detailed waste-rock dyke model created as part of the Smart Exploration project from the southern end of the Särkijärvi pit based on geological mapping, GigaPan images and a 3D photogrammetry model. The reflection seismic, GPR and magnetic data have very different scales and these different data are suitable for different purposes in mineral exploration and mine planning at Siilinjärvi. The carbonatite-glimmerite ore is associated with elevated magnetic total field values and at a larger scale the deposit could possibly be followed with magnetic surveys. With reflection seismic method, the large geological structures can be imaged at depth, and the data could be used for detailed planning of a new open pit. The higher resolution GPR measurements could then be implemented in the operating phase of the mine in a more routine manner to aid creation of reliable production prognoses.
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Using Three-Component Data for Seismic Interferometry Studies at the Kylylahti Mine, Eastern Finland (2019)The reflection seismic surveying method is useful when conducting mineral exploration in the crystalline bedrock because of its good depth extent and resolution. However, the traditional experiments with active sources are expensive and difficult to carry out, especially in remote areas or in conservation areas where mineral exploration is limited due to environmental reasons. Recently, a number of theoretical advances have proven that passive soundings utilizing ambient seismic noise can provide new opportunities for seismic imaging and contribute to data generation for reflection seismic surveys, without the need for explosive or vibratory sources. One of the most promising new methods is seismic interferometry (SI), where the impulse response between two receivers is reconstructed by correlating their signals with each other. COGITO-MIN is a joint project between the University of Helsinki, the Geological Survey of Finland, Polish Academy of Sciences, and industrial partners with the aim of investigating and developing new cost-effective seismic exploration methods in the crystalline bedrock. Within the framework of the project, a passive seismic experiment was carried out in which 45 three-component geophones were deployed for a month in the vicinity of the polymetallic Kylylahti Mine in Polvijärvi, northern Karelia, where the mining operator is the Swedish metal company Boliden. The original purpose of these geophones was to collect data suitable for detecting underground cavities related to underground nuclear explosions. The institute that collected the data was CTBTO (Comprehensive Test Ban Treaty Organization) whose task is to monitor the treaty in the pre-ratification stage. The purpose of this Master's thesis was to develop an SI workflow for the three-component data and to investigate the method's performance in an area where local geology is known after nearly 40 years of exploration and consequent mining operations. The specific scientific objectives of the thesis are (1) to demonstrate the usefulness of collecting three-component data in conjunction with or instead of single-component data, (2) to assess the noise-based SI methods used in previous studies and to improve their stability in the crystalline bedrock, and (3) to investigate the possibilities of SI from an operational perspective. Seismic velocities obtained through laboratory measurements were merged with geological and density models of the target area provided by Boliden. The resulting velocity and density grids were then used as the basis for waveform modelling, and the results from SI were validated against them. The starting point for SI was the noise-driven approach where 'each sample matters'. The interferometric workflow is built on the Seismic Unix suite together with self-written algorithms that are based on theoretical evaluations. SI is followed by an imaging workflow, which provides the basis for the reflectivity profiles. The thesis work focuses on five components of the Green's tensor and the vertical, radial and transverse component of the impulse response. With the horizontal components, one can access the S-wave patterns in addition to the P-waves. As a specialty, the so-called sign bit normalization (SBN) method was also tested. The technique involves destroying much of the amplitude information of the original seismograms by only retaining the sign bit of each sample. According to the results outlined in this thesis, SBN can make it easier to image the weak reflectors of the subsurface. This type of seismic interferometry seems particularly suitable for the early stage of mineral exploration, where the explorer does not yet fully understand the target they are studying. The most important advantage of seismic interferometry, however, is its cost effectiveness, and its potential for reducing risks for the environment.
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(2020)The seismic reflection methods produce high-resolution images from the subsurface, which are useful in structural studies of geology. Northern Finland features a complex Precambrian geological history, including massive extension and compression stages, which has been extensively studied. The xSoDEx survey is the most recent seismic survey carried out in northern Finland by the Geological Survey of Finland (GTK). The XSoDEx concluded four survey lines, which are located in Central Lapland Greenstone Belt (CLGB) in Sodankylä, Lapland. This thesis aims to find out whether the strong reflections shown in the xSoDEx Alaliesintie reflection profile, underneath the outcropping Archaean basement indicate a lithological contact or a fault zone. The Alaliesintie profile is characterized by Koitelainen intrusion, Archaean outcrops, and layers of younger Paleoproterozoic group rocks. The work was carried out in stages, with the use of the SKUA - GOCAD 3D modeling software. The four stages are: 1. Create a 3D geological model based on the Alaliesintie reflection section and geological bedrock observations. 2. Use gravity and magnetic geophysical data from the study area to improve model reliability. 3. Use the geological 3D model and petrophysical data to build a synthetic seismic forward mode. 4. Analyze and evaluate the modeling result for understanding the possible origins of the reflections. In the geological 3D model, I presented that the reflection would present lithological contacts and that the Archean bedrock would have folded and partly overthrust on top of the younger Proterozoic rocks. The seismic forward model is used as an experiment to test the geological 3D model’s lithological contact respondence to the synthetic seismic signal and to discover the possible reflector underneath the Archaean basement. The results present that the seismic forward model can be used to perform the reflections and that the geological 3D model presented similar reflections in the seismic forward model comparing to the original Alaliesintie reflection data.
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