Argo Jõeleht

GEOTHERMAL MODELLING OF THE LITHOSPHERE IN THE CENTRAL BALTIC SHIELD AND ITS SOUTHERN SLOPE

Lithospheric temperature and heat flow density (HFD) were studied in the central Baltic (Fennoscandian) Shield and its subsurface continuation to the south, along a transect trending from eastern Finland to southern Estonia. The transect represents an example of a low HFD (≤ 30 mW m−2) Archaean craton on a thick (150–190 km) lithosphere surrounded by Early and Middle Proterozoic mobile belts on a thinner (110–150 km) lithosphere with slightly elevated HFD (35–55 mW m−2). Numerical 2-D conductive models were constructed in which peridotite solidus temperatures were assigned to those depths which correspond to the seismically determined lithosphere/asthenosphere boundary. This technique was found to reduce the effect of uncertainties in heat production and thermal conductivity values on the simulation results. Upper crustal heat production values for the Finnish terrain were taken from published geochemical analyses of outcropping rocks. For the Estonian terrain new heat production values were measured from core samples representing nineteen deep boreholes. Middle and lower crustal lithologies were estimated with the aid of the deep seismic VPVS data, and corresponding heat production values were adapted from global xenolith averages and from data for granulites cropping out in other Precambrian areas. The results of the modelling suggest that the lithosphere and Moho depth variations are only weakly reflected in the measured surface heat flow data, which are mainly controlled by heat sources in the upper crust. The simulated heat flow densities at 50 km depth (approximately at the Moho) are relatively low and range from 12 mW m−2 at the Archaean northeastern end to 19 mW m−2 on the Proterozoic southwestern end of the transect. Simulated temperatures at 50 km depth increase from northeast to southwest, ranging from 450–550°C in eastern Finland to about 650°C in Estonia. Sensitivity of the simulations to parameter changes was studied by varying the heat production and thermal conductivity values. The extreme values for the Moho temperature estimates thus obtained may be about 50 K lower or 100 K higher than the values above. The corresponding sensitivity of the Moho HFD is about ±6 mW m−2 and of the surface HFD ±5–20 mW m−2, respectively.

Thermal properties of granulite facies rocks in the Precambrian basement of Finland and Estonia

Abstract We present results of heat production, thermal conductivity and P-wave velocity measurements of 252 rock samples from five granulite facies areas in Finland and Estonia. These compositionally mainly intermediate Archaean and Palaeoproterozoic metamorphic rocks have relatively high heat production values. Mean values averaged by areas range from 0.57 to 2.24 μW m −3 . The lowest values are in the Varpaisjarvi area, which is the oldest and most mafic and where the highest metamorphic pressure occurred (8–11 kbar), whereas the highest heat production is found in the Turku granulite belt where the metamorphic pressure was 4–6 kbar. The heat production decreases with the increase of the metamorphic pressure. However, a general numerical relationship cannot be presented because of considerable variations in heat production data. There is no significant difference in the metamorphic temperatures of the studied granulites and therefore no dependence on temperature is found. Further, our data suggest no relationship between heat production and P-wave velocity. The mean thermal conductivity of granulites varies from 3.0 to 3.5 W m −1 K −1 . Slightly elevated thermal conductivity values in the Varpaisjarvi and Lapland granulite areas can be attributed to higher sillimanite and quartz contents, respectively.

Physical properties of Vendian to Devonian sedimentary rocks in Estonia

Abstract Measurements of petrophysical properties of sedimentary rocks in Estonia are reported. We measured 364 samples in order to find out how the rock properties vary between and within the rock types, and which factors control these rock properties. The sampled boreholes cover different strata ranging from Vendian to Devonian in age, and represent Estonia laterally as well. Lithologically the analysed samples mainly represent carbonate rocks (limestone, dolomite, marl), but terrigeneous rocks (sandstone, siltstone, claystone) were also measured. Within the rock group, porosity is the main factor controlling thermal conductivity, saturated density, P-wave velocity and electric resistivity. From the point of view of rock properties, limestone and marl exhibit a homologous line where they represent low and high porosity fields, respectively, and thus could be treated together. The same applies for dolomite and dolomitic marl. P-wave velocity shows slight anisotropy, being in lateral direction 6–11% higher than in vertical direction, whereas electric resistivity shows no systematic change. The lateral variations of the rock properties for the same stratigraphic unit in Estonia are mainly due to lithological changes. As a rule, increase of the clay content in a rock increases porosity, susceptibility and natural remanent magnetization (NRM). All studied sedimentary rocks are paramagnetic with low NRM.

Palaeomagnetism of Middle Ordovician Carbonate Sequence, Vaivara Sinimäed Area, Northeast Estonia, Baltica

The hill range of Vaivara Sinimäed in northeast Estonia consists of several narrow east- to northeast-trending glaciotectonic fold structures. The folds include tilted (dips 4–75°) Middle Ordovician (early Darriwilian) layered carbonate strata that were studied by mineralogical, palaeomagnetic, and rock magnetic methods in order to specify the postsedimentational history of the area and to obtain a better control over the palaeogeographic position of Baltica during the Ordovician. Mineralogical studies revealed that (titano)magnetite, hematite, and goethite are carriers of magnetization. Based on data from 5 sites that positively passed a DC tilt test, a south-easterly downward directed component A (Dref = 154.6°± 15.3°, Iref = 60.9°± 9.7°) was identified. The component is carried by (titano)magnetite, dates to the Middle Ordovician (Plat = 17.9°, Plon = 47.3°, K = 46.7, A95 = 11.3°), and places Baltica at mid-southerly latitudes. Observations suggest that in sites that do not pass the tilt test, the glaciotectonic event has caused some rotation of blocks around their vertical axis.

Simulation of the hydrogeologic effects of oil-shale mining on the neighbouring wetland water balance: case study in north-eastern Estonia

The water balance of wetlands plays an integral role in their function. Developments adjacent to wetlands can affect their water balance through impacts on groundwater flow and increased discharge in the area, and they can cause lowering of the wetland water table. A 430 km2 area was selected for groundwater modelling to asses the effect of underground mining on the water balance of wetlands in north-eastern Estonia. A nature conservation area (encompassing Selisoo bog) is within 3 km of an underground oil-shale mine. Two future mining scenarios with different areal extents of mining were modeled and compared to the present situation. Results show that the vertical hydraulic conductivity of the subsurface is of critical importance to potential wetland dewatering as a result of mining. Significant impact on the Selisoo bog water balance will be caused by the approaching mine but there will be only minor additional impacts from mining directly below the bog. The major impact will arise before that stage, when the underground mine extension reaches the border of the nature conservation area; since the restriction of activities in this area relates to the ground surface, the conservation area’s border is not sufficiently protective in relation to underground development.RésuméLe bilan hydrique joue un rôle fondamental dans le fonctionnement des milieux humides. Des activités contigües peuvent impacter l’écoulement de la nappe, causer une décharge accrue et un abaissement de son niveau. Une zone de 430 km2 a été sélectionnée pour modélisation de nappe et évaluation de l’effet de l’exploitation souterraine sur le bilan hydrique d’une zone humide du Nord-Est de l’Estonie. Un périmètre naturel protégé (incluant la tourbière de Selisoo) se trouve à 3 km d’une exploitation d’huile de roche mère. Deux scénarios correspondant à différentes emprises de la future exploitation ont été simulés et comparés à l’état actuel. Les résultats montrent que la conductivité hydraulique verticale de sub-surface est critique vis à vis de l’assèchement potentiel de la zone humide conséquente à l’exploitation. Un impact significatif sur le bilan hydrique de la tourbière de Selisoo sera causé par l’approche de l’exploitation mais l’impact additionnel de l’exploitation sous la tourbière elle même sera mineur. L’impact majeur surgira avant ce stade, quand la mine souterraine atteindra la limite de la zone de protection naturelle ; comme la restriction des activités dans cette zone concerne la surface du sol, la conservation de sa bordure n’est pas une protection suffisante vis à vis du développement souterrain des travaux.ResumenEl balance de agua de los humedales juega un rol integral en su función. Los desarrollos adyacentes a humedales pueden afectar su balance de agua a través de impactos sobre el flujo de agua subterránea y el incremento de la descarga en el área, y ello puede causar el descenso del nivel freático del humedal. Un área de 430 km2 fue seleccionada para modelar del agua subterránea y evaluar el efecto de la explotación subterránea sobre el balance de agua de un humedal en el noreste de Estonia. Un área de conservación natural (que abarca el pantano de Selisoo) está dentro de un radio de 3 km de una mina subterránea de esquistos bituminosos. Se modelaron dos escenarios futuros de explotación con diferentes extensiones areales y se compararon con la situación actual. Los resultados muestran que la conductividad hidráulica vertical del subsuelo es de importancia crítica para el drenaje potencial del humedal como resultado de la explotación. Un impacto significativo sobre el balance de agua del pantano de Selisoo será causado por la aproximación a la mina, pero serán solo un impacto menor adicional proveniente de la explotación directamente debajo del pantano. El mayor impacto surgirá antes de esa etapa, cuando la extensión de la mina en el subsuelo alcance el límite del área de conservación natural; puesto que la restricción de actividades en este área se relaciona a la superficie del terreno, el límite del área de conservación no es lo suficientemente protectora en relación al desarrollo subterráneo.摘要湿地水平衡在其功能上页页着整体作用。比页湿地的页页通页页地 下 水流的影响和增加的页页可影响水平衡,可降低湿地水位。页页一个 430平方公里的区域页行地下水模页,页价地下页采页页沙尼页页北部湿 地水平衡的影响。自然保页区就页于地下油页岩页3公里之内。页不同区 域范页的两个未来页采方案页行了模页并与页在的情况页行了页比。页 果页示地下的垂直水力页水系数页页采中潜在的湿地排水至页重要。接近 页山就会页Selisoo沼页水平衡页生重要影响,但直接在沼页之下的页采只 有很小的页外影响。主要影响出页在页个页期前,即地下页山延伸到自然 保页区的页界。因页本地区的活页限制与地表有页,因此,保页区的页界 不能足页防页地下的页页。ResumoO balanço hídrico das zonas húmidas desempenha um papel fundamental na sua função. Desenvolvimentos adjacentes às zonas húmidas podem afetar o seu balanço hídrico através de impactes sobre o fluxo de água subterrânea e o aumento da descarga na área, podendo causar o rebaixamento do nível da água subterrânea na zona húmida. Foi selecionada uma área de 430 km2 para modelação das águas subterrâneas, a fim de avaliar o efeito da exploração mineira subterrânea no balanço hídrico da zona húmida, no nordeste da Estónia. A área de conservação da natureza (englobando o pântano Selisoo) fica a 3 km de uma mina subterrânea de xisto betuminoso. Dois cenários futuros de exploração mineira, com diferentes extensões de área de exploração, foram modelados e comparados com a situação atual. Os resultados mostram que a condutividade hidráulica vertical do subsolo é de importância crítica para a potencial secagem das zonas húmidas, como resultado da mineração. A aproximação dos trabalhos mineiros causará um impacte significativo sobre o balanço hídrico no pântano Selisoo, mas apenas existirão impactes adicionais menores da mineração diretamente por baixo do pântano. O maior impacte vai surgir antes dessa fase, quando a extensão subterrânea da mina atingir a fronteira da área de conservação da natureza; uma vez que a restrição de atividades nesta área se relaciona com a superfície do solo, a conservação da fronteira da área não é suficientemente protetora em relação ao desenvolvimento dos trabalhos subterrâneos.

Determination of electromagnetic wave velocity in horizontally layered sedimentary target: A ground-penetrating radar study from Silurian limestones, Estonia

Ground-penetrating radar (GPR) is a non-destructive geophysical technique to obtain information about shallow subsurface by transmitting electromagnetic waves into the ground and registering signals reflected from objects or layers with different dielectric properties. The present GPR study was conducted in Võhmuta limestone quarry in Estonia in order to describe the relationship between GRP responses to the variations in petrophysical properties. Sub-horizontally oriented cores for petrophysical measurements were drilled from the side wall of the quarry. The GPR profiles were run at the sloped trench floor and on the top of side wall in order to correlate traceable reflections with physical properties. Based on three techniques: (i) hyperbola fitting, (ii) wide angle reflection and refraction (WARR), and (iii) topographic, a mean electromagnetic wave velocity value of 9.25 cm ns−1 (corresponding to relative dielectric permittivity of 10.5) was found to describe the sequence and was used for time-to-depth conversion. Examination of radar images against petrophysical properties revealed that major reflections appear in levels where the changes in porosity occur.