Impact structures and events - a Nordic perspective     

Henning Dypvik Jueri Plado Claus Heinberg Eckart Hakansson Lauri J. Pesonen Birger Schmitz Selen Raiskila 《《幕》》2008,31(1):107-114

Impact cratering is one of the fundamental processes in the formation of the Earth and our planetary system, as reflected, for example in the surfaces of Mars and the Moon. The Earth has been covered by a comparable number of impact scars, but due to active geological processes, weathering, sea floor spreading etc, the number of preserved and recognized impact craters on the Earth are limited. The study of impact structures is consequently of great importance in our understanding of the formation of the Earth and the planets, and one way we directly, on the Earth, can study planetary geology.
The Nordic-Baltic area have about thirty confirmed impact structures which makes it one of the most densely crater-populated terrains on Earth. The high density of identified craters is due to the level of research activity, coupled with a deterministic view of what we look for. In spite of these results, many Nordic structures are poorly understood due to the lack of 3D-geophysical interpretations, isotopeor other dating efforts and better knowledge of the amount of erosion and subsequent tectonic modifications.
The Nordic and Baltic impact community is closely collaborating in several impact-related projects and the many researchers （about forty） and PhD students （some seventeen） promise that this level will continue for many more years. The main topics of research include geological, geophysical and geochemical studies in combination with modeling and impact experiments. Moreover, the Nordic and Baltic crust contains some hundred suspect structures which call for detailed analysis to define their origin.
New advanced methods of analyzing geophysical information in combination with detailed geochemical analyses and numerical modeling will be the future basic occupation of the impact scientists of the region. The unique Cretaceous/Tertiary boundary （K-T） occurrences in Denmark form an important source of information in explaining one of the major mass extinctions on Earth.  相似文献   

On the mean zonal and meridional circulations and the flux of moisture in the northern hemisphere during the summer season   总被引：6，自引：0，他引：6  

Lauri A. Vuorela  Ilkka Tuominen 《Pure and Applied Geophysics》1964,57(1):167-180

Summary The mean zonal and meridional wind components of the northern hemisphere at different pressure levels for the summer season June–August have been determined and the mean meridional mass circulation has been computed as a function of latitude. From the mass circulation the meridional flux of moisture is computed for the latitudinal belt 0°–45° N. Using the horizontal divergence of this flux the average difference between precipitation and evapotranspiration from the earth's surface is evaluated.  相似文献   

Radiometer studies of low-salinity sea ice     

Martti Tiuri  Martti Hallikainen  Antti Lääperi 《Boundary-Layer Meteorology》1978,13(1-4):361-371

Theoretical and experimental studies of ultra-high-frequency (UHF) and microwave radiometry to sense remotely the thickness and other characteristics of low-salinity (less than one per mille) sea ice are described. The experimental studies used a 600-MHz and a multichannel, multibeam, 4.7-GHz radiometer carried by a helicopter to measure the brightness temperature of different types of sea ice in the Gulf of Bothnia in 1975 and 1976.The developed theory and experiments show that the brightness temperature of low-salinity sea ice oscillates as a function of ice thickness at UHF frequencies. Due to higher attenuation, the oscillations quickly die away at 4.7 GHz and the brightness temperature reaches an almost constant value when the thickness exceeds a few decimetres. By using a three-channel UHF radiometer with suitably selected center frequencies, the oscillations can be reduced and the effective brightness temperature will grow more linearly with the ice thickness.Experiments show that ice ridges appear as thick ice at 600 MHz and as thinner ice at 4.7 GHz, thus allowing them to be detected by radio techniques.  相似文献   

Geometric test field calibration of digital photogrammetric sensors   总被引：1，自引：0，他引：1  

Eija Honkavaara  Eero Ahokas  Juha Hyypp  Juha Jaakkola  Harri Kaartinen  Risto Kuittinen  Lauri Markelin  Kimmo Nurminen 《ISPRS Journal of Photogrammetry and Remote Sensing》2006,60(6):387

Test field system calibration will be a fundamental part of the future photogrammetric production line. Accurate calibration and performance evaluations are necessary for fully assessing the stability and accuracy of digital sensing techniques. In this paper, a method of comprehensive geometric calibration in a test field has been developed and empirically tested using eight image blocks collected with three UltraCamD digital large format photogrammetric cameras. Permanent photogrammetric test fields form the basis of the method. Important components of the method are determination of system parameters, evaluation of systematic errors, and assessment of geometric accuracy. The results showed that UltraCamD images contained systematic deformations that could not be modeled with single lens additional parameter models. Good point determination accuracy was obtained despite the systematic errors; the typical accuracy was 2–3 μm in image space in the horizontal coordinates and 0.05–0.09‰ of the object distance in height. One of the cameras had significantly poorer performance. In the worst cases, the horizontal accuracy was 5 μm in image space and the height accuracy was 0.18‰ of the object distance. The analog cameras gave better results than the UltraCamD, but the development of appropriate mathematical models for UltraCamD as well as improvements in digital sensors may change the situation in the near future.  相似文献   

Footprint Evaluation for Flux and Concentration Measurements for an Urban-Like Canopy with Coupled Lagrangian Stochastic and Large-Eddy Simulation Models   总被引：1，自引：1，他引：0  

Antti Hellsten  Sofia-M. Luukkonen  Gerald Steinfeld  Farah Kanani-Sühring  Tiina Markkanen  Leena Järvi  Juha Lento  Timo Vesala  Siegfried Raasch 《Boundary-Layer Meteorology》2015,157(2):191-217

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Magnetic classification of stony meteorites: 1. Ordinary chondrites     

Pierre ROCHETTE  Leonardo SAGNOTTI  Michle BOUROT‐DENISE  Guy CONSOLMAGNO  Luigi FOLCO  Jrme GATTACCECA  Maria Luisa OSETE  Lauri PESONEN 《Meteoritics & planetary science》2003,38(2):251-268

Abstract— We present a database of magnetic susceptibility measurements on 971 ordinary chondrites. It demonstrates that this parameter can be successfully used to characterize and classify ordinary chondrite meteorites. In ordinary chondrites, this rapid and non‐destructive measurement essentially determines the amount of metal in the sample, which occurs in a very narrow range for each chondrite class (though terrestrial weathering can result in a variable decrease in susceptibility, especially in finds). This technique is particularly useful not only for a rapid classification of new meteorites, but also as a check against curation errors in large collections (i.e., unweathered meteorites, the measured susceptibility of which lies outside the expected range, may well be misclassified or misidentified samples). Magnetic remanence, related to magnetic field measurements around asteroids, is also discussed.  相似文献   

Rock magnetic and paleomagnetic study of the Keurusselkä impact structure,central Finland     

Selen RAISKILA  Johanna SALMINEN  Tiiu ELBRA  Lauri J. PESONEN 《Meteoritics & planetary science》2011,46(11):1670-1687

Abstract– There are 31 proven impact structures in Fennoscandia—one of the most densely crater‐populated areas of the Earth. The recently discovered Keurusselkä impact structure (62°08′ N, 24°37′ E) is located within the Central Finland Granitoid Complex, which formed 1890–1860 Ma ago during the Svecofennian orogeny. It is a deeply eroded complex crater that yields in situ shatter cones with evidence of shock metamorphism, e.g., planar deformation features in quartz. New petrophysical and rock magnetic results of shocked and unshocked target rocks of various lithologies combined with paleomagnetic studies are presented. The suggested central uplift with shatter cones is characterized by increased magnetization and susceptibility. The presence of magnetite and pyrrhotite was observed as carriers for the remanent magnetization. Four different remanent magnetization directions were isolated: (1) a characteristic Svecofennian target rock component A with a mean direction of D = 334.8°, I = 45.6°, α₉₅ = 14.9° yielding a pole (Plat = 51.1°, Plon = 241.9°, A₉₅ = 15.1°), (2) component B, D = 42.4°, I = 64.1°, α₉₅ = 8.4° yielding a pole (Plat = 61.0°, Plon = 129.1°, A₉₅ = 10.6°), (3) component C (D = 159.5°, I = 65.4°, α₉₅ = 10.7°) yielding a pole (Plat = 21.0°, Plon = 39.3°, A₉₅ = 15.6°), and (4) component E (D = 275.5°, I = 62.0°, α₉₅ = 14.4°) yielding a pole (Plat = 39.7°, Plon = 314.3°, A₉₅ = 19.7°). Components C and E are considered much younger, possibly Neoproterozoic overprints, compared with the components A and B. The pole of component B corresponds with the 1120 Ma pole of Salla diabase dyke and is in agreement with the ⁴⁰Ar/³⁹Ar age of 1140 Ma from a pseudotachylitic breccia vein in a central part of the structure. Therefore, component B could be related to the impact, and thus represent the impact age.  相似文献   

Asteroid taxonomic signatures from photometric phase curves     

Dagmara Anna Oszkiewicz  Edward Bowell  L.H. Wasserman  Karri Muinonen  Antti Penttilä  Tuomo Pieniluoma  David E. Trilling  Cristina A. Thomas 《Icarus》2012,219(1):283-296

We explore the correlation between an asteroid’s taxonomy and photometric phase curve using the H, G₁₂ photometric phase function, with the shape of the phase function described by the single parameter G₁₂. We explore the usability of G₁₂ in taxonomic classification for individual objects, asteroid families, and dynamical groups. We conclude that the mean values of G₁₂ for the considered taxonomic complexes are statistically different, and also discuss the overall shape of the G₁₂ distribution for each taxonomic complex. Based on the values of G₁₂ for about half a million asteroids, we compute the probabilities of C, S, and X complex membership for each asteroid. For an individual asteroid, these probabilities are rather evenly distributed over all of the complexes, thus preventing meaningful classification. We then present and discuss the G₁₂ distributions for asteroid families, and predict the taxonomic complex preponderance for asteroid families given the distribution of G₁₂ in each family. For certain asteroid families, the probabilistic prediction of taxonomic complex preponderance can clearly be made. In particular, the C complex preponderant families are the easiest to detect, the Dora and Themis families being prime examples of such families. We continue by presenting the G₁₂-based distribution of taxonomic complexes throughout the main asteroid belt in the proper element phase space. The Nysa–Polana family shows two distinct regions in the proper element space with different G₁₂ values dominating in each region. We conclude that the G₁₂-based probabilistic distribution of taxonomic complexes through the main belt agrees with the general view of C complex asteroid proportion increasing towards the outer belt. We conclude that the G₁₂ photometric parameter cannot be used in determining taxonomic complex for individual asteroids, but it can be utilized in the statistical treatment of asteroid families and different regions of the main asteroid belt.  相似文献   

Causes, effects and control of seasonal frost action in railways     

Antti Nurmikolu  Mika Silvast 《寒旱区科学》2013,5(4):0363-0367

Seasonally cold climate and resulting frost action set great demands to railway track substructure in order to maintain track geometry. Challenges culminate on high-speed lines, where the tolerances for roughness are the tightest. Problems may result in highly increased track maintenance and need for temporary speed restrictions. The causes of frost action can be associated with subsoil, subballast or ballast. The major concern in frost protection is to avoid the freezing of frost susceptible subsoil by using sufficient thickness of subballast and relying on non-frost-susceptible subballast material. This paper provides an overview of the main research findings on the role of ballast, subballast and subsoil in frost action. In new construction the material specifications, design procedures and construction methods have been developed to ensure adequate performance of track substructures, but special challenges exist in managing existing tracks that were not designed for modern requirements. In order to perform cost-effective and sustainable track maintenance, it is necessary to recognize the problem areas and define the root-causes of problems. For locating the problem sections and defining the causes of defects, a sophisticated analysis based on integration of track geometry and ground penetrating radar (GPR) data has been developed and is summarized in this paper.  相似文献   

Does the zooplankton prey availability limit the larval habitats of pike in the Baltic Sea?     

Meri Kallasvuo  Maiju Salonen  Antti Lappalainen 《Estuarine, Coastal and Shelf Science》2010

The objectives of this study were to (1) investigate whether the availability of suitable zooplankton prey limits the distribution of the coastal larval areas of pike (Esox lucius) in two archipelago areas of the northern Baltic Sea and (2) compare the availability of zooplankton prey in spring between different types of coastal littoral habitat. According to the results, reed belt habitats formed by Phragmites australis constitute hot spots for zooplankton prey in the coastal ecosystem. During the spring, reed-covered shores of the inner archipelago maintained more than 10 times higher densities of copepods and cladocerans, the preferred prey for larval pike, compared to the other studied shores. Temperature conditions were also most favourable in the reed belt habitat. Thus, the reed belts of the inner and middle archipelago were shown to form the best habitat for larval pike in the coastal area of the northern Baltic Sea, and this was also the only habitat where pike larvae were found. Our results suggest that the poor survival and recruitment of pike in the outer archipelago, however, cannot exclusively be explained by sub-optimal feeding conditions of the larvae. There are also other important factors, presumably connected to the exposure to the open sea, that affect the distribution of the pike larvae. Our results, however, highlight the importance of sheltered coastal reed belt shores as reproduction habitat for spring-spawning fish in the northern Baltic Sea. Further, this study disproves the assumption that the seaweed bladder wrack (Fucus vesiculosus) forms a reproduction habitat for pike in the coastal area.  相似文献