Weak-contrast approximation of the elastic scattering matrix in anisotropic media     

Bjørn Ursin  Geir U. Haugen 《Pure and Applied Geophysics》1996,148(3-4):685-714

The plane-wave reflection and transmission coefficients at a plane interface between two anisotropic media constitute the elements of the elastic scattering matrix. For a 1-D anisotropic medium the eigenvector decomposition of the system matrix of the transformed elasto-dynamic equations is used to derive a general expression for the scattering matrix. Depending on the normalization of the eigenvectors, the expressions give scattering coefficients for amplitudes or for vertical energy flux.Computing the vertical slownesses and the corresponding polarizations, the eigenvector matrix and its inverse can be found. We give a simple formula for the inverse, regardless of the normalization of the eigenvectors. When the eigenvectors are normalized with respect to amplitudes of displacement (or velocity), the calculation of the scattering matrix for amplitudes is simplified.When the relative changes in all parameters are small, a weak-contrast approximation of the scattering matrix, based on the exactly determined polarization vectors in an average medium, is obtained. The same approximation is also derived directly from the transformed elasto-dynamic equations for a smooth vertically inhomogeneous medium, proving the consistency of the approximation.For monoclinic media, with the mirror symmetry plane parallel to the interface, the approximative scattering matrix is given in terms of analytic expressions for the non-normalized eigenvectors and vertical slownesses. For transversely isotropic media with a vertical axis of symmetry (VTI) and isotropic media, explicit solutions for the weak-contrast approximations of the scattering matrices have been obtained. The scattering matrix for amplitudes for isotropic media is well known. The scattering matrix for vertical energy flux may have applications in AVO analysis and inversion due to the reciprocity of the reflection coefficients for converted waves.Numerical examples for monoclinic and VTI media provide good agreement between the approximative and the exact reflection matrices. It is, however, expected that the approximations cannot be used when the symmetry properties of the two media are very different. This is because the approximation relies on a small relative contrast between the eigenvectors in the two media.Presented at the Workshop Meeting on Seismic Waves in Laterally Inhomogeneous Media, Castle of Trest, Czech Republic, May 22–27, 1995.  相似文献   

Palaeoclimatic studies in South Shetland Islands,Antarctica, based on numerous stratigraphic variables in lake sediments     

Svante Björck  Hannelore Håkansson  Siv Olsson  Lena Barnekow  Jan Janssens 《Journal of Paleolimnology》1993,8(3):233-272

The hitherto longest found lake sediment sequence on Byers Peninsula, Livingston Island, South Shetland Islands, was analysed with respect to lithology, chronology, diatoms, Pediastrum, pollen and spores, mosses, mineralogy, and sediment chemistry. During the ca. 5000 year long development the sediments were influenced by frequent tephra fall-outs. This volcanic impact played a major role in the lake's history during two periods, 4700–4600 and 2800–2500 BP, but was of importance during the lake's entire history with considerable influence on many of the palaeoenvironmentally significant indicators. The large and complex data set was analysed and zonated with different types of multivariate analysis. This resulted in a subdivision of the sequence into 8 time periods and 21 variables. Redundancy analysis (RDA) of this data set, both without and with the tephra periods, and with 4–6 of the variables as explanatory environmental variables, reveal that climatic/environmental signals are detectable. The palaeoclimatic picture that emerged out of the tephra noise suggests that the first 100 years were characterized by mild, humid conditions. This was followed by a less mild and humid climate until ca. 4000 BP when a gradual warming seems to have started, coupled with increased humidity. These mild and humid conditions seem to have reached an optimum slightly after 3000 BP. At ca. 2500 BP a distinct climatic deterioration occurred with colder and drier conditions and long seasons with ice cover. This arid, cold phase probably reached its optimum conditions at ca. 1500 BP, when slightly warmer conditions might have prevailed for a while. Except for the modern sample with rather mild climate, the last 1400 years seem to have been fairly arid and cold, and the effects of the frequent volcanic activity during this period is only vaguely seen in the records.  相似文献   

Solar noise storms coordinated observations: May 16–24, 1981     

C. Mercier  Ø. Elgarøy  A. Tlamicha  P. Zlobec 《Solar physics》1984,92(1-2):375-381

We present the main results obtained from the coordinated observations of solar radio noise storms organized by C.E.S.R.A. in May 1981. They concern the structure and polarization of radio sources, and the relation with the photospheric magnetic field. A model of coronal magnetic field, accounting for the observations, is briefly discussed.  相似文献   

Structure and oscillations in quiescent filaments from observations in He i λ10830 å     

Yi Zhang  Oddbjørn Engvold  Stephen L. Keil 《Solar physics》1991,132(1):63-80

Observations of two quiescent filaments show oscillatory variations in Doppler shift and central intensity of the He i 10830 Å line.The oscillatory periods range from about 5 to 15 min, with dominant periods of 5, 9, and 16 min. The 5-min period is also detected in the intensity variations, after correction for atmospheric effects. Doppler shifts precede intensity variations by about one period. The possibility that the oscillations are Alfvén waves is discussed.The Doppler signals of the filament form fibril-like structures. The fibrils are all inclined at an angle of about 25° to the long axis of the filament. The magnetic field has a similar orientation relative to the major direction of the filament, and the measured Doppler signals are apparently produced by motions along magnetic flux tubes threading the filament.The measured lifetimes of the small-scale fibrils of quiescent disk filaments are very likely a combined effect of intensity modulations and reshuffling of the structures.  相似文献   

Geophysical and geochemical evidence of Proterozoic collision in the western marginal zone of the Baltic Shield     

Josef A. Zuber  Björn Öhlander 《International Journal of Earth Sciences》1990,79(1):1-11

New continental crust was formed in the Svecofennian domain of the Baltic Shield c. 1.9 Ga ago. Approximately 0.1–0.15 Ga later, new crust accreted to the SW part of the Shield. In this paper an attempt is made, on the basis of gravity measurements and lithogeochemistry, to describe the tectonic processes responsible for the continental growth c. 1.75–1.8 Ga ago. The Transscandinavian Granite Porphyry Belt (TGPB) separates the Svecofennian domain from the polymetamorphic terrain of the SW Swedish gneiss region. Red orthogneisses occurring immediately west of the TGPB are the deformed equivalents of the TGPB type granitoids, while grey orthogneisses, displaying a tonalitic-granodioritic trend and situated further west, were generated in a »volcanic arc« environment. The TGPB granitoids and the red SW Swedish gneisses represent a transition from this volcanic arc type rock to contemporaneous »within-plate« type granites intruded in the Svecofennian crust. The volcanic arc was forced against the Svecofennian crust in which large tensional fracture zones ensued with strike directions normal to the collision front. In such tensional environments the »withinplate« type granites were generated. In the collision zone the crust was down-warped, and huge amounts of granitic melts were generated at the base of the crust. This TGPB Magma rose upwards utilizing the fracture zone between the arc rocks, generated slightly earlier, and the Svecofennian crust. A relatively thin upper part of the TGPB that spread laterally westwards became strongly deformed during the collision (i.e. the red SW Swedish gneisses), while the major deep-reaching TGPB root zone that was not completely solidified yet, acted as a buffer against the foliation front.

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Zusammenfassung Vor 1,9 Milliarden Jahren kam es zur Neubildung von kontinentaler Kruste im svecofennischen Bereich des Baltischen Schildes. Ungefähr 100–150 Millionen Jahre später wurde im Südwesten des Schildes neue Kruste hinzugefügt. In diesem Artikel wird auf der Basis von Gravimetriemessungen und Lithogeochemie der Versuch unternommen die tektomschen Vorgänge, die zu diesem 1,75–1,8 Milliarden Jahre alten Krustenzuwachs führten, zu beschreiben.Der Transskandinavische-Granit-Porphyr-Gürtel (Transscandinavian-Granite-Porphyry-Belt/TGPB) trennt das Svecofennium von der polymetamorphen, im Südwesten Schwedens gelegenen Gneis-Region. Ein direkt westlich des TGPB gelegenes Vorkommen roter Orthogneise entspricht den deformierten TGPB Granitoiden. Graue Orthogneise, die weiter im Westen aufgeschlossen sind, zeigen eine mehr tonalitische bis granodioritische Zusammensetzung und werden auf einen vulkanischen Inselbogen zurückgeführt. Die TGPB Granitoide und die roten südwest-schwedischen Gneise stellen einen Übergang von den Inselbogen-Vulkaniten zu den zeitgleichen »Intra-Platten-Graniten« der svecofennischen Kruste dar. Der Inselbogen kollidierte mit der svecofennischen Kruste, es entstanden großräumige Bruchzonen mit Streichrichtungen senkrecht zur Kollisionsebene. Während des Zustands der hohen Druckspannung des Gebietes intrudierten die »Intra-Platten-Granite«. Innerhalb des Kollisionsbereiches wurde die Kruste nach unten gebogen, und so entstanden an der Basis der Kruste große Mengen granitischen Magmas. Dieses TGPB Magma stieg entlang der Störungszone innerhalb der Inselbogengesteine, die nur wenig älter sind, und der svecofennischen Kruste, auf. Nur ein, von relativ geringer Mächtigkeit, weiter westlich gelegener Teil des TGPB, die roten südwest-schwedischen Gneise, wurde während der Kollision intensiv deformiert. Dagegen war der Hauptanteil der tiefreichenden TGPB Wurzelzone noch nicht vollständig erstarrt und wirkte deshalb wie eine Pufferzone gegen die Schieferungsfront.

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Résumé De la croûte continentale nouvelle s'est formée il y a 1,9 Ga dans le domaine des Svecofennides (Bouclier baltique). Environ 100 à 150 Ma plus tard, de la croûte nouvelle s'est accrétionnée à la bordure sud-ouest du bouclier. Cette note basée sur des mesures de gravité et la lithogéochimie, présente un essai d'analyse des processus tectoniques responsables de cette croissance continentale d'âge 1,75 à 1,8 Ga. Le «Transcandinavian Granite Porphygry Belt» (TGPB) sépare le domaine svécofennien des gneiss polymétamorphiques du sud-ouest de la Suède. Immédiatement à l'ouest de TGPB affleurent des orthogneiss rouges qui représentent l'équivalent déformé de granitoïdes du TGPB, tandis que des orthogneiss gris de tendance tonalitique-granodioritique, situés plus à l'ouest, ont été engendrées dans un environnement d'arc volcanique. Les granitoïdes du TGPB et les gneiss rouges du sud-ouest de la Suède représentent une transition entre ces produits d'arc volcanique et les granites intra-plaque de même âge intrudés dans la croûte svécofennienne. L'arc volcanique a été accrétionné à la croûte svécofennienne avec production dans celleci de grandes fractures d'extension perpendiculaires au front de collision. C'est dans ce domaine en extension que les granites intra-plaque se sont mis en place. Dans la zone de collision, la croûte s'est incurvée vers le bas et de grandes quantités de liquides granitiques ont été engendrées à la base de la croûte. Ces magmas TGPB sont montés à la faveur de la zone fracturée entre les roches de l'arc engendrée un peu plus tôt, et la croûte svécofennienne. Seule une fraction supérieure relativement mince du TGPB, développée vers l'ouest, a subi une déformation importante au cours de la collision, pour former les gneiss rouges du sud-ouest de la Suède; par contre, la partie principale de la racine profonde du TGPB, qui n'était pas encore entièrement solidifiée, a joné le role tampon en avant du front de foliation.

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, 1,9 100–150 - . - (Transscandinavian Granite-Porphyry-Belt - TGPB) , - . TGPB , , , - , . TGPB - . , , . («within plate» type granites) , . . TGPB , , . TGPB, , - , . TGPB, , .

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The dependence of the frequency spectra of the interplanetary magnetic-field fluctuations on the direction of the mean field     

Jörn E. Kunstmann 《Earth, Moon, and Planets》1978,18(1):91-104

The frequency spectra of the interplanetary magnetic field fluctuations are the projection of their wavenumber spectra onto one dimension. Only the frequency spectra can be measured by spacecrafts. It is studied how their measured size depends on the direction of the mean fieldB ₀, which structures the symmetry of the fluctuations relative to the solar wind system. It is specialized for the slab model, Alfvén waves, magneto-acoustic waves and the isotropic case. For the slab model the frequency spectra are proportional to , whereq is the spectral index and the angle betweenB ₀ and the radial direction. For the diffusion coefficientK _TT the relation holds.  相似文献   

Distribution of lunar impacts by objects in parabolic low inclination orbits     

Nils Aall Barricelli  Ragnar Thorbjørnsen 《Earth, Moon, and Planets》1978,18(4):479-490

This paper presents a computer investigation extending to the case of parabolic orbits, an earlier investigation conducted by Barricelli and Metcalfe (1969) on lunar impacts by external low eccentricity satellites as a means to interpret the asymmetric distribution of lunar maria. Parabolic orbits can be approximated by two kinds of objects:

1. High eccentricity external satellites may, near periapsis, approach the Moon with orbital velocity and other characteristics closely resembling those of a parabolic orbit.
2. Asteroids and meteoroids approaching the Earth-Moon system with a low velocity may have moved in a nearly parabolic orbit when they reached the lunar distance from the Earth at the time when the impacts which carved the lunar maria took place.

The investigation gives, therefore, not only additional information relevant to the interpretation of the distribution of lunar maria by the satellite impacts hypothesis (in this case high eccentricity ones), but also information about the alternative hypothesis (Wood, 1973) that asteroid impacts rather than satellite impacts were involved.  相似文献   

The fine structure of prominences     

Oddbjørn Engvold 《Solar physics》1976,49(2):283-295

The fine structure of nonspot prominences are studied from H filtergrams. The size of the smallest prominence structures increases with height above the chromosphere. Some prominences contain structures close to 1/2 arc second, which is the spatial resolution in the present data. The effective thickness of many nonspot prominences ranges between 4 × 10⁷ cm and 1.5 × 10⁸ cm. An apparent downward directed motion is observed in the majority of the prominences. No preferred direction of the motion is seen in regions composed of comparatively large diffuse structures. Some bright threads are visible for 1 hr and longer. Bright knots have an average observed lifetime of about 8 min. The process of condensation and subsequent destruction of prominence fine structure appears to take place on a very short time scale compared to the life time of the regions where prominences may exist. The observed H brightness of the prominences in the present data may be accounted for as scattered chromospheric radiation.  相似文献   

Late Holocene sea-level changes and isostasy in western Denmark     

W. Roland Gehrels  Katie Szkornik  Jason R. Kirby  William A. Marshall  Jørn B.T. Pedersen 《Quaternary Research》2006,66(2):288-302

Cores and exposed cliff sections in salt marshes around Ho Bugt, a tidal embayment in the northernmost part of the Danish Wadden Sea, were subjected to ¹⁴C dating and litho- and biostratigraphical analyses to reconstruct paleoenvironmental changes and to establish a late Holocene relative sea-level history. Four stages in the late Holocene development of Ho Bugt can be identified: (1) groundwater-table rise and growth of basal peat (from at least 2300 BC to AD 0); (2) salt-marsh formation (0 to AD 250); (3) a freshening phase (AD 250 to AD 1600?), culminating in the drying out of the marshes and producing a distinct black horizon followed by an aeolian phase with sand deposition; and (4) renewed salt-marsh deposition (AD 1600? to present). From 16 calibrated AMS radiocarbon ages on fossil plant fragments and 4 calibrated conventional radiocarbon ages on peat, we reconstructed a local relative sea-level history that shows a steady sea-level rise of 4 m since 4000 cal yr BP. Contrary to suggestions made in the literature, the relative sea-level record of Ho Bugt does not contain a late Holocene highstand. Relative sea-level changes at Ho Bugt are controlled by glacio-isostatic subsidence and can be duplicated by a glacial isostatic adjustment model in which no water is added to the world's oceans after ca. 5000 cal yr BP.  相似文献   

T-distributed Random Fields: A Parametric Model for Heavy-tailedWell-log Data1     

Jo Røislien  Henning Omre 《Mathematical Geosciences》2006,38(7):821-849

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