Sul carattere delle tempeste ionosferiche osservate a Buenos Aires

Riassunto L'A. espone in forma sintetica i risultati di uno studio sulle caratteristiche delle perturbazioni ionosferiche in Buenos Aires. Durante i mesi invernali la fase positiva delle perturbazioni nella regione F2 appare notevolmente ampia e si nota una stretta correlazione negativa fra la foF2 e la componente orizzontale del campomagnetico terrestre.

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Summary The Author reports synthetically the results of an investigation concerning the characteristics of ionospheric storms at Buenos Aires. In winter, the positive phase of the storms in the F2 region appears considerably large, and a close negative correlation is observed between the foF2 and the horizontal component of the terrestrial magnetic field.

     

Improvement of a hypoplastic model to predict clay behaviour under undrained conditions

A shortcoming of the hypoplastic model for clays proposed by the first author is an incorrect prediction of the initial portion of the undrained stress path, particularly for tests on normally consolidated soils at isotropic stress states. A conceptually simple modification of this model, which overcomes this drawback, is proposed in the contribution. The modified model is applicable to both normally consolidated and overconsolidated soils and predicts the same swept-out-memory states (i.e., normal compression lines) as the original model. At anisotropic stress states and at higher overconsolidation ratios the modified model yields predictions similar to the original model.     

Evidence of rainfall variations in Southern Brazil from trace element ratios (Mg/Ca and Sr/Ca) in a Late Pleistocene stalagmite

Trace element ratios in the Bt2 stalagmite from Botuverá cave, Southern Brazil, are explored as a proxy for changes in the local rainfall recharge during the last 116 ky. BP Mg/Ca and Sr/Ca ratios, measured with an electron microprobe, are significantly positively correlated with one another throughout the entire record, and vary in a way that is very consistent with variations of δ¹⁸O in the same speleothem during the last glacial period. We suggest that prior calcite precipitation in the vadose zone of the cave system is the main factor affecting the incorporation of Mg and Sr into calcite of the stalagmite. This interpretation is supported by trace element correlation patterns and by results from a hydrochemistry study performed in a cave located in the same region and in a similar environmental setting. Therefore, we conclude that higher (lower) Mg/Ca and Sr/Ca values are associated with lower (higher) levels of recharge into the karstic aquifer, as such conditions lead to an increase (decrease) in the volume of calcite precipitated in the unsaturated zone above the cave during dry (wet) climate periods.Trace element variations point to generally dryer (wetter) conditions during lower (high) phases of summer insolation in the southern hemisphere. These periods coincide with decreased (increased) activity of the South American summer monsoon, as revealed by δ¹⁸O stalagmite records. In addition trace element variations show that rather wet conditions persisted throughout most of the last glacial period from approximately 70 to 17 ky BP. We suggest that during this period the glacial boundary conditions, especially ice volume buildup in the northern hemisphere, played an important role for monsoon rainfall intensification in the region.     

The earthquake of 250 a.d. in Augusta Raurica, A real event with a 3D site-effect?

The Roman city Augusta Raurica is located East of Basel, Switzerland. One important topic of the city’s history concerns the hypothesis of an earthquake striking the city in the middle of the third century a.d. This idea had been formulated according to archaeological features and findings, but had not been tested so far. A selection of the archaeological features were reviewed and dated in order to test the hypothesis of a single event. However, archaeological investigations do not draw a conclusive picture; it could not be proven that all features of possible destruction date to the same event. Detailed seismological investigations were performed. These included geological and geotechnical mapping of the unconsolidated sediments. Important parameters such as the thickness and composition of the unconsolidated sediments, the terrain topography and the topography of the bedrock surface were mapped. Ambient vibration H/V measurements provided the fundamental frequency of resonance for the unconsolidated sediments. The velocity of shear waves traveling through sediments is the controlling parameter for amplification of seismic waves. This material property is estimated using the relation between the ellipticity of the fundamental mode Rayleigh wave and the H/V curve. From all information we compiled a three-dimensional model of the surface geology. This model is used to simulate earthquake ground motion and amplification effects in the city, and to map the variability of the amplification. In the part of the city where possible earthquake damage was recognized, amplification occurs in the frequency band of building resonance (2–8 Hz). In the other part of the city amplification occurs much above the building’s resonance. From 1D modelling we estimate a difference in spectral amplification of about a factor of 2.5 to 3 between the two parts of the city. This corresponds approximately to a difference in macroseismic intensity of one unit. 3D modelling showed a large variability of ground motion within very close distance in the part of the city where possible earthquake damage was recognized. The maximum amplification reaches values up to a factor of nine, which is due to 3D effects and the choice of using vertically incident waves. Finally, all paleoseismological findings in the area of Basel were reviewed in order to find indications of a large event in the time-period of interest. Paleoseismological findings provide no hints to a large earthquake in the third century. If we assume that an earthquake caused at least part of the identified damage in Augusta Raurica, we have to assign to this event a magnitude Mw of about 6.0 or even lower, that is much smaller than the value of 6.9 that is actually in the Swiss earthquake catalogue. The earthquake source of this event must then be very close to the site of Augusta Raurica and a strong site-effect occurred in one part of the city.     

Large displacement behaviour of a structural model with foundation uplift under impulsive and earthquake excitations

This paper considers the dynamical behaviour of a structural model with foundation uplift. The equations of motion of the system considered are derived for large displacements thus allowing for the eventual overturning of the system. The transition conditions between successive phases of motion, derived in terms of the specific Lagrangian co‐ordinates used in the formulation of the equations of motion, present innovative aspects which resolve some previously inexplicable behaviour in the structural response reported in the literature. The dynamical behaviour of the model is considered under impulsive and long‐duration ground motions. The minimum horizontal acceleration impulses for the uplift and the overturning of the system are evaluated in analytical form. The sensitivity of the model to uplifting and to overturning under impulsive excitations is established as a function of few significant structural parameters. Numerical applications have been performed changing either the structural parameters or the loading parameter, in order to analyse several dynamical behaviours and also to validate the analytical results. For earthquake ground motions the results, reported in the form of response spectra, show that linearized models generally underestimate, sometimes significantly, the structural response. Copyright © 2003 John Wiley & Sons, Ltd.     

3D Hybrid Ray-FD and DWN-FD Seismic Modeling for Simple Models Containing Complex Local Structures

Hybrid approaches find broad applications wherever all-in-one modelling of source, path, and site effects is too expensive. Our new 3D hybrid approach allows to compute the seismic wavefield in elastic isotropic models containing a complex local structure embedded in a large, but considerably simpler, regional structure. The hybrid modelling is realized in two successive steps.In the 1^st step, the ray or discrete wave number (DWN) method is used to compute the seismic wavefield due to the source and simple regional structure. The complex local structure is not present. Thus, the excitation contains the source and regional path effects. The time history of this wavefield (excitation), recorded at the points of so called excitation box, is stored on a disk. The excitation box envelopes a small portion of a computational domain.The 2^nd step of the hybrid method, now containing the complex local structure, is computed by finite differences (FD) inside the excitation box and its close vicinity. The excitation from the 1^st step is now used to inject the 1^st step wavefield into the 2^nd step computation. After that, the hybrid combination of the 1^st and 2^nd steps contains the source, regional path, and local structure effects at reasonably lower computational costs than in case of all-in-one modelling.The 3D ray-FD method is tested on models in which the locally complex structure is the well-known Volvi lake basin, embedded in various 1D structures. The wavefield is excited by the point source situated outside the basin. Although the structure outside the excitation box may be less dimensional (2D, 1D, homogeneous), the whole problem is actually 3D due to the 3D features of the structure inside the excitation box, 3D shape of the excitation box, and arbitrary source — excitation-box configuration. Simple (1D) structures outside the excitation box allow for comparison with the alternative hybrid DWN-FD results. However the ray method is suitable for computation of 3D regional structures outside the excitation box. The results from both approaches show a very good agreement for realistic crustal and local structural models.     

Fortschritte in der Stratigraphie des mährischen (ostsudetischen) Devons

Zusammenfassung Die vorliegende Arbeit faßt die Ergebnisse neuer Forschungen, die während der letzten zehn Jahre durchgeführt worden sind, zusammen. Im Devon werden drei Haupttypen der Faziesentwicklung unterschieden: die Entwicklung des Mährischen Karstes (Schwellenfazies mit einer Transgression im Givet und vorwiegenden Kalksteinen), die Übergangsentwicklung (Transgression im Oberems, Schiefer im Eifel, ab Givet Kalksteine, Schiefer im Famenne) und die Drahany-Entwicklung (Transgression im Siegen — Unterems, im ganzen vorwiegend Schiefer und Initialvulkanite). Die oberdevonische Schichtenabfolge ist durchweg vollkommen und stellenweise setzt die Kalksedimentation ohne Unterbrechung vom Devon bis zur Pericyclus-Stufe des Unterkarbons fort.Die jungkaledonische Faltung hinterließ im Gebiet des mährischen Devons keine nachweislichen Spuren. Die Devontransgression drang über den älteren Untergrund von N gegen S in der Zeitspanne Unterdevon — Unterfrasne vor. Die unterdevonischen Faunen sind vorwiegend rheinischen Charakters, die eifelischen weisen eine engere Beziehung zur böhmischen (herzynischen) Devonentwicklung auf und bezeugen eine breite Faunenmigrationsmöglichkeit (eine ähnliche Lage ist auch im Falle der wärmeliebenden Flachwasserfaunen vom Givet — Frasne zu beobachten). Im höheren Oberdevon erfolgt, besonders in den Gebieten der Kalkentwicklung, eine Faziesdifferenzierung. Die grundlegende, durch den Auftritt der Kulmfazies bezeichnete Faziesänderung fällt erst ins Unterkarbon. In den nichtmetamorphosierten Devon wurde fast keine bedeutsame Diskordanz zwischen dem Devon und Unterkarbon nachgewiesen; die Faltung beider Komplexe verlief gleichzeitig erst in der sudetischen oder asturischen Phase. Die bretonische Faltungsphase kam wahrscheinlich nur im metamorphen.Devon in der Nähe des Kristallins am O- und NO-Rand des Böhmischen Massivs zustande. Die Metamorphose sowie die tektonischen Störungen des Devons nehmen im allgemeinen von O gegen W zu, was die Bedeutung der variszischen Gebirgsbildung und Metamorphose bei der Gestaltung der Einheiten besonders im NO-Teil des Böhmischen Massivs unterstreicht.

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The present paper summarizes the results of investigations carried out in these ten years. In the Devonian three main types of development have been distinguished: the development of Moravian Karst (shallow-water facies with the marine transgression in Givetian and a prevalence of limestones), the Transition development (with the transgression in Upper Emsian, and shale- [Eifelian], limestone- [since Givetian], shale- [Famennian] sedimentation) and, the Drahany development (the transgression in Siegenian-Lower Emsian, general prevalence of shales and initial volcanics). The sequence of the Upper Devonian is everywhere complete, in places carbonate sedimentation continues without break up to the Pericyclus stage of the Lower Carboniferous.The Late Caledonian folding did not leave any distinct traces in the Moravian Devonian. The Devonian transgression proceeded over the older basement from the N to the S during the time-span of Lower Devonian — Lower Frasnian. The Lower Devonian faunas have prevailingly the Rhinish character, whereas those of Eifelian show a closer relationship to the Bohemian (Hercynian) development, attesting to a possibility of wide migration of fauna (similarly as the shallow-water thermophile Givet-Frasnian faunas). During the Famennian a differenciation of facies takes place, particularly in the areas of limestone development. In the sections paleontologically well evidenced the main facies change expressed by the onset of the Culm facies does not occur before the Lower Carboniferous. In the unmetamorphosed Devonian no unconformity between the Devonian and Lower Carboniferous has anywhere been proved, both the units were folded simultaneously in the Sudetic or Asturian phase. The Bretonian folding can be postulated only for the metamorphosed Devonian complexes occurring near to the marginal crystalline areas in the E and NE of the Bohemian Massif. There, also the facies change (beginning of flysch sedimentation) started probably sooner. The degree of metamorphism and tectonic disturbance generally increases from the E to the W, which emphasizes the significance of Variscan orogeny and metamorphism for the development of geological units, particularly in the north-eastern part of the Bohemian Massif.

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Résumé L'article résume les nouveaux résultats des recherches entreprises au cours des dix années dernières. Dans le Dévonien trois types principaux du développement de faciès furent distingués: celui du Karst de Moravie (faciès géoanticlinal avec transgression pendant le Givétien et la prédominance de calcaires), celui de transition (transgression pendant l'Emsien supérieur, schistes de l'Eifelien, calcaires depuis le Givétien, schistes au Famennien) et le développement de Drahany (transgression pendant le Siegénien — Emsien inférieur, prédominance générale des schistes et des volcanites initiaux). La succession du Dévonien supérieur est partout complète et par endroits la sédimentation des calcaires se poursuit sans lacune depuis le Dévonien jusqu'à l'étage Pericyclus du Carbonifère inférieur. Dans la région du Dévonien de Moravie le jeune plissement Calédonien n'a pas laissé de traces prouvables. La transgression dévonienne avançait sur le socle plus ancien dans la direction nord—sud pendant l'intervalle Dévonien inférieur — Frasnien inférieur. Les faunes du Dévonien inférieur ont pour la plupart un caractère rhénan, les faunes eifeliennes se trouvent en relation plus étroite avec le développement tchèque (hercynien) du Dévonien et prouvent la possibilité d'une migration de faunes étendue (une situation analogue existe chez les faunes néritiques thermophiles giveto-frasniennes). Dans le Famennien il se produit, surtout dans les régions du développement calcaire, une différentation de faciès. Le changement des faciès principaux, marqué par le début du faciès culmien, appartient, dans les profils mieux vérifiés du point de vue paléontologique, jusqu'au Carbonifère inférieur. Dans le Dévonien non métamorphosé une discordance entre le Dévonien et le Carbonifère inférieur ne fut prouvée nulle part; le plissement simultané des deux formation n'avait lieu que dans la phase sudétienne ou asturienne. Le plissement de la phase bretonienne se produisait probablement seulement au Dévonien métamorphisé à la proximité du Cristallin à la périphérie est ou sud-est du Massif de Bohême, où même le changement de faciès (commencement de la sédimentation du flysch) avait probablement commencé plus tôt. Le métamorphisme et les accidents tectoniques du Dévonien s'accroissent en général dans la direction est—ouest, ce qui souligne la signification de l'orogénèse et du métamorphisme varisque pendant la formation des unités, surtout dans la partie nord—est du Massif de Bohême.

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Gaussian Processes for history-matching: application to an unconventional gas reservoir

The process of reservoir history-matching is a costly task. Many available history-matching algorithms either fail to perform such a task or they require a large number of simulation runs. To overcome such struggles, we apply the Gaussian Process (GP) modeling technique to approximate the costly objective functions and to expedite finding the global optima. A GP model is a proxy, which is employed to model the input-output relationships by assuming a multi-Gaussian distribution on the output values. An infill criterion is used in conjunction with a GP model to help sequentially add the samples with potentially lower outputs. The IC fault model is used to compare the efficiency of GP-based optimization method with other typical optimization methods for minimizing the objective function. In this paper, we present the applicability of using a GP modeling approach for reservoir history-matching problems, which is exemplified by numerical analysis of production data from a horizontal multi-stage fractured tight gas condensate well. The results for the case that is studied here show a quick convergence to the lowest objective values in less than 100 simulations for this 20-dimensional problem. This amounts to an almost 10 times faster performance compared to the Differential Evolution (DE) algorithm that is also known to be a powerful optimization technique. The sensitivities are conducted to explain the performance of the GP-based optimization technique with various correlation functions.     

The origin of shallow landslides in Moravia (Czech Republic) in the spring of 2006

At the end of March 2006, the Czech Republic (CZ) witnessed a fast thawing of an unusually thick snow cover in conjunction with massive rainfall. Most watercourses suffered floods, and more than 90 shallow landslides occurred in the Moravian region of Eastern CZ, primarily in non-forested areas. This region, geologically part of the Outer Western Carpathians, is prone to landslides because the bedrock is highly erodible Mesozoic and Tertiary flysch.The available meteorological data (depth of snow, water equivalent of the snow, cumulative rainfall, air and soil temperatures) from five local weather stations were used to construct indices quantitatively describing the snow thaw. Among these, the Total Cumulative Precipitation (TCP) combines the amount of water from both thawing snow and rainfall. This concurrence of rain and runoff from snow melt was the decisive factor in triggering the landslides in the spring.The TCP index was applied to data of snow thaw periods for the last 20 years, when no landslides were recorded. This was to establish the safe threshold of TCP without landslides. The calculated safe threshold value for the region is ca. 100 mm of water delivered to the soil during the spring thaw (corresponding to ca. 11 mm day^− 1). In 2006, 10% of the landslides occurred under or at 100 mm of TCP. The upper value of 155 mm covered all of the landslides.