A frequency and velocity-dependent impedance method for prediction of rail/foundation dynamics

This paper presents an efficient numerical tool for the prediction of railway dynamic response.A behavior calibration of the infinite Euler-Bernoulli beam resting on continuous viscoelastic foundation is proposed.Constitutive laws of the discrete elements are determined for a rectilinear ballasted track.A three-dimensional model coupled with an adaptive meshing scheme is employed to calibrate the beam model impedances by finding the similarity between the output signals using the genetic algorithm.The model shows an important performance with significant reduction in computational effort.This study emphasizes the major impact of the excitation characteristics on the parameters of the discrete models.     

Origin of spheroidal chert nodules, Drunka Formation (Lower Eocene), Egypt

Chert nodules of the Drunka Formation (Lower Eocene) are mostly spherical, have diameters from 40 to 120 cm, are quasi-uniformly spaced 2–3 m apart in the plane of bedding, have concentric internal structure and, except for rare small (<6 cm) solid chert nodules, are less than 85% chertified. Nodules formed after moderate alteration of limestone by meteoric water (δ¹⁸O_calcite = –4 to –8‰) at shallow (<100 m) burial depths; more extensive alteration of limestone (δ¹⁸O = –10 to –16‰) by meteoric water followed nodule growth. Chertification was by low-temperature meteoric water (δ¹⁸O_quartz = +18‰ in margins to +22‰ in nuclei) at shallow burial depths. Meteoric water may have invaded the Drunka Formation in association with shelf progradation during the Early Eocene, or during the development of a Middle Eocene unconformity. Replacement of carbonate mud by microcrystalline quartz was the dominant chertification process, but fossils were replaced in part by fine-grained equant megaquartz, quartzine and chalcedony; the last of these occurs in places as beekite. Opal A-secreting marine organisms are the inferred source of silica, but none are preserved. There is no compelling evidence of an opal-CT precursor, so quartz may have formed by direct precipitation. Self-organization processes of enigmatic character established the spacing pattern of the nodules and also the Liesegang-banded internal structure of the chert nodules. Nodules grew chiefly by diffusive supply of silica, although one locality has elongate nodules that grew when there was some porewater advection. Chertification patterns and δ¹⁸O values of both calcite and quartz indicate that nodule growth was complex and variable. Some nodules probably grew from the centre outwards. Many nodules, however, initially grew simultaneously across the entire nodule, but late-stage growth was predominantly at the outer margins or at selective internal sites.     

Preliminary aspects of rainfall rates in the north east of Spain

Summary A series of charts of rainfall rates recorded by means of a Jardí pluviograph in Barcelona, Spain, and covering a period of 20 years have been digitised. The time intervals of exceedance of certain pre-set rainfall rate thresholds were computed and expressed in percentages of the whole 20-year period. The average worst month period and the absolute worst month were defined and computed. Results are found to agree well with those obtained elsewhere in Southern Europe; they are also compared with some published results for Kew. Frequency-distribution of rates as well as time of occurrence are presented. Rainfall amounts pertaining to several rainfall rate categories have been analysed in order to obtain the ratio of convective to total rainfall amount. Comparison of this ratio with results of some other authors is inconclusive.

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Zusammenfassung Eine Anzahl von Tabellen über Niederschlagsraten, die in Barcelona (Spanien) durch einen Jardí Niederschlagsmesser über eine Zeitspanne von 20 Jahren aufgezeichnet wurden, werden bearbeitet. Die Andauer von gewissen festliegenden Niederschlagsraten wurde berechnet und in Prozenten von der gesamten 20jährigen Zeitspanne ausgedrückt. Die durchschnittliche Dauer im schlechtesten Monat, sowie der absolut schlechteste Monat werden definiert und berechnet. Die Ergebnisse stimmen im allgemeinen mit solchen anderer südeuropäischer Gebiete überein; sie werden mit schon veröffentlichten Ergebnissen aus Kew verglichen. Häufigkeits- als auch Verteilungsraten sowie die Eintrittszeit der Ereignisse werden angegeben. Die Niederschlagsmengen, die jeweils zu verschiedenen Niederschlagsraten gehören, wurden analysiert, um das Verhältnis des konvektiven zum gesamten Niederschlag zu ermitteln. Ein Vergleich dieses Verhältnisses mit den Ergebnissen anderer Autoren sind noch nicht abgeschlossen.

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With 7 Figures     

Interannual variability of cut-off low systems over the European sector: The role of blocking and the Northern Hemisphere circulation modes

Summary An earlier developed multidecadal database of Northern Hemisphere cut-off low systems (COLs), covering a 41 years period (from 1958 to 1998) is used to study COLs interannual variability in the European sector (25°–47.5° N, 50° W–40° E) and the major factors controlling it. The study focus on the influence on COLs interannual variability, of larger scale phenomena such as blocking events and other main circulation modes defined over the Euro-Atlantic region. It is shown that there is a very large interannual variability in the COLs occurrence at the annual and seasonal scales, although without significant trends. The influence of larger scale phenomena is seasonal dependent, with the positive phase of the NAO favoring autumn COL development, while winter COL occurrence is mostly related to blocking events. During summer, the season when more COLs occur, no significant influences were found.     

Stereotomography assisted by migration of attributes

Depth velocity model building remains a difficult step within the seismic depth imaging sequence. Stereotomography provides an efficient solution to this problem but was limited until now to a picking of seismic data in the prestack time un-migrated domain. We propose here a method for stereotomographic data picking in the depth migrated domain. Picking in the depth migrated domain exhibits the advantage of a better signal-to-noise ratio and of a more regular distribution of picked events in the model, leading to a better constrained tomographic inverse problem. Moreover, any improvement on the velocity model will improve the migrated results, again leading to improved picking. Our strategy for obtaining a stereotomographic dataset from a prestack depth migration is based on migration of attributes (and not on a kinematic demigration approach!). For any locally coherent event in the migrated image, migration of attributes allows one to compute ray parameter attributes corresponding to the specular reflection angle and dip. For application to stereotomography, the necessary attributes are the source/receiver locations, the traveltime and the data slopes. For the data slope, when the migration velocity model is erroneous, some additional corrections have to be applied to the result of migration of the attributes. Applying these corrections, our picking method is theoretically valid whatever the quality of the migration velocity model. We first present the theoretical aspects of the method and then validate it on 2D synthetic and real seismic reflection data sets.