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  • 1
    ISSN: 1420-9136
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Summary An irregular wave group (here called pseudospherical), the existence of which is connected with the velocity boundary at which the velocity decreases discontinuously, is investigated. A schlieren modelling device was chosen for a model investigation of this wave since it permits the investigation of wave fields inside the measured models. The model consisted of two layers of transparent gels, the source lay in the layer of higher velocity. The measurements have shown that an irregular wave in the layer of lower velocity exists only in a certain region along the boundary; its wave front has a spherical form and its intensity decreases rapidly with increasing distance of the source from the boundary. The wave always comes only after the regular refracted wave which conforms with the ray theory. These properties correspond to the properties of a wave first described byOtt [1]3) andBrekhovskikh [2]. In the conclusion of the present paper the possibilities of recording pseudospherical waves in seismology are outlined.
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  • 2
    Electronic Resource
    Electronic Resource
    Amsterdam : Elsevier
    ISSN: 0031-9201
    Source: Elsevier Journal Backfiles on ScienceDirect 1907 - 2002
    Topics: Geosciences , Physics
    Type of Medium: Electronic Resource
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  • 3
    ISSN: 1573-1626
    Source: Springer Online Journal Archives 1860-2000
    Topics: Architecture, Civil Engineering, Surveying , Geosciences , Physics
    Notes: Summary Following the recommendation of the European Seismological Commission (Copenhagen 1966) to carry out comparative measurements on simple seismic models with a prescribed velocity-depth distribution (test models), an attempt was made to compare results obtained with different model techniques. In the present paper, model investigations with the first two proposed test models carried out by 2- and 3-dimensional model techniques are described. Seismogrammes, travel-time curves and amplitude-distance curves of the models investigated are given. A quantitative comparison of the results shows a satisfactory agreement.
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  • 4
    ISSN: 1573-1626
    Source: Springer Online Journal Archives 1860-2000
    Topics: Architecture, Civil Engineering, Surveying , Geosciences , Physics
    Notes: Zusammenfassung Bei Verfolgung der Entstehung und Formierung der durch eine explosive Kugelquelle erregten Druckwellen steht die Frage ihres Frequenzgehalts im Vordergrund. Der Frequenzgehalt dieser Wellen wurde durch die Abhängigkeit ausgedrückt (2), die die Änderungen der relativen Breite des Amplitudenspektrums (1) vom Gewicht und von der Entfernung der Quelle angibt. Aus(2) erfolgt, dass der Frequenzgehalt der Druckwellen mit wachsender Entfernung und Grösse der Ladung exponential sinkt, wobei die Steilheit dieses Abfalls für höhere Grössen der Ladung kleiner ist. Das Herabsetzen des Frequenzgehalts dieser Wellen verursacht, dass die maximate spektral Dichte in grösseren Entfernungen von der Quelle markanter wird.
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  • 5
    ISSN: 1573-1626
    Source: Springer Online Journal Archives 1860-2000
    Topics: Architecture, Civil Engineering, Surveying , Geosciences , Physics
    Notes: Summary The paper deals with a study of the mean stress field (σM) and its gradient (δσM/δx) in uniaxially loaded two-dimensional models of small thickness with respect to other dimensions, in which two inhomogeneities, e.g., a circular opening and a linear slit have been made. Particular attention was paid to the stress field development in the treated models durin gthe change of the mutual geometric configuration of both the inhomogeneities in question. The problem was analysed theoretically by means of Muskhelishvili's potentials of the two-dimensional theory of elasticity. To compare the results of calculations, the schlieren method was used to visualize and record the studied gradients of stress fields in physical models. The results of the calculations were calibrated by laboratory tests. In this way the individual model situations were classified from the point of view of their resistance to the applied load.
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  • 6
    ISSN: 1573-1626
    Source: Springer Online Journal Archives 1860-2000
    Topics: Architecture, Civil Engineering, Surveying , Geosciences , Physics
    Notes: Summary An analytical migration method is developed for inverting wide angle seismic reflection travel time data with proper migration. In this method the concept of effective velocity (V eff ) in the vertically inhomogeneous earth has been used, which takes into account the refraction effect. It is shown that the migration process in this case becomes extremely rapid as curved ray paths are replaced by equivalent straight paths. A post migration ‘sliding correction’ is found to be necessary, which arises due to the non-collinearity of the recording point, reflection point and the image point in the equivalent straight path geometry for dipping reflectors. Calculations for some representative dips show that, for the depths and recording ranges of interest in DSS, the vertical component of the sliding correction is negligible compared to the horizontal component. However, even the horizontal component is not significant for reflectors with gentle dips. The effect of certain simplifying assumptions in the estimation of V eff is shown to be within the error limits of velocity determination. A computer programme package has been written for carrying out the migration. The method yields more accurate results in various theoretical and field cases than the conventional graphical wave front methods which are at present widely used for crustal reflection work.
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  • 7
    ISSN: 1573-1626
    Source: Springer Online Journal Archives 1860-2000
    Topics: Architecture, Civil Engineering, Surveying , Geosciences , Physics
    Description / Table of Contents: Summary The dependence between Pn-wave velocities and the surface heat flow, temperature at the core-mantl boundary and thickness of the Earth's crust for continents (Europe, Asia, North America and Australia) was investigated statistically in connection with the problem of lateral inhomogeneities in the upper mantle. The relations obtained were compared with those determined under laboratory conditions. The conclusion is that temperature and pressure effects may provide additional explanations of the regional variations of Pn-wave velocities observed in most continents.
    Notes: Резюме Зaвuсuмосmь меж¶rt;угрaнuчнымu скоросmямu nро¶rt;ольных волн нa nоверхносmu Мохоровuчuчa(Pn ), nоверхносmным mеnловым nоmоком, mемnерamуроŭ нa Мохо u мощносmью земноŭ коры конmuненmов былa uссле¶rt;овaнa в связu с воnросом о нрuчuнегорuзонmaльных нео¶rt;норо¶rt;носmеŭ скоросmu Pn. Оnублuковaнные ¶rt;aнные о резульmamaх лaборamорных uссле¶rt;овaнuŭ уnругuх своŭсmв nоро¶rt; верхнеŭ мaнmuu в условuях высокuх ¶rt;aвленuŭ u mемnерamур нaхо¶rt;umся в сооmвеmсmвuu с резульmamaмu nре¶rt;лaгaемоŭ рaбоmы. Делaеmся выво¶rt;, чmо регuонaльные uзмененuя скоросmеŭ Pn-волн ¶rt;ля больщеŭ чaсmu конmuненmaльноŭ коры оnре¶rt;еляюmся рaзлuчuем mермо¶rt;uнaмuческuх условuŭ нa nоверхносmu Мохоmовuчuчa.
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  • 8
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: An approximate hybrid approach to computing high-frequency body wave synthetic seismograms in 2-D and 3-D laterally varying layered structures containing thin transition layers is suggested. It combines the ray method with the matrix (reflectivity) method. The ray method is applied to thick layers with smooth variations of velocity and the reflectivity method is applied to thin transition layers. Alternatively, the method of summation of Gaussian beams is used instead of the ray method in the hybrid code. An algorithm and the relevant program package BEAM87, designed for such hybrid computations in 2-D laterally varying layered structures containing one laterally varying thin transition layer, are briefly described. The thin transition layer may represent a region of a high velocity gradient, a laminated region, a region of a low Q, etc. It is simulated by a stack of very thin layers. The accuracy of the hybrid computations is tested on 1-D models by the comparison with reflectivity-method computations. The hybrid method yields sufficiently accurate results for transition layers, the thickness of which is smaller than one half of the prevailing wavelength of the wavefield under consideration, particularly for small angles of incidence. For reflected waves, the best accuracy is obtained for subcritical reflections, but the accuracy is lower for critical and overcritical reflections. The accuracy in the critical region is improved, if the Gaussian beam summation method is used instead of the ray method in the hybrid scheme. Numerical examples of synthetic body wave seismograms for a laterally varying model containing thin transition layers of various types are presented.
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  • 9
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: A concept of a factorized anisotropic inhomogeneous (FAI) medium is introduced. In the FAI medium, the position dependent density normalized elastic parameters aijkl(Xi) can be factorized in the following sense: aijkl(Xi) =f2(xi)Aijkl, where Aijkl are constants, independent of Cartesian coordinates xi, and f(xi) is an arbitrary continuous function of xi. Thus, all the density normalized elastic parameters aijkl(xi) in the FAI medium depend on the coordinates xi in the same way, but the generality of anisotropy and of inhomogeneity is not restricted. The factorization of aijkl(xi) leads to a factorization of certain important ray theory expressions and equations (eikonal equation, etc.), and to particularly simple ray tracing and dynamic ray tracing. For certain types of FAI media, these equations may even be solved analytically or semianalytically.
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  • 10
    ISSN: 1573-1626
    Source: Springer Online Journal Archives 1860-2000
    Topics: Architecture, Civil Engineering, Surveying , Geosciences , Physics
    Notes: Summary The dynamic focal parameters of a series of strong earthquakes, recorded by the FBV broadband velocigraph at KHC (Kašperské Hory, Czechoslovakia), are estimated. The influence of specific wave pattern features on the estimation accuracy is studied. Polarization analysis is used to demonstrate the complicated interference character even of record portions corresponding to a single teleseismic wave group. The values of the dynamic focal parameters do not depend significantly on the length of the analysed record portions and are sufficiently consistent with those reported by Riznichenko [2]. The possibility of estimating dynamic focal parameters from recording of standard classes B (intermediate band) and C (long-period) seismographs is pointed out.
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