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  • 1
    Electronic Resource
    Electronic Resource
    Springer
    Pure and applied geophysics 149 (1997), S. 541-551 
    ISSN: 1420-9136
    Keywords: Key words: Synthetic seismograms, absorption and dispersion.
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract —An algorithm has been developed to compute the dispersive and dissipative seismic response using FUTTERMAN’S (1962) third attenuation-dispersion relationship. In the computation, frequency-dependent velocity and quality factor Q have been used but in the case of the nondispersive synthetic seismogram, frequency-independent velocity has been used. The model’s parameters are density, phase velocity, quality factors and thicknesses of the layers. Dispersive and nondispersive synthetic seismograms have been computed with and without absorption for a layered earth geological model. Fast Fourier transform (FFT) technique has been adopted for converting the frequency domain response into the time domain. The frequency spacing, Δf = 0.976 Hz, has been considered to avoid the aliasing effect. The results have revealed changes in the reflected waveforms in the frequency domain as well as in the time domain for absorption and dispersion cases. It is also concluded that dispersion reduces the arrival time and this effect is increasing with the travel time. The effect of constant Q on the seismic response has also been studied.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    Springer
    Pure and applied geophysics 151 (1998), S. 47-61 
    ISSN: 1420-9136
    Keywords: Key words: 2.5-D Acoustic wave equation, simulation and results.
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract. —Endeavors to simulate the 3-D seismic wave propagation effects in the 2-D environment have stimulated the development of 2.5-D simulation techniques. Liner (1991) has developed a damped 2.5-D acoustic wave equation using the 3-D Green's function for a constant density medium. This paper presents the finite-difference simulation and study of various numerical artifacts, using Liner's acoustic wave equation. Comparisons of second-order as well as fourth-order accurate 2.5-D and 2-D acoustic wave simulation results have been given. The snapshots at different times and zero off-set response of a salt dome model have been computed and strong reflections and diffraction phases have been reported. Also, the analysis of amplitude behaviors and computational time for Liner's (1991) and Williamson and Pratt's (1995) 2.5-D wave equations, with and without the last term (1/t 2),reveals that this last term is superfluous as suggested by Stockwell (1995). The effectiveness of Sponge transmissive as well as Clayton and Engquist (1980) absorbing boundary conditions has been studied. A modified Ricker wavelet, the second derivative of the convolution of Gaussian function and a polynomial window, has been used as a source. It has been found that the stability condition and the requirement of the number of grid points per wavelength to avoid grid dispersion are the same as for the 2-D acoustic cases.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    Springer
    Pure and applied geophysics 144 (1995), S. 59-77 
    ISSN: 1420-9136
    Keywords: P-SV wave solution and simulation
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Numerical solution of the scalar and elastic wave equations has greatly aided geophysicists in modeling seismic wave fields in the complicated geologic structures containing hydrocarbons and hence increases the geologic interpretation. Finite-difference method offers a versatile approach to compute synthetic seismograms numerically for given subsurface complex geological structures. To avoid the spatial derivative of the elastic parameters and density, elastodynamic equation (first-order hyperbolic equation) has been solved using the Lax-Wendroff scheme. A numerical finite-difference modeling program has been developed for the P-SV wave using the above solution. A line source with a time delay of 0.015s and dominant frequency of 120 Hz has been utilized in the simulation. In order to avoid the large values of the displacement vector in the source region,Alterman andKaral's method (1968) has been utilized. Horizontal and vertical component synthetic seismograms have been computed for two different geological models with and without oil and gas bearing zones. It has been concluded from the response that a finite-difference technique not only yields the relative arrival times but also accounts for the variation in amplitude and phase according to the elastic impedance contrast across the interfaces. It should come as no surprise to learn that in spite of the limitation of this numerical method, the scheme has provided a valid response for the thin layer, high acoustic impedance contrast and the pinch out.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    [s.l.] : Nature Publishing Group
    Nature 199 (1963), S. 1246-1247 
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] THE Lower Greensand, in which the Hythe and Folkestone Beds are the principal members, is a shallow marine deposit dating from the Lower Aptian to Lower Albian. The principal British outcrops lie in the Isle of Wight, southern Dorset, Wiltshire, the Weald, East Anglia. The Weald outcrop continues ...
    Type of Medium: Electronic Resource
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  • 5
    Electronic Resource
    Electronic Resource
    Amsterdam : Elsevier
    Physica C: Superconductivity and its applications 209 (1993), S. 421-427 
    ISSN: 0921-4534
    Source: Elsevier Journal Backfiles on ScienceDirect 1907 - 2002
    Topics: Physics
    Type of Medium: Electronic Resource
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  • 6
    Electronic Resource
    Electronic Resource
    Amsterdam : Elsevier
    Journal of Physics and Chemistry of Solids 44 (1983), S. 61-69 
    ISSN: 0022-3697
    Source: Elsevier Journal Backfiles on ScienceDirect 1907 - 2002
    Topics: Chemistry and Pharmacology , Physics
    Type of Medium: Electronic Resource
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  • 7
    Electronic Resource
    Electronic Resource
    Amsterdam : Elsevier
    Physica C: Superconductivity and its applications 167 (1990), S. 669-676 
    ISSN: 0921-4534
    Source: Elsevier Journal Backfiles on ScienceDirect 1907 - 2002
    Topics: Physics
    Type of Medium: Electronic Resource
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  • 8
    Electronic Resource
    Electronic Resource
    Amsterdam : Elsevier
    Physica C: Superconductivity and its applications 168 (1990), S. 599-604 
    ISSN: 0921-4534
    Source: Elsevier Journal Backfiles on ScienceDirect 1907 - 2002
    Topics: Physics
    Type of Medium: Electronic Resource
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  • 9
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 65 (1989), S. 4028-4032 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We have studied the nature of the interfaces between silicon dioxide and germanium-rich layer (SiO2/GexSi1−x) in Ge+-ion implanted and oxidized Si (100) samples. A series of implants were used to study the interfacial roughness as a function of implant dose. Oxidations were carried out at temperatures both above and below the viscous flow point of silicon dioxide (SiO2). In each case, the SiO2/GexSi1−x interface was found to be sharper on an atomic scale compared to the oxide/silicon (SiO2/Si) interface in virgin Si formed after similar oxidation treatments. An enhancement in oxidation was found for the implanted samples over the virgin silicon samples. This enhancement was greater for oxidation carried out at 900 °C for 30 min compared to that at 1000 °C for 20 min. At the higher implant doses, predominantly 60° misfit dislocations were observed at the GexSi1−x/Si interface. The mechanism of oxidation leading to these phenomena is discussed in detail.
    Type of Medium: Electronic Resource
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  • 10
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 81 (2002), S. 841-843 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: InxGa(1−x)N/GaN multiquantum-well light-emitting diodes (LEDs) having periodic thickness variations (TVs) in InxGa(1−x)N active layers exhibit substantially higher optical efficiency than LEDs with uniform InxGa(1−x)N layers. In these nanostructured LEDs, the thickness variation of the active layers is found to be more important than the In composition fluctuation in quantum confinement of excitons (carriers). Detailed scanning transmission electron microscopy-atomic number Z contrast analysis, where image contrast is proportional to Z2 (Z being the atomic number), was carried out to investigate the variation in thickness as well as the spatial distribution of In. In the nanostructured LEDs, there are short-range thickness variations (SR-TVs) (3–4 nm) and long-range thickness variations (LR-TVs) (50–100 nm) in InxGa(1−x)N layers. It is envisaged that LR-TV is key to quantum confinement of the carriers and enhancement of the optical efficiency. We propose that the LR-TV is caused by two-dimensional strain in the InxGa(1−x)N layer below its critical thickness. The SR-TV may be caused by In composition fluctuation. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
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