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Diffraction Effects (diffraction + effects)

Distribution by Scientific Domains

Engineering	50%

Selected Abstracts

A Maslov-propagator seismogram for weakly anisotropic media

GEOPHYSICAL JOURNAL INTERNATIONAL, Issue 1 2002
Georg Rümpker
Summary We introduce a formalism to calculate shear-wave seismograms for weakly-anisotropic and inhomogeneous media. The method is based on the combination of the forward-propagator method, which accounts for shear-wave interaction along a single reference ray, and the Maslov ray-summation, which incorporates amplitude and phase information from neighbouring rays to account for waveform and diffraction effects at caustics and in shadow regions. The approach is based on the assumption that the multiply split shear waves, on the way to a given receiver, travel along a common ray path that can by obtained from ray tracing in an isotropic reference medium (i.e. the common-ray approximation). The forward propagator and the Maslov amplitude are expressed with respect to radial and transverse coordinates (perpendicular to the ray propagation direction) that are defined uniquely by the initial conditions. Local polarizations and slownesses of the fast and slow shear-waves in the direction of propagation are obtained from the eikonal equation. The Maslov-propagator phase is given by the average shear-wave traveltime along the reference ray. Phase advances and delays of individual shear wave components are accounted for by the propagator. The geometrical-spreading information required for the Maslov integration is supplied by dynamic ray tracing in the isotropic reference medium. In the high-frequency limit effective phase functions are defined to assess the validity of the Maslov propagator phase information. For a homogeneous isotropic reference medium, we find good agreement with exact Maslov phase functions for anisotropic perturbations of up to 20 per cent. As a numerical application we consider effects of inhomogeneous anisotropy in a shear-wave cross-hole survey. The variations of the transversely-isotropic medium require 2-D slowness integrals. The method can handle discontinuities of the fast polarization along the ray path and also for neighbouring rays which is important for the slowness integration. Smooth transitions between isotropic and anisotropic regions along the ray path can be accounted for without the need to switch between numerical formulations. [source]

The performance analyses of multitarget CMA adaptive array considering mutual coupling and diffraction effects

INTERNATIONAL JOURNAL OF ADAPTIVE CONTROL AND SIGNAL PROCESSING, Issue 7 2007
Jin Xu
Abstract The performance of a multitarget constant modulus algorithm (MU-CMA) adaptive array located on a regular conducting plate is studied in this paper. The effects of mutual coupling (MC) between array elements and diffraction caused by the conducting plate are taken into account. A hybrid method of equivalent edge current method (ECM) and moment method (MM) is employed in electromagnetic calculation to investigate the distortion of initial array pattern. We compare the capture property of three well-known MU-CMA arrays, respectively: multitarget least-squares constant modulus array (MT-LSCMA), multitarget decision-directed array (MT-DD) and least-squares despread respread multitarget constant modulus array (LS-DRMTCMA). Simulation result shows that: (i) the distorted initial pattern leads to the descending of the signal catch performance of MT-LSCMA and MT-DD; (ii) only the LS-DRMTCMA can work correctly due to its stronger anti-jamming ability in the presence of MC and diffraction. Copyright © 2006 John Wiley & Sons, Ltd. [source]

An enhanced Markov chain based model for the narrowband LMS channel in built-up areas

INTERNATIONAL JOURNAL OF SATELLITE COMMUNICATIONS AND NETWORKING, Issue 2 2005
F. Perez-Fontán
Abstract In this paper, a technique to derive the transition probabilities for a Markov chain model for the land mobile satellite (LMS) channel in built-up areas is presented. This technique contributes to improving empirically derived parameters in that it can account for elevation and street orientation effects as well as building density. Physical-statistical or ,virtual city' techniques are used to relate observed edification statistics to signal attenuation statistics. This methodology can also be applied to generating correlated time-series for simultaneous links to a constellation of satellites from the same mobile terminal. The proposed approach also allows the generation of enhanced time-series which take into account diffuse multipath and diffraction effects. Copyright © 2005 John Wiley & Sons, Ltd. [source]

On the pre-metric foundations of wave mechanics I: massless waves

ANNALEN DER PHYSIK, Issue 4 2009
D.H. Delphenich
Abstract The mechanics of wave motion in a medium are founded in conservation laws for the physical quantities that the waves carry, combined with the constitutive laws of the medium, and define Lorentzian structures only in degenerate cases of the dispersion laws that follow from the field equations. It is suggested that the transition from wave motion to point motion is best factored into an intermediate step of extended matter motion, which then makes the dimension-codimension duality of waves and trajectories a natural consequence of the bicharacteristic (geodesic) foliation associated with the dispersion law. This process is illustrated in the conventional case of quadratic dispersion laws, as well as quartic ones, which include the Heisenberg,Euler dispersion law. It is suggested that the contributions to geodesic motion from the non-quadratic nature of a dispersion law might represent another source of quantum fluctuations about classical extremals, in addition to the diffraction effects that are left out by the geometrical optics approximation. [source]