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Reflection Points (reflection + point)
Selected AbstractsWhat is DMO coverage?GEOPHYSICAL PROSPECTING, Issue 6 2000Ralf Ferber ,Coverage' or ,fold' is defined as the multiplicity of common-midpoint (CMP) data. For CMP stacking the coverage is consistent with the number of traces sharing a common reflection point on flat subsurface reflectors. This relationship is not true for dipping reflectors. The deficiencies of CMP stacking with respect to imaging dipping events have long been overcome by the introduction of the dip-moveout (DMO) correction. However, the concept of coverage has not yet satisfactorily been updated to a ,DMO coverage' consistent with DMO stacking. A definition of constant-velocity DMO coverage will be proposed here. A subsurface reflector will be illuminated from a given source and receiver location if the time difference between the reflector zero-offset traveltime and the NMO- and DMO-corrected traveltime of the reflection event is less than half a dominant wavelength. Due to the fact that a subsurface reflector location is determined by its zero-offset traveltime, its strike and its dip, the DMO coverage also depends on these three parameters. For every surface location, the proposed DMO coverage consists of a 3D fold distribution over reflector strike, dip and zero-offset traveltime. [source] A reflector at 200 km depth beneath the northwest PacificGEOPHYSICAL JOURNAL INTERNATIONAL, Issue 1 2001S. Rost SUMMARY We present an analysis of precursors to PP produced by underside reflections from discontinuities in the upper mantle beneath the NW Pacific. The events used for this study occur in the western Pacific Rim (New Zealand, Fiji, Tonga, Solomon, New Guinea, Philippine Islands) and are recorded at the short-period Yellowknife Array (YKA) in northern Canada. The source,receiver combination results in PP reflection points which allow us to study the upper mantle structure in a corridor from the Hawaiian Islands to the Kuril subduction zone. To detect the weak precursors in the time window between the P arrival and the PP onset and to identify them as PP underside reflections, special array techniques are used. Our analysis indicates a reflector at a depth of ,200 km beneath the northwestern Pacific. This reflector shows strong topography of some tens of kilometres on length scales of several hundred kilometres, complicating the detection of this reflector in global or regional stacks of seismograms. Different models for the impedance jump across the reflector, the thickness and the possible fine structure of the reflector are modelled using synthetic seismograms and are compared with the data. The thickness of the reflector has to be less than 7 km and the P wave impedance contrast has to be larger than 5.0,6.5 per cent to be detected by this study. This corresponds to a P -velocity jump of ,4 per cent assuming the PREM density model. [source] The evolution and redefining of ,CAL': a reflection on the interplay of theory and practiceJOURNAL OF COMPUTER ASSISTED LEARNING, Issue 1 2010R. Hartley Abstract This article comments on how the core idea of the computer as an assistant to teaching and learning became reconfigured through changing technologies, pedagogies and educational cultures. Early influential researchers in computer assisted learning (CAL) made strong but differing links to theories and representations of learning, showing a relevance to pedagogy through innovative projects. Amid controversy, the educational potential of CAL became recognized and hardware,software developments stimulated the involvement of teachers in shaping applications and practices within contexts that favoured a constructivist student focus. Further advances in technology gave students greater autonomy in the style and management of learning, and enabled CAL to be redefined as a participative and collaborative enterprise. Institutions responded through supports and structures in ways that suited their wider educational policies. Technological developments (and controversies) continue to extend and reshape the applications of CAL, and this reflection points to the significance of the interplay between theory and practice in this evolving and redefining process. [source] Asymptotic expansions of multiply scattered surface currentsPROCEEDINGS IN APPLIED MATHEMATICS & MECHANICS, Issue 1 2007*Article first published online: 29 FEB 200, Fatih Ecevit We have recently uncovered the convergence characteristics of multiple scattering iterations for "two-dimensional" as well as "three-dimensional scalar (acoustics)" scattering models in the high-frequency regime. As we have demonstrated, a most distinctive property of these latermodels, compared to their two-dimensional counterparts, is the dependence of corresponding asymptotic expansions on the relative angle of rotation between the principal axes of the successive reflection points of the optical rays. Concerning the case of fully "three-dimensional vector (electromagnetic)" scattering problems, here we show that the vectorial nature of the problem, in turn, gives rise to new additional complex structure. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] | ||||||||||