Migration velocity analysis is a method devoted to the evaluation of both reflectivity and background velocity models, associated with the high and low wavenumber components of the model, respectively. Inversion velocity analysis is one of its improved versions, leading to more stable background velocity updates. Still, the impact of the user parameters should be understood for an optimal update of the background velocity. We show that a sign reversal of the background velocity gradient could occur when the selected surface offset range or the space lag range is too small. We derive the theoretical limits and check their consistency through simulations in a simple model with a single interface. These guidelines determine the necessary ranges of surface offsets and space lags for a proper update of the background velocity model. We discuss their applicability on the Marmousi model. Artefacts in the retrieved background velocity model are observed when the guidelines are not satisfied. 相似文献
Recent studies to assess very long-term seismic hazard in the USA and in Europe have highlighted the importance of the upper
tail of the ground-motion distribution at the very low annual frequencies of exceedance required by these projects. In particular,
the use of an unbounded lognormal distribution to represent the aleatory variability of ground motions leads to very high
and potentially unphysical estimates of the expected level of shaking. Current practice in seismic hazard analysis consists
of truncating the ground-motion distribution at a fixed number (εmax) of standard deviations (σ). However, there is a general lack of consensus regarding the truncation level to adopt. This paper investigates whether
a physical basis for choosing εmax can be found, by examining records with large positive residuals from the dataset used to derive one of the ground-motion
models of the Next Generation Attenuation (NGA) project. In particular, interpretations of the selected records in terms of
causative physical mechanisms are reviewed. This leads to the conclusion that even in well-documented cases, it is not possible
to establish a robust correlation between specific physical mechanisms and large values of the residuals, and thus obtain
direct physical constraints on εmax. Alternative approaches based on absolute levels of ground motion and numerical simulations are discussed. However, the choice
of εmax is likely to remain a matter of judgment for the foreseeable future, in view of the large epistemic uncertainties associated
with these alternatives. Additional issues arise from the coupling between εmax and σ, which causes the truncation level in terms of absolute ground motion to be dependent on the predictive equation used. Furthermore,
the absolute truncation level implied by εmax will also be affected if σ is reduced significantly. These factors contribute to rendering a truncation scheme based on a single εmax value impractical. 相似文献