关键词: 深海水体, 速度建模, 海底反射, 走时反演, Fréchet导数, 海森矩阵

Abstract: Using ideas from computational electromagnetics and the basic procedure from the domain decomposition method, we present a biaxial parabolic approximation method for enlarging the angle domain in which the parabolic approximation to the acoustic wave equation is multiple applied and that can be used for migrating data with super-wide angles. Mathematically, the biaxial approximation is a special case of the multiaxial parabolic approximation, and thus can be straightforwardly extended to multiaxial approximations. In detail, we first divide the subsurface into several fan-shaped subdomains and use the axis of each subdomain as the local wave propagation direction. Then, we make a parabolic approximation to the wave equation in each subdomain with respect to the corresponding symmetry axis and compute the forward-propagating wave alternately using a high-order parabolic approximated wave equation in two subdomains. Finally, we use the computed forward-propagating wave as the boundary condition for calculating the back-scattered wave and Green's function by applying the thin slab approximation. Compared with most one-way methods, the proposed method can deal with the strong velocity contrast in the horizontal direction at a large scattering angles. Moreover, compared with certain full-wave methods that have been published in recent years, the proposed method is considerably more efficient and sacrifices only a little in terms of accuracy. Numerical results from comparisons with models of different complexities demonstrate that our method is cost-effective and has considerable potential for use in seismic modeling and imaging.

Key words: deep sea water body, velocity model building, seabed reflection, traveltime inversion, Fréchet derivative, Hessian matrix