IMAGING OF OFFSET VSP DATA WITH AN ELASTIC ITERATIVE MIGRATION SCHEME

Citation
R. Mittet et al., IMAGING OF OFFSET VSP DATA WITH AN ELASTIC ITERATIVE MIGRATION SCHEME, Geophysical prospecting, 45(2), 1997, pp. 247-267
Citations number
14
Categorie Soggetti
Geochemitry & Geophysics
Journal title
ISSN journal
00168025
Volume
45
Issue
2
Year of publication
1997
Pages
247 - 267
Database
ISI
SICI code
0016-8025(1997)45:2<247:IOOVDW>2.0.ZU;2-4
Abstract
VSP data are usually acquired in order to obtain high-resolution image s of complex structures in reservoirs and near boreholes. We present a n elastic iterative migration scheme which has few limitations regardi ng the complexity of the geology, and where the macromodel for both P- and S-wave velocities is automatically improved and updated at each i teration. We avoid wavefield separation (up/down and P/S) and the simp lifying assumptions of small dips underlying most such methods. The mi gration scheme is based on elastic inversion theory. The wavefield ext rapolation is based on a high-order, coarse-grid, finite-difference so lution to the elastic two-way wave equation. At each iteration, the ma cromodel is updated using a gradient method, in which the gradient is computed by correlation of forward-modelled fields with back-propagate d residual fields. The first iteration of the migration scheme is equi valent to elastic reverse-time migration with an imaging condition sim ilar to Claerbout's principle. Both P- and S-wave reflections contribu te to the images. In numerical examples with both synthetic and real o ffset VSP data, we find that increasing the number of iterations impro ves the image quality. Images based on both P- and S-wave energy give more near-well information and higher spatial resolution than images o n only acoustic energy. In the real data example we show that the iter ative migration scheme can image a relatively complex geological struc ture. A fault and a small graben intersecting the well can be identifi ed.