Imaging of offset VSP data with an elastic iterative migration scheme

Abstract

VSP data are usually acquired in order to obtain high-resolution images of complex structures in reservoirs and near boreholes. The authors present an elastic iterative migration scheme which has few limitations regarding the complexity of the geology, and where the macromodel for both P- and S-wave velocities is automatically improved and updated at each iteration. They avoid wavefield separation (up/down and P/S) and the simplifying assumptions of small dips underlying most such methods. The migration scheme is based on elastic inversion theory. The wavefield extrapolation is based on a high-order, coarse-grid, finite-difference solution to the elastic two-way wave equation. At each iteration, the macromodel is updated using a gradient method, in which the gradient is computed by correlation of forward-modelled fields with back-propagated residual fields. The first iteration of the migration scheme is equivalent to elastic reverse-time migration with an imaging condition similar to Claerbout`s principle. Both P- and S-wave reflections contribute to the images. In numerical examples with both synthetic and real offset VSP data, they find that increasing the number of iterations improves the image quality. Images based on both P- and S-wave energy give more near-well information and higher spatial resolution than images on only acoustic energy.more » In the real data example the authors show that the iterative migration scheme can image a relatively complex geological structure. A fault and a small graben intersecting the well can be identified.« less