VSP data can provide a high-resolution reservoir image due to relatively lower wavefield distortion in the overburden. However, imaging VSP data using conventional primary-only migration suffers from poor illumination, imaging artifacts and low reliability, especially away from the well. Here, we are proposing the full wavefield migration (FWM) approach to image VSP data which aims at estimating the true-amplitude angle-dependent reflectivity of the subsurface using the primaries, surface and internal multiples. The FWM algorithm is recursive in depth and iteratively incorporates the nonlinear transmission effect at each depth level, followed by wavefield updating at each depth level. The use of the full wavefield in imaging the VSP data can enhance illumination and image reliability. The algorithm falls in the category of full waveform inversion – i.e. it explains every sample of the input data – using reflectivity as the parameters to be determined. It involves a constrained least-squares inversion approach, where all reflection energy is explained. Based on synthetic data studies, in this paper, we illustrate the advantage of using all multiples in this inversion-based migration scheme. We also demonstrate the potential of using deviated wells, as opposed to true vertical wells, for obtaining high resolution image of the complex reservoirs.


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