Summary

Gaussian beam migration is an effective and robust depth imaging method, which overcomes the disadvantage of Kirchhoff migration in imaging multiple arrivals and has no steep-dip limitation of one-way wave equation migration. But its imaging quality depends on the initial beam parameter, which makes the beam width increasing and wave-front spreading with the propagation of the central ray, resulting in poor migration accuracy at depth, especially for exploration areas with complex geological structures. Aiming at this problem, we present an adaptive focused beam method for shot-domain prestack depth migration in this paper. Using the input velocity field information, we firstly derive an adaptive focused beam parameter, which makes a seismic beam focused along the whole ray and improves the wave-field construction accuracy in both the shallow and the deep regions. Then, we introduce this parameter into the Gaussian beam migration, which not only improves imaging quality of deep reflectors but also makes the shallow small-scale geological structures well-defined. Besides, using the amplitude-preserved extrapolation operator and deconvolution imaging condition, the concept of amplitude-preserved imaging has been included in our method. Typical numerical examples and the field data processing result demonstrate the validity and adaptability of our method.

Introduction

Gaussian beam migration (GBM) is an elegant and effective depth imaging method, which not only retains the advantage of ray-based migration, such as flexibility and efficiency, but also has a comparable imaging accuracy with wave equation migration. Ever since the basic GBM frame was presented by Hill (1990, 2001), it has been extended to irregular topographic conditions (Gray 2005; Yue et al, 2010; Yang et al, 2014) and true-amplitude migration (Gray and Bleistein, 2009). In addition, many new seismic beam imaging methods spring up, such as fast beam migration (Gao et al, 2006, 2007), focused beam migration (Nowack, 2008) and Laser beam migration (Xiao et al, 2014. Most of them, however, are based on the GBM frame and use a constant initial beam parameter, which makes a seismic beam focused either at the initial position or at a certain depth. Thus, the imaging accuracy varies along the central ray, which is highest at the focus point and gradually decrease as moving away from the focus point.

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