Summary

Based on the full-wave equation, reverse time migration (RTM) can handle wave propagation in all directions without angle limitation. However, the wide-angle capability of full-wave propagator creates artifacts in migrated image if the conventional imaging condition (zero-lag cross-correlation) is applied. In this paper, we propose a new imaging condition to attenuate these artifacts by decomposing the full wavefields at every image location to local plane waves of different directions, followed by constructing the image by correlating certain combinations of the plane-wave components. Synthetic examples demonstrate that this imaging condition can effectively remove the undesired artifacts in both simple and complex model.

Introduction

Reverse time migration (RTM) has spurred much interest in recent years because of the increased imaging challenges posed by complex subsurface targets and affordable computer resources such as Linux clusters.

As a wave equation technique using full-wave equation, the RTM can handle not only multi-arrivals but have no dip limitation, enabling imaging of overturned reflections. However, the RTM is known for producing low-wavenumber artifacts, which often mask the geological structures. Those artifacts result from the unwanted cross-correlation of head waves, diving waves and backscattered waves at the imaging step (Yoon et al., 2004). These events are particularly serious where high velocity contrasts or high velocity gradient exist.

Several approaches have been proposed to suppress the image artifacts resulted from conventional imaging condition. In post-stack RTM, these artifacts can be effectively suppressed by matching the impedance of the media, i.e., utilizing the non-reflecting wave equation (Baysal et. al, 1984) or by smoothing the velocity model to reduce the reflections (Loewenthal, et. al, 1987). However, both approaches become less effectively under the prestack scenario, because it is hard to simultaneously match non-reflection conditions for broad incident angles. Guitton et al. (2006) used different post-imaging filters to remove the image artifacts, e.g., Laplacian filter, derivative filter and least square filter. Fletcher et al. (2005) attenuated reflections at boundaries by introducing a directional damping term to the non-reflecting wave equation during propagation.

Angle related image conditions have been tested by several authors to be a more effective method to suppress the image artifacts. Under this category, Yoon et al. (2004) used an angle-related weighting function to sort the source and receiver waves based on their incoming direction. The angle information is calculated from Poynting vectors of source and receiver waves. Only the energy actually related to the reflections is kept in the imaging. Xie and Wu (2006) proposed a full-wave coupled with one-way method to solve the image problem, where the wavefield generated by the full-wave method is decomposed into one-way components at different directions. Then the image condition is applied to the proper combinations of the wave components for imaging. Liu et al. (2007) also explored the angle-domain image condition. Denli and Huang (2008) extended the method to the elastic wave case.

In the above mentioned angle-domain image conditions, the wave propagation direction is extracted using the differentiation method. In a complex model, different wavefields often coexist and overlap with each other.

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