Summary

Using a simple salt layer elastic model with both flat and steep faults at the subsalt area, we demonstrated the concepts and method of selected C-path (converted-wave path) imaging to improve the subsalt illumination and reduce migration artifacts. First we investigate the survey efficiency of different C-paths through energy budget evaluation for different incident and scattering paths. Then we test the image quality for different migration C-paths. By using C-paths with at least one S-segment inside the salt body, the blind area of pure P-wave imaging for steep subsalt faults having dip larger than the critical angle (370) was eliminated. Using different C-path combinations can also substantially reduce the elastic migration artifacts caused by path-mismatches between the data and migration. Further study for eliminating the conversion mismatch artifacts is needed.

Introduction

The large velocity contrast across the sedimentary/salt interfaces, which are often irregular in shape, is a major obstacle that prevents seismic waves from penetrating the salt body, resulting in poor subsalt images. Especially for the steep faults beneath salt body, the deadly critical/postcritical reflections of P-waves may put these reflectors totally in the blind zone of P-wave survey. It is known that the converted-wave can penetrate the salt body and reach the blind zone of P-wave survey (Purnell, 1992; Li et al., 1998; Jones and Gaiser, 1999). Previous attempt of using converted-wave path (C-path) imaging to improve the Pwave illumination adopted scalar wave propagators for both P- and S-waves and demonstrated the idea using a post-stack migration of the 2D SEG/EAGE salt model (Wu et al., 2001). Although the steep subsalt reflectors, missing in the conventional P-wave imaging, appeared in the Cpath image, however, strong artifacts in the image may be caused by the use of scalar propagator along the C-paths. Also the use of scalar wave propagator for elastic wave extrapolation is dynamically incorrect. In this study we expand the previous study and address two issues in prestack migration using converted-wave paths. First we investigate the survey efficiency of different C-paths through energy budget evaluation for different incident and scattering paths; second, we demonstrate the concept and method of prestack C-path imaging by using a simple saltlayer model. As an elastic wave propagator, we developed a hybrid one-way propagator using a combination of R/T (reflection-transmission) method and an elastic Born modeling operator.

ENERGY PARTITION OF DIFFERENT C-PATHS
FOR SUBSALT SURVEY

In this study, we are mainly concerned with subsalt imaging in the poorly illuminated region of conventional imaging methods. Especially we are targeted at steep subsalt reflectors which are totally invisible by the conventional P-P imaging. These reflectors have only weak elastic parameter contrasts from the surrounding media. Let us first look at the energy partition between P and S scattering (reflection) as a function of incident angles (Figure 1). We know that for these steep faults, the recorded P-to-S reflections from targets are mainly from near-normal, incident waves due to the acquisition aperture limitation. This conclusion is for the conventional CCP (common-conversion-point) migration.

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