Analysis of Steep Dip Subsalt Seismic Illumination Via Ray Tracing and Finite-Difference Modeling: A Gulf of Mexico Case Study
Lee Seyon, Annaliese 1987-
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This thesis aims to demonstrate the feasibility of improving the seismic depth image of the steeply dipping “attic” regions of the salt feeder of a complex salt body in the deep-water Gulf of Mexico. In order to assess the feasibility of improving the image, the first priority is to determine whether the current data acquired will illuminate the target reservoirs and their trap, given the complexity of the model and the limitations of the acquisition method. My study illustrates and confirms the cause of the previously observed low illumination zone and the possibility of improving the image by incorporating longer offsets or converted waves for imaging. Firstly, interactive ray tracing and acoustic finite-difference modeling (FDM) are used to assess the low illumination of the steeply dipping salt feeder. Secondly, ray tracing illumination maps and 2D acoustic FDM synthetic images are generated to simulate the impact of full azimuth geometries with source-receiver offsets of up to 8 km and up to 14 km, respectively, in order to determine the benefits of the longer offsets to imaging the subsalt steep dip areas. Thirdly, the contribution of converted waves to imaging the feeder is assessed using elastic FDM and imaging. The results show that it is impossible to image the steeply dipping portion of the salt root using only P-wave primary reflections, due to the post-critical reflection of the primary off the steep salt feeder. To see if we can improve the imaging of the steep dip areas by increasing the acquisition offset, the ray tracing result shows no improvement due to limitations of the method, but the more reliable waveform modeling results show significant improvement in the illumination of steep dips. The improvement is due to the use of prismatic waves in imaging the steep dip attic region of the salt feeder. Investigation into the contribution of converted waves to imaging the subsalt steep dip areas indicates that elastic imaging has the potential to fill the steep dip illumination holes in the images produced using conventional surface-acquired P-wave data. This modeling study confirms the benefit of acquiring longer offset to improve steep dip subsalt imaging, particularly for using non-primary energy and a sufficiently accurate velocity model.