Least-Squares Migration Technique Improves Imaging and Inversion Offshore China

Abstract

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper IPTC 19285, “Improving Imaging and Inversion Through Least-Squares Q-Kirchhoff APSDM: A Case Study From Offshore China,” by Chenghai Jiao, Jianfeng Yao, and Keat Huat Teng, CGG, et al., prepared for the 2019 International Petroleum Technology Conference, Beijing, 26–28 March. The paper has not been peer reviewed. Copyright 2019 International Petroleum Technology Conference. Reproduced by permission. Least-squares migration (LSM) has become an increasingly important imaging tool. Recently, several case studies have shown that LSM provides greatly improved seismic imaging. However, only a few examples reveal its advantages in both imaging and amplitude-vs.-offset (AVO) inversion. In this paper, the authors propose a least-squares Q migration (LSQM) method that combines the benefits of both LSM and Q prestack depth migration (QPSDM) to improve the amplitude fidelity and image resolution of seismic data. Introduction Seismic data can be considered an outcome of a forward modeling experiment with an unknown Earth velocity model and absorption Q model. Several major factors can affect the resolution and amplitude fidelity of the resulting seismic images, namely the acquisition con-figuration, absorption of the anelastic Earth, and the complexity of the subsurface velocity model. With regard to the first factor, seismic images can suffer from limited migration aperture and nonuniform illumination. This illumination issue can be addressed through image-domain single-iteration LSM. With regard to the second and third factors, it is important to include Q absorption in the velocity model-building process and the migration algorithm, and to use least-squares means to control the compensation of energy loss through LSQM. In the South China Sea, complex faulting geology poses a significant challenge to imaging with narrow-azimuth towed streamer data. Over the past decade, Q tomography and Q migration have been developed to tackle the issue with Q absorption. However, because both the Q compensation level and the noise level of seismic data usually increase with travel time and frequency, high-frequency noise and migration swings are often overamplified in Q migration. The issues with amplitude distortion associated with fault shadows and smeared fault images with a limited usable aperture can be coped with better through the use of LSM, and seismic image resolution can be increased by compensating Q effect with travel time and frequency. The high-frequency noise generated from QPSDM can be attenuated better through LSM if Q can be brought into the process of LSM. To fully exploit the merits of LSM and QPSDM, seismic imaging can be improved through LSQM. The authors apply LSQM to a narrow-azimuth marine-towed streamer survey acquired over an area with complicated faulting structures and significant absorption in the South China Sea. Using image-domain single-iteration LSQM and reservoir-oriented processing, the authors demonstrate that the previously mentioned problems can be addressed, with the result of improved AVO inversion at the wells.