成像域最小二乘偏移成像方法及西北超深层探区应用

经典最小二乘偏移通常采用波形残差目标泛函, 通过精确匹配地震波形实现反射率定量反演。然而, 在实际地震数据处理过程中, 高精度的地震子波和偏移速度模型难以获得, 且迭代算法收敛不稳定, 计算量过大, 因此很难取得显著效果。提出了一种面向实际数据处理的成像域最小二乘逆时偏移方法, 基于宏观速度场和子波基本频带特征, 构建全局空变点扩散函数, 在成像域采用高维空间反褶积反演算法实现高分辨率成像。所提方法避免了对地震波形的精确匹配, 充分考虑点扩散函数在不同波数成分的照明特征, 因此对地震子波和偏移速度精度的依赖度不高。同时无需迭代计算, 直接获取成像结果, 计算效率高, 在三维超深层地震勘探中具有应用潜力。西北探区两块不同储层类型的超深层实际地震数据应用结果表明, 该方法可以提高成像分辨率, 改善断裂-缝洞构造的成像效果, 提升串珠的识别能力, 为西北超深层碳酸盐岩增储上产提供了技术支撑。

The classical least-squares migration is implemented based on a waveform misfit function in the data domain to accomplish quantitative inversion of subsurface reflectivity.However, it is hard to achieve remarkable results due to unknown wavelet, inaccurate migration velocity, and unacceptable computational cost.To address these issues, we provide a practical least-squares reverse-time migration method in the image domain.This method adopts a background velocity field and a given wavelet with real frequency band to compute the globally spatial-varying point-spread function, and uses an image-domain high-dimensional spatial deconvolution algorithm for high-resolution imaging.This method sidesteps seismic waveform misfit, and focuses on the illumination of the point-spread function at different wavenumbers.As a result, the accuracy of the seismic wavelet and migration velocity does not make a strong impact on imaging.Moreover, one-iteration imaging with high computational efficiency facilitates its application to 3D seismic exploration in ultra-deep zones.Two case studies dealing with different reservoir types in northwestern China demonstrate that this method can improve imaging resolution for the description of fracture-cave structures and thus offer technical support to reserves and production increase in ultra-deep carbonate reservoirs.