Multifocusing seismic imaging of complex geological structures

Abstract

We introduce two highly effective stacking approaches, 2.5D multifocusing (MF) and 3D generalized spherical multifocusing (GSMF) algorithms, to improve stacked seismic sections. The proposed methods can be applied to crooked-line or 3D seismic data with arbitrary recording geometry from areas with complex near-surface/subsurface to generate either a 2D crooked stacked section or a 3D stacked volume. Both methods simultaneously correct for 3D normal moveout and 3D azimuth-dependent dip moveout, and transform multi-coverage prestack seismic data into a stacked section that is equivalent to a synthesized zero-offset wavefield. In addition, the 3D GSMF approach accounts for the elevation and spatial coordinate of all source and receiver positions, so can be applied for seismic data collected in areas with irregular topography. The 2.5D MF moveout operator is accurate for quasi-hyperbolic reflections, whereas, the 3D GSMF traveltime surface is a more general higher-order moveout operator with a closed-form implicit formulation that can account for non-hyperbolic moveout. The proposed methods efficiently extract valuable 3D structural information when applied to crooked-line/3D seismic surveys. The introduced methods are data-driven algorithms and can perform automatically with a coherence-based global optimization search. We also deployed an efficient processing sequence for applying the developed methods, which mainly consists of building super common-midpoint bins, wavefield attributes analysis, multifocusing moveout correction, data enhancement/stacking, and prestack/poststack dip-independent RMS velocity analysis and migration. Our wavefield analysis uses a multidimensional differential evolution algorithm to simultaneously determine optimal wavefield parameters, which improves the efficiency and accuracy of the estimation. The performance of the proposed methods is tested using 3D synthetic data with both 3D and crooked-line surveys over various curvatures ranging from low (gently curved interface or planar layer with curvature of zero) to high (spherical reflector or point diffractor). The numerical tests demonstrate that the new approximations extract dips from seismic data accurately and are accurate for gently to highly curved interfaces beneath low (multiple homogeneous layers) to relatively high heterogeneous overburden (vertical/horizontal background velocity with a gradient of 1/s for the 2.5D MF method and vertical background velocity with a gradient of 2/s for the 3D GSMF method). Applying the 2.5D MF approach to the Larder Lake crooked-line transect with subsurface complexity, as well as a real data set collected in a thrust-belt area, focused the steeply dipping reflections more coherently and mapped new reflections that are not visible in the conventional results. Application of the 3D GSMF method on a 3D low-fold real seismic dataset, acquired over a complex thrust-belt area with rugged terrain, yielded high-resolution and an accurate stacked seismic volume with a high signal-to-noise ratio, compared to the conventional 3D stacking.