Angle‐domain common‐image gathers for migration velocity analysis by wavefield‐continuation imaging

Abstract

We analyze the kinematic properties of offset-domain Common Image Gathers (CIGs) and Angle-Domain CIGs (ADCIGs) computed by wavefield-continuation migration. Our results are valid regardless of whether the CIGs were obtained by using the correct migration velocity. They thus can be used as a theoretical basis for developing Migration Velocity Analysis (MVA) methods that exploit the velocity information contained in ADCIGs.We demonstrate that in an ADCIG cube the image point lies on the normal to the apparent reflector dip, passing through the point where the source ray intersects the receiver ray. Starting from this geometric result, we derive an analytical expression for the expected movements of the image points in ADCIGs as functions of the traveltime perturbation caused by velocity errors. By applying this analytical result and assuming stationary raypaths, we then derive two expressions for the Residual Moveout (RMO) function in ADCIGs. We verify our theoretical results and test the accuracy of the proposed RMO functions by analyzing the migration results of a synthetic data set with a wide range of reflector dips.Our kinematic analysis leads also to the development of a new method for computing ADCIGs when significant geological dips cause strong artifacts in the ADCIGs computed by conventional methods. The proposed method is based on the computation of offsetdomain CIGs along the vertical-offset axis (VOCIGs) and on the "optimal" combination of these new CIGs with conventional CIGs. We demonstrate the need for and the advantages of the proposed method on a real data set acquired in the North Sea.

Migration-velocity Analysis Using Image-space Generalized Wavefields

“…The subscript hg corresponds to subsurface offsets computed by shifting along the apparent geological dip α. a) Underestimated velocity, and b) overestimated velocity. Modified from Biondi and Symes (2004). perm/.…”

High Resolution/High Fidelity Seismic Imaging and Parameter Estimation for Geological Structure and Material Characterization

“…This demonstrated the significance of acquisition aperture correction in true-reflection imaging. For wave-equation based migration methods, the subsurface angle-domain common-image gathers (CIGs) can be obtained either by decomposing the propagated wavefield before imaging ("data-space methods") (e.g., de Bruin et al, 1990;Mosher et al, 1997;Prucha et al, 1999;Mosher and Foster, 2000;Wu and Chen, 2002;Xie and Wu, 2002;Chen et al, 2006), or by prestack imaging at zero time but not at zero offset ("image-space methods") (e.g., Rickett andSava, 2001, 2002;Biondi and Shan, 2002;Sava and Fomel, 2003;Biondi and Symes, 2004;Biondi 2007;Rosales et al, 2008). In the image-space method, the subsurface offset-domain CIGs are firstly computed and then transformed to the subsurface angle-domain CIGs.…”

Residual moveout of 2D multiples in angle‐domain common‐image gathers

“…The residual moveout of primaries in ADCIGs, under the approximation of stationarity of the rays (local constant velocity) is given by (Biondi and Symes, 2004):…”

“…For primaries, we can estimate the true depthz using the migration depth at normal incidence z 0 (Biondi and Symes, 2004) as…”

“…When they are migrated with the wrong velocity, their residual moveout in ADCIGs can be approximated, to first order, by the equations given in Biondi and Symes (2004). For a flat reflector, their approximation reduces the residual moveout of the primaries as a function of aperture angle, to a tangent squared.…”