Automated 3D tomographic velocity analysis of residual moveout in prestack depth migrated common image point gathers

Abstract

Residual moveout analysis of prestack depth migrated common image point gathers is used for velocity model building in areas of complex geology. Where the velocity varies smoothly, the model is commonly built from stacking velocity analysis of the residual curvature on the gathers after conversion back to time. Where the velocity varies more rapidly, the model must be built instead by projecting the residual depth errors back over individually traced raypaths in a full tomographic inversion in depth. In this paper we show results of the latter method applied to a Gulf Coast data set. The 3D field example was run in a highly automated fashion. The depth errors (residual moveout) were first picked in batch mode throughout a sparsely sampled volume of common image point gathers and then minimized through a global inversion of all picks at all depths simultaneously. The inverse problem was constrained with preconditioning to solve for the smoothest part of the velocity field first. This automated approach improves turnaround by minimizing human intervention.

Integrating Complementary Tools for Depth Imaging

“…In these areas, grid-based tomography is an effective tool for updating velocities (Woodward et al, 1998). Significantly flatter migrated gathers can often be obtained by using iterative grid tomography in which length scales of the velocity updates are progressively reduced for stability and convergence.…”

Tomographic residual curvature analysis: The process and its components

“…Second is building a linear system of tomographic updating equations that incorporate raypaths traced through the initial model with the data residual errors, which come either from discrepancies between observed traveltimes and raytraced traveltimes through the initial model (traveltime tomography) or from residual moveout on migrated CIG's (RCA). Third is solving the equations by back-projecting these errors along the raypaths, either to minimize the traveltime discrepancies or to flatten the events in either depth or time (Bishop et al, 1985;Van Trier, 1991, Woodward et al, 1999Zhou et al, 2001).…”

Advanced reprocessing and imaging: enhancing legacy surveys

“…Each tomography iteration used multi-parameter (also referred to as non-parametric) RMO picking. This type of picking has been used since the inception of 3D CIP tomography (Woodward et al, 1998). This picking method is particularly important in the presence of short scale lateral velocity variations, such as those observed in the Browse basin that are associated with overburden features and extensive faulting.…”

“…smoothing approach (Woodward et al, 1998). The dip information was derived by estimating the 3D geologic dip and azimuth from an intermediate depth migrated stack.…”