Imaging of complex fault-fold structures in a mountainous setting

In areas of complex structural geology, with folds, thrusts and imbricate structures which are typical of many mountain belts around the world, the assumption that the common midpoint (CMP) stack represents a zero-offset section is no longer valid. This is attributed to the strong lateral changes in velocity and coincident events that have different stacking velocities. Similar problems are also encountered in areas of salt mobilization and extensional faulting (Fagin 1991). Under such circumstances, migration after stack will no longer produce a true image of the subsurface, and depth or time migration before stack becomes necessary (Yilmaz 1987; Gray 1998). Furthermore, the conventional assumptions of elevation static corrections are often violated in areas with large topographic variations which are typical of fold-thrust belts. In such environments, pre-stack migration from topography is found to produce the best images of steeply dipping, shallow reflectors (e.g. Gray & Marfurt 1995; Lines et al. 1996; Wu et al. 1998). The objectives of this paper are to demonstrate which migration algorithms perform best for imaging these complex fault-fold environments, evaluate the sensitivity of different migrations to variations in the velocity model, examine the effect of rugged topography on the migration result and to determine which structural styles are likely to be the most difficult to image. This knowledge is of importance to the petroleum industry, as many hydrocarbon traps occur in complex fold-thrust environments.