The fault shadow problem: Its nature and elimination

Abstract

The fault shadow problem is what explorationists call the zone of unreliable seismic imaging in the footwalls of faults. Although it can occur in all types of faults, the term is usually applied to extensional faults. These problems are common-place, but much current exploration still relies on images in which they remain. In this article I present an example of the fault shadow problem from the Wilcox trend (Eocene/Paleocene) of south Texas. The stratigraphic-velocity alternations in this area cause a distinctive suite of problems in conventional time imaging. Explorationists can cite numerous examples of wells drilled on time highs, but significantly off structure in depth. Moreover, imaging often degrades precipitously at levels below the Middle Wilcox even when wells indicate an uncomplicated structure. I’ll begin by demonstrating, through schematic and synthetic examples, the nature of these problems and their elimination through prestack depth imaging. A real data example will then show identical effects.