Seismic Imaging Beneath Thrust Blocks

Abstract

In a proof of concept study, conditions under which turning ray fault plane reflections can be applied to imaging sediments beneath an overthrust block are investigated through modeling. In a simplified overthrust model, conventional primary reflections and fault plane reflections are compared as ways of imaging sediments beneath the block. Velocities chosen for basement and sediments in the model correspond broadly to those for igneous rocks, elastics, and carbonates of the Anadarko Basin, Oklahoma. The acquisition geometry required in order to utilize fault plane reflections is considered, and amplitude behaviors associated with vertical velocity gradients of.2,.5, and.7 ft/sec per ft are investigated through ray tracing. In the case of a vertical velocity gradient of.7 ft/sec per ft, the simplified model is modified to include a syncline beneath an overthrust. When the vertical velocity gradient is.7 ft/sec per ft, amplitudes for the fault plane reflection become substantially larger than those for the primary reflection, which suggests that recording the fault plane reflection may be useful in imaging subthrust sediments in some situations. Challenges in the acquisition and processing of actual fault plane reflections are discussed.