Image mispositioning due to dipping TI media: A physical seismic modeling study

A scaled physical model was constructed to investigate the magnitudes of imaging errors incurred. by the use of isotropic processing code when there is seismic velocity anisotropy present in the dipping overburden. The model consists of a block of transversely isotropic (TI) phenolic material with the TI axis of symmetry dipping at an angle of 45 degrees. Its scaled thickness is 1500 m, and it is intended to simulate the dipping elastic sequences found in many fold-thrust belts. A piece of isotropic Plexiglas, affixed to the underside of the anisotropic block, has a step function in it to simulate a target reef edge or fault. The anisotropy parameters of the material are delta = 0.1 and epsilon = 0.24. On zero-offset data the imaged position of the target is shifted laterally 320 m in the updip direction of the beds, whereas on time- and depth-migrated multichannel sections the shift is 300 m. The lateral shift is offset dependent, with the amount of shift in any common-midpoint gather decreasing from 320 m on the near off-sets to 280 m on the far offsets. Prestack depth-migration velocity analysis based upon obtaining consistent depth images in the common-offset domain results in the base of the anisotropic section being imaged 50 m (about 3%) too deep.