Imaging below dipping anisotropic strata

A historical account of research into seismic anisotropy and research into seismic migration illustrates the progression of geophysical study towards identifying and solving the problem of imaging geologic structures beneath dipping anisotropic strata. With this goal in mind, I developed an anisotropic depth migration algorithm and methodology, demonstrated its effectiveness at correcting for imaging and positioning problems on seismic data, and applied this technology in reducing oil-and-gas exploration risk.; Numerical modelling predicted that the lateral-position error on structures below dipping anisotropic strata would be a maximum when the overburden dip is near 45°. The magnitude of the lateral-position error is approximately equal to Thomsen's anisotropic parameter ϵ multiplied by the thickness of the dipping anisotropic strata. The other anisotropic effect I modelled was the smear effect, where different source-receiver offsets image the same structure at different locations, thereby smearing the structural image. The smear is larger than Thomsen's ϵ multiplied by the thickness of the anisotropic overburden for dips between 15° and 30°.; For algorithm development, I modified a Kirchhoff migration algorithm already in use for seismic imaging in the Canadian thrust belt to handle corrections for seismic velocity anisotropy. Further modification of the velocity-model-building tools allow the creation of anisotropic velocity models for depth migration. Anisotropic velocity model building involves definition of the geologic dip and Thomsen's anisotropic constants. Traditional migration-velocity diagnostics guide the interpretation of the anisotropic velocity model.; Three case histories that apply the methodologies developed for this thesis illustrate the improvements in position accuracy and imaging of subsurface structures when anisotropic effects are corrected in depth migration. The final case history addresses the ultimate purpose of this research: to reduce risk in hydrocarbon exploration. The location for a natural-gas development well was chosen from seismic sections processed using the assumption of isotropic velocities. Drilling in front of the leading edge of the exploration target revealed the incorrect position of the target structure on these processed seismic data. Anisotropic depth migration positioned the subsurface structure accurately. The structural position interpreted on the new seismic section led to additional drilling on the structure and a successful natural-gas well.