Polar anisotropy from walkway VSPs

Abstract

Polar anisotropy, also called vertical transverse isotropy (VTI), causes a dependence of velocity on propagation direction resulting from layering or microstructure. Shales commonly exhibit this type of anisotropy, and the magnitude of the effect can be large. A stack of isotropic layers will also exhibit the effects of polar anisotropy; such a medium is then said to be effectively anisotropic. It is well known that polar anisotropy can have a large effect on seismic imaging and neglecting its effect causes mispositioning and degraded focusing of reflections. It can also impact AVO. Demands on polar anisotropy measurements are different depending on the problem. For correct seismic imaging (migration), average or effective properties are needed; for AVO interpretation, local properties are also important. Both types of polar anisotropy measurements are possible with walkaway (multioffset) vertical VSP data.