The effect of resistivity anisotropy on earth impulse responses

Abstract

Resistivity anisotropy arises through a variety of scales from micro (e.g. grain size, pore water connectivity) to macro (e.g. laminated sand-shale sequences). For general anisotropy the physical property under consideration may vary in all three spatial directions. The simplest problems involve transverse anisotropy where resistivity at a point in any direction in a plane differs from the value perpendicular to the plane. We are here concerned solely with transverse anisotropy with a vertical axis of symmetry (TIV) so that resistivity at a point has a constant magnitude in any horizontal direction. Induction logs, laterolog and LWD (logging-while-drilling), at least in vertical wells, may be used to examine TIV in particular and these well log results often differ from indirect determinations of resistivity through DC resistivity and general EM surveying. Much of the earlier EM literature considered resistivity as isotropic but there is now great emphasis on the inclusion of anisotropy in modeling and inversion studies. In this paper we consider the effects of transverse anisotropy (specifically TIV) on the earth?s electromagnetic impulse and step responses.