The Effect Of Resistivity Anisotropy On Earth Impulse Responses

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