The magnetotelluric phase over 2‐D structures

Abstract

In 1985 Fischer gave a simple explanation for the behaviour of the E‐polarization magnetotelluric phase over a 2‐D dike. This phase behaviour is a direct consequence of the rearrangement of the current flow in a 2‐D structure as opposed to the flow in a simple uniform half‐space. In spite of its usefulness it is shown that the simple phase rule is only qualitatively correct. For B‐polarization it seemed at first that the simple phase rule would not remain valid. However, it is found that if the very different current distributions for E‐ and B‐polarization are correctly interpreted, the simple phase rule retains its validity qualitatively. Arguments are given for the very general validity of this rule at the surface of any 2‐D structure, even in the presence of complicated topography. The rule is best formulated in terms of two statements: (a) the magnetotelluric phase is a continuous function over any structure, even across outcropping resistivity contrasts; and (b) where the current is drawn to greater depths the phase will rise, and where the current is concentrated near the surface the phase will drop. Copyright © 1992, Wiley Blackwell. All rights reserved