An interpretive analysis of magnetotelluric data

Abstract

A simple analysis to determine the dimensionality of magnetotelluric resistivity variations, to separate the near‐surface resistivity variation from the variations at depth, and to find the relative resistivity variations with depth is described. Parameters derived from the magnetotelluric impedance tensor are mapped in pseudosection and planar view for two‐dimensional (2-D) model data and for survey data from Montana. These data illustrate procedures that can qualitatively map complex resistivity structures in the lateral distance‐frequency domain or, equivalently, the lateral distance‐scaled vertical distance domain. Phase‐dependent parameters have better vertical resolution than parameters related to scalar apparent resistivity, but the latter allow estimates of the resistivities encountered. Phase parameters for the Montana data provide a semiquantitatively accurate cross‐section of a geologically known anticlinal structure. This suggests a means of subsurface structural mapping with magnetotelluric data. The use of these parameters with sufficiently dense and accurate data provides a first‐order indication of structure for subsequent quantitative modeling.