An Interpretive Analysis of Magnetotelluric Data

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 pseudo-section 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, equivalents, 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.