The Inversion of Magnetotelluric Data and the Elimination of Topographic Effects Through Modeling

Rough topography may cause severe distortion in imaging based on AMT data acquired with the electric field oriented perpendicular to geologic strike, i.e. TM-mode data. Topographic peaks create high-angle conductive distortion while topographic valleys create high-angle resistive features in TM-mode AMT data. Topographic distortions will be carried through to produce high-angle conductive or resistive artifacts in inversion models unless the imaging procedure accounts for the distortion. SCSINV 1-D resistivity-depth images are affected by 2-D topography, while SCS2D 2-D inversions with models including a topographic profile do not.

TE-mode magnetotelluric data, with the electric field oriented parallel to geologic strike, is distorted less by 2-D topography than is TM-mode data. However collecting TE-mode is generally impractical when collecting closely spaced data along survey lines oriented perpendicular to geologic strike. Continuous AMT production is optimized when electric-field dipoles are positioned along survey lines to collect TM-mode data. In contrast, aligning electric field dipoles perpendicular to survey lines to collect TE-mode data is time consuming (and expensive). As a result, most closely sampled AMT data are collected in the scalar TM-mode. Consequently, this paper focuses mostly on the effects of topography upon the interpretation of TM-mode AMT data, although some TE-mode results are included.