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

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Paper –  [pdf] AMT_Topo

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.

But collecting TE-mode data 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.