Using synthetic magnetotelluric data to evaluate the efficiency of methods based on local-regional decomposition of the impedance tensor

Abstract

A characteristic simplified resistivity model of the earth’s crust has been compiled, containing a three-dimensional conductive sedimentary depression in a resistive basement. Two variants of the model are considered: with a uniform near-surface part and with multiple local near-surface inhomogeneities. Using three-dimensional modeling, synthetic magnetotelluric sounding (MTS) data were calculated using a system of profiles. In the data for the second variant of the model, a widespread effect of near-surface distortions is observed, leading to a static shift in the amplitude MTS curves, but not affecting the phase curves. We applied methods for separating local and regional effects in MT data, based on the corresponding decomposition of the impedance tensor. It is shown that, under the conditions of the applicability of this decomposition, the Bahr’s and phase tensor methods make it possible to confidently determine the principal directions (azimuths) associated with regional structures (with a sedimentary depression) and not distorted by local effects (of near-surface inhomogeneities). The effectiveness of using maps of principal directions, polar diagrams of the phase of impedance tensor component and of the phase tensor components, as well as phase tensor ellipses for characterizing regional resistivity structures is demonstrated.