A simple form of MT impedance tensor analysis to simplify its decomposition to remove the effects of near surface small-scale 3-D conductivity structures

Magnetotelluric (MT) is a natural electromagnetic (EM) technique which is used for geothermal, petroleum, geotechnical, groundwater and mineral exploration. MT is also routinely used for mapping of deep subsurface structures. In this method, the measured regional complex impedance tensor (Z) is substantially distorted by any topographical feature or small-scale near-surface, three-dimensional (3-D) electrical inhomogeneity. The effects of this local galvanic distortion should be separated and removed from the regional response to improve the accuracy and reliability of the data interpretation. In this paper, it is attempted to introduce an effective form of tensor analysis to facilitate the process of GB (Groom-Bailey) tensor decomposition on MT data. This approach was used to recover the regional response of conductivity structures beneath 12 MT sounding sites of a sedimentary basin in South Australia. The results of this study clearly indicate that the regional structures beneath these sites are two-dimensional (2-D) and their strike are mainly in NS (±100) direction which are geologically supported. The obtained results also show that the distortion parameters of the surficial bodies are fairly constant for the whole frequency band or its two or, at most, three subsets. In addition, the low misfit values between the measured impedances and those produced by a hypothetical 3D/2D model confirm that the regional structures beneath all these 12 MT sites are 2-D and some local surficial 3-D features are superimposed on them.