Magnetotelluric strike rules

Abstract

The distortion of the magnetotelluric impedance tensor by complex “near‐surface” structure leads to leakage between the elements of the tensor. The magnetotelluric impedance tensor for our principal model, which has both a local and a regional strike, can be written in the long‐period limit as a sum of the regional, undistorted impedance and a perturbed impedance. The latter can be written as a product of a local distortion (which can be regarded as thin‐sheet distortion in the long‐period range) tensor and the regional impedance. Local and regional strikes are found by rotating the impedance tensor into directions in which diagonal elements are proportional and column elements are proportional, respectively. The regional impedance tensor is calculated assuming that the strikes are uniquely defined. An example from a crystalline area with well conducting fracture zones illustrates the model concepts. A weighted least‐squares procedure is used for the estimation of distortion parameters.

The first magnetotelluric image of the lithospheric-scale geological architecture in central Svalbard, Arctic Norway

“…1) becomes simpler and only the off-diagonal elements have values that are significantly different from zero when the tensor is rotated along the strike direction. In our case, a regional N308E strike direction was identified, with 908 ambiguity (Simpson & Bahr 2005;Chave & Jones 2012), using Bahr's phase-sensitive (Bahr 1991) and Q-function (Zhang et al 1987;Smirnov & Pedersen 2009) methods as dimensionality tools (Fig. 2a, b).…”

A magnetotelluric investigation of the Scandinavian Caledonides in western Jämtland, Sweden, using the COSC borehole logs as prior information

“…For MT data, Zhang's strike and distortion model (Zhang et al 1987), Bahr's phase-sensitive skew (Bahr 1991) and the phase tensor (Caldwell et al 2004) were used for dimensionality and strike analyses.…”

“…Moreover, when we computed misfit values √ Q using Zhang et al's distortion model (e.g. Zhang et al 1987;Smirnov & Pedersen 2009;Bastani et al 2011) with fixed strike angles (e.g. Fig.…”

“…In this configuration, the diagonal and off-diagonal elements of any column of the distorted impedance tensor are related to the corresponding regional off-diagonal impedance tensor element in strike coordinates by real and frequency independent distortion coefficients (Zhang et al 1987;Groom & Bailey 1989). In subsequent inversion, the diagonal elements of this configuration are disregarded, because they do not contain additional information on the regional structure as compared to the off-diagonal elements.…”

Magnetotelluric Tensor Decomposition: Insights from Linear Algebra and Mohr Diagrams

“…In co-ordinates aligned with the surficial geology the distortion amounts to scaling the electric fields by different amounts, g 1 and g 2 , in directions parallel and perpendicular to the surficial strike (Zhang et al, 1987). Distortion matrix d then has the form…”