On galvanic distortion of regional 3-D MT impedances On galvanic distortion of regional three-dimensional magnetotelluric impedances

Abstract

Summary In magnetotelluric (MT) studies, the observed response function (the MT impedance) usually suffers from galvanic distortions due to near-surface inhomogeneities. Removal of these effects is essential to obtain an accurate model of the subsurface electrical conductivity structure. Galvanic distortion is usually expressed by a simple real tensor multiplying the undistorted regional impedance. The problem still remains of how to solve the ensuing linear equations in order to determine the distortion tensor and then to obtain the undistorted impedance. The methods presented and widely applied in previous works assume two-dimensionality for the undistorted impedance. This paper proposes a method that employs a relationship between the spatial derivatives of the horizontal electric field and the vertical geomagnetic component, which can be directly derived from Faraday’s law. The identity derived from the relationship is written using the vertical magnetic transfer function, the impedance, and the spatial derivatives of the impedance and horizontal magnetic transfer functions. The present method determines the real distortion tensor so that the identity is satisfied. Therefore, the method has two major advantages: (1) galvanic distortion that is to be removed from the impedance tensor is clearly defined, and (2) the method is applicable even when the regional structure is 3-D.