On galvanic distortion of regional three-dimensional magnetotelluric impedances

In magnetotelluric (MT) studies, the observed response function (the MT imp edance) usually suffers from galvanic distortions due to near-surface inhom ogeneities. Removal of these effects is essential to obtain an accurate mod el of the subsurface electrical conductivity structure. Galvanic distortion is usually expressed by a simple real tensor multiplying the undistorted r egional impedance. The problem still remains of how to solve the ensuing li near equations in order to determine the distortion tensor and then to obta in the undistorted impedance. The methods presented and widely applied in p revious works assume two-dimensionality for the undistorted impedance. This paper proposes a method that employs a relationship between the spatial de rivatives of the horizontal electric field and the vertical geomagnetic com ponent, which can be directly derived from Faraday's law. The identity deri ved from the relationship is written using the vertical magnetic transfer f unction, the impedance, and the spatial derivatives of the impedance and ho rizontal magnetic transfer functions. The present method determines the rea l distortion tensor so that the identity is satisfied. Therefore, the metho d has two major advantages: (1) galvanic distortion that is to be removed f rom the impedance tensor is clearly defined, and (2) the method is applicab le even when the regional structure is 3-D.