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

Abstract

S U M M A RYIn magnetotelluric (MT) studies, the observed response function (the MT impedance) usually su¡ers from galvanic distortions due to near-surface inhomogeneities. Removal of these e¡ects 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 ¢eld 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 satis¢ed. Therefore, the method has two major advantages: (1) galvanic distortion that is to be removed from the impedance tensor is clearly de¢ned, and (2) the method is applicable even when the regional structure is 3-D.

Use of ssq rotational invariant of magnetotelluric impedances for estimating informative properties for galvanic distortion

“…Here, the period range was selected to sense a structure existing between 10 and 100 km in depth. Therefore, any small structures confined in the near-surface layer of a few kilometers or less, which is shallower than the inductive scale length of present interest, are considered to be galvanic distorters (Utada and Munekane 2000). We further assume for simplicity that galvanic distorters have…”

“…In addition, the static shift could be corrected with the TEM data with some limitations (see Watts et al 2013;Tournerie et al 2007;Wilt and Williams 1989), e.g., when the heterogeneity is smaller than the transmitter loop. Utada and Munekane (2000) attempted to solve this problem by introducing Faraday's law as a constraint, but the solution was not practical. The numerical examples presented here show that the concept of the apparent gain can be used to approximate the site gain in the assumed situation.…”

“…Although it is possible to perform global induction studies using geomagnetic observatory data, there are significant differences among existing inverted models (e.g., Kelbert et al 2009;Kuvshinov and Semenov 2012;Semenov and Kuvshinov 2012). Most likely, such attempts may include biases due to the nonuniformity of their site distributions or false images resulting from spatial aliasing (Utada and Munekane 2000) because the distributions of existing geomagnetic observatories and MT observation sites are spatially nonuniform. More importantly, the EM induction method is generally sensitive to the conductivity beneath each observation site.…”

“…Here, we consider the case in which MT data are obtained from an array of observations on land. The solution of this problem is not straightforward because MT data on land are usually affected by galvanic distortion, i.e., an alteration in the MT impedance due to near-surface small-scale heterogeneity that is smaller than a typical site spacing and confined to be shallower than the inductive scale length of interest (Ledo et al 1998;Utada and Munekane 2000;Bibby et al 2005). In other words, the physical dimensions of the distorting bodies are smaller than their inductive scale length and also that of the host.…”

2-D Versus 3-D Magnetotelluric Data Interpretation

“…For 2-D data, application of the techniques proposed by Zhang et al (1987), Jones (1988), Bailey (1989, 1991), Bahr (1991), Groom and Bahr (1992), Jones and Groom (1993), Chakidri et al (1992), Chave and Smith (1994), Smith (1995), Spitzer (2001), and Ledo et al (2002b) usually suffice. For 3-D data, the methods proposed by Ledo et al (1998), Utada et al (2000), and Garcı´a and Jones (2002) are advocated. A complete review of different aspects and techniques can be found in the reviews by Jones (1983), Jiracek (1990) and Ogawa (2002).…”