Magnetotelluric Soundings on a Stratified Earth with Two Transitional Layers
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Pure and Applied Geophysics
Magnetotelluric Soundings on a Stratified Earth with Two Transitional Layers LINJIANG QIN1,2 Abstract—Theoretical magnetotelluric (MT) soundings are investigated for a stratified (five-layered) Earth model consisting of two transitional layers with conductivity varying linearly with depth, and three homogeneous layers with constant conductivity. The analytical expressions for the tangential electric and magnetic fields as well as the surface impedance are derived in terms of Airy functions. The effect of the thicknesses of the two transitional layers and the interlayer between them on the MT responses (apparent resistivity and impedance phase) is examined in detail. Keywords: Magnetotelluric soundings, transitional layer, conductivity, stratified Earth.
1. Introduction The magnetotelluric (MT) method is used to image the subsurface conductivity structures of Earth by simultaneously recording the surface electric and magnetic fields in mutually perpendicular directions induced by natural electromagnetic waves resulting from the interaction of Earth’s magnetic field with the solar wind (Tikhonov 1950; Cagniard 1953; Tikhonov 1986). Before the rapid development of computers, the interpretation of MT responses was mainly dependent on a one-dimensional (1D) model consisting of many layers, with constant conductivity in each layer and sharp boundaries. However, this assumption is not sufficient to describe the
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Key Laboratory of Submarine Geosciences, Second Institute of Oceanography, Ministry of Natural Resources, Hangzhou 310012, People’s Republic of China. E-mail: qinlinjiang@ 126.com; [email protected] 2 Southern Marine Science and Engineering Guangdong Laboratory (Zhuhai), Zhuhai 519080, People’s Republic of China. 3 Key Laboratory of Geoscience Big Data and Deep Resource of Zhejiang Province, School of Earth Sciences, Zhejiang University, Hangzhou 310027, People’s Republic of China. E-mail: [email protected]
and CHANGFU YANG3 conductivity distribution of the Earth. As was pointed out by Mallick and Roy (1968), the boundaries between layers in the geological section in many instances are transitional rather than sharp in nature. A number of analyses in the field of resistivity well logging have also provided much evidence to support this statement. Therefore, it is necessary to study the theoretical MT fields of a 1D model with a transitional layer. A transitional layer refers to a layer in which conductivity (or its reciprocal, resistivity) varies with depth. The analytical MT soundings of a 1D model with a transitional layer have been investigated in detail by various authors. For example, Mallick (1970) and Kao and Rankin (1980) investigated the MT soundings of a 1D layered model with a transitional layer in which the conductivity varied linearly as a function of depth. Berdichevsky et al. (1974) treated the MT fields over models of a transitional layer in which resistivity decreased exponentially with depth. The MT fields of a vertical inhomogeneous Earth having resistivit
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