Influence of two-dimensional electrical anisotropy on dimensionality analysis of magnetotelluric data
The magnetotelluric(MT)method is one of the main methods for imaging the electrical resistivity structure of the whole lithosphere.Dimensionality analysis of MT data can infer the properties of the subsurface geoelectric structure and is fundamental in determining the most suitable modeling method(1D,2D or 3D).However,when the subsurface exhibits electrical anisotropy,its effect on dimensionality analysis is relatively less studied and more unknown.Here,based on two-dimensional theoretical models with electrical anisotropy,we used WAL criterion,phase tensor and induction vector to study the effect of anisotropy on dimensionality analysis of MT data.According to the real dimensional information of our models and the results of dimensionality analyses,isotropic inversions were subsequently carried out to study the influence of electrical anisotropy on the inversions.Our research shows that the azimuthal anisotropy and the structural complexity of electrical anisotropy are important factors that influence the dimensionality analysis.Azimuthal anisotropy can result in apparent"false three-dimensionality".According to two-dimensional isotropic inversions,joint inversions of TE+TM modes show an electrical structure interleaved by high and low resistivity anomalies in the true anisotropic body region.The differences in the separate inversions of the TE and TM modes are closely related to the distribution of electrical anisotropy.The inversion results of the TM dataset are more reasonable,while inversions of the TE dataset from the responses of azimuth anisotropy models show a"T"-shaped low-resistivity anomaly.According to the 3D inversions indicated by dimensional analysis,inversion of the tipper can effectively identify the change of azimuth angle,while the impedance is susceptible to the distortion of electrical anisotropy.Our study suggests that dimensionality analysis of MT data with identifying and considering electrical anisotropy will be a key research point in future,which can help to improve the interpretation accuracy of real data.
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