3. EM/GPR Imaging Technologies

Analyses of the Magnetotelluric Impedance: Comparison and Assessment

Nguyen Thanh Van and Levan Anh Cuong

Department of Geophysics ,University of Natural Sciences,VietNam Nationnal University-HoChiMinh City, Vietnam.

Abstract

Magnetotelluric analysis is one of the methods to research inhomogeneity of 2D and 3D electric environments,whose depths are about tens kilometers. Explaining MT data is to get useful arbitrary parameters from a general MT impedant tensor.These processes are analysed by using different methods such as Eggers method, Latorraca method, conventional method and Mohr cycles method. First, the basic idea of the conventional data analysis is to set rotation Z. The rotation angle is chosen so that Z approximates in some sense the form for a two-dimensional (2D) subsurface conductivity distribution,with zero elements on the diagonal.When 3D distortions exist, quantitative interpretation only based on the off-diagonal elements of the conventionally rotated impedance tensor is incomplete because these off-diagonal elements are insensitive to the tensor trace. In order to solve this problem, Simons, Lilley, Eggers and Latorraca defined a complete set of parameters suitable for recognition of three-dimensionality. Simons Spritz's approach gives less information (six parameters). The Eggers's parameters are rather similar to Latorraca's (eigh parameters) but the beginning ideas to establish them are different.The principal axes of polarization ellipses of eigenvector pair (Ei, Hi) are perpendicular in Eggers method. On the other hand, the principal axis direction for the E and H vectors are not at right angles as in the biorthgonal analysis in the Latorraca method. Because of invariant phases of eigenvalues of tensor Z in Eggers's method while phases of eigenvalues of tensor Z in Latorraca's method are variant (althought they vary a little), Eggers method is more effective. The Mohr circles construction (Lilley's method) gives seven invariants of a tensor which explain the properties of 1D, 2D, 3D environment clearly and easily understandable. So, we use Eggers method and Mohr circles method to analyse 3D Model data and Benluc_VietNam experiment data order to draw helpful geology information

Last modified: Fri May 19 18:51:52 2006