An investigation of the proposed WALDIM criteria to identify electrical anisotropy in complex geological region; case study: continental margin

In an electrically anisotropic media, current density  is not aligned with the electric field  and varies with the E-field direction. This can be considered as the spatial aliasing effects aroused by the subsurface structures whose individual dimensions are smaller than the inductive length scale of diffusively propagating EM fields. The role of these structures is particularly significant in tectonically active regions, where tectonic processes induced penetrating fabrics. Accordingly, identification, characterization and interpretation of electrical anisotropy have large implications to understand evolutionary aspects of geological structures, evaluate the economic resources and interpret hydrological flows (Wannamaker, 2005). Marti (2014) provides a comprehensive review of the works conducted to recognize electrical anisotropy imprints during MT data analysis and also different strategies used to model this property in case studies.
Dimensionality analysis is a preliminary stage in MT data interpretation procedure to recover the strike direction of the regional geo-electric structure, characterize the distortion effects of superficial conductive bodies and also to adopt an appropriate modeling approach (1D, 2D or 3D), coincident with the intrinsic dimension of the measured data. The application of the preliminary dimensionality tools, Swift’s and Bahr’s skews for synthetic and real MT data affected by anisotropy shows that they are disable to identify the anisotropy footprints and distinguish between structural and anisotropy strike directions (Heise and Pous, 2001). Weaver et al., (2000) suggested a family of rotationally invariant parameters characterizing the dimensionality properties of the underlying geo-electric structures. Marti et al. (2009, 2010) published the WALDIM code based on these invariants and extended them to provide proper conditions from which isotropic and anisotropic structures could be differentiated. The main criteria proposed by the WALDIM code to differentiate anisotropic media from isotropic ones are as follows:
The WAL rotational invariant values indicate a 2D regional structure, while the strike directions estimated from the first and second columns of impedance tensor are inconsistent. This situation is mentioned as “3D/2D anisotropy” in subsequent table and figures.
We report here on the application of this scheme to analyze the dimensionality of MT responses from some principal anisotropic models representing complex geological settings at continental margins and also for MT data from an active continental margin in South-Central Chile, where the presence of electrical anisotropy has been previously recognized (mainly from geomagnetic transfer functions).
In electrical anisotropy modeling resistivity is represented as a symmetric, positive definite tensor which can be diagonalized employing Euler’s elementary rotations to obtain its principal directions and their corresponding resistivities (principal resistivities: ρ_xx, ρ_yy, ρ_zz). These directions are known as the strike (α_s), dip (α_D) and slant (α_L) anisotropy angles. The non-zero values of these angles and the specified relationships between principal resistivities would determine the type and geometry of the electrical anisotropy. We restricted our study to uniaxial, azimuthal anisotropy, where α_s≠0, α_D= α_L= 0 and also ρ_xx= ρ_zz≠ ρ_yy.
Model responses were calculated employing the algorithm of Pek and Verner, 1997. The proposed models of geological settings are selected so that their complexity is gradually increasing. Dimensionality analysis results for the synthetic model responses and real data are depicted in figures (2, 3 and 4) and (6 and 7), respectively. The results indicate that the proposed criteria is slightly firm in the sense that they could not identify electrical anisotropy in the presence of galvanic distortions caused by superficial conductive structures and complexities of regional structures.