Probing 1-D electrical anisotropy in the oceanic upper mantle from seafloor magnetotelluric array data

SUMMARY Electrical anisotropy in the oceanic upper mantle can only be imaged by seafloor magnetotelluric (MT) data, and arguably provides important clues regarding the mantle structure and dynamics by observational determinations. Here, we attempt to probe the electrical (azimuthal) anisotropy in the oceanic mantle by analysing recent seafloor MT array data from the northwestern Pacific acquired atop 125–145 Ma seafloor. We propose a method in which an isotropic 1-D model is first obtained from seafloor MT data through an iterative correction for topographic distortions; then, the anisotropic properties are inferred as deviations from the isotropic 1-D model. We investigate the performance of this method through synthetic forward modelling and inversion using plausible anisotropic 1-D models and the actual 3-D bathymetry and topography of the target region. Synthetic tests reveal that the proposed method will detect electrical anisotropy in the conductive upper mantle or electrical asthenosphere. We also compare the performance of the proposed scheme by using two rotational invariant impedances and two topographic correction equations. The comparison reveals that using different rotational invariants and correction equations provides relatively consistent results, but among the rotational invariants, the sum of squared elements (ssq) impedance yields better recovered results for topographically distorted data than the determinant impedance. An application of the method to seafloor MT array data sets from two areas in the northwestern Pacific reveals the possible presence of two layers of electrical anisotropy in the conductive mantle (<100 Ω-m) at depths of ∼60–200 km. The anisotropy is estimated to be more intense in the shallower layer for both areas. On the other hand, the estimated anisotropic azimuth (defined as the most conductive direction) and the depth to the interface between the two layers are different between the two array areas separated by a small horizontal distance of ∼1000 km in spite of their similar seafloor ages. The most conductive directions are aligned neither with the current absolute plate motion direction nor with the fastest direction of seismic azimuthal anisotropy. The inferred electrical anisotropy features may result from array-scale (∼1000 km) mantle dynamics, such as small-scale convection, which might affect the electrical and seismic properties differently, although there remains the possibility that some portions of these features are explained by laterally heterogeneous mantle structures.

Invariant TE and TM impedances in the marine magnetotelluric method

SUMMARY The magnetotelluric (MT) impedance tensor has a nil diagonal when one of the axes of the coordinate system coincides with the strike of a 2-D structure. In general, real data are full tensors either because of 3-D effects or measurements not aligned to the geological strike. The usual practice to adapt the field tensor to the 2-D assumption is to rotate to a new system of coordinates. In most cases, there is no single angle of rotation that warranties that the diagonal elements become zeros as in the ideal 2-D case. Even maximizing the off-diagonal elements does not necessarily produce a nil diagonal. Consequently, the 2-D inversions proceed by neglecting whatever there is left in the diagonals. In this work, we explore an alternative that places no constraints on direction but assures a nil diagonal. We use two rotational invariants that compact the four elements of the tensor into only two and reduce in 2-D to the TE and TM impedances. These are obtained readily by solving a quadratic equation. We explore four different scenarios: (1) using the invariants, (2) rotating the tensor perpendicular to the profile, (3) rotating to the average maximum orientation for each station and (4) maximizing the off-diagonal elements of the tensor for each site, frequency to frequency. These approaches were applied to 3-D synthetic and field data. The field data correspond to two marine magnetotelluric surveys in the Gulf of California. In one of them, there is no information on the instrument orientation because the compasses failed. In this case, the rotational invariants come handy to overcome the problem. In the other survey, there was orientation information and the 2-D inversions illustrate the better performance of the invariants relative to the traditional approaches.