Modelling 2D Magnetotellurics in Anisotropic Media using Esys-Escript

Anisotropy in the electrical conductivity of the subsurface can highly disturb measured Magnetotelluric (MT) responses in comparison to expected observations under isotropic assumption, hence it can lead to difficulties in the interpretation of MT observations. In order to further understand low frequency electomagnetic (EM) wave propagation through the Earth in the presence of anisotropy and - in a second step - to obtain better inversion results of MT survey data, a robust modelling code producing accurate simulation results needed to be developed. We present open source Python package Esys-Escript. It is designed to solve mathematical modelling problems using the finite element method (FEM). It is especially suitable for geophysical modelling and inversion and has been successfully applied to gravity, direct current resistivity, and isotropic MT problems. Its key idea is to formulate the problem in terms of a general partial differential equation (PDE) with proper boundary conditions. By providing PDE template to simplify the task of python programming it allows geoscientists to focus on solving the actual modelling problems without being experts in numerical modelling. This object-oriented programming style creates a flexible and easy-to-use interface for users to tackle their tasks. PDEs will be then solved on structured or unstructured 2D or 3D meshes. To facilitate the modelling of complex and geologically realistic structures, meshes are read directly by Esys-Escript after building up by using tools like Gmsh. Esys-Escript has been used to simulate 2D isotopic MT responses in frequency domain with a good agreement with published analytical solutions. Extending from this work, we present an implementation of the MT forward modelling for anisotropic media using Esys-Escript. In order to validate our code, we show anisotropic MT modelling results with the comparison of analytical solutions under an axially anisotropic setting. Then we add more examples displaying Escript MT modelling results under the assumptions of azimuthal and dipping anisotropic settings and more general anisotropy cases.