The effect of seafloor topography on magnetotelluric fields: an analytical formulation confirmed with numerical results

Abstract

SUMMARY Magnetotelluric fields and impedances are distorted at undulating interfaces. An analytical formulation is presented to calculate magnetotelluric effects in the presence of a sinusoidal interface. In contrast to previous analytical approaches, this formulation is not based on perturbation theory. It is applicable to observations both on land and on the seafloor. Electric and magnetic fields, as well as apparent resistivities and phases, are calculated on the interface. The topographic distortion on land mainly influences the TM mode data where the electric field is perpendicular to the geological strike. Both the TM mode and the orthogonal TE mode data are distorted on the seafloor. Systematic parameter tests indicate which modes are independent of the period and the conductivity contrast, provided that the induction depths are large relative to the amplitude of the topography. The differing physics of the seafloor and land surface is illustrated by plotting current streamlines. For the land model, contour lines diverge below a hill and converge below a valley. For the seafloor model, electric currents mainly flow in the conductive sea water. Contour lines converge above a hill and diverge above a valley. Numerical results, derived from finite-element modelling, support the analytical solutions. Streamlines of the electric current, derived from a model for the Central Andes, illustrate the connection between a graphical display of electromagnetic fields and an algebraic sensitivity analysis of the magnetotelluric impedance tensor.