An improved forward modeling method for two-dimensional electromagnetic induction problems with bathymetry

Abstract

Recently, electromagnetic observations have become common not only on land but also on the seafloor. In particular, thanks to the development of instruments for use in shallow seas, one can conduct observations along land-sea arrays. However, since there is a large contrast in conductivity between sea water and rocks in the crust and the mantle, we have to pay more attention to the accuracy of forward solvers used for electromagnetic induction problems, including bathymetry. In this paper, we develop a two-dimensional forward code using triangular finite elements, and confirm the accuracy of the new code by TM mode responses. The accuracy of the improved solver was tested by comparison with an analytical solution in a hemi-cylindrical geometry. We also show that triangular elements are more reliable than rectangular elements in determining conductivity structures beneath land-sea arrays. Our results indicate the importance of precisely discretized bathymetry and the accuracy of spatial derivatives of electromagnetic field components, especially in the vicinity of coastlines.

Three‐Dimensional Time Domain Simulation of Tsunami‐Generated Electromagnetic Fields: Application to the 2011 Tohoku Earthquake Tsunami

“…These demands are readily met by our unstructured tetrahedral mesh. It is also noticeable that realistic representation of bathymetry by triangular/tetrahedral grids is more reliable than so‐called “staircase expression” for accuracy of magnetic fields at the seafloor (e.g., Minami & Toh, ). Furthermore, the curvature of the Earth can be expressed by our mesh, which improves estimation of large‐scale electric current circuits stemming from tsunami propagation.…”

Two‐dimensional simulations of the tsunami dynamo effect using the finite element method

“…We adopted simulations in the time domain, because transient waves such as solitary waves cost a lot to reproduce in the frequency domain. Furthermore, we adopted FEM because unstructured triangular meshes can express actual bathymetry, which may virtually be impossible using rectangular meshes [ Minami and Toh , ]. Our simulation method consists of two steps.…”

Automated airborne EM anomaly picking and 3D model fitting

“…Wilson et al, (2012) and Yang and Oldenburg (2012) attempt 3D inversion using over parameterised blocky models, with millions of voxels. There are several disadvantages to the over parameterised approach: 1) it requires somewhat arbitrary stabilisation techniques such as "smoothing" or minimal departure from an assumed starting model 2) it consumes very significant computer time and memory resources and, most importantly 3) if using rectangular voxels, it cannot properly model thin conductors with dips or strikes much different from 0 or 90, with the worst cases at 45 (Minami and Toh, 2012).…”