Three-dimensional time-domain electromagnetic modelling using a compact finite-element frequency-stepping method

A class of time-domain electromagnetic modelling problems is solved using a frequencystepping solution to the vector diffusion equation. The model consists of a heterogeneous structure embedded in the bottom layer of a two-layer half-space. The compact finite-element method (CFEM) is used for the vector Helmholtz equation part of the solution. The CFEM combines finite-element and integral-equation techniques in a direct, rather than an iterative, solution. The partial use of an isoparametric formulation allows the modelling of a dipping heterogeneous zone without a staircase effect. For sheet-like models, the technique is consistent with modelling results for resistivity contrasts of up to 300: 1. As an illustrative application, the responses of vertical and dipping prisms, in contact with and below a conductive overburden, are compared.