3-D electromagnetic modelling and inversion incorporating topography

Abstract

Topographic effects are known to influence electromagnetic (EM) field measurements, but are rarely taken into consideration in interpretation. In this paper, 3-D modelling and inversion schemes that allow topography to be included are presented. The modeling method is based on a staggered-grid finite difference method that uses the biconjugate gradient method preconditioned with an incomplete Cholesky decomposition to solve the system of equations. In addition, to accelerate the solution, the static divergence correction procedure is used. This proves to be essential in dealing with extremely high conductivity contrasts between the earth and air. Model examples include magnetotelluric, controlled-source ground EM, and airborne EM responses to 3-D hills. The accuracy of the scheme is demonstrated by comparison with 2-D finite-element and semi-analytical solutions. Once modelling of topographic responses is possible, it becomes easy to modify existing inversion algorithms so that topography can be incorporated. Synthetic examples for ground and airborne EM surveys show that the inversion recovers the resistivity structures well with little influence from topography.