Simulation Of Airborne Electromagnetic Measurements In Three Dimensional Environments

Abstract

A 3-D frequency domain EM modeling code has been implemented for helicopter electromagnetic (HEM) simulations. A vector Helmholtz formulation for the electric fields is employed to avoid problems associated with the first order Maxwell`s equations numerically decoupling in the air. Additional stability is introduced by formulating the problem in terms of the scattered electric fields which replaces an impressed dipole source with an equivalent source that possesses a much smoother spatial dependence and is easier to model. In older to compute this equivalent source, a primary field arising from dipole sources in a whole space must be calculated where ever the conductivity is different than that of the background. The Helmholtz equation is approximated using finite differences on a staggered grid. After finite differencing, a complex-symmetric matrix system of equations is assembled and preconditioned using Jacobi scaling before it is solved using the quasi-minimum residual (QMR) method. In order to both speed up the solution and allow for larger, more realistic models to be simulated, the scheme has been modified to run on massively parallel architectures. The solution has been compared against other I-D and 3-D numerical models and is found to produce results in good agreement. The versatility of the scheme is demonstrated by simulating a survey over a salt water intrusion zone in the Florida Everglades.