A 3-D frequency domain EM modeling code has been implemented for helicopter<br>electromagnetic (HEM) simulations. A vector Helmholtz formulation for the electric fields is<br>employed to avoid problems associated with the first order Maxwell’s equations numerically<br>decoupling in the air. Additional stability is introduced by formulating the problem in terms of the<br>scattered electric fields which replaces an impressed dipole source with an equivalent source that<br>possesses a much smoother spatial dependence and is easier to model. In order to compute this<br>equivalent source, a primary field arising from dipole sources in a whole space must be calculated<br>where ever the conductivity is different than that of the background.<br>The Helmholtz equation is approximated using finite differences on a staggered grid. After<br>finite differencing, a complex-symmetric matrix system of equations is assembled and<br>preconditioned using Jacobi scaling before it is solved using the quasi-minimum residual (QMR)<br>method. In order to both speed up the solution and allow for larger, more realistic models to be<br>simulated, the scheme has been modified to run on massively parallel architectures. The solution<br>has been compared against other 1-D and 3-D numerical models and is found to produce results in<br>good agreement. The versatility of the scheme is demonstrated by simulating a survey over a salt<br>water intrusion zone in the Florida Everglades.


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