Three-dimensional forward modeling and characterization of the responses of the ground-airborne frequency-domain electromagnetic method

Abstract

Investigation of three-dimensional forward modeling for the ground-airborne frequency-domain electromagnetic (GAFEM) method and the characterization of its responses are presented. The electric field Helmholtz equation in the frequency domain is discretized by the Galerkin weighted residual method. The secondary electric field is the unknown to be solved for, discretized using edge-element basis functions. The edge-element basis functions are also used as the weight functions in the Galerkin approach. The solution of the discretized system is then obtained by a direct matrix-equation solver. To verify the effectiveness of the forward-modeling method and the accuracy of the corresponding code for computing the EM fields and reproducing their physical behavior, the numerical solutions computed using the presented method are compared against the equivalent solutions calculated from other techniques for a 3-D conductivity model. The good agreement between the various numerical methods and codes for this model demonstrates the accuracy of the responses computed by the method presented here. The method is then used to analyze the characteristics of GAFEM responses by comparing the magnetic fields and transfer functions (i.e., the magnetic field components Hx, Hy and Hz, and the transfer functions Tzx = Hz / Hx and Tzy = Hz / Hy) for different source-receiver configurations, altitudes of the observations and frequencies for several 3-D earth models. The aim is to improve the understanding of GAFEM responses and to determine the preferred measurement components and survey parameters. The responses at different frequencies of a complex model that includes topography are also investigated to study the characteristics of GAFEM responses of realistic earth models.