Four dimensional visualization of EM fields for a helicopter EM system

Abstract

Three different types of models are considered: an isotropic, homogeneous half-space, an anisotropic half-space, and a 3D earth. For this example, we consider only the horizontal coplanar (HCP) coil configuration of the Fugro HEM systems. The numerical results and the animation show that the EM field travels more slowly in the earth than in the air, and with greater attenuation. The EM amplitude decays and the phase shifts both with the horizontal distance from the transmitter and with vertical depth (as well as time). In an isotropic earth, the current flow for an HCP transmitter is horizontal; in an anisotropic earth it is dipping and focusing in the conductive direction, offering the possibility to identify the anisotropic character of the earth. The EM distribution for a dipping, conductive layer clearly distinguishes the current channeling in the conductive structure. In comparison to the current distribution, the magnetic field is less sensitive to the inhomogeneous resistivity distribution in the earth (e.g., layering, anisotropy, or 3D structures). By introducing the sliced contours into the EM vector presentation, the “smoke ring” effect of EM diffusion is clearly displayed. From the animated presentations in this paper, we also observe the continuous polarity change of the EM field, as it propagates through the earth.