TIME‐DOMAIN ELECTROMAGNETIC DIFFUSION AND IMAGING DEPTH FOR AIRBORNE ELECTROMAGNETIC DATA

To study the electromagnetic (EM) diffusion of a time-domain airborne electromagnetic (AEM) system, we first calculate the frequency-domain EM field inside the earth based on the continuity boundary condition and then transform it into time-domain via a Fourier transform. We calculate the EM fields induced by a step pulse for a homogeneous half-space and a two-layer earth model and display the EM diffusion in the earth as 2D vectors or 3D animated time-varying contours (like the “smoke ring”). These time-domain current rings display the true EM diffusion, because they demonstrate the spatial variation and decay of underground EM fields with time. The study of the EM current rings shows that the EM diffusion is strongly influenced by the resistivity structure inside the earth. In a conductive earth, the current ring diffuses slowly but decays fast, while in a resistive earth it diffuses fast but decays slowly. The induced current for a vertical magnetic dipole forms a single current ring that propagates with time outward and downward, while for a horizontal magnetic dipole, the induced current forms two stacked current rings in the underground, diffusing with time into the deep earth. Based on the EM current ring, we find that the imaging depth for AEM data is about 0.55 times the EM diffusion depth. By showing the underground induced current as vectors and 3D time-varying animated contours, the EM diffusion and smoke ring are clearly observed. An animated EM smoke ring offers more information than a static contour of current density. The relation between imaging depth and diffusion depth is further confirmed by the smoke rings. Study on EM smoke ring can not only offer an insight into EM diffusion in the earth, but can also assist in airborne EM data interpretation.