Evaluation of terrain effects in AEM surveys using the boundary element method

Abstract

In mountainous areas, electromagnetic terrain effects are readily observed in the course of VLF (14–20 kHz) measurements made on the surface and constitute a serious source of geological noise that affects the collected data. One may, therefore, inquire whether similar effects will be observed during the course of conventional helicopter‐towed electromagnetic (HEM) surveys as the frequency of the newer systems is increased beyond the lower regions of the audio range. To answer the question, we have evaluated the terrain effects that would be observed with a conventional HEM system in a number of simple cases. The operating frequency chosen for most of the numerical simulations was 8 kHz, while the topographic features investigated were taken to be two‐dimensional. The calculations were done using the boundary element method of solving the appropriate integral equations. Accuracy of the numerical solutions was shown to vary from 1 percent for a half space to 10 percent for a shallow valley where the verification was done on a laboratory scale model. For the models investigated, the amplitude of the computed secondary fields shows a distinct correlation with the overflown topography. Surprisingly, however, the phase of the secondary field remains invariant and so may be reliably used to compute the resistivity of the terrain below the aircraft.