Topographic effects in frequency-domain helicopter-borne electromagnetics

Abstract

Many helicopter-borne electromagnetic (HEM) surveys have been carried out in mountainous areas. However, there have been few studies of the effects of topography on the HEM response reported in the literature. We simulate the response to topography using a staggered-grid finite-difference method. Modelling shows that a hill produces a high-resistivity anomaly over its top, and a low-resistivity anomaly over its foot, when the magnetic field response is transformed into the apparent resistivity. In order to reduce these effects, a simple correction procedure is presented and tested on synthetic data. Results indicate that the corrected data do not reproduce the effects of the actual resistivity structure accurately enough to permit quantitative interpretation assuming a flat-earth model. The reason for this is that the geometrical relationship between the coil system and the subsurface structure changes. The most rigorous and accurate approach to interpreting HEM data with topographic effects is to incorporate a forward-solution scheme, capable of modelling topography, into inversions. A 3D inversion method is successfully tested on synthetic data.