Since the responses in frequency-domain helicopter-borne electromagnetic (FDHEM) are affected by topographic variations, especially in mountainous terrains, few number of improvements have taken into account certain surface elevation variations along the survey to damp EM response perturbations caused by the surface geometry. We simulate the response to topography using a 2.5-D finite element method. Modelling illustrates a hill produces a high-resistivity anomaly, while the foot where the slope inclination in the hill changes leads to a low-resistivity anomaly; conversely, it seems that low-resistivity anomaly is a natural response that is elicited from the data are obtained over the valley. Moreover, valley sides causes high-resistivity anomaly. It is therefore necessary to include topography in the modelling and inversion procedure to recover resistivity model with true surface features of earth. The example of a topographical field FDHEM data inversion shows the efficacy of the presented approach to appraise the subsurface resistivity structure.