The Effect of Topography on ZTEM Inversions

ZTEM is an airborne passive source electromagnetic system surveying in frequency domain (30 Hz - 720 Hz). In this paper we study the effect of topography on 2D and 3D ZTEM modelling and inversion results. In the synthetic study, the 3D model is introduced and the predicted data are calculated considering the 3D topographic feature (hill) on the Earth’s surface. Then the predicted data are inverted using MTZ3D (developed at Geophysical Inversion Facility, University of British Columbia) with and without topography consideration. The inversion results are compared to the true model. In the field study, ZTEM observed data are compared to data predicted for 100 Ohm m halfspace with considerable topography. The modeling is done in 2D. The 2D response over uniform half-space should be zero, however due to the presence of the topography the amplitude of the predicted half-space response becomes dominant, compared to the response of conductors. It is further shown that when topographic features are neglected, the inverse models recover false geoelectrical structures. These false electrical structures include near-surface electrical conductors consistent with the positive topographic features (hills) and electrical resistors, consistent with the depressions. The effect is increasingly notable with greater volume of the topographic inhomogeneity.