3D inversion of ZTEM data for uranium exploration

Abstract

We present a Gauss-Newton-based 3D inversion method for airborne ZTEM (Z-axis Tipper Electromagnetic) data to define resistivity structure relating to uranium deposits in Athabasca Basin of Saskatchewan, Canada. The geophysical targets in this region can be represented by conductive plunging dykes in a resistive basement beneath a thick, more resistive overburden. We demonstrate using synthetic examples the effectiveness of the inversion method for detecting and delineating the target dykes and discuss how the inversion results are affected by various factors. It is shown that the dykes can be well imaged to depths more than 2 km even for the data from 200-m receiver height, provided the flight line is oriented perpendicular to the strike, and that the inversion results are relatively robust to the choice of the starting model. It is also shown that topographic effects are not serious for detecting the dykes at depth, because topographic effects are more significant at higher frequencies, while the sensitivity to the dykes increases with decreasing frequencies. One important finding is that if the flight line is oblique to the strike, the dependence of the starting model increases and the overall resolution decreases, compared to the 2D case, due to 3D effects.