Variable-depth magnetization mapping; application to the Athabasca Basin, northern Alberta and Saskatchewan, Canada

Total magnetic-field data, from the Athabasca basin of northern Saskatchewan and Alberta, Canada, have been inverted to produce a magnetization map of the sub-Athabasca crystalline basement. Since the basement topography is variable, the problem is nonlinear and an extra degree of freedom in the solution is caused by the existence of a distribution of magnetization (the annihilator) that produces no external magnetic field. I outline an iterative frequency-domain inversion scheme, which is based on an approximation to the true partial derivative matrix for the linearized problem. This approximation causes each iteration to be equivalent to a simple frequency-domain deconvolution.Modeling of selected anomalies allows determination of the magnetization at a number of points in the study area. These values are then used to determine the amount of annihilator to be added to the general solution found from the inversion. The procedure automatically corrects for the effects of variable attenuation of anomalies due to changes in basement depth. Thus, magnetization units and geology that are correlated in areas of outcrop can be extended beneath the sedimentary cover to provide improved geologic mapping control.