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UBC Theses and Dissertations

Cooperative and parametric strategies for 3D electromagnetic inversion McMillan, Michael S. G.


Airborne electromagnetic (AEM) data is commonly collected for detecting buried natural resources, and this technique is sensitive to subsurface electrical resistivity distributions. The subsequent process of 3D AEM inversion constructs a physical property model from this data in order to better understand the size and shape of potential hidden resources. This thesis is designed to develop practical strategies to improve 3D AEM inversion accuracy in geologic settings where AEM data sets produce inconsistent or unsatisfactory inversion results. In this research, two overarching problematic scenarios are examined. First, in regions where an AEM survey overlaps with other electromagnetic data sets, a novel cooperative approach is introduced. This method is first tested on synthetic data where instead of producing an inversion model from each data set, the cooperative algorithm finds one consistent 3D resistivity model with improved resolution. The approach is then applied to field data over a high-sulfidation epithermal gold deposit where similar improvements are observed. The second scenario relates to improving 3D AEM inversions over thin conductive anomalies, a common geophysical target for copper and gold deposits. A new parametric inversion is developed using skewed Gaussian ellipsoids to represent target bodies. The approach is general but applied to frequency and time-domain AEM data with one or multiple anomalies. Combined with a voxel algorithm, the parametric inversion forms a hybrid approach capable of resolving thin conductive targets with smooth surrounding features. This hybrid technique is tested on synthetic data over conductive targets in a resistive background, and consistently produces models that are easier to interpret compared to voxel inversions alone. Field examples from a volcanogenic massive sulfide and an orogenic gold deposit highlight the practical nature of this method to image conductive mineralization with increased precision. The thesis concludes by analyzing a setting where multiple spatially overlapping AEM data sets exist over narrow conductive anomalies. Here, parametric, cooperative and voxel inversions are applied together to generate a consistent 3D resistivity model with thin targets and smooth background features. This section includes a discussion about potential pitfalls of such an approach when incompatible field measurements are encountered.

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