| Description |
The induced polarization (IP) phenomenon has been observed in numerous geologic structures, typically mineralized ore bodies. In polarizable structures a relationship exists between applied frequency and observed conductivity. In particular, conductivity can be observed to increase as applied frequency increases. This relationship is readily identifiable in IP data; however, the relationship may be present in any frequency domain measurement. Additionally the relationship may be unique for different mineralized geologic structures and may provide information to identify the type of structure being surveyed. The spectral IP method is a powerful tool for mineral exploration. Using the spectral IP method, variations in frequency-dependent complex conductivity, which correspond with different mineral targets, have been observed. Until recently, this method was applied as a ground based geophysical survey method only. However, frequency-domain airborne electromagnetic (FDAEM) measurements may contain information similar to that found in a ground-based IP survey, namely variations in complex conductivity, i.e., conductivity with both real and imaginary value. In this thesis, we examine the possibility of inverting for spectral resistivity in helicopter electromagnetic data. Our study is based both on forward modeling and inversion of synthetic FDAEM data for models with both complex frequency-dependent conductivity and models without. Comparison of electromagnetic synthetic data suggests a difference exists between synthetic modeled data with frequency-dependent conductivity and synthetic data with frequency-independent conductivity. Inversion of the synthetic data suggests recovery of the frequency-dependent variations complex conductivity parameters in the synthetic models is possible. The differences in conductivity are sensitive not only to the range of FDAEM measurements, but also to synthetic ground-based measurements. A numerical modeling study demonstrates the possibility of developing an airborne version of the spectral IP method. The results of the synthetic model study suggest the technique of solving for complex conductivity within the inversion domain may be applied to field data. In this thesis, we apply complex conductivity inversion to two different field collected frequency domain helicopter electromagnetic (FDHEM) data sets. The data are collected over a kimberlite pipe and over an orogenic gold target. Both are conductive targets and therefore make excellent targets for conductivity inversion. Variations in frequency-dependent complex conductivity behavior in both inversion targets are observed. The inversion results for both targets suggest recovery of frequency-dependent complex conductivity is possible in real, as opposed to synthetic, data. v |
