IPRAMA: INDUCED POLARIZATION FOR RAW MATERIALS
As the global economy transitions to cleaner energy, the ability to accurately map mineral deposits has become a key challenge to achieve decarbonization goals. The development of innovative and non-invasive exploration methods is essential to ensure the sustainable exploitation of natural resources, minimizing environmental impact and respecting local communities. Among the useful indirect techniques that can be applied to describe the ground physical properties, this thesis focuses on the Airborne Induced Polarization (AIP) method. Electromagnetic (EM) methods—both airborne (AEM) and ground-based—are known to be sensitive to induced polarization (IP) effects when surveying chargeable grounds. Although the relevance of IP phenomena in EM data has been demonstrated in numerous studies, these effects are frequently overlooked in standard exploration workflows. This is largely due to the challenges associated with recognizing IP signatures in EM data space and the complexities involved in incorporating them into inversion frameworks. The modelling of IP effects in EM data often leads to non-unique solutions which increase the model ambiguity and decrease its interpretability: the relationship to both geology and galvanic chargeability become unclear, and the complexity of IP modelling is not repaid. This thesis investigates various aspects of airborne induced polarization, addressing both methodological challenges and applied case studies across multiple spatial scales. These include a continental-scale study using fixed-wing AEM systems in Australia, a mine-scale investigation in Spain, and a greenfield exploration program in Portugal. In the latter, AIP-derived anomalies identified from airborne data informed the design and execution of targeted ground-based follow-up surveys. Together, these case studies illustrate the potential of AIP methods to enhance subsurface characterization in chargeable terrains and contribute to more effective mineral exploration strategies. The first study is developed to assess the TEMPESTTM fixed-wing sensitivity to IP effects, first on synthetics and then on real data. This system is currently flying the entire Australia to map the Country’s ground resistivity and being able to model also its chargeability would add an important layer of information to geological description and mineral targeting. We first theoretically demonstrate the system sensitivity to AIP effects by computing and mapping one-hundredth-thousand forward responses to define the parametric range of sensitivity to AIP of the method. Then, moving to real data, the synthetic results have been confirmed comparing the modelling results with other helicopter-borne platforms and inverting a portion of 20,000-line km of Australia with ground-proved consistency of the retrieved chargeability models with geology. The application scale is then reduced, from continental to deposit-scale, and a comparison between ground galvanic Direct Current Induced Polarization (DCIP) and airborne inductive chargeability is carried. The comparison has been done within an active mining project in Spain where a helicopter-borne EM survey overlaps 17 ground DICP profiles. These have been first independently modelled, proposing a multi-mesh approach for AEM data and a consistent modelling for ground DCIP, showing a good comparability between the retrieved models. Then, to uniquely solve the ground chargeability through the merging of the sensitivities of the methods, a joint inversion considering the IP effects is carried between the two and a unified chargeability model is obtained. Through the joint modelling a shared field of sensitivity with the galvanic method is shown and the AIP capability to resolve the ground chargeability consistently with the ground methodology has been demonstrated. Finally, an AIP survey for a greenfield exploration project in Portugal has been designed to define chargeable targets on which follow-up on the ground. Here, after a feasibility study, two AEM surveys have been flown with different base frequencies, 25 Hz and 12.5 Hz, to improve the near surface resolution and the IP effects recording, respectively . After an independent and joint modelling of the two considering the IP effects, the ground DCIP follow-up has been defined and acquired over an airborne-defined anomaly. The ground results confirmed the airborne model and the approach, from the feasibility to the integration, has been effective. The airborne and galvanic datasets have then been jointly modelled, merging two different inductive base frequency surveys and a three decades DCIP dataset to resolve the ground resistivity and chargeability. From a general point of view, this work shows that the chargeability models retrieved from different sources (inductive/galvanic) and acquired at different scales is consistent when properly treated. As implication, a regional/continental-scale chargeability mapping can be a valuable tool to define where to develop a next exploration step with a proved physical reliability and with consistency with the galvanic method.
Airborne Induced Polarization, Airborne Electromagnetic, Joint inversion, Mineral Exploration, Critical Raw Material, Green Transition, Settore GEOS-04/B - Geofisica applicata
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