Modelling of airborne Full Tensor Magnetic Gradiometry using data from the INFACT project

The measurement of the magnetic field has been a “backbone” geophysical method in mineral exploration since the 17th century. The existing instrumentation that measures Total Magnetic field Intensity (TMI) are a routinely used in ground, borehole and airborne surveys. In the TMI intensity data it is possible to observe certain signatures of magnetised objects, but retrieval of both magnetisation intensity and shape of 3-D magnetised objects from TMI can be difficult due to the vector nature of magnetisation and fundamental non-uniqueness of potential fields interpretation. Moreover, the presence of magnetic material in the host rock and/or presence of remanent magnetisation are challenges for TMI data interpretation.

Full Tensor Magnetic Gradiometry (FTMG) measurements provide the complete description of the magnetic field and hence an opportunity to get more information on the size, shape and material property of the magnetic rock mass. This is because the signatures in magnetic field originating from a specific magnetic object is observed in all independent components of magnetic field gradient tensor and thus, joint analysis of these tensor components constrains the number of possible magnetic models that fit the same data. In addition, observing the full tensor of magnetic field makes it possible to estimate the remanent magnetization with respect to the induced magnetization field if no a-priori information of remanent magnetization is available.

Highly sensitive magnetometers based on SQUID (Superconducting QUantum Interference Devices) technology has been successfully adopted in FTMG airborne measurements during the past decade. This achievement has given magnetic methods a new opportunity in terms of purely magnetic data modelling. In our work the benefits of interpretation of tensor airborne FTMG data are demonstrated through forward modelling and inversion with the grid search multiobjective global optimisation. As a case study, we consider airborne FTMG data measured with Supracon® JESSY STAR system in Northern Finland during the INFACT project.

Acknowledgements: This study has been done in the framework of EU Horizon 2020 funded INFACT project (webpage: https://www.infactproject.eu).