ACT tech talk: What we can and cannot know from unconstrained inversion of regional magnetic field data

Title: What we can and cannot know from unconstrained inversion of regional magnetic field data

Presenter: Clive Foss / CSIRO Mineral Resources

Date and time: 4pm (AEST time), 27th Sep 2023

Registration: https://us02web.zoom.us/webinar/register/WN_4o61GZALT9e7daAhyQKDzg#/registration

Abstract:

For many years regional magnetic field data acquired by Geoscience Australia and State and Territory geological surveys has enabled and transformed geological mapping across Australia where many areas are beneath extensive cover and/or pervasive deep weathering. As computing power and availability have increased by orders of magnitude the same data that so successfully supports geological mapping is being re-purposed for building continuous three-dimensional magnetisation models. These models are in some cases accepted by their users in confidence that they are true representations of the subsurface achieved by spectacular powers of computing. However, while aeromagnetic surveys perform extremely well in mapping the horizontal locations and extents of magnetisations, recovery of models of subsurface magnetisation is severely restricted by extensive non-uniqueness. Magnetic field data is so useful for geological mapping because of the dominant expression of shallow magnetisations, in many cases directly beneath a basement unconformity. The sharp curvature of these field variations carries all the reliable information in the magnetic field data. Deeper magnetisations may cause the bulk of amplitude changes in the magnetic field without giving rise to diagnostic curvature of the field. These parts of the magnetisation cannot be reliably assigned to a specific depth or depth range. In space-filling voxel inversions this task is achieved by depth-weighting functions included in the inversion algorithms. It is these functions, not the distribution of magnetisation in the ground, that determine the depth distribution of magnetisation in the models.

I propose separation of features of sharp curvature that carry the most reliable source information (that I term ‘sweet spots’) from the remaining, much less informative field variations. This results in subsurface models that are much sparser in their apparent level of detail. It may seem a negative message, but it is not, because the distilled information can be treated with much higher confidence than continuous models in which it is not clear which aspects can be trusted and which cannot. I use examples of Australian regional magnetic field data to demonstrate analysis and interpretation of sweet-spots suitable for estimation of depth to magnetisation and sweet-spots suitable for estimation of magnetisation direction.      

Biography

Clive is a senior principal research scientist in CSIRO Mineral Resources where he works mostly on magnetic field inversion and interpretation. He has a BSc and PhD from two Earth Science departments where he learnt to integrate studies of physics and geology. His particular interest since his PhD (a long time ago) on Archean rocks of Southern Africa is in the magnetic field expression of remanent magnetisation and how direction of magnetisation can be recovered from magnetic field data. After his PhD Clive taught exploration geophysics at the University of Malaya in Kuala Lumpur where living between the geographic and magnetic equators provoked an interest in low inclination magnetic fields. Clive then moved to Bandung to work for the Bureau of Mineral Resources (BMR, now Geoscience Australia) on the AIDAB funded Indonesia – Australia Geological Mapping Project in Kalimantan. Following that Clive returned to Kuala Lumpur to work as a consultant and for ARK Geophysics based in Kuala Lumpur providing services in gravity and magnetic methods for petroleum companies throughout Southeast Asia. In 1995 Clive moved to Sydney, Australia to work with Encom Technology both contributing to the ModelVision development team and acting as senior consultant on gravity and magnetic projects worldwide. In 2009 Clive moved to his present position in CSIRO.