April Technical Talk

BCGS April Technical Talk – Thursday April 17th, 2014

Speaker: Peter Fullagar, Fullagar Geophysics

Title: 3D magnetic modelling and inversion incorporating self-demagnetisation and interactions

Date/Time: Thursday April 17th, 2014 at 4:30pm.

Location: Room 451, 409 Granville St (UK Building at Granville and Hastings), Vancouver

Abstract:

Self-demagnetisation can significantly reduce the amplitude and modify the shape of the response from highly magnetic bodies. The direction of magnetisation rotates in a manner determined by the shape and orientation of the body. Furthermore, when highly magnetic bodies are in close proximity, the magnetisation induced in one body is affected by the magnetisations in all the others. When modelling highly magnetised bodies, it is important to take both self-demagnetisation and interactions into account. Inverting for magnetisation vector has become popular recently. However, this is not a substitute for physically valid magnetic modelling. Magnetisation inversion is highly nonunique, with the result that any particular solution must be interpreted with care. An inverted magnetisation rotated from the ambient field is not necessarily indicative of remanence. Moreover, relating an inverted in situ magnetisation to rock properties is often problematic. In this respect, magnetisation inversion complicates ground truthing. Potential field modelling and inversion software “VPmg” has been upgraded to account for self-demagnetisation within, and interaction between, 3D magnetic bodies. Remanence can be taken into account. The algorithm computes H-field perturbations at the model cell centres in two stages: initialisation and optimisation. During initialisation, a first estimate for the H-field perturbation is derived from the demagnetisation tensor computed for each cell.

During optimisation, the H-field perturbation is refined iteratively via an inversion procedure. The algorithm has been validated for homogeneous spheres, spheroids, slabs, and cylinders. It has also reproduced magnetic interactions between two horizontal cylinders, published by Hjelt (1973). Explicit verification for complex heterogeneous bodies requires a suitable independent algorithm for benchmarking. The application to inversion in highly magnetic environments is illustrated on field data examples.

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