New methods for interpretation of magnetic vector and gradient tensor data I: eigenvector analysis and the normalised source strength
David A. Clark
+ Author Affiliations
- Author Affiliations
CSIRO Materials Science and Engineering, and CSIRO Earth Science and Resource Engineering, PO Box 218, Lindfield, NSW 2070, Australia. Email: David.Clark@csiro.au
Exploration Geophysics 43(4) 267-282 https://doi.org/10.1071/EG12020
Submitted: 6 April 2012 Accepted: 3 August 2012 Published: 27 September 2012
Abstract
Acquisition of magnetic gradient tensor data is likely to become routine in the near future. New methods for inverting gradient tensor surveys to obtain source parameters have been developed for several elementary, but useful, models. These include point dipole (sphere), vertical line of dipoles (narrow vertical pipe), line of dipoles (horizontal cylinder), thin dipping sheet, and contact models. A key simplification is the use of eigenvalues and associated eigenvectors of the tensor. The normalised source strength (NSS), calculated from the eigenvalues, is a particularly useful rotational invariant that peaks directly over 3D compact sources, 2D compact sources, thin sheets and contacts, and is independent of magnetisation direction. In combination the NSS and its vector gradient determine source locations uniquely. NSS analysis can be extended to other useful models, such as vertical pipes, by calculating eigenvalues of the vertical derivative of the gradient tensor. Inversion based on the vector gradient of the NSS over the Tallawang magnetite deposit obtained good agreement between the inferred geometry of the tabular magnetite skarn body and drill hole intersections. Besides the geological applications, the algorithms for the dipole model are readily applicable to the detection, location and characterisation (DLC) of magnetic objects, such as naval mines, unexploded ordnance, shipwrecks, archaeological artefacts, and buried drums.
Key words: dipole localisation, eigenvalues, eigenvectors, magnetic field vector, magnetic gradient tensor, magnetisation, normalised source strength.
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