Locating geologic contacts with magnitude transforms of magnetic data

Abstract

Locating geologic contacts by mapping lateral magnetization changes provides a variety of useful geologic information. Magnetic contacts will often correspond to lithologic boundaries, while the shapes and textures of contact maps can also indicate rock type and the style of deformation present. Well-established methods for contact mapping are the horizontal gradient magnitude of the pseudogravity (PSG-hgm) and the total field (TF-hgm). More recent mappers include the analytic signal (AS), the 3-D local wavenumber (LW) and the horizontal gradient magnitude of the tilt (TI-hgm). PSG-hgm and TF-hgm generally produce coherent contact trends but the locations are dependent on the contact dip and its magnetization direction. The AS, LW and TI-hgm methods produce contact locations that are independent of dip and magnetization. Magnitude transforms of the anomalous magnetic field offer an alternative approach to contact mapping since only the field components or the field component derivatives are used, in contrast with other methods that require total field derivatives, possibly up to second order. The magnitude transforms are also dip- and magnetization-independent. Contact maps determined for the magnitude transforms are compared with those from the PSG-hgm and TF-hgm methods for a recently-flown survey over a poorly-exposed Precambrian Shield area in Manitoba, Canada. The T transform map (magnitude of the anomalous magnetic field) is similar to that from PSG-hgm. The other magnitude transforms perform less well because of sensitivity to noise and gridding artefacts, resulting in less coherent trends. None of the transforms are able to resolve the same detail that TF-hgm does. Contact maps from magnitude transforms are therefore only to be preferred over PSG-hgm and TF-hgm maps in areas of high remanent magnetization and gently dipping geology.